IZDAJAJO ŽELEZARNA JESENICE. RAVNE, ŠTORE IN INŠTITUT ZA KOVINSKE MATERIALE IN TEHNOLOGIJE LJUBLJANA REVIJA JE PREJ IZHAJALA POD NASLOVOM ŽELEZARSKI ZBORNIK Navodila avtorjem za pripravo člankov za objavo v reviji Kovine, zlitine, tehnologije V letu 1992 uvajamo nov način tehničnega urejanja in priprave za lisk revije Kovine, zlitine, tehnologije. Da bi pocenili tiskarske stroške, skrajšali čas od prejema članka do njegove objave in prepustili avtorju končno odgovornost za morebitne neodkrite tipografske napake, smo se v uredništvu odločili, da izkoristimo možnosti, ki jih danes nudi namizno založništvo. Za oblikovanje in pisanje člankov smo izbrali TgX oziroma IAT]?X sistem, ki je za pisanje tehničnih člankov in knjig v svetu najbolj razširjen. TgX oziroma IATjt;X oblikovalnik besedil je izdelan za skoraj vse vrste računalnikov, od IBM PC kompatibilnih računalnikov, Apple Macintosh računalnikov, Atarijev, pa do velikih računalnikov. Besedila, oblikovana v IM j.-N-u, so enostavno prenosljiva, saj imajo obliko ASCII zapisa. Kodiranje naših šumnikov je enotno rešeno, tako da lahko pošljete članek, napisan v IAT]?X-u, kamorkoli po svetu, pa z njimi ne bo težav. Zato naprošamo avtorje, če je le mogoče, da napišejo svoje članke z IATpX oblikovalnikom besedil, sicer pa naj nam poleg besedila na papirju pošljejo vsaj disketo z. običajnim ASCII zapisom besedila brez kakršnih koli drugih ukazov za formatiranje. Vsebina članka Kako naj članek izgleda vsebinsko, naj si avtorji ogledajo v starih izdajah Železarskega zbornika. Vsak članek pa mora vsebovati: • slovenski in angleški naslov članka. • imena ler naslove avtorjev, • povzetka v angleščini in slovenščini, • reference, ki naj bodo v besedilu članka označene z zaporednimi številkami, primer1-5. Način citiranja članka: avtor, inicialkam naj sledi priimek, naslov članka, ime revije, letnik, strani, leto. Način citiranja knjige: avtor, naslov, založnik in kraj izdaje, lelo, po potrebi poglavje ali strani. Besedilo članka naj bo razdeljeno na razdelke (označene z zaporednimi številkami) in po potrebi še na pod-razdelke (označene z decimalno številko, kjer celi del označuje razdelek. Slike Vse slike naj bodo na posebnih listih papirja, z jasno označeno številko slike. Slike naj bodo označene z zaporednimi številkami povsod v članku. Originali za vse vrste slik naj bodo ostri in brez šuma. Risbe naj bodo narisane s črnim na belem ozadju. Vse oznake in besedila na risbah naj bodo v istem jeziku kot besedilo članka in dovolj velike, da omogočajo po-manjšanje slike na 8 cm. Le izjemoma lahko slika sega čez obe koloni besedila (16.5 cm). Fotografije so lahko katerekoli običajne dimenzije, na svetlečem papirju in z dobrim kontrastom. Mikroskopska in makroskopska povečevanja označite v podpisu na sliki, še bolje pa z vrisanjem ustrezne skale na fotografiji. Za vsako sliko naj avtor predvidi, kam naj se slika v besedilu članka uvrsti, kjer naj se nahaja ustrezen podnapis z zaporedno številko slike (na primer: ''Slika 3 prikazuje...", nikakor pa ne: "Na spodnji sliki vidimo..."). Tabele Avtor naj se izogiba zapletenih tabel z mnogo podatki, ki bralca ne zanimajo, posebej še, če so isti podatki tudi grafično ponazorjeni. Nad vsako tabelo naj se nahaja zaporedna števila tabele s pojasnilom. Tabele naj bodo povsod v članku označene z zaporednimi številkami. Pisanje besedil na računalniku Avtorje naprošamo, da pri pisanju besedil na računalniku upoštevajo naslednja navodila, saj le-ta precej olajšajo naše nadaljnje delo pri pripravi za tisk: • ne puščajte praznega prostora pred ločili (pikami, vejicami, dvopičji) in za predklepaji oziroma pred zaklepaji, • puščajte prazen prostor za vsemi ločili (pikami, vejicami, dvopičji)—razen decimalno piko, • pišite vse naslove in besede z majhnimi črkami (razen velikih začetnic in kratic), • besedilo naj ne vsebuje deljenih besed na koncu vrstice. Če avtor pripravlja ilustracije na računalniku, ga naprošamo, da priloži datoteke s slikami na disketo z besedilom članka, s pojasnilom, s katerim programom so narejene. Pisanje v I^Tj?X-u Uporabljajte article style. sicer pa se držite vseh IATEX konvencij. Vse matematične izraze, imena spremenljivk in podobno (razen SI enot) pišite v matematičnem okolju. Uporabljajte že vgrajene fonte. med pripravo za tisk jih bomo zamenjali z ustreznimi PostScript fonti. Krtačni odtis Krtačni odtis—končna podoba članka—bo poslan avtorju v končno revizijo. Avtorja naprošamo, da čim hitreje opravi korekture in ga pošlje nazaj na uredništvo. Hkrati naprošamo avtorje, da popravljajo samo napake, ki so nastale med stavljenjem članka. Če avtor popravljenega članka ne vrne pravočasno, bo objavljen nepopravljen, kar bo tudi označeno. Uredništvo KOVINE ZLITINE TEHNOLOGIJE Izdajajo (Published by): Železarna Jesenice, Železarna Ravne, Železarna Štore in Inštitut za kovinske materiale in tehnologije Ljubljana Izdajanje KOVINE ZLITINE TEHNOLOGIJE delno sofinancira: Ministrstvo za znanost in tehnologijo UREDNIŠTVO (EDITORIAL STAFF) Glavni in odgovorni urednik (Editor): Jožef Arh, dipl. ing. Uredniški odbor (Associate Editors): dr. Aleksander Kveder, dipl. ing., dr. Jože Rodič, dipl. ing., prof. dr. Andrej Paulin, dipl. ing., dr. Monika Jenko, dipl. ing., dr. Ferdo Grešovnik, dipl. ing., Franc Mlakar, dipl. ing., dr. Karel Kuzman, dipl. ing., Jana Jamar Tehnični urednik (Production editor): Jana Jamar Lektorji (Lectors): Cvetka Martinčič, Jana Jamar Prevodi (Translations): prof. dr. Andrej Paulin, dipl. ing., dr. Nijaz Smajič, dipl. ing. (angleški jezik), Jožef Arh, dipl. ing. (nemški jezik) NASLOV UREDNIŠTVA (EDITORIAL ADRESS): KOVINE ZLITINE TEHNOLOGIJE, Železarna Jesenice d.o.o., 64270 Jesenice, Slovenija Telefon: (064) 81 441 Telex: 37 219 Telefax: (064) 83 397 Žiro račun: 51530-601-25734 Stavek in prelom: Igor Erjavc, Tisk: Gorenjski tisk, Kranj, Oblikovanje ovitka: Ignac Kofol Fotografija na ovitku: Franci Sluga IZDAJATELJSKI SVET (EDITORIAL ADVISORY BOARD): Predsednik: prof. dr. Marin Gabrovšek, dipl. ing.; člani: dr. Božidar Brudar, dipl. ing., prof. dr. Vincenc Čižman, dipl. ing., prof. dr. D. Drobnjak, dipl. ing., prof. dr. Blaženko Koroušič, dipl. ing., prof. dr. Ladislav Kosec, dipl. ing., prof. dr. Josip Krajcar, dipl. ing., prof. dr. Alojz Križman, dipl. ing., dr. Karel Kuzman, dipl. ing., dr. Aleksander Kveder, dipl. ing., prof. dr. Andrej Paulin, dipl. ing., prof. dr. Z. Pašalič, dipl. ing., prof. dr. Ciril Pelhan, dipl. ing., prof. dr. Viktor Prosenc, dipl. ing., prof. dr. Boris Sicherl, dipl. ing., dr. Nijaz Smajič, dipl. ing., prof. dr. J. Sušnik, dr. Leopold Vehovar, dipl. ing., prof. dr. Franc Vodopivec, dipl. ing. Po mnenju Ministrstva za znanost in tehnologijo Republike Slovenije št. 23-335-92 z dne 09. 06. 1992 šteje KOVINE ZLITINE TEHNOLOGIJE med proizvode, za katere se plačuje 5-odstotni davek od prometa proizvodov. Contents Vsebina N. Smajič: Computer Simulation and Optimization of VOD Treatment Računalniška simulacija in optimiranje VOD obdelave................... 313 F. Vodopivec: Microafloying of Steel Mikrolegiranje jekla..................................... 319 J. Črnko: The Dependence of the Heat Energy Consumption upon the Working Intensity and the Frequency of the Isolation Maintenance of a Pusher-type Furnace Ovisnost utroška toplinske energije od intenziteta rada i učestalosti održavanja izolacije potisne peči ......................................... 329 B. Arzenšek, B. Šuštaršič, G. Velikajne, I. Kos, K. Zalesnik, F. Marolt: Tool-steel Wire Dravving at Elevated Temperatures Vlečenje žice iz orodnih jekel pri povišanih temperaturah................. 333 P.D. Odesskij, N. Kudajbergenov, L. Kosec, F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation Odpornost gradbenih jekel proti nastanku in širjenju razpoke............... 337 M. Gojič, M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina: Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti................... 343 M. Jenko, A. Kveder, S. Spruk, L. Koller: Chromizing of Iron Difuzijsko kromanje železa ................................. 351 Letno kazalo......................................... 357 Computer Simulation and Optimization of VOD Treatment Računalniška simulacija in optimiranje VOD obdelave N. Smajič, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana Mathematical model and the software CAPSS (Computer Aided Produc tion of Stainless Steel) developed as a pad of URP-C2-2566 research program were used for computer simulation of EAF-VOD-CC stainless steelmaking technology line. Basic aim of model testing was to optimise VOD treatment with emphasis on obtaining the maximum productivity at the lowest possible thermal load of VOD ladle and EAF tap temperature. It was concluded that only computer controlled oxygen blowing can secure maximum productivity at the acceptable thermal load of VOD ladle and lowest EA furnace tap temperature. V okviru petletnega raziskovalnega programa URP-C2-2566 izdelani model in računalniški program CAPSS (Computer Aided Production of Stainless Steel) je bil uporabljen za računalniško simulacijo EOP-VOD-KL tehnologije za izdelavo nerjavnih jekel. Osnovni namen modelnih poskusov je bil optimiranje VOD obdelave s posebnim poudarkom na zagotavljanju maksimalne produktivnosti VOD naprave ob najmanjši možni toplotni obremenitvi VOD ponovce in minimalni temperaturi preboda. Ugotovili smo, da le računalniško programirano pihanje zagotavlja maksimalno produktivnost ob še sprejemljivi toplotni obremenitvi VOD ponovce in minimalni temperaturi preboda. 1 Introduction The production in metallurgical industries was for a very long time based dominantly on experience. However, the development of theory of metallurgical processes and metallurgical thermodynamics couplcd with small-size and yet povverful computers has made it possible to combine the experience with theoretical knovvledge, which can result in a signilicant improvement of operation perfomiance and raising techno-economic production parameters to an essen-tially higher lcvel. The application of computers in steelmaking has become indispensable due to high competition and ever increasing demands for lovver priče, better quality and narrower toler-ances of steel properties such as strength, ductility, hot and cold deformability, chemical composition, corrosion resistance, etc. The concept of TQC (Total Quality Control) which excludes any refuse is based on absolutely reliablc control of quality. At present TQC is obligatory for ali high priced products. Consequently, it is imperative also in the manufacture of stainless steel since the value of a heat of high quality stainless steel can amount up to 300,000 USS. Naturally, there is no room for any allowable refuse despite the fact that technological regulations at present rec-ognize allowable waste. Conventional technologial regulations deal vvith imaginary "average" heat and "normal" con-ditions. For that reason and because of great value, each heat of stainless steel must be regarded as unique i.e., it must be processed in a specific way taking into account actual initial state and conditions (chemical composition, temperature, VOD ladle state, pumping sy.stem state, etc.). There is no such thing as standard process parameter. Every de-viation of initial conditions from some imaginary standard can and must be compensated for by adequate adaptation of process parameters. Such a complex assignment can suc-cessfully be carried out only by computer aid. The work was canied out as a part of the projeet Stainless Steel vvhich is aimed at the reduetion of production costs, improvement of the quality of stainless steel, development of new high quality and extra clean e.g. superfer-ritic stainless steels and the optimization of EAF-VOD-CC tehnological line operation. The research was concentrated to vacuum oxygen de-carburization rate, which should be inereased as much as possible since VOD treatment is serious bottleneck of the technological line particularly when producing ELI (Ex-tra Low Interstitials) steel. In addition better control of melt temperature and lovver carbon, nitrogen and phospho-rus content especially in čase of extra clean steel must also be considered in any optimization of VOD treatment. 2 Computer simulation and model tests Main part of the research was carried out in the form of computer simulation of VOD treatment which is a key part of the technological line. The model tests performed by use of CAPSS (Computer Aided Production of Stainless Steel) vvere undertaken • to inerease productivity by shortening of VOD treatment, • to improve techno-economic production parameters and • to determine most rational and expedient measures to be taken in order to improve the present technology 2.1 CAPSS The PC oriented software CAPSS has been developed in Mathematical Modelling and Computer Simulation Department of IMT (Institute of Metals and Technologies) Ljubljana on the basis of a sophisticated mathematical model elaborated through extensive theoretical studies and investigations1-10. The model has been devised primarily for thermodynantical analysis of the Fe-Cr-C-Si-Mn-Al-Ti-O-N system in ntolten state which is essential in the production of stainless steel. CAPSS integrates thermodynamic principles, industrial experience, theory of metallurgical processes and expert knowledge required for the economic and massive production of high quality austenitic, ferritic and superferritic conventional, ELC and ELI (Extra Low Carbon, Interstitials) stainless steel. Thermodynamical data and published informations on VOD operation results were obtained from reference11-33. CAPSS vvas calibrated by a posteriori analysis of a number of heats which had been produced in steelvvorks Jesenice to determine some entpirical unnteasurable constants typical for the steelworks. CAPSS was developed for: • Off-line control of VOD (Vaeuum Oxygen Decarbur-ization) unit of EAF-VOD-CC technological line for the production of stainless steel - to reduce the operating costs - to inerease the productivity and - to secure the high quality of produced steel • Research and development purposes - to perform model tests by simulation of VOD operation in order to determine the effect of a change in process variables (oxygen blovving rate, oxygen and argon comsumption, lime ad-dition, etc.) or initial conditions (e. g. melt composition, temperature, etc.) on operation results (productivity, production costs, quality of steel produced, etc.) - to optimize production technology and - to develop new grades of stainless steel • Education and training purposes - for training of technical personnel in newly installed VOD units or steelvvorks and - for education of University students 2.2 Computer control ofVOD treatment Advantages of the computer control of VOD treatment are nunterous and evident. Firstly, intmediate and continuous inspeetion of the process of vaeuum oxidation and development of melt temperature. It offers the possibility to stop oxygen blovving at exactly right tirne i.e., at desired final carbon content. The progress and rate of oxidation are continuous^ monitored and controlled in order • to prevent melt temperature from exceeding allovvable upper limit, • to assure the lovvest heat loading of ladle lining and • to attain the maximum productivity and the lovvest pos-sible loss of chromium. Computer control is benelicial also for reduetion stage of VOD treatment since it • offers monitoring of the reduetion of chromium from slag and temperature changes, • assures exact calculalion of proper addition of lime and reducing means and • helps to synchronize the operation of whole technological line EAF-VOD-CC and especially of VOD unit and consequent continuous casting machine. For research and development purposes simulation of imag-inary or real but future heats knovvn also as model test is particularly useful. Similar model tests performed in the form of simulation of real past heats i.e., a posteriory anal-ysis is very instruetive in education and training of technical personnel. It can reveal errors and vvrong decisions made in the past during VOD treatment of a given heat. Simulation and analysis of previous heats can be extensive and include ali heats manufactured e.g. in the last month or year. Such an analysis is valuable for evaluation of the existent tech-nology. It can help to find out and determine oscillations of techno-economic parameters (productivity, specific con-sumption of energy and raw materials, etc.) due to instable technology, unsuitable technical regulations, poor perfor-mance of technical personnel or inadequate maintenance. Particularly valuable is analysis of extreme heats i.e., ex-ceedingly good and bad operation results. It can result in significant technological improvements and more suitable regulations. By ali means it is first and obligatory step to-vvard the introduetion of computer control of steelmaking. 3 Optimisation of VOD treatment bv computer simulation 3.1 ELI stainless steel When producing ELI e.g., superferritic stainless steel deni-trogenization of melt proceeds under deep vaeuum (approx. 100 Pa) at a high carbon content. Of course, such a vaeuum can be made only in especial and additional stage of vaeuum treatment vvithout oxygen blovving. There can be one, tvvo or even more denitrogenization steps vvhich depends on initial melt temperature and antount of heat losses. The extent and kinetics of this intermediary denitrogenization treatment can also be determined by model tests. Therefore, the efficiency of one stage and the necessity for multistage denitrogenization treatment to obtain required total (C+N) content can be established. As regards the oxygen blovving technique there are tvvo methods namely, uniform and "smart" oxygen blovving. In final stage of oxidation decar-burization rate is very small since the most part of oxygen is uitilized for harmful oxidation of chromium. Therefore, it should be smart and useful to decrease oxygen blovving rate gradually tovvard the end of decarburization. Uniform oxygen blovving is another blovving technique vvhich applies steady blovving rate e.g., 900 m3/h and does not take into account rapid and continuous inerease in the chromium/carbon activity ratio occuring in the final stage of decarburization. This change of thermodynamic conditions results in a drastically reduced decarburization rate and inereased chromium oxidation rate vvhich is accompanied vvith steep rise in melt temperature. Consequently, the final stage of decarburization is most delicate since VOD ladle lining must be prevented from thermal overloading. On the other side computer simulation in the form of model tests airned at the optimization of VOD treatment should also optimize the productivity. The most appropriate compro-mise betvveen high productivity and temperature limitation can be find out only by the use of computer simulation and model tests. Model tests carried out by the use of CAPSS vvere planned to compare "smart" and uniform blovving (900 m3/h) to find out the advantages and deficiencies of each. "Smart" blowing Uniform 900 m3/h "Smart" blovving Uniform 900 m3/h "Smart" blovving Uniform 900 m3/h Time min. —> Figure 1. Influence of blovving technique on VOD productivity. Slika 1. Vpliv načina pihanja na produktivnost VOD naprave. As can be seen from fig. 1 "smart" blovving character-ized by continuously diminishing blovving rate tovvard the end of decarburization, as compared to uniform 900 nt3/h rate, results in a serious drop of productivity i.e., prolonga-tion by 40 mins. of the tirne required for decarburization from 1.2% C to 0.02% C. Quite unexpectedly, fig. 2 shovvs that adaptation of blovving rate to the continuous change in thermodynamic conditions as applied by "smart" blovving does not reduce the extent of chromium oxidation to slag. There is no change in final chromium content vvhich clearly does not depend on oxygen blovving method. Uniform and intense oxidation vvith 900 m3/h blovving rate results in the reduction of time required for VOD treatment by 40 mins. as can be seen from fig. 3. Hovvever, this increase in productivity could be too expensive since max-imum melt temperature vvould exceed prescribed 1700°C limit. Fig. 4 shovvs that blovving technique exerts practically no influence on the volume and kinetics of denitrogeniza-tion. Because of a higher melt temperature at uniform blovving favorable influence of temperature on denitrogenization of stainless steel can be seen. E 3 S o ■C O 17.20 16.80 16.60 16.40 16.20 1450 ........................................................................................................................................................................................ 0 20 40 60 80 100120140160180 200220 Time min. ---> Figure 3. Effect of blovving method on melt temperature. Slika 3. Vpliv načina pihanja na potek temperature taline. 3.2 Conventional stainless steel The manufacture of ELI stainless steel e.g., superferritic steel differs from production technology for converttional stainless steel by intermediate stop of oxygen blovving fol-lovved by special usually 15 mins. long denitrogenization step at a high carbon content (approx. 1.0% C). Initial carbon content of melt planned for the manufacture of com- The attention has ben focused at the productivity, extent of chromium oxidation to slag, maximum temperature at the end of oxidation and volume and kinetics of denitrogenization. Initial melt composition and temperature vvas 17.0% Cr, 1.20% C, 0.1%. Si, 0.5%. Mn, and 1550°C, respectively. Final carbon content should be lovver or equal to 0.02 vvt.% . C. VOD treatment should be as short as possible to obtain 1 high productivity of VOD unit hovvever, melt temperature should not exceed 1700° degrees. The results of model . tests are presented in figs. 1^}. ^ 15 Figure 2. Effect of blovving method on the oxidation of chromium. Slika 2. Vpliv načina pihanja na obseg in potek oksidacije kroma. 220 0) 3 a E Q) K 15.60 0..........20........40........60.......80......YoO 120 140160 180 200 220 Time min. -- > 1500 A C O -Q i-<0 O 0 0 Iiiiiii iii iuii lintn li i li i n iii i ii i ii i ii i m i m i iiimiii 111 lil ni hi iiii n lil i iii li ni i ii li ii i ii i n hi i ii' iti ii i ti i m iif 0 15 30 45 60 75 90 105 Time min. — > "smart" uniform controlled A E S o v. -C O 15.60 liiuinniiiiiniiniimiiniiiiii................................................I ..................... 0 15 30 45 60 75 90 105 Time min. —> "smart" uniform controlled Figure 4. Infiuence of blowing technique on denitrogenization. Slika 4. Vpliv načina pihanja na potek in hitrost razdušičenja. mon stainless steel is therefore significantly lower in order to shorten VOD treatment. Final carbon conten of common steel is higher which also helps to reduce the time required for vacuum oxygen decarburization. Special denitrogenization stage is not necessary since low nitrogen content is not preseribed for common stainless steel. Except for ELC grades technological proeess for common stainles steel does not include (tnal vacuum degassing stage followed by re-duetion. Therefore vacuum decarburization treatment can be performed also by controlled blowing rate in addition to "smart" and uniform blovving. The need for additional i.e., controlled oxygen blowing appears as a consequence of vvider range wherefrom proeess parameters can be se-lected. The tirst aim of controlled blowing technique is to maximize the productivity of EAF-VOD-CC production line by shortening duration of VOD treatment which is the slowest stage and therefore VOD unit acts as a bottleneck. The problem is serious particularly in čase of UHP electric furnace coupled with VOD unit. Hovvever, maximization of productivity by optimization of VOD must take into account follovving requirements: • EAF tap temperature should be as low as possible, » highest allovvable temperature at tire end of oxidation is 1700° C and • melt temperature at the end of VOD treatment should coiTespond to the requirentents of continuous casting (CC). Exact solving of the problem is not possible. A compro-mise is needed between the contradictory requircments men-tioned. Computer simulation i.e., a series of model tests carried out during tapping, ladle transport and preparation for VOD treatment is only possible. Figs. 5, 6 and 7 present results of model test planned to determine most appropri-ate controlled blowing technique in order to optimize VOD treatment. Figure S. Optimization of VOD productivity by seleetion of proper blouing technitjue for common stainless steel. Slika 5. Optimiranje produktivnosti z izbiro načina pihanja pri izdelavi klasičnega nerjavnega jekla. As seen from fig. 5 "smart" blovving requires longest time for decarburization. There is no essential difference in decarburization time at uniform and controlled blowing technique (101 mins. and 102 mins., respectively). 1750 A O o a E <]> 1700 1650 1600 1550 1500 i 1 450 *—IIHMIlllMI........—um—TT------r-------------------- 0 15 30 45 60 75 90 105 Time min.—> -"smart" uniform controlled Figure 6. Intluenee of blou ing method on chromium oxidation. Slika 6. Vpliv načina pihanja na oksidacijo kroma v žlindro. In ali three cases chromiunt content at the end of treatment is the same as seen in fig. 6. This holds for both oxidation and reduetion stage. Hovvever, it can be seen that during processing chromium content of melt is on the average lovvest at controlled blowing because of lower initial temperature. 350 300 A 1 1 E 250 CL 0. š 200 C 0) CD 0 150 i: § 100 "V. 50 LriifiniiHiiiiiiiiiiiiii« 0 20 40 60 80 100 120140 160 180 200 220 Time min.—> "Smart" blovving Uniform 900 m3/h Figure 7. Influence of blovving method on themial load of VOD ladle. Slika 7. Vpliv načina pihanja na toplotno obremenitev VOD ponovce. At uniform 900 m3/h oxygen blowing rate (fig. 7) melt temperature at the end of oxidation just exceeded the al-lowable limit (1700° C). Heat load of VOD ladle is lowest at controlled blowing which is optimized to obtain praeti-cally the same productivity (102 vs. 101 rnins.) at EAF tap temperature reduced by 20° C. 4 Conclusion By the use of PC oriented software CAPSS which had been developed as a part of the research programme URP-C2-2566 computer simulation of stainless steelmaking EAF-VOD technology was carried out. A series of model tests was perfornted to investigate the influence of three different oxygen blowing methods on the productivity of 90 ton VOD unit, chromium losses with slag, volume and kinetics of denitrogenization, and themial load of VOD ladle. Based on the rcsults obtained following conclusions can be dravvn. • Common oxygen blowing technique known as "smart" blowing does not make it possible to reduce chromium losses. Hovvever, it has very negative influence on the productivity of VOD unit and consequently whole steelmaking EAF-VOD-CC technological line. • Uniform oxygen blowing with constant blowing rate does not inerease chromium losses to slag. By proper seleetion of blowing rate this blowing method can re-sult in a higher productivity hovvever, VOD ladle lining can be thermally overloaded. • Best results can be achieved by computer controlled blovving rate which ensures highest possible produc-tivity at lovvest tap temperature and thermal load of VOD ladle. 5 References 1 N. Smajič: Mathematical Model for EAF-VOD Stainless Steelmaking, Proceedings of 6. CENIM, Madrid, October 1985. N. Smajič: Modelle Thermodinamique de 1'Elaboration de 1'Accier inoxydable suivant le Procede VOD, Proceedings of 25. JAS, St. Etienne,1986. N. Smajič: Verifikacija matematičnega modela za računalniško vodenje EOP-VOD tehnologije izdelave nerjavnih jekel, Žel. Zbornik, 1986, 3. 4 N. Smajič: Študij kinetike vakuumskega razdušičenja su-perferitnih talin, Poročilo Metalurškega inštituta v Ljubljani, N 89-007. 1989. N. Smajič: Superferitna nerjavna jekla. Žel. zbornik, 1988.2. 6 N. Smajič: Vakuumsko razdušičenje nerjavnih jekel, Žel. zbornik. 1990, 1. ' N. Smajič: Production D'Acier Ferritique Inox Avec Car-bone et Azote Tres Bas Par La Technologie EAF-VOD, Bulletin du Cercle de Metaux, 29ems Journees 'Etudes de Metaux, Avril 1990, Saint Etienne. 8 N. Smajič: Vakuumsko razdušičenje nerjavnih jekel, Žel. zbornik, 24, 1990, št. 1. 9 N. Smajič: CAPSS, Demonstration on Technova 92 International, Graz 1992. 10 N. Smajič: Integralni matematični model EP-VOD tehnologije, Poročilo IMT N. 92-09, Ljubljana, 1992. 11 Mc Coy et al., Electr. Furn. Conf. Proc. 21, 1963, 17-26. 12 Hilty. D.C. cit. v Handbook of Stainless Steel, 1977, 3-32. 13 Peckner, D.: Handbook of Stainless Steel, Mc Gravv Hill Book Inc., N.Y. 1977, 3-26. 14 Otto J. et al., Stahl und Eisen 96, 1976, 1939-45. 15 Otto J.. Disertation, Aachen 1975. 16 M. Schmidt et al., Stahl und Eisen, 88, 1968, 153. 17 H. Bauer et al, ibid. 90, 1970. 725. 18 C. Wagner: Thermodynamics of Alloys, Addison-Wesley, Cambridge, 1952. 19 J.F. Elliot et al., Thermochemistry for Steelmaking, vol. 2, Addison-Wesley, London 1963. 20 Ban-ya et al.. Tetsu-to-Hagane, 60, 1974, 1443. 21 F. Tsukamoto, Transactions of ISIJ, 26, 1986, 273-81. 22 B.I. Leonovič et al., Metalli, 1980, 4. 23 Fujio Is hi i et al., Tetsu-to-Hagane 68, 1982, 946-55. 24 K. Mori, Transactions of ISIJ, 28, 1988, 246-261. 25 N.P. Vladimirov et al., Metalli, 5, 1973, 89-95. 26 M. Murakami et al., Transactions ISIJ, 27, 1987. 27 K. Ishihara et al., Proceedings of 100111 ISIJ Meeting, October, 1980, 836. 28 K. Mori et al., Tetsu-to-Hagane, 52. 1966, 1443-45. 29 V.P. Luzgin et al.. Izvestija VUZ, Čem. Metal. 9. 1963, 50-54. 30 J.V. Javojskij, Izvestija VUZ, Černaja Met. 1977. 7. 31 E.T. Turkdogan et al.. Trans. Metali. Soc. A IME, 1963, 227. It p k 64270 JESENICE, Cesta Železarjev 8 - telefon: (064) 81 -341, 81-441, 84-262 telefax (064) 83-395-telex 37-219,37-212zeljsn - telegram Železarna Jesenice □ debelo, srednje in tanko pločevino □ hladno valjane trakove in pločevino □ dinamo trakove in pločevino □ nerjavne trakove in pločevino □ vlečeno, brušeno in luščeno jeklo □ valjano in vlečeno žico □ patentirano žico □ pleteno patentirano žico za prednapeti beton □ hladno oblikovane profile □ kovinske podboje za vrata □ dodajni material za varjenje □ žičnike □ tehnične pline STORITVE □ prevaljanja, vlečenja, iztiskanja in toplotne obdelave pločevine in žice □ tehnične dejavnosti: elektro, strojne, konstrukcijske, obrtne in tehnične Microalloying of Steel Mikroiegiranje jekla F. Vodopivec, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana Mechanisms of the effect of microalloying on strength and toughness of steels. Influence on grain size, precipitation hardening, processes of hot deformation, and economv of microallovina with Al Nb, i/, and Ti. Mehanizmi vpliva mikrolegiranja na trdnostne lastnosti in žilavost jekla. Vpliv na velikost zrn, izločilno utrditev, procesi vroče deformacije in gospodarnost mikrolegiranja z Al, Nb, V in Ti. 1 Introduction The microalloying of steel is a technology which has been intensively developed for about 25 years, and it exploits the theoretical know!edge on mechanisms of precipitation hardening, grain size control, and deformation of steel. The term "microalloying" is used because steels are alloyed with up to 0.05% of various elements, and an important influence on the following characteristics and properties is achieved: • austenite and ferrite grain size are diminished, and because of it yield stress, strength, and toughness are in-creased while the ductile/brittle fracture transition temperature is diminished; • precipitation hardening is achieved, this increases the yield stress and strength of steel, diminishes the toughness and increases the ductile/brittle fracture transition temperature; • the hardenability is improved and austenite/ferrite transition temperature is lowered; • the susceptibility of steel to strain ageing is eliminated; • the content of dissolved oxygen and sulphur in steel are diminished and the purity of steel is improved; • the shape and composition of non-metallic inclusions are changed and the isotropy of properties and the machinability of steel are improved; • the texture in non oriented electrical sheets is improved and the energy losses diminished. Microalloying elements are: aluminium, base element for steel deoxidation and for the decrease of oxygen is solution, niobium, titanium, vanadium, zirconium, boron, calcium, tellurium, antimony, tin, nitrogen, and in some cases also sulphur and lead. In this paper only microal-loying elements in the narrower sense will be discussed, i.e. those which influence the microstructure, strength and toughness of steel: aluminium, niobium, titanium, vanadium, and nitrogen which are in various combinations the basic constituents of high-strength structural steels and modem machine-building steels. In order to understand better the influence of microalloying elements on the mechanical properties and the hot vvorking process it is necessary to know the processes and reactions in steel involving these elements, and their compounds with nitrogen and carbon which form precipitates called in the following as disper-soide phases. The influenced processes are austenite grain growth, precipitation hardening in ferrite, and recrystalliza-tion of austenite during hot rolling. 2 Size and stability of austenite grains The first condition for the formation of small ferrite grains during the cooling of steel are small austenite grains and are obtained either by recrystallization of austenite after hot rolling at a relatively low temperature if a suitable delay of austenite grain grovvth is achieved during the hot vvorking, or during the cooling after normalization. Austenite grains grow through migration of boundaries, which can be hindered or stopped if the boundary is pinned to precipitates of dispersoide phases. When migration progresses, at first a concavity is formed at the precipitate, then the grain boundary envelopes it and finally bypasses it. This process requires an additional energy. The driving force for the growth is the tendency of material to reach a minimal total energy (E,) through the change of the shape and the size of grains, and is obtained by the minimal specific surface energy of grains. The total energy consists of the volume (E v) and the surface component (Ep). Ev is proportional to the grain volume, thus to D3, if D is linear dimension of grain, vvhile the surface component is proportional to D2. Schematically it can be written Es = I< D2 + I\\D3. The specific energy is thus: El = 1 D2 D + 1 E.g.: for D = 1, EJD = 2; for D = 2, E,/D = 1.5; for D = 3, Es/D = 1.33, etc. Thus total energy is the lower the coarses is the grain size. The prevention of the migration of a grain boundary is achieved when the distance among the precipitates is below a critical value. Instead of the distance betvveen the precipitates, which is difficult to be measured, the more easily measurable precipitation size (d) and volume part of dispersoide phase (/) are used in the analytical treatment of grain grovvth. The relationship between the grain size— D, the volume part of precipitates—/, and their size—d is according to Zener1 given by the equation: D _ 4d J ~ 3f The above equation vvas further developed for the grovvth of austenite grains in structural steel by Gladmann and Pickering2. They have assumed that in grain grovvth the energy D \Z 2 J is released. In the equation S—represents the boundary migration, y—the surface energy of austenite, and Z—the ratio betvveen the size of a growing grain and the average grain size in the matrix. It is evident that the grain growth is possible only if Z > 4/3, otherwise the growth energy change is positive and a spontaneous process is not possible. An exception represents the cases of very great growth driving force, e.g. a grain shapes which greatly deviates from the equilibrium. The equation was further transformed into the expression eonnecting the critical size of precipitates, c/j., with other easily nteasurable parameters: d i-. The equation shows that grains grow with the growth of the critical size and the dccrease of the content of precipitates, as vvell as with the increasing non-uniformity of grain size. A spontaneous grovvth is initiated the easier the greater is the initial non-uniformity of austenite grain size. For better understanding it can be mentioned that after one-hour of heating of a Cr-Ni carburizing steel at 920°C, i.e. before anormal grain grovvth, the ratio of maximal and minimal grain size is Z = 3.18. By the same quantity of the dispersoide phase the precipitates are the more efficient the smaller is their size, i.e. the greater is their volume density and thus the smaller is their mutual distance. Precipitates are not completely stable at the grain grovvth temperature and grow at prolonged an-nealing tinte and especially at higher temperatures loosing the pinning efficiency. In structural steel the precipitates of sizes below 10 nni represent a low hindrance for grain grovvth because of their instability caused by the high ratio of surface to the total energy. Efficient precipitates are formed by dispersoide phases vvhich are dissolved in austenite at the heating of steel before the hot rolling or forging. As steels are becoming cool, the solubility of dispersoide phase is diminished, and precipitates are formed vvith size depending on the temperature and the length of isothcrmal annealing. During the transformation and the recrystallization the grovvth rate of ali grains is not uniform, single grains grovv faster, reach a lovver total energy, become more stable, and at sufficient temperature grovv at the expense of their neigh-bours. This is the explanation why a microstructure of grains of different size is found in normalized steel vvith a too lovv quantity of the precipitates for complete pinning of the migration of austenite grain boundaries. The grains can grovv also by coalescence if their space orientation is similar and are parted by lovv-anglc boundaries. This occurs in textured materials. The boundary migrating at the extent of a neighbour grain is concave. A simplificd explanation is that the atoms on the concave side are on average more tinte bound in the crystal lattice. The migration of crystal boundary is produced by the difference in the number of atoms vvhich are deplaced over the grain boundary because of thermal oscillation. The number of jumps front the convex to the concave side of the boundary is equal to the number of jumps in the reverse dircction, but on the concave side more of oscillating atoms are retained. This produces a flovv of atoms from the convex to the concave side, i.e. the shift of crystal boundary in the opposite dircction. The theoretical cxplanation for the migration process of a crystal boundary tovvards the centre of curvature is found in ref.3, vvhere it is suggested that the driving force for the boundary migration is the decrease of surface energy. The rate of migration is described by a parabola of the form D = Do + A'1/n vvith D0—an initial grain size, D—the grain size after an annealing time t, and n—the grovvth exponent. Theoreti-cally n is 2 vvhile empirically the values betvveen 2 and 4 vvere measured. The connection betvveen the grain size (D) and the yield stress (Re) is given by the Hall-Petch equation Re = Re0 + KD-1'2 vvith Re0 as a constant depending on the contposition and the microstructure of steel . The constant K is a measure for the hindering effect of crystal boundaries on the mobility of dislocations. In Fig. 1 taken from the ref.5 the relation betvveen the grain size, expressed by D1?2 and the ASTM number, and the yield stress of steel vvith 0.17% C and 0.8% Mn is shovvn. The decrease of the grain size from ASTM number 5 to ASTM number 10, obtained through the microalloying produces an increases in yie!d stress of steel for about 50%. This increase takes plače at an increased toughness and a decreased ductile/brittle fracture transition temperature (Tp) at notch toughness test. The proposed relation is ^r = T0 + K D -1/2 with To and K constants depending on the contposition and the microstructure of the steel4. Figure 1. Relation betvveen the grain size expressed as D , the ASTM number, and the yield stress of steel vvith 0.17% C and O.S^i. Mn (Ref.5). Slika 1. Odvisnost med velikostjo zm izraženo kot D-1/2 in ASTM razredom ter mejo plastičnosti jekla z 0.17% C in 0.8% Mn. Po viru5. The movement of dislocations in the lattice is hindered by the Peierls-Nabarro force (rpn), and each crystal bound-ary produces an additional obstacle for the movement. The 5 10 15 Grain size d~!/J v mm 10 11 12 13 ASTM number kp/mm mm"1'' total force (Ts) essential for the movement of dislocations in polycrystalline material is: Ts = Tpn + K D~1'2 A piling-up of dislocations at the grain boundary is re-quired in order to accumulate a sufficient driving force for the penetration of dislocation into the neighbour grain with a different space orientation6. 3 Dispersoide phases Dispersoide phases are carhides and nitrides, very fre-quently also carbonitrides since carbides and nitrides of mi-croalloying elements are mutually completely soluble. The composition of dispersoides depcnds on the amounts of mi-croalloying elements, nitrogen, and carbon in steel. If the content of microalloying elements, nitrogen or carbon is too high, dispersoide phases can form already in the melt or during the solidification of steel. The size of precipitates in this čase is 100 nm or more, accordingly small is their volume density, and low the hindering effect at standard size of austenite grains. In microalloyed steel in which the content of microalloying elcmenLs for the most part does not exceed 0.05%, the sufficient volume density of precipitates is not achieved if they are formed in the melt or during the solidification. In this čase they are enriched on the solidification interfaces or in eutectic clusters, vvhich decreases the ductility of the steel. Such example represent A1N and Nb(CN) formed during the solidification of steel manufactured in electric are furnace7, vvith a high content of nitrogen, aluminium, and niobium. The mechanism of grovvth of precipitates involves the solution of small particles vvith a greater specific surface energy and the diffusion of microalloying components on coarse, more stable particles. The grovvth of precipitates, often named as Ostvvald ripening, is deseribed by the Wagner8 equation df ~ d30 = 16 aDCV 9 RT -t vvith dt precipitate diameter after the annealing time t do precipitate diameter in time 0 u surface tension betvveen the matrix and pre- cipitate D diffusivity of constitutive atoms C concentration of constitutive atoms V molar volume of precipitate R gas constant T absolute temperature The equation shovvs that the rate of precipitate grovvth will be at constant other conditions the faster, the faster is the diffusivity, the greater is the concentration of constitu-tive atoms in solution, and the higher is temperature. Thus the ideal dispersoide is that vvith the lovvest solubility of constituents, and vvith the lovvest diffusivity of microalloy-ing element, since the diffusivity of nitrogen and carbon in interstitial solution is very fast. Let us assume that the steel contains 0.03% A1N vvhich ensures the austenite grains size after normalization of 6-7 ASTM number9. Fig. 2 piesents the calculated A1N precipitate size for such a steel after one hour fiolding at various temperatures, the content of aluminium nitride, the volume density of precipitates, and the relative austenite grain size. The solubility product used for the calculation is in good agreement vvith the A1N solubility determined for Cr-Ni carburizing steel9. If the heating temperature of steel is inereased from 900 to 950° C, the same hindering effect can be obtained vvith an about 40% higher content of nitride, vvhile above 1000°C the pinning effect of aluminium nitride is very rapidly diminished. 0.15 i.0,10 300 200 100 ,0.05 OL o r 30 1000 1100 Temperature °C Figure 2. Relation betvveen the annealing temperature and the precipitate size. the number of precipitates per unity of volume, the content of A1N in solution, and the austenite grain size. The theoretical AIN content is 0.03%. The calculation is based on the A1N solubility product given in ref.2. Slika 2. Odvisnost med temperaturo žarjenja in velikostjo izločkov, številom izločkov na enoto prostornine, količino AIN v raztopini in velikostjo zm austenita. Teoretična vsebnost AIN 0.03%. Izračun je izvršen na osnovi topnostnega produkta za AIN v viru2. In microalloyed steel usually there are 2 or 3 grain grovvth inhibitors; AIN, Nb(CN), TiC and VN. The presence of precipitates formed by the addition of 0.03% niobium to the steel vvith 0.10-0.20% C ensures grain sizes of ASTM number 10-11 after normalization. The most frequent dispersoide is aluminium nitride (AIN) vvhich is found in ali steels deoxidized, and thus mi-croalloyed vvith aluminium. The solubility of AIN and of other dispersoide phases in austenite in struetural steels is given by the solubility product. Ref.10 gives the follovving solubility product for aluminium nitride log(Al x N) = -6770/T + 1.48 In the above equation N and Al represent the vveight content of both elements in solution in the steel, and T is the temperature in K. According to ref. 10, 11, and solubility products for other dispersoide phases are 12 the log(Ti x C) = — 10475/T + 5.33 ti log(Ti x N) = - 8000/T + 0.32 12 log (V x C) = - 9500/7" + 6.72 11 log(V x N) = - 8330/T + 3.46 10 log(Nb x C + N) = - 6770/7" -f 2.26 10 In some references also other equation for the solubility of dispersoides is found but they do not differ significantly from the above given. The solubility of ali the dispersoides, but of vanadium carbide, in austenite with up to 0.2% C and 0.01% N is small and at the normalizing temperature less than 10% of the quantity at the temperature of about 1200° C. On the contrary, the solubility of vanadium carbide in austenite is very high, and this dispersoide is completely dissolved already at about 900°C in steel with 0.2% C. Other dispersoides are very stable because of the low solubility at the normalizing temperatures and the inhibition of grain growth is diminished only at higher temperatures. During the cooling from the solubility temperature and at isothermal holding during such cooling the formation of precipitates is very slow (Fig. 3) due to slow formation of nuclea though the solid solution is highly oversaturated13. The explanation for the slow formation of nuclea is the great dilution since the content seldont exceeds 0.03% vvhich e.g. represents 3 atoms of titanium per 10000 atoms of iron. The number of atoms of microalloying elements is thus very lovv and consequently the rate of formation of sufficient statistic aggregations of atoms from vvhich precipitation nuclea are formed is very slovv. The kinetics of the precipitation during the holding after direct cooling from the solubility temperature is a slovv parabola highly different from that describing the formation of precipitates in austenite quenched from the solubility temperature and then reheated (Fig. 3). The kinetics of A1N formation is in this čase a step parabola indi-cating that the rate of grovvth of precipitates is determined by the diffusion rate of aluminium on the nuclea formed during the reheating of steel, due to the high oversatura-tion because of the cooling to ambient temperature or to the double transition of the austenite/ferrite phase boundary on vvhich the solubility of A1N is changed strongly. dium (Fig. 4). The specific vveight of niobium carbide is higher than that of aluminium nitride, the vveight solubility of both in austenite is similar, thus the same vveight content of niobium in austenite gives less precipitates. Conse-quently, if seems probable that niobium hinders the migra-tion of boundaries also by some other mechanism, e.g. by a segregations on grain boundaries vvhich produces a greater number of precipitates on these boundaries as it could be expected from the average niobium content in steel. Ref.14 presents micrographies shovving that the boundaries or sub-boundaries of austenite grains are marked vvith strings of precipitates vvhich confirm the possibility of an intercrys-talline segregation of niobium. The size of austenite grains is thus related to the thermal deformation history of steel. Ref.15 describes a bimodal size distribution of precipitate after the rolling of niobium steel from 1050°C vvhich can also be explained supposing an intercrystalline segregation of niobium. 30 o. u o 20 10 \ Č 4320 ~~--- \ \ ^^ -V i * - —a- Nb 0,05 0.10 Content of Nb and V . % 0.15 Figure 4. Relation betvveen the amounts of niobium and vanadium in steel and the size of austenite grain after half-hour and 8-hour austenitizing at 920°C. Basic steel composition: 0.18% C, 0.95% Mn, 0.28% Si, 1.0% Cr and, belovv 0.002% Al (Ref.16). Slika 4. Odvisnost med količino niobija in vanadija v jeklu in velikostjo zm austenita po polurni in 8 umi austenitizaciji pri 920°C. Osnovna sestava jekel: 0.18% C, 0.05% Mn, 0.28% Si, 1.0% Cr. pod 0.002% Al,. Po viru16. Figure 3. Kinetics of A1N precipitation after various thermal history of steel vvith 0.11% C, 0.49% Mn, 0.029% Al, and 0.0063% N. Slika 3. Kinetika precitipacije A1N po različni temiični zgodovini jekla z. 0.11% C. 0.49% Mn, 0.029% Al in 0.0063% N. It must be mentioned that niobium if its concentralion exceeds about 0.035% and at high nitrogen contents—the limit is at about 0.012%, is bound during the solidification process into a carbonitride very rich in nitrogen and practi-cally unsoluble during heating the steel before the rolling7. Niobium bound in this phase is lost as active microalloy-ing element, thus the microalloying vvith niobium in steel molten in electric are fumace is economical only up to about 0.03%. In CrMn čase hardening steel by already 0.02% Nb the same stability and size of austenite grains is achieved as vvith the same amount of aluminium or vvith 0.1% vana- 4 Microalloying and precipitation hardening The precipitation hardening is one form of dispersion hardening, i.e. hardening caused by a nevv phase vvhich is found in small quantities in the metallic matrix. The general ex-pression describing the relations betvveen the quantity of precipitates (/), their size (d), the shear modulus (G), the Burgers veetor of dislocations (b), and inerease of strength (ARt) was proposed by Hombogen17 in the follovving form ARr = A-921 d K is a constant vvith a value I\ = 1 for a polycrystalline material and uniformly distributed spheroidal particles of the nevv phase. The hardening is proportional to the third root of the quantity of precipitated phase and inversely proportional to the particle size of that phase. It is thus more strongly dependant on the size than on the quantity of precipitates. The precipitation hardening is stable only till the shear moduls is not diminished because of the temperature change or the precipitates do not hinder the moving of dislocations. In microalloyed steel the precipitates formed at 0.029AI, 0,0063 N 1300Nb), d =0.C )5pm with Nb as niobium content in vveight %, d—the size of niobium carbonitride precipitates, and ARe—the increase of yield stress. The exponent at the niobium content proves that the expression was derived through simplification of the Hornbogen equation. A disadvantage of the expression is the lack of parameters considering the temperature of formation of precipitates and the shear modulus, thus it can be used only for heat treatment by quenching and ageing at a selected temperature. Fig. 5 presents the influence of precipitates size at constant niobium content, and the content of niobium at constant size of 50 nm precipitates on the hardening effect. Already a small amount of niobium is efficient if present in steel in small precipitates. The increase of the content of niobium does not improve the hardening effect to an economica!ly justified extent. Fig. 5 further proves that precipitates of an average size of 25 nm, vvhich can be found in microalloyed steel after normalizing, cause hardly a hardening effect. Practically only a part of precipitation hardening effect can be industrially exploited hovvever it is not negligible 0 0.01 0.02 Precipitates size d. pm t_t_i_i 0 0,02 0,04 0,06 Content of Nb,% Figure 5. Relation betvveen the size of precipitates in steel vvith 0.03% Nb or NbC concentration in 5 nm precipitates, and the increase of yield stress. Slika 5. Odvisnost med velikostjo izločkov v jeklu z 0.03% Nb oz. količino NbC v izločkih z velikostjo 5 nm in povečanjem meje plastičnosti. from the vievvpoint of the material strength. E.g. a small change in basic composition of the steel vvith a yield stress above 350 N/mm2, and microalloying vvith niobium and vanadium can give a yield stress above 470 N/mm2, vvhere precipitation hardening due to formation of vanadium carbide during the cooling of steel after normalizing contributes for about 50 N/mm2 . As already mentioned, the precipitates formed in the approximate temperature range 570 to 620° C are efficient. At lovver temperatures the formation of precipitates is too slovv due to the slovv diffusion of vanadium, and it could be exploited only by a longer annealing or in a very slovv cooling vvhich is economic only in coils. The effects of the diminution of grain size and of the precipitation hardening on the yield stress are additive, their influence on the other tvvo very important properties, the notch toughness and the ductile/brittle fracture transition temperature, is opposite. Diminished grain size increases the toughness and decreases the transition temperature vvhile the precipitation hardening has an opposite effect. Fig. 6 presents, according to data in ref.4, some relations vvhich confirm the above conclusions for a standard as normalized Nb-V microalloyed steel. By thermal treatment, e.g. by normalizing and through the rate of cooling, a rather different relation betvveen the yield stress and the toughness transition temperature can be achieved, even an unaxcept-able transition temperature can be obtained vvhich nullifies ali the advantages of microalloying. 5 Microalloying and hot deformation Most steel products are manufactured by hot rolling vvhen the steel cross section is reduced from pass to pass at drop-ping temperature till a final thickness of plate, strip or bar is obtained. A similar sequence of events is found by forgirig only the sequence of partial deformations is less uniform. During the rolling process the steel is cooled partially by radiation and the convection into the surroundings, and par-tially by contact vvith the cooling vvater, rolls, hammers or 400 300 E E ■ 200 tr 100 100 ARe N/mm2 : 17,36 + 1.81 D"v2 Re=f(d) 250 r+80 VPT= f (IU) TT = f (D) 0,15 C, 0,46 Si. 1,16 Mn , 0 036 V, 0,015 Nb normahsed L_I________i 12 10 6 ASTM number 4 +60 +40 +20 -4 o X 20 -I -40 o o O) i_ ZD a O) CL E (D C O Cn c a -60 20 40 60 80 Gram size D, jjm 100 Relationships grain size - yield stress grain size - transition temperature precipitation hardening - - transition temperature Figure 6. Relation between the grain size in as normalized steel, the precipitation hardening, and the yield stress increase due to the precipitation hardening (ARe), and the notch toughness transition temperature (PT). Slika 6. Odvisnost med velikostjo kristalnih zm v normaliziranem jeklu, oz. izločilno utrditvijo in mejo plastičnosti povečano zaradi izločilne utrditve (ARe) ter prehodno temperaturo žilavosti (PT). other cooler parts of equipnient. Because of the successive deformations the steel is in deformed state for some time and it contains a great number of point and line defects. The rate of diffusion processes in the deformed ntatrix is very fast. Jonas and covvorkers21 22 found that the nucleation rate of precipitates was for an order of magnitude faster during the deformation, and that the rate of precipitates growth in deformed austenite was for two orders and during the deformation even for three orders of magnitude greater than in not deformed or in recrystallized austenite. The static recrystallization vvhich eliminates from austenite the deformation energy delayed for a few seconds corresponds thus to a 100 or even 1000 sec. long annealing. Any component which delays the recrystallization thus highly accelerates the proeess of precipitaton but only as long as austenite remains unrecrystallized. As soon as the recrystallization is finished the rate of precipitation is diminished again. E.g. in labo-ratory rolling of 12 mm plates from 60 mm billets the steel remained between the rolls for 0.47 seconds, and total time of rolling was 70 seconds. Let us assume a great rate and an uniform precipitation in the period when steel is deformed betvveen the rolls. The rate of precipitates growths is de-seribed by approximate cubic parabola which can be simpli-fied for a rough evaluation into the expression t/'/'5 % Kt, t being the time. The calculation shows that the ratio of precipitates size in unrecrystallized austenite (dan) and in recrystallized austenite (dar) is dan/dar = 4.5. If steel is rolled by uncompleted intetpass recrystallization and if it contains a small quantity of microalloying elements the unhomogenity of microstructure represented by the number of anormally grown grains is the greater the lower is the rolling temperature9,2 because of the unhomogenity of precipitation during the rolling. During the rolling of low and medium alloyed steel with an austenite microstructure static recrystallizaton is the basic proeess for the elimina-tion of deformation energy. Dynamic softening processes and static recovery are virtually negligible. In absence of interpass recrystallization static recovery rapidly eliminates the deformation hardening, and it does not change the size of austenite grains. Niobium is the microalloying element which has the strongest delaying effect on the rate of static recrystallization of austenite. Two explanations are pro-posed for the mechanism of the effect of niobium. Tite first elaims that the effect is linked to niobium in solid solution in austenite24. The proeess of static recrystalization is initiated on the grain boundaries, it seems thus that the inhi-bition of formation of recrystallization nuelea on boundaries is connected to the presence of niobium at these boundaries. Fig. 7 shows that the temperature of completed interpass static recrystallization of austenite is already by 0.02% niobium inereased for about 100° C in the CrMn carburizing steel. The weight content of 0.02% of niobium means that the solution tn austenite contains appr. 1.1 niobium atom per 104 iron atoms. A logic conclusion is that sueh a dilution could hardly influence the proeess linked to shifts of iron atoms and it seems justified to conclude that the austenite grains boundaries are richer in niobium due to a segregation. This explanation is supported also by the fact that small amounts of niobium improve the nardenability of steel through the delaying the nucleation of ferrite below the transformation temperature. Thus niobium can hinder the formation of recrystallization nuelea and ferrite by a similar mechanism. o: CrMn cc ise harder R o r ing steel u - O O 6 G- / i P —m— / / / /' /F / ^__ 6 E E in cz 4 o t_ OT O!-o*a-U---. V - ---------10 1200 1100 1000 900 800 Initial rolling temperature, °C Figure 7. Influence of the initial rolling temperature on the ratio length/width (R) and on the number of unrecrystallized austenite grains (P). Steel G: 0.14% C. 1% Mn, 0.85% Cr, 0.02% Nb, and 0.0078% N; steel F: 0.16% C, 1.1% Mn, 0.98%: Cr, 0.025% Al, and 0.0095% N. Slika 7. Vpliv začetne temperature valjanja na razmerje dolžina/širina (R) in na število nerekristaliziranih zm austenita (P). Jeklo G: 0.14% C, 1% Mn, 0.85% Cr, 0.02% Nb in 0.0078% N; jeklo F: 0.16% C, 1.1% Mn, 0.98% Cr, 0.025% Al tn 0.0095% N. The second hypothesis links the influence of niobium on the recrystallization on precipitates formed during the deformation. Two questions are not explained by this hy- pothesis: why the precipitates of other microalloying elements, e.g. TiC, and VN and A1N, vvhich are also formed during the deformation, hinder the static recrystallization of austenite to a much lesser extent (Fig. 8), and why the recrystallization process takes plače when the content of niobium in solid solution is diminished below a limit of about 0.005% due to the formation of precipitates. Both explanations of the effect of niobium are found in recent papers on microalloying, and it is left to the reader to chose the more probable significance weighting the significance of empirical findings. 300 0,050 0,100 0.150 0.200 Content of elements.0/« 0.250 Figure 8. Influence of the content of various microalloying elements on the hindering temperature of static recrystallization of austenite. Slika 8. Vpliv vsebnosti raznih mikrolegimih elementov na temperaturo zaustavitve statične rekristalizacije austenita. In Fig. 9 the effect of rolling temperature on the content of A1N and NbC in various steels, and on the ferrite grain size given as intercept length by rolling 15 mm plates from 60 mm billets in 7 passes is shown. Ali the steels were heated to 1200°C before the rolling. In steel vvith-out niobium where the interpass recrystallization of austenite is fast and complete, only few precipitates are formed during the rolling and the influence of temperature on the arnount of precipitates is hardly perceivable because at de-creasing rolling temperature the content of A1N formed during the rolling is very slowly inereased. The precipitation behaviour in niobium steel is significantly different because austenite remains between passes for longer time unrecrys-tallized, or the quantity of austenite which does not re-crystallize at ali betvveen passes is inereased, and thus the precipitation is faster. Below a limit temperature austenite remains completely unrecrystallized between passes and the precipitation is accelerated to sueh extent that practically ali A1N and NbC are precipitated in the relatively short rolling time of 1 minute. On the base of the processes of recrystallization and of precipitation tvvo technologies of rolling of microalloyed steel were developed. In thermomečhanical rolling the slabs are rolled to a thickness which is 30-50% greater than the final thickness of plates, the rolling is stopped till steel temperature drops belovv about 950°C, and then the plates are rolled to the final thickness in scveral passes, the number de-pends on the strength of the rolling stand, and cooled in air. Defomied austenite is during the eooling very rapidly trans-formed into finegrained ferrite and pearlite25, while A IN and NbC precipitates hinder the growth of ferrite grains after the transformation. A finegrained microstructure with high strength and toughness is obtained, and i! supports the precipitation hardening with VC during the air eooling of plates with an acceptable deteriorating effect on noteh toughness. 0 _L _L 1200 1100 Initial rolling 1000 900 temperature , °C Figure 9. Relation betvveen the initial temperature of tvvo steel vvith a similar basic composition, one microalloyed vvith niobium, the amounts of A1N and NbC precipitated during the rolling and the grain size after air eooling. Slika 9. Odvisnost med začetno temperaturo dveh jekel s podobno osnovno sestavo, eno pa mikrolegirano z niobijem na količino A1N in NbC, ki sta nastala med valjanjem in na velikost zrn po ohladitvi na zraku. This technology is applied for steel microalloyed vvith niobium and vanadium vvhich can achieve yield stresses up to 500 N/mm2 at earbon contents belovv 0.15%. Similar properties are obtained vvith the combination of rolling in less controlled temperature range and normalization anneal-ing. Fig. 10 represents the various hardening mechanisms in normalized steel microalloyed vvith aluminium, niobium, and vanadium. The advantages of microalloying can be exploited also through the rolling process vvith controlled recrystallization. This method demands an exact harmonization of steel composition vvith the degree of deformation and the pass se-quence, since the temperature and the per pass deformation must enable the complete interpass recrystallization, and si-multaneously also the formation of precipitates vvhich hinder the grovvth of recrystallized austenite grains. Fig. 11 presents mechanical properties of three steel of similar composition vvhich vvere rolled under conditions of controlled recrystallization. In both microalloyed steels much better properties are achieved than in the comparative steel dovvn to about 800° C when the transformation of austenite during the rolling, the deformation hardening, and the formation of texture during rolling start to occur. By the same rolling conditions the microstructure of vanadium steel is more ho-mogeneous because of less of microstructural unhomogene-ity due to the or incompleted interpass recrystallization. At stili lovver rolling temperatures the deformation hardening, 20mm plate from steel with 0,18 C ; 0.4 Si , UMn ; 0.02P , 0.025AI, 0.0042Nb 0,06V, 0.12Cr , 0,21 Cu , 0,10Ni, normalised yield stress 488N/mm2, grain size ASTM numberl! 500 VC NbC Al N Cu , Cr, Ni. P. Si precipitation hardening diminution of grain size substitution hardening 10 7. 9 K 9 5 24 C -perlite " C and N interstitlaf hardening n Natural yield stress and y uncontrolled irnpurities Figure 10. Constitution of yield stress in a 20 mm sheet of normalized microalloyed steel with 0.18% C, 0.4% Si, 1.4% Mn, 0.025% Al, 0.042% Nb, 0.06% V, 0.12% Cr, 0.21% Cu, and 0.10% Ni. Slika 10. Zgradba meje plastičnosti v 20 mm pločevini iz normaliziranega mikrolegiranega jekla z 0.18% C, 0.4% Si, 1.4% Mn. 0.025% Al, 0.042% Nb, 0.06% V. 0.12% Cr, 0.21% Cu in 0.10%- Ni. microstructural nonhomogeneities, and strain anisotropy in-crease, therefore a too low finishing temperature has a harm-ful effect. An even higher strength can be achieved by a proper coiling temperature since slow cooling enables a greater precipitation hardening. 6 Economy of microalloying Microalloying is the more efficient the more dispersoide is dissolved at heating before hot vvorking. The quantity of dissolved dispersoide is the greater the closer are the concentrations of the constituting elements to the stoichio-metric ratio; e.g. the amount of dissolved A1N in austenite will be the greatest if the weight contents of aluminium and nitrogen in steel are 2 : 1. A too great deviation of one or the other element causes that less dispersoide is dissolved in austenite, and less precipitates are formed during the rolling and the cooling. This explains why at high aluminium contents, over 0.04%, austenite grains are coarser and less stable than at a lower content of aluminium and at the same content of nitrogen about 0.01 %. For the stability of austenite grains the nature of precipitates is not important, only their number and stability matter, or more correctly, the number of precipitates per unity of volume of austenite. Theoretically the presence of about 0.03% A1N or of a corresponding quantity of other dispersoides of precipitates of equal size is needed to at-tain a sufficient stability of austenite grains. The contents of microalloying elements and of dispersoide phases giving equal volume densities of precipitates are given in Table 1 for the most frequent dispersoides. Aluminium assures a sufficient density of precipitates already at the lovvest content, while the highest content is required in the čase of vanadium carbide since this carbide is much more soluble E E cn c o; i_ i/i a> 10 c < 45" .20 Figure 2. Contoured double-cantilever double axially notched probe. Slika 2. Dvojno konzolno vpela klinasta epruveta z dvema osnima zarezama. UD 0Dk Figure 3. Cylindrical probe vvith concentric fatigue crack—(type 2 according to GOST 25.506-85). L = length betvveen the clamped parts of probe in llie tensile testing machine. L = 5D; d = (0.6 to 0.7)D: Li > 7D; /0 = 0.5(D - d) > h + 1.5 mm; /o > 3.71 g a; DK = D - 2h = (0.65 io 0.85)D. Slika 3. Cilindrična epruveta s koncentrično utrujenostno razpoko—(lip 2 po GOST 25.506-85). L = razdalja med deloma epruvete, ki se vpneta v trgalni stroj L = 5D\ d = (0.6 do 0.7)D; L\ > 7D: /0 = 0.5(D - d) > h + 1.5 mm; (0 > 3.7 tg a\ DK = D - 2h - (0.65 do 0.85)D. 200 L| a CL -120 "80 T (°C) -40 Figure 5. Fracture toughness of structural steel plate. Dependance of 1\ ic on temperature, plate thickness 20 mm. Probe from Fig. 2. 1) Hardened and tempered molybdenum-alloyed steel: 0.12% C, 0.54% Si. 1.05% Mn, 0.5% Cr, 1.47% Ni, 0.12% V, 0.24% Mo, 0.011% Al, 0.022% N, 0.025% S 2) The same steel, plate thickness 40 mm, Rp = 710 MPa 3) Hardened and tempered manganese-silicon steel (0.1% C, 1.48% Mn, 0.9% Si, 0.031% S, 0.021% P); Rp = 435 MPa 4) Steel above, rolled; Rp = 350 MPa 5) Hot rolled low-carbon steel (0.16% C, 0.24% Si, 0.65% Mn, 0.025% S. 0.025% P); Rp = 265 MPa. Slika 5. Lomna žilavost pločevine iz jekla za gradbene konstrukcije. Odvisnost /\ /c od temperature, debelina pločevine 20 mm. Fpruveta slika 2. 1) Poboljšano legirano jeklo z molibdenom: 0.12% C, 0.54% Si, 1.05% Mn, 0.5% Cr, 1.47% Ni, 0.12% V, 0.24% Mo, 0.011% Al, 0.022% N, 0.025% S 2) Isto jeklo, debelina pločevine 40 mm, Rp = 710 MPa 3) Poboljšano mangan-silicijevo jeklo (0.1% C, 1.48% Mn. 0.9% Si, 0.031% S, 0.021% P); Rp = 435 MPa 4) Jeklo (3) valjano; Rp = 350 MPa 5) Vročevaljano maloogljično jeklo (0.16% C, 0.24% Si. 0.65% Mn, 0.025% S, 0.025% P); 265 MPa. Figure 4. Scheme of crack tip vvith the zone of plastic deformation. Slika 4. Shema razpoke s cono plastične defomiacije. GOST 9454-77 in distilled vvater and in 3% NaCl vvater solution according to RM SEV niethod (Corrosion protec-tion in building engineering. Corrosion cracking of high-strength armature steel. Investigation methods, 1986). Moving rate of the tensile-tester clamping javvs vvas 2 • 10"8 mm/min vvhich vvas sufficient for completing test in one day. Also impact toughness vvith U and V-notched probes (according to GOST 9454-78) vvas measured simul-taneously vvith the uniaxially loaded tensile tests according to GOST 1797-84 vvith (lat probes of the same thickness as investigated plate. 4 Results of Tests Static testing gave a series of relations valid for the crack stability in structural steel (Figs. 5 to 8). It shovvs that in steel of standard purity and rational microstructure the fracture toughness value increases vvith the increased strength and vvith the transition from ferrite-pearlite microstructure POVRŠINA CONA PLASTIČNE DEFORMACIJE CONA PLASTIČNE DEFORMACIJE VRH RAZPOKE VRH RAZPOKE If -196 -122 -80 T (°C) Figure 6. Fracture toughness and the size of plastic deformation zone in normalized steel (0.17% C, 1.56% Mn, 0.4%, Si, 0.11% V. 0.015%. N, 0.008%- S. 0.07% P). Sulphide inclusions are modified by addition of RF., curve (1) vacuum treated steel; R,, = 460 MPa. Slika 6. Lomna žilavost in velikost cone plastične defomiacije v normaliziranem jeklu (0.17% C, 1.56% Mn, 0.4% Si, 0.11% V, 0.015%. N, 0.008% S. 0.07% P). Sullidni vključki so modificirani z dodatkom RZ, (1) jeklo vakuumirano; Rp = 460 MPa. 5 600 650 680 T (°C) Figure 7. Dependance of fracture toughness and the size of plastic deformation zone on tempering temperature for molybdenum-alloyed steel (0.10% C, 0.37% Si, 1.16% Mn, 3.1% Cr, 1.0% Ni. 0.34% Mo. 0.016% S. and 0.04% P), plate thickness 20 mm. probe type from Fig. 3. Slika 7. Odvisnost lomne žilavosti in velikosti cone plastične deformacije od temperature popuščanja za jeklo legirano z molibdenom (0.10% C. 0.37% Si, 1.16% Mn, 3.1% Cr. 1.0% Ni. 0.34% Mo, 0.016% S in 0.04% P) debelina pločevine 20 mm. epruveta si. 3. to microstructures obtained by hardening and tempering (Fig. 5). Plate thickness reduces the A'/e value (Fig. 5) due to the reduced plastic deformation zone (?'r). The investiga-tion results also indicate that fracture toughness of high-strength steel depends on type, amount and distribution of non-metallic inclusions, mainly sulphides (Fig. 6). Pure steel (0.008% S) excells the steel vvith standard amount of sulphur both in the respect of fracture toughness and in size of plastic deformation zone at crack tip ()••/•)• This confirms the influence of inclusions, on vvhich decohesion takes plače (formation of voids), on the conditions of crack initiation *. In heat-treated steels the fracture toughness A'/c is abruptly reduced if tempering temperature is reduced from 650 to 600°C (Fig. 6). The reason is in changed mech-anism vvhich controls the crack initiation. The tvvo-stage process connected to formation of microvoids is substituted by an energy undemanding mechanism of local destruetion vvhich is detailed deseribed elsevvhere4. The reduced tempering temperature reduces the I\ j <• value measured by static loading (Figs. 6 and 7) vvhich is in contradiction vvith the hitherto ideas, especially vvith the changed size of plastic deformation zone rj. This can be explained by applied testing methods vvhich did not allovv a suitably high microstructural sensitivity of parameters deseribing the crack stability in ductile steel vvith rational microstructure. This is confirmed vvith tvvo ad-ditional cases of unsufiicient microsh-uctural sensitivity in estimating A'/c value vvith static tests. Table 2 presents the relation betvveen the fracture toughness of manganese-silicon steel and the chemical contposition and the hardening temperature. Steel santples vvith various chemical com-positions vvere hardened at optimal temperature of 930°C, and at 1050°C. This temperature vvas chosen in order to determine the influence of overheating. In ali the cases the steel samples vvere tempered at 650° C. Investigation results indicated that concentrations of al-loying elements had a small influence on mechanical properties. Overheating of steel highly deteriorates the Charpy-test values, transition the impact toughness value vvhich vvas practically halved. K k; value is not extra highly influenced by overheating; the obtained differencies vvere belovv 5% and they are in the region of measuring errors. The second čase is connected to the seleetion of thermo-mcchanical treatment in manufacturing 50 mm plate vvith yield stress Rp > 450 MPa made of microalloyed man-ganese steel (Table 3). Steel vvas quenched from the rolling temperature and tempered at 650°C. Initial and final rolling temperatures, and the quenching temperature (i.e. interval betvveen ftnished rolling and quenching in vvater) vvere var-ied. Specifications TMT 1, 2, 3, and 4 in the mentioned table represent various regintes of thermomechanical treatment. 1\ jc values vvere measured vvith CTS probes (Fig. 1) having plate thickness. The highest temperature at vvhich the A'/c value vvas correctly measured vvas —40° C. Table gives the dissipation of results of three tests. Table also suggests the seleetion of optimal regime of treatment vvhich enables the yield stress above 490 MPa at simultaneously the highest toughness transition temperature, i.e. TMT-1. Simultaneously it is evident that fracture toughness values (A'/c,-40 and A"/c,-7o) do not enable to judge vvhich thermomechanical treatment is optimal. These measurements only reliably indicate that TMT-4 treatment (hot rolling) is the most unusitable one. The mentioned cases shovv that parameters of linear fracture mechanics are microstructural^ not enough sensitive to enable the regarding seleetion of tested steels. The most probable reason is the high ductility of structural steel; in this čase the ductility of high-strength steel vvhich have oversized plasticity zone at crack tip in static P.D. Odesskij. N. Kudajbergenov, L. Kosec. F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation Table 2. Some parameters of bnttle-fracture resistance of manganese-silicon steels with yield stress Rp > 390 MPa, plate thickness 20 mm Chemical composition of steel % Quenching temperature (°C) Mechanical properties Rv (MPa) KCV~ ,u (J/cm2) (°C) T ' — ' UXJ" JXIC (MPa m1/2) C Mn Si S P 0.09 1.42 0.28 0.030 0.022 930 429 64 -20 165 0.09 1.42 0.28 0.030 0.022 1050 432 29 +20 160 0.09 1.30 0.60 0.022 0.019 930 417 84 180 0.09 1.30 0.60 0.022 0.019 1050 414 41 +20 175 0.09 1.42 1.03 0.030 0.023 930 435 69 -20 175 0.09 1.42 1.03 0.030 0.023 1050 445 41 +20 170 j—Toughness transition temperature determination was based on 50% tough fracture n—Probe in Fig. 2. —70°C was the higliest temperature on which A'/c could be estimated Table 3. Mechanical properties of 50 mm thick plate of microalloyed manganese steel (0.19% C. 1.58% Mn. 0.48% Si, 0.07% V, 0.024%. S) Steel treatment Rv (MPa) XXX (°C) 7^50 <°C) K C V"70 (J/cm2) A-4U ic A'"™ I c (MPa m1/'-') Hot rolled 420 -70 -20 49 55-76 42-55 Hardened and tempered 500 -60 +20 31 61-82 52-67 TMT 1 520 -100 -40 92 67-103 52-64 TMT 2 570 -60 -20 38 52-91 42-64 TMT 3 620 0 32 64-106 39-67 TMT 4 635 -20 +20 28 45-64 27^8 in—Determined by impact toughness criterion .39 J loaded probes though ali the testing conditions described by corresponding standards were fulfilled. Microstructural depcndance of investigated parameters becomes more pronounced in more severe testing conditions vvhich reduce the extent of plastic deformation and the size of plastic deformation zone at the crack tip. This occurs during testing in corrosive media (Fig. 7). In these tests the K\c value increases vvith the increased tempering temperature of steel. The obtained result is the consecjuence of a high density of disordered dislocation loops. Such a structure is essentially less resistant to stress corrosion2, especially if hydrogen embrittlement is developed. In steel vvith such a structure, the KCjc value is essentially lovver than the A'/c one (Fig. 8). Behaviour of hardened and tempered steel during load-ing is characterized by its substructure vvhich is formed in recovery of ferrite and in the beginning of recrystallization. Materials vvith such a structure are not sensitive to stress corrosion; therefore there it is valid: A'/c — ^/c (Fig. 8). Another way to increase the structural sensitivity of A'/c 316 the impact tests vvhere loading rates are increased for six orders of magnitude. In this čase the extent of plastic deformation is reduced to suitable amount, also size of plastic deformation zone at the crack tip is abruptly reduced, and it becomes dependant on steel microstructure. This finding can be confirmed vvith the investigations of steel of big pipelines vvhich measured A"/c values vvere close to the K ic values determined by static testing (Table 4). In static loading the CTS probe vvas applied vvhile for impact tests Charpy test probe vvith fatigue crack vvas used. Steels vvith equal A"/c values exhibit the same sequence £ <2 Figure 8. Dependance of fracture toughness and the size of plastic defonmation zone on tempering temperature for microalloyed manganese steel (0.14% C, 1.64% Mn, 0.52% Si, 0.07% V, 0.007% N, 0.031% S, 0.013% P). KcIC—fracture toughness for tests in corrosive medium. Slika 8. Odvisnost lomne žilavosti in velikosti cone plastične deformacije od temperature popuščanja za mikrolegirano manganovo jeklo. (0.14% C, 1.64% Mn, 0.52% Si, 0.07% V, 0.007% N, 0.031% S. 0.013% P). K'fj—lomna žilavost pri preizkusih v korozijskem mediju. Table 4. Fracture toughness and the size of plastic deformation zone on crack tip Steel treatment Plate thickness Chemical composition Grain size RP /Oc rT Kfc rd t (mm) C S (pm) MPa MPa1/2 mm MPa1/2 mm Microalloyed Mn steel, hardened vvith A1N, 12 0.17 0.012 9 418 80 4.2 13.7 0.15 normal ized Microalloyed Mn steel, hardened vvith VN, 12 0.13 0.011 6 425 80 4.1 22.6 0.14 normalized Mn steel alloyed vvith small • amounts of Mo and Nb, 16 0.13 0.011 4 590 84 3.1 37 0.29 controlled rolled after impact testing. Investigations also showed the advan-tage of controlled rolled plates. Microstructural sensitivity of fracture parameters vvas also essentially increased, espe-cially the Kjc and values vvhich are in a good correla-tion vvith the grain size. 5 Analysis of Results The described testing results shovv that the A'/c value mea-sured at static loading is not sufficiently microstructurally sensitive property (Figs. 6 and 7, Tables 2 and 3). The needed sensitivity can be achieved by increased test sever-ity or vvith more demanding tests applying corrosive media or impact loads (Fig. 7, Table 4). Microstructural sensitiv-ity vvas increased if the extent of plastic deformation vvas reduced. As a rule, at least three of the follovving con-ditions must fulftlled in that čase: temperature belovv 0°C, dynamic tensile load, stress raisers must be present in struc-ture, dimensional factor (great cross section, and the like), and unsuitable steel microstructure. In these cases the parameters of crack stability measured in the conditions of highly limited plastic deformation give good description of fracture conditions. These parameters vvere vvell applied in engineering design of structures. As an example, the crack propagation in the vvall of main pipeline will be described. Crack vvas initiated in the vveld on the line of fusion penetration inside the pipeline, and then it propagated tovvards the extemal surface. The I\'/c values and the critical crack lengths lr in the heat affected zones for various steel are revievved in Table 5. Critical size of defect vvas calculated by expression3: vvhere p is pressure, R pipe radius, and t vvall thickness. The obtained results shovv that crack in manganese-silicon steel becomes unstable at low temperatures, and it starts spontaneously to propagate in the axial direction at relatively small penetration into the pipe vvall. At those temperatures the use of pipes made of the mentioned steel is not allovved. After controlled rolling the critical size of crack becomes greater than the vvall thickness. Thus the stable crack vvhich reaches the pipe surface hinders the sponta-neous propagation of crack. At the given temperature the pipe made of the third steel (cited in Table 5) is safe. Table 5. Fracture characteristics of steel in heat affected zone of pipeline vveld Steel treatment Plate thickness ,.<*<-(30) IC ,(-60) ' C (mm) (MPa m1/2) (mm) Mn-Si steel, normalized 12 24 3 Microalloyed (V) Mn steel, controlled rolled 17 81 24 Mn steel, microalloyed vvith V and Nb, controlled rolled 14 97 32 Fatigue crack vvas normal to the plate surface Determination the fracture toughness at static load at -60°C (A/c > 100 MPanr /2) for manganese-silicon steel indicates the safety of that steel, but unfortunately the prac-tical experiences did not confirm it. Thus the parameters of linear fracture mechanics measured in the conditions of very limited plastic deformation did not exhibit only high structural sensitivity but they are also useful in engineering design of brittle-fracture resistance in the cases vvhen they enough accurately describe the mechanism and conditions for brittle fracture of certain steel. 6 References 1 Werkstoffkunde Eisen und Stahl; Teil 1; Grundlagen der Festigkeit. der Zahigkeit und der Bruchs; Verlag Stahleisen, Diisseldorf, 1983 2 Tylkin M.A.. Bolšakov V.I., Odesskij P.D.: Struktura i svojstva stroiteFnoj stali; Moskva, Metallurgija. 19X3 3 Knott J.: Mikromehanizmy razrušenija i treščinostojkost' konstrukcionnyh splavov; Mehanika razrušenija. (prevod iz angl.), Moskva, Mir. 1979. (101-130) 4 Smith E., et al.: Lokalizacija plastičeskogo tečenija 1 treščinostojkost vysokopročnyh materialov. Mehanika razrušenija, (prevod iz angl.); Moskva. Mir. 1980. (124— 147) 5 Duffy A.R., et al.: Praktičeskie primery rasčeta na sopro-tivlenie hrupkomu razrušeniju truboprovodov pod davle-niem, Razrušenie, 5; Moskva, Masinostroenie. 1977. (146-209) Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti M. Gojič, M. Balenovič, Željezara Sisak "IRI" d.o.o., Sisak L. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana L. Vehovar, Inštitut za kovinske materiale in tehnologije, Ljubljana L.J. Malina, Metalurški fakultet Sisak, Sisak Recently great attention is given to investigations related to oil - and natural gas exploitation, especially from the aspect of material selection used for production of tubular equipment. In these conditions tubes destructions caused by mechanical damages are less frequent than destructions caused by various corrosion crack forms and especially corrosion cracks in sulphide conditions, with H2S as a dominant component. Results of mechanical properties and evaluation of tendency of seamless tubes in as rolled and heat treated condition, produced from medium-carbon Mn-V steel to hydrogen embrittlement by method of cathode polarization at the constant current density of 4.0 m A/cm2 are presented in this paper besides the above stated possible tube destruction forms in the conditions of oil - and natural gas exploitation. The fractographic analysis of fractured samples surfaces after cathode polarization was also performed by scanning electron microscope. It was found that tubes in the as rolled condition (no heat treatment) shovv great tendency to hydrogen embrittlement with the values of the calculated embrittlement index of 86.4%. By normalization (900" C/30 min. air) of tubes, resistance to hydrogen embrittlement vvas not improved compared with the as rolled condition. The heat treatment (quenching + tempering) resulted in a great resistance of tubes to hydrogen embrittlement with the calculated embrittlement index of 25.1%. The stated results are proved by the fractographic analysis of morphology of fractures that are explicitly brittle for tubes without heat treatment, but tough for quenched and tempered tubes. Therefore it can be concluded that from the aspect of mechanical properties and corrosion resistance the most suitable tubes for use in the oil - and gas wells are those quenched and tempered at high temperatures (70CP C). Danas se u svijetu velika pažnja poklanja istraživanjima vezanim uz problematiku eksploatacije nafte i zemnog plina, posebno s aspekta izbora materijala koji se koriste za izradu cijevne opreme. Propadanja cijevi u tim uvjetima uslijed mehaničkih oštečenja su manje cesto nego propadanja usiijed raz Učiti h oblika korozijskih raspucavanja, a posebno korozijskog raspucavanja u sulfidnim uvjetima gdje je H2S dominantna komponenta. U radu su pored navedenih mogučih oblika oštečenja cijevi u uvjetima eksploatacije nafte i zemnog plina prikazani rezultati mehaničkih svojstava kao i ocjena sklonosti valjanih i toplinski obradenih bešavnih cijevi iz srednjeugljičnog Mn-V čelika prema vodikovoj krtosti metodom katodne polarizacije kod konstantne gustoče struje od 4.0 m A/cm2. Takoder je provedena na scanning elektronskom mikroskopu i fraktografska analiza prelomnih površina uzoraka nakon katodne polarizacije. Utvrdeno je da cijevi u valjanom stanju (bez toplinske obrade) pokazuju veliku sklonost prema vodikovoj krtosti s vrijednostima izračunatog indeksa krtosti od 86.4%. Normalizacijom (900' C/30 zrak) cijevi otpornost na vodikovu krtost nije se poboljšala u usporedbi s valjanim stanjem. Medutim toplinskom obradom (kaljenje + popušatnje) postignuta je velika otpornost cijevi prema vodikovoj krtosti s izračunatim indeksom krtosti od 25.1%. Navedene spoznaje dokazuje i fraktografska analiza morfologije preloma koji su za cijevi bez toplinske obrade izrazito krti cijepajuči dok su žilavi za zakaljene i popuštene cijevi. Prema tome, ovaj rad dokazuje da su s aspekta mehaničkih svojstava i korozijske otpornosti za primjenu u naftnim i plinskim bušotinama najpogodnije cijevi koje su zakaljene i popuštene na visokim temperaturama (70CPC). 1 Introduction Due to ever increasing demand for energy a great attention is given to investigations related to problems connected to oil and natural gas exploitation, especially to the selection of material for tubular equipment. Thirty five years ago medium quality steels having 400 MPa yield strength were used for tubes and pipes in natural gas and oil exploitation from 1000-3000 meters depths. Nowadays the exploitation proceeds not only in higher depths (10000 m) but in more severe conditions also because of the presence of highly corrosive acidic gas (H2S, CO2), chlorides, high pressure and temperature, etc. Therefore, material for tubular ware have to satisfy high requirements for both mechanical and corrosion properties. Cracking of tubular equipment in the presence of sulphide is particularly frequent corrosion phenomenon. The selection of appropriate steel and the heat treatment of fi-nal goods to obtain optimal compromise of mechanical and corrosion properties are used as countermeasure1-6. Different possihle tube damage in oil and natural gas exploitation with special emphasis on SSCC (sulphide stress corrosion cracking) as a form of hydrogen embrittlement is described. Samples made from hot rolled seamless tube manufac-tured from medium carbon low alloyed Mn-V steel vvere in-vestigated Cathodic polarization follovving heat treatment carried out under laboratory conditions vvas used to deter-mine susceptibility to hydrogen embrittlement and evaluate corrosion properties. 2 Forms of oil pipe damage Tubes and pipes used in natural gas and oil exploitation are known as Oil Country Tubular Goods (OCTG). OCTG is divided into tubing, casing and drill tube used in vertical direetion for pumping, extemal proteetion and drilling, re-spectively. In a vvider sense OCTG includes also pipe line used for transport purposes mainly in horizontal direetion. Despite complex stresses pure mechanical failure is less frequent as compared to corrosion failure. Economic op-eration of oil vvells is often dependent on proper selection of tube material. The control of corrosion has become one of main factors in the produetion of energy from geoen-ergetic sources. Corrosion failure of natural gas and oil exploitation equipment is very serious problem from safety as vvell as economic vievvpoint since the costs in oil indus-try amount to several hundreds millions of USS per year. Material failure due to corrosion is also associated vvith produetion breaks. Corrosion problems in oil and gas ex-plotation equipment7~10 can appear in several forms: • vveight loss due to corrosion, • localized corrosion knovvn as pitting, • corrosion fatigue, • galvanic corrosion, • stress corrosion and • sulphide stress corrosion cracking (SSCC) Stress corrosion cracking caused by sulphide is very frequent since typical oil and gas exploitation environment besides chlorides, sulphates, carbonates, C02 and moisture contains considerable amount of H2S also vvhich can reach up to 30%. Hydrogen sulphide attack increases general corrosion, erosion corrosion in turbulent media, stress corrosion cracking, corrosion fatigue of tubes and equipment at bottom of drilled vvells etc. Stress corrosion cracking in the presence of sulphide may appear in tvvo forms: • hydrogen induced cracking (HIC) • sulphide stress corrosion cracking (SSCC) Hydrogen induced cracking is characteristic for OCTG equipment made from lovv alloyed steel vvith ferrite—perlite microstructure and 700 MPa tensile strength. It can occur even in the absence of external stress. This form of corrosion results from atomic hydrogen absorbed on microstruc-tural defecLs (hydrogen traps) vvhich recombines into molec-ular hydrogen. Sulphide stress corrosion cracking occurs in OCTG equipment made from high strength steel as a form of hy-drogen embrittlement. Hydrogen embrittlement has been vvell knovvn and frequently observed in metals for quite a long time. It is caused mostly by corrosion, galvanisation or leaching associated vvith the generation of atomic hydrogen vvhich under certain conditions can diffuse into crystalline lattice resulting in the hydrogenization of metal. In the beginning the effect of hydrogen vvas attributed to stress corrosion. Recently the similarity betvveen hydrogen embrittlement and certain types of stress corrosion, partic-ularly SSCC8-10 has been pointed out. There is no unique theory capable of explaining ali phe-nomena associated vvith hydrogen attack because it depends on a number of factors e.g. type of steel, its microstructure, electrolyte, etc. At present, the proposed mechanisms of hydrogen attack are based on inerease in the inner pres-sure, surface adsorption, decohesion, inerease or decrease in plasticity and the formation of hydrideu. Acidic enviro-ment in natural gas and oil exploitation enhances hydrogen embrittlement and SSCC as its particular form because of • the presence of H2S at lovv pH value of media, • sulphides vvhich inerease the amount of hydrogen dif-fusing into the crystalline lattice of metal and • tendency for the localization of anodic part of corrosion reaction vvhich promotes initial cracks. There are tvvo sources of atomic hydrogen; the inner gener-ated by manufacture and heat treatment of steel and the ex-terior resulting from the effect of definite environment. During application of material hydrogen may be adsorbed from gaseous phase in molecular form vvith subsequent dissoci-ation into atoms or by electrochemical dissolving of liquid phase i.e. surrounding corrosive media vvhich takes plače during the exploitation. In this čase hydrogen is formed in molecular or atomic form. The overall reaction of hydrogen formation is in acidic solutions: 2H30++2e~ — H2 + 2H20 (1) in caustic solutions: 2H20+2e~ — H2 + 20H~ (2) The transport of HjO+ ion (front now on H+ ) or H20 moleculc to electrode surface and subsequent formation of adsorbed hydrogen atoms can be described by (3) and (4): H++e" - Hads (3) H20 + e- - Hads + OH- (4) Irrespective on the nature of solution, hydrogen is adsorbed on metal electrode surface. To be continuous the electrochemical process requires permanent presence of Hais on electrode surface from vvhere it can be removed in one among three ways12,13: • by catalytic recombination (Volmer-TafeTs mecha-nism) vvhere both the adsorption and desorption take plače simultaneously: H++e" - Haiis (5) Hads + Hads — H2 (6) • by Volmer-Heyrowski's mechanism of electrochemical desorption vvhere desorption results from the reduetion Table 1. Chemical composition of the Heat (T) and billet (A ) Steel Sample Composition in wt. % C Mn P S Si V Mo Al Cr Mn-V T 0.33 1.10 0.017 0.004 0.23 0.21 - 0.027 _ K 0.33 1.14 0.025 0.005 0.29 0.23 0.02 0.04 0.08 of H+ ion or H2O molecule according to (7), (8) and (9): H+ + e" H+ + Hads + e" H20 + Hads + e- Hads H2 H2 + OH" (7) (8) (9) • by emission mechanism where adsorbed hydrogen atoms vaporize from electrode surface: H Uds H (10) Consequently, hydrogen atoms adsorbed on the surface (Hads) can recombine into either harmless gaseous hy-drogen through (6) vvhich is bubbled out of solution or diffuse into metal and enhance embrittlement: Hads —* Hahs It can be concluded that sulphide stress corrosion crack-ing is caused by absorbed hydrogen. The dissolving of iron and the presence of H2S in vvater solution create conditions for increase in hydrogen content of steel14. Usually, only a small part of hydrogen generated on cathode diffuses into metal. Diffusion rate depends on numerous faetors, e.g. the type of steel or alloy, its composition and previous thermo-mechanical treatment, nature of electrode surface, type of electrolyte, its composition, cathode current density, etc. Hydrogen embrittlement is often used in evaluation of the effect of hydrogen on steel at room temperature vvhich results in a loss of ductility (reduetion in elongation and contraction), decrease in tensile strength and enhanced brit-tleness. 3 Experimental Mn-V steel billet of cfl 135 • 420 mm dimensions vvas pro- duced under laboratory conditions on Metallurgical Institute Hasan Brkič, Zenica. The billet vvas hot rolled into 060.3 -4.83 mm pumping oil tube (tubing) under industrial conditions in Seamless Tube Mili of Željezara (Iron and Steelvvorks) Sisak. Table 1 presents Heat (T) and a control (K) analysis of the steel. Samples cut from rolled tube vvere subjeeted to heat treatment (annealing, annealing + tempering, quenching + tempering) in electric resistance chamber furnace. Inves-tigation of mechanical properties vvere canied out on IN-STRON type 1196 machine using samples prepared according to ASTM standard. Brinell or Rockvvell C hardness vvas determined depending on the sample hardness. Corrosion resistance vvas measured by the method of cathodic polarization vvhich is knovvn as one among the most appropriate for determination of relative susceptibility to hydrogen embrittlement. After cleansing vvith acetone the samples for cathodic polarization vvere put in the elec-trochemical celi of ZWIC'K 50 kN tensile machine (fig. 1) and subjeeted to static load of 80% of yicld stress. 1 - POSTOE .[ I M! MANI/ A n KIDAL 2- tajus 11 KioAuti -i ■ H I K I R (?K E 111 1 S K A r F L I JA - -UZORAK 0T0PINA 0 M n T LJ EI I KTR0DE / 1 IJGGINOVA KAPI| ARA REFLRENTNOM E LEKIR000M ISPIIŠIANJE OTOPINE P0KAZIVAČ ',111 10 -1' I '-.Ar 11 -REGULATOR BRZINI 12 ZAUSTAVI JANJE RA,7 VLAČ t N JA H IKLJUL IVAN JE RAZVLAEENJA ] i. -POVRAT Ni H0Q 1 S-U TE Z NA KI A I NU lb-PREKIU A POGONA , DA! IC E ref elektioda proTuelektroda rodna elektroda Figure 1. Schematic illustration of equipment for hydrogen emhriltlement evaluation by cathodic polarization method. Slika 1. Shematski prikaz aparature za ocjenu vodikove krtosti metodom katodne polarizacije. Sample of Mn-V steel vvas used for vvorking electrode and saturated calomel electrode (SCE) situated in Luggin's capillary tube as a reference. Tvvo graphite Johnson Matthey electrode of 6.5 nun diameter vvere used as counter electrode. The solution used vvas IN H2S04 vvith addition of 10 mg/l As20? used to aetivate the generation of Hads- The solution vvas deaerated by nitrogen blovving for 30 min. For cathodic polarization WENKING potentiostat modeli 68 FR 0.5 vvas used and constant 4 niA/cm2 current den-sity vvas applied. After 2 hours of polarization samples vvere taken out of the celi and immediately tested on INSTRON machine at very lovv deformation rate of 2.4- 10~4s_1. The overall tensile test lasted for 3-4 rnins. Aftervvard the cross seetion of fraetured surface vvas measured to determine the contraction. Due to the loss of ductility (contraction) embrittlement index F vvas calculated from: F = Z o Zn — • 100 vvhere: Z0 Z1 contraction before polarization contraction after polarization Table 2. Results of mechanical testing of Mn-V steel tube samples in as rolled and heat treated state. Sample Heat treatment Re Rm A2" Hardness, HB: API MPa MPa % I II III IV grade 2 - 605.5 759.5 25.5 216 216 211 213 L-80 900°C/30' air 20 + 670° C/60' air 535.0 657.0 24.6 222 215 229 235 J-55 870°C/30' vvater 21 + 640° C/60' air 794.0 836.0 21.2 276 278 276 278 P-105 870730' vvater 23 + 700760' air 692.0 723.0 23.4 232 234 255 231 C-90 2 ?0/Jm I----1 — trakasta feritno-perlitna mikrostruktura — a) valjano stanje — popušteni martenzit — b) K: 870"C/30 voda + P: 700°C/60 zrak Figure 2. Microstructure of tubing Mn-V steel in rolled (a) and heat treated (b) condition. Slika 2. Mikrostruktura cijevi iz Mn-V čelika u valjanoni (a) i toplinsko obradenoni (b) stanju. 4 Results Results of mechanical and metallographic investigation Investigation of mechanical properties (tensile strength, yield stress and strain) vvere carried out on two tube sam- ples in as rolled state and another tvvo in heat treated state. Ring-like 060.3 '1.83-30 mm samples vvere used for Brinell or Rockvvell C (for quenched samples) hardness measure-ment using three impressions per quadrant (in the middle of sample wall). Average values of mechanical properties for as rolled and different heat treated state are seen in table 2. Mechanical properties of Mn-V steel tubes in as rolled state i.e. vvithout heat treatment correspond to L-80 API grade of corrosion resistant OCTG wares. Annealing treatment (900°C/30' in air) follovved by tempering at 670°C produces lovver quality OCTG vvares vvith mechanical properties correponding to J-55 API grade. Quenching in vvater coupled vvith subsequcnt tempering at 640° C yields OCTG vvares of higher mechanical properties (P-105 grade) vvhich are not desired because of poor resistance to SSCC, i.e. a high susceptibility to hydrogen embrittlement. Hovvever, tempering at 700°C results in API C-90 grade correspond-ing to corrosion resistant OCTG vvares. Mechanical properties measured are in accordance vvith corresponding microstructures as can be seen on fig. 2. Results of cathodic polarization tests Since the determination of susceptibility to hydrogen embrittlement by catodhic polarization is based on the loss of ductility caused by absorbed hydrogen, samples vvere sub-jected to tensile test vvith 2.4 • 10~4s~' deformation rate immediately after the polarization. Embrittlement index F was calculated from equation (11) taking into account con-traction measured before (Zu) and after (Z\) polarization. The results are given in table 3. Histograms given on figs. 3 and 4 present the change in contraction and embrittlement index, respectively for as rolled and heat treated Mn-V steel sample caused by cathodic polarization. Samples in as rolled state and that in annealed state shovv high susceptibility to hydrogen embrittlement i.e. a great decrease in contraction (fig. 3) and high index of embrittlement (fig. 4). Samples annealed and subsequently tempered at 670° C have significantly lovver embrittlement (F = 28.1% only). The best resistance to hydrogen embrittlement (F = 25.1%) have samples vvhich vvere quenched and tempered at 700° C because of highly tempered martensite microstructure as seen in fig. 2b. Fracture surface of samples after cathodic polarization vvas analysed by electron scanning microscope as seen in fig. 5. Fracture surface of Mn-V steel samples in as rolled state after cathodic polarization 100 80 60 - 40 20 0 [Že K + P VS N N+P t o p L i n s k a obrada Figure 3. Concentralion change resulting from cathodic polarization of Mn-V steel in different heat treatment conditions VS-rolled. Heat treatment . N: 900°/30' air . N + P 900°/30' air + 670°/60' air . K + P: 870°/30' water + 700°/60' air Slika 3. Promjena kontrakcije uslijed katedne polarizacije za različita stanja toplinske ohrade Mn-V čelika. VS—valjano stanje. Toplinska obrada • N: 900°/30' zrak . N + P 900°/30' zrak + 670°/60' zrak • K + P: 870°/30' voda + 700°/60' zrak 100 80 - 60 - 40 - 20 - VS N N + P K + P toplinska obrada Figure 4. Embrittlement index change resulting from cathodic polarization of Mn-V steel in different heal treatment conditions VS—rolled. Heat treatment • N: 900°/30' air • N + P 900°/30' air + 670°/60' air . K + P: 870°/30' vvater + 700°/60' air Slika 4. Promjena indeksa krtosti uslijed katodne polarizacije za različita stanja toplinske obrade Mn-V čelika. VS—valjano stanje. Toplinska obrada . N: 900°/30" zrak • N + P 900°/30' zrak + 670°/60' zrak • K + P: 870°/30' voda + 700°/60' zrak shovvs typical brittle fraeture (fig. 5a) vvhereas quenched and highly (700°C) tempered samples shovv duetile fraeture (fig. 5b) coiresponding to a lovv index of embrittlement (fig. 4). 5 Discussion Despite the fact that mechanical properties of Mn-V steel seamless tube samples in as rolled state correspond to API L-80 grade of corrosion resistant OCTG vvares, the cathodic polarization at 4.0 mA/cnr current density displayed great susceptibility to hydrogen embrittlement. As a result of cathodic polarization the contraction dropped from initial (as rolled state) 56.9% to 7.7% corresponding to embrittlement index F = 86.4% (figs. 3 and 4). The susceptibility to hydrogen embrittlement of Mn-V steel tubes in as rolled state is also seen (fig. 5a) from brittle fraeture surface. It results from strip like ferrite-perlite microstructure vvith elongated inclusions (fig. 2a) vvhich is favorable for accu-mulation of the critical antount of hydrogen required for the initiation of cracking. From the vievvpoint of resistance to SSCC viz. hydrogen embrittlement, elongated sulphide inclusions (especially MnS vvhich act as hydrogen trap) and high tendency for segregation of ntanganese (martensite and bainite islands observed in microstructure) are unfavorable and have a dontinant influence on corrosion resistance of Mn-V steel in as rolled state. The annealing treatment carried out (900° C/30' air) on tested tubes did not intprove the resistance to hydrogen embrittlement since strip like ferrite-perlite strueture vvith prevailing influence of MnS inclusions aeting as hydrogen traps vvas preserved. On the contrary, tempering (670°C/60' air) of annealed tubes resulted in considerable inerease of the resistance to hydrogen embrittlement since martensite and bainite islands vvere removed. In respect to mechanical properties the heat treatment combined of quenching and tempering at temperatures vvithin 640-700° C range yielded J-55, P-105 and C-90 (table 2) API grades. Cathodic polarization test of tubes corresponding to API C-90 grade shovved high resistance to hydrogen embrittlement vvith only 25.1%' reduetion in contraction. The obtained high resistance to hydrogen embrittlement is illustrated also by fig. 5b shovving duetile nature of fraeture surface. As comparcd to samples in as rolled state the microstructure of heat treated sample tubes of Mn-V steel instead of bands of ferrite-perlite vvas composed of honto-geneous highly tempered martensite ntarked by high duc-tility and capacity for accumulation of higher amounts of energy generated e.g. by Zappfe's mechanism11 of hydro-gen embrittlement. Since MnS vvas observed in heat treated samples also, it is evident that highly tempered martensite reduces harmful influence of MnS on corrosion properties of OCTG vvares. Table 3. Loss of ductility of Mn-V steel as detemiined by the method of cathodic polanzation. Samplc Heat treatment Re Applied ^0 Zi i F MPa stress % % mA/cnr % 2-3 - 599.1 0.8 Re 56.9 7.7 4.0 86.4 20N-4 900° C/30'air 594.5 0.8 Re 61.1 10.9 4.0 82.0 900°C/30' air 20-5 + 670°C/60' air 519.4 0.8 Re 67.2 48.3 4.0 28.1 870°C/30' vvater 23-4 + 700°C/60' air 681.8 0.8 R, 70.1 52.5 4.0 25.1 — krti cijepajuči prelom — Uzorak 2—3 (valjano stanje) — žilavi prelom — Uzorak 23-4 )K: 870°C/30 voda + P: 700"C/60 zrak) Figure S. Fracture moiphology of specimens form Mn-V steel in rolled (a) and heat trealed (b) condition after cathodic polarizatton. Slika S. Morfologija preloma uzoraka iz Mn-V čelika u valjanom (a) i toplinsko obradenom (b) stanju nakon katodne polarizacije. Summarv Based on the investigation of susceptibility to hydrogen embrittlement of seamless Mn-V steel tubes utilized in natural gas and oil industry the following conclusions can be de-rived. • Mechanical properties of Mn-V steel tubes in as rolled state with higly oriented ferrite-perlite microstructure correspond to L-80 API grade. • Beside J-55 and P-105, C-90 API grade conforming to corrosion resistant OCTG vvares was also attained by heat treatment (annealing + tempering, quenchung + tempering) of tubes. • Cathodic polarization of tubes in as rolled state shovved small resistance to hydrogen embrittlement since embrittlement index F vvas 86.4%. • The resistance to hydrogen embrittlement vvas not im-proved by annealing (900°C/30' air) (F = 82.0%) as compared to as rolled state. • Based on brittle fracture surface revealed by fractographic analysis of samples subjected to cathodic polarization it vvas established that Mn-V steel tubes in annealed or as rolled state are not suitable for the use in oil industry. • High resistance to hydrogen embrittlement proven by comparatively small embrittlement index (F = 25.1%0 and ductile nature of fracture surface vvas ac-quired by quenching and tempering at a high temperature (700° C). • In respect to both mechanical and corrosion properties Mn-V steel tubes quenched and temepered at a high temperature are suitable for the use in natural gas and oil vvells. Chromizing of Iron Difuzijsko kromanje železa M. Jenko, A. Kveder, Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 61001 Ljubljana S. Spruk, L. Koller, IEVT, Teslova 30, 61111 Ljubljana In the paper, the theoretical aspects of CVD processes of iron chromizing and the comparison vvith the PVD process, developed by the authors for professional electronic industry are presented. V sestavku so podani teoretični vidiki CVD postopkov difuzijskega kromanja železa in primerjava s PVD postopkom, ki so ga ovtorji razvili za potrebe profesionalne elektronike. 1 Introduction Corrosion is one of the most frecjuent and the most unde-sired processes on the surface of metals and alloys. Since corrosion is a surface reaction, ali types of protective coat-ing must be involved to change the behaviour of metallic component in the surface composition. This change can be achieved by addition of a different material or in the form of outer skin, vvhich provides a barrier betvveen the body and the surrounding corrosive medium. The form of coating is the most common; it includes paints, plastics, metals deposited by electroplating etc. It is also possible to modify the chemical composition of the surface to be pro-tected, by diffusion of a suitable metal or an element into it vvhich vvill combine vvith the parent metal or alloy and pro-vide the required resistance to the corrosive medium. Such formed surface alloys are called diffusion coatings. The di-mensional change of the protected specimen is smaller than the thickness of the effective surface alloy and it may be neglected. Chromium diffusion—chromizing is probably one of the most versatile types of diffusion coatings and it is applied to achieve resistance to corrosion, thermal oxidation and abrasion for iron, steel, stainless steel, nickel and its alloys, molybdenum, tungsten and its alloy, etc. 2 Technological development of chromizing The first attempts to achieve a chromium rich surface on iron by the diffusion process vvere made by Kelly in 1923". Iron specimens vvere buried into chromium povv-der and treated in reducing atmosphere. A chromium rich layer, about 125 /tm thick, formed after 4 hours heating at 1300°C, vvas a protecting layer vvith good adherence to the underlying metal, resistant to corrosion, as vvell as to thermal oxidation and, therefore, very interesting for vvide commercial use. Similar investigation vvas made by H.S. Cooper in 19382. The process of chromizing vvas applied in a re-ducting atmosphere at the temperature of 1300°C, lasting 24 hours; the thickness of chromized layer vvas 250 /im. A high processing temperature vvas disadvantage of both procedures. The chromizing process has undergone considerable development changes over the years and it has been the subject of careful and detailed studies. A major achivement vvas the introduction of volatile halides. L.H. Marshall developed the first CVD (chemical vapour deposition) procedure of chromizing, using the volatile halides at the processing temperature of 1050° C. Modern chromizing processes like DAL, BDS, etc. are based on the above mentioned principle3,4. Simultaneously, the first experiments of vacuum diffusion chromizing vvere performed by Hicks as early as 1932. Particles of pure iron vvere buried in a chromium povv-der and heated for 96 hours at 1200° C in a vacuum of 4 • 10"2 mbar. Eight years later, Cornelius and Bollenrath obtained similar results in their experiments; chromium con-centration profiles vvere determined by the X-ray analy-sis. Further, this process vvas described by Gorbunov and Dubinin12. In Slovenia the vacuum chromizing process (PVD— physical vapour deposition) has been developed at the Institute for Electronics and Vacuum techniques together vvith the Institute of Metals and Technologies4, 17 and has been used for diffusion chromium coating of iron parls of mag-netic circuit for miniature relays. 3 Fe-Cr constitution diagram The iron-chromium constitution diagram is shovvn in Figure 1. At approximatelly 1000°C, it can be seen that the austenite microstructure of the iron remains unchanged until a con-centration of approximately 12% chromium is reached vvhen chromium is deposited and it diffuses invvards. At higher chromium concentrations, the microstructure becomes ferritic; continuation of chromizing causes moving of the al-pha/gamma boundary into interior. During cooling the ferrite surface layer remains unchanged, vvhile inner austenite is transformed into ferrite. This recrystallization of inner region vvith less than 12% Cr causes that boundary vvith 12% Cr is good visible, Figure 2. The depth to vvhich extends the 12%- Cr boundary is taken as the thickness of chromized layer, Figure 3. Since the rate of diffusion of chromium is greater in ferrite than in austenite, there is a rapid rise in the chromium concentration of the coating tovvards the surface, and be-yond the 12% Cr boundary there is a sharp concentration drop at the ferrite/austenite boundary. Grain boundary diffusion occurs too, but it has a little effect on the coat thickness, Figure 4. 1900 1800 1700 1600 Figure 3. Microsections of vacuum chromized iron samples at: 1050°C (a) 3 hours, (b) 8 hours (c) 12 hours, 1100°C (d) 3 hours, (e) 8 hours, (f) 12 hours, 1150°C (g) 3 hours. (h) 8 hours. Nital etched. Slika 3. Metalografski posnetki vakuumsko kromanih vzorcev želeZa pri: 1050°C (a) 3 ure, (b) 8 ur (c) 12 ur, 1100°C (d) 3 ure. (e) 8 ur, (0 12 ur, 1150°C (g) 3 ure, (b) 8 ur. Jedkano z nitalom. Effective coating o o 1500 1400 1300 1200 1100 1000 900 800 700 600 3NS: __i* i_i_* i——-— 0 10 20 30 40 50 60 70 80 90 100 % Cr Figure 1. Iron—Chromium diagram5. Slika 1. Fazni diagram Fe—C5. 1 MRG: 750 IBIDIFF CR 1B, Figure 2. a) Micro-section of vacuum chromized sample. A sharp o/phase boundary is visible (nital etched). b) Cr concentration profile of the same sample. Slika 2. a) Metalografski posnetek vakuumsko kromanega vzorca železa; vidna je ostra fazna meja alfa/gama (jedkano z nitalom). b) Koncentracijski profil kroma posnet z elektronskim mikroanalizatorjem na istem vzorcu. The alloyed layer is generally called a coating, but it must be clearly distinguished from the eoatings produced by electroplating and spraying processes, since there is no diffusion. The chromized coating represents an inseparable part of the treated specimen, the composition is changing from the surface to the core. 4 Mechanism and kinetics of cromizing the iron 4.1 Chromizing technic/ue with volatilc halidcs In these processes chromium is brought to the surface of the iron heated to 900-1150° C as a gaseous compound, e.g. chromium chloride, where it is deposited in atomic form by a chemical reaction. O 12% a y Distance from surface Figure 4. Chromium concentration profile in o-FeCr layer. Slika 4. Gradient kroma v kromani plasti. In many chromizing techniques, chromium chloride is applied and an atmosphere containing hydrogen is main-tained in the reaction chamber. The deposition of chromium on iron is described vvith the follovving equations: Interchange Reduction Fe + CrCb = FeCl: + Cr CrClj + H2 = 2HC1 + Cr • Dissociation CrCl2 = Cl2 + Cr (D (2) (•'I) In the reaction (1) an atom of iron is removed from the surface for each deposited chromium atom. Since iron and chromium atoms are similar in vveight and size, there occur only slight mass and dimensions changes of iron specimens after the treatment. The reaction is reversihle and the equi-librium chromium concentration at the surface depends on the relative vapour pressures of iron and chromium chlo-rides in gaseous phase. Reactions (2) and (3) are catalysed by the iron surface. Theoretically the surface chromium concentration may ap-proach 100 per cent, but since it is assumed that the cat-alytic activity of the iron surface drops with the increasing chromium content, the concentration of chromium is limited. The mass and dimension change are equivalent to the amount of deposited chromium. Generally, the volatile halides are used for transport of chromium atoms to the surface of iron, where they are ad-sorbed and diffuse imvards. In Figure 5 the layout of BDS (Becker, Daeves, Stein-berg) proeess, a typical CVD proeess, is shovvn. 2. Chromium migration from the surface inwards into the specimen expressed by the interdiffusion coefficient D: D = 2.08exp(-243000/RT) (7) Termoregulation Oegassing Sample Figuro S. The layout of BDS (Becker. Daeves, Steinberg) proeess, typieal CVD proeess, is shovvn. Slika 5. Shematičen prikaz CVD-BDS postopka. (b) Vacuum chromizing There are tvvo possible processes of supplying an iron surface vvith the chromium atoms: • transfer due to the close contact of iron surface and chromium granulate enabling the surface diffusion of Cr • absorption of Cr vapour through the formed gaseous phase In vacuum chromizing the grovvth of a — FeCr layer is controlled by tvvo processes: 1. The arrivai and condensation of C r atoms on the surface of the specimen given by the condensation rate w: (4) /g cm-2s"7(5) vvhere ak is the condensation coefficient; pCr (mbar) is vapour pressure; M is Cr molecular mass and T is absolute temperature (K). The decisive quantity is pcr, and its temperature dependence is being described by p = 11.743exp(-394000//žr) /mbar/ (6) (R is the gas constant in JK-1mol-1) 0 2 10 12 14 t/h Figure 6. Thickness of the chromized layer, d, and the mass inerease, W, as a funetion of chromizing time t. a) experimental results b) calculated values. Slika 6. Debelina vakuumsko kromane plasti d in narastek teže W v odvisnosti od časa t a) eksperimentalni rezultati b) izračunane vrednosti. By increasing the temperature 7', p inereases more rapidly than D as the evaporation enthalpy of chromium AHevap = 394 kJmor1 is higher than the aetivation energy for the diffusion Edij = 243 kJmol-1. This circumstance leads to three dif-ferent a-FeCr layer grovvth rates. (a) At lovv temperatures 950 < 6 < 1050°C the slovvest proeess is the Cr condensation. Ali con-densed Cr atoms are transported immediately by diffusion from the surface invvards. Therefore the layer grovvth rate is linearly proportional to the condensation rate w: w = Dt or d = Vwt (b) At high temperatures 0 > 1150°C, vvhen p is high enough, the slovvest proeess is the diffusion, leading to the parabolic law w = Dt d = vVTDt w (Trn 0 2 8 10 12 K t/h Table 1. Values applied in the evaluation of the thickness. d, and ueight inerease, W, of u-FeCr layers given in Figure 6b. D-interdiffusion eoeffieient, p-equilibrium Cr vapour pressure, u>-condensation rate of chromium, ta, da—critical time eoresponding o-FeCr layer thickness when the linear growth rate changes into the parabolic one. °C 900 950 1000 1050 1100 1150 1200 1250 D (cm1,-1) 1.52 x 10-11 4.20 X 10~u 1.07 x 10"10 2.56 x 10~10 5.72 X 10-10 1.21 x 10"9 2.43 x 10~9 4.65 x 10-9 p (mbar) 1.0 X 10~7 8.0 X 10"7 3.8 x 10-6 1.5 x 10-5 5.7 X 10"5 1.9 X 10"4 5.9 x 10"4 1.7 x 10-3 W (gcm-2s-') 9.23 X tO"10 7.24 X tO"9 3.36 X 10"8 1.3.5 x 10"7 4.88 X 10-7 1.62 X 10"6 4.8S X 10"6 1.36 x 10-3 ta (S) 4.32 x 107 1.94 X tO6 2.3 x 105 3.4 x 104 5829 1150 247 59.31 (h) 1200 538.8 63.9 9.52 1.6 0.319 0.069 0.0165 da (^m2) 362.4 127.60 70 41.9 25.8 16.6 10.9 7.3 At these values of T the linear rate appearing in the early stages of growth cannot, be detected. (c) In the intermediate region 1050 < 0 < 1150°C the thickness of o-FeCr layer begins to inerease linearly with time. The grovvth rate changes to a parabolic low at the critical time t, which corre-sponds to the critical thickness d. The calculated grovvth rates of a-FeCr layers are shovvn in Figure 6b. Table 1 contains ali necessary data; p is obtained from the equation (6), w from the equation (5) assuming a — 1, D from the equation (7), and da from the relationships given in Figure 6a. ! GORBUNOV ? KUBASCHEVSKI 3 BRf WER PAZUHIN 5 KOVALENKO 6 THIS W0RK 7 H 0 NIG Preal = CiC2C3p (8) vvhere: c i co takes in account the residual atmosphere; c3 takes in account the surface/granulate ratio. 5 Conclusion The results of this investigation shovv that none of CVD processes is suitable for the proteetion of iron parts for mag-netic circuit in miniature hermetic relays. For this purpose PVD process of vacuum chromizing vvas developed. With this procedure the maximal chromium eontent of 15% Cr at the surface vvas obtained, enough for corrosion proteetion in corrosive media vvhich are demanded by MIL-R-39016 and MIL-R-5757. PVD process assures the optimal magnetic properties, a very lovv coercivity and a good weldability and additionally, it is an environment friendly process. 1000 1/00 '100 v. .10 1800 Figure 7. Temperature dependence of equilibrium Cr vapour pressure p according to various references"-16. Slika 7. Pami tlak kroma v odvisnosti od temperature po podatkih različnih avtorjev11-16. The real Cr vapour pressure preai is equal to the equi-librium pressure p only if the experimental conditions are carefully chosen: pra < 10"4 mbar, vvhile the surface of the chromium granulate has to be as clean as possible and the ratio of specimens surface and the granulate amount must be adequate. If these conditions are not correctly chosen then prcai can be expressed by p multiplied by three correction coefficients c i, C3 < 1: A —tXI H v- 1 |i____| -1 1 dvostopenjska rotacijska črpalka 2 rr.enlnik srednjega vakuuma 3 turbomolekularna črpalka 4 ■ ventil 5 - merilnik visokega vakuuma b - izolacija 7 grelec 8 škatla iz nerjavnega jekla 9 kremenčeva cev takes in account the portion of oxidized surface of granulate; Figure 8. A schematic diagram of PVD—vacuum chromizing procedure7. Slika 8. Shematičen prikaz PVD—vakuumskega difuzijskega postopka kromanja. 6 References 1 N.A. Lockington, in Corrosion, Vol. 2, Principles of Applying Coatings by Diffusion, Buttervvorths, London 1978. 2 R.L. Samuel, N.A. Lockington, Met. Treat. 18. 354. 407, 440 (1951). 3 G. Becker, K. Daeves, F. Steinberg, Stahl und Eisen 61. 289 (1941). 4 M. Jenko, A. Kveder, R. Tavzes, E. Kansky, J. Vac. Sci. Technol. A3. 6. 2657(1985) E. Kansky, M. Jenko. Vaeuum 37, 1/2, 81 (1987). M. Jenko. R. Tavzes. E. Kansky. J. Vac. Sci. Technol A5/IV 2685 (1987). A.H. Sully. E.A. Brandes, Chromium, Chpt. 7, 258, But-terworth. London (1967). H. Cornelius, s. R Bollenrath, Arch. Eisenhuttenwesen, 15. 145 (1941). L.C. Hicks, Trans. AIME. 113. 13, 163 (1934). T. P. Ho ar. EA. Croom. J. Iron Steel Inst. 169, 101 (1951). R.E. Honig, D.A. Kramer. RCA Rev. 30. 285 (1969). N.S. Gorbunov. Diffusion coatings on iron and steel. Is-rael program for Sci. translations. Jerusalem 1962. 13 V.A. Pazuhin, A.J. Fišer, Vakuum v metallurgii, Metal-lurgizdat, Moskva (1956). 14 V.E Kovalenko, Teplofizičeskie Procesy i Elektrovaku-umnye Pribory, Sovetskoe radio, Moskva (1957). 15 J.L. Margrawe, The Characterization of High Temperature Vapours, John Wiley and Sons, New York (1967). 16 O. Kubaschewski, E.L. Evans, C.B. Alcock, Metallurgi-cal Thermochemistry, Pergamon, Oxford (1967) 1' M. Jenko, A. Kveder, R. Tavzes, Postopek za protikorozi-jsko ščitenje majhnih kosov iz čistega železa ob hkratnem doseganju najboljših mehkomagnetnih lastnosti. Številka patenta: P-19470. KOVINE ZLITINE TEHNOLOGIJE, 26, 1992, 1-4 1. Kronološko kazalo Ažman Slavko: Razvoj in problematika mikrolegiranih jekel za petrokemično industrijo.....KZT 26 (1992) 1-2, 015-022 Tolar Miha, J. Lamut: Racionalizacija in optimiranje proizvodnje v jeklarni Bela .... KZT 26 (1992) 1-2, 023-029 Triplat Jože: Razvoj tehnologije izdelave nerjavnih jekel v Železarni Jesenice od leta 1984 naprej..................... .............................KZT 26 (1992) 1-2, 030-033 Ploštajner Henrik, V. Prešern, G. Todorovič: Optimizacija tehnologije izdelave in odlivanja jekla v Železarni Store..... .............................KZT 26 (1992) 1-2, 034-037 Lamut Jakob, F. Pavlin, A. Poklukar: Taljenje vložka s plinom v jaškasti kupolni peči . KZT 26 (1992) 1-2, 038-040 Vižintin J.: Razvoj in pomen tribologije doma in v svetu ... .............................KZT 26 (1992) 1-2, 041-048 Legat Franc: Toplotna obdelava verig s poudarkom na indukciji.....................KZT 26 (1992) 1-2, 049-052 Rodič Alenka, J. Žvokelj, F. Legat, S. Krivec: Vpliv kemijske sestave na lastnosti jekel za verige po toplotni obdelavi . .............................KZT 26 (1992) 1-2, 053-057 Vojvodič-Gvardjančič Jelena: Nizkotemperaturna meja uporabnosti mikrolegiranih jekel s stališča lomne mehanike. . .............................KZT 26 (1992) 1-2, 058-062 Dobi D.: Uporaba instrumentiranega Charpyja pri razvoju jekel .............................KZT 26 (1992) 1-2, 063-073 Kosec Ladislav, N. Igerc, B. Kosec, B. Godec, B. Urnaut: Temperaturna utrujenost jekel.. KZT 26 (1992) 1-2, 074-078 Kejžar Rajko: Oplemenitenje površin z navarjanjem in metalizacijo..................KZT. 26 (1992) 1-2, 079-084 Kolenko Tomaž, B. Glogovac, D. Novak, D. Žagar, B. Omejc: Konceptualna rešitev procesnega vodenja ogrevanja vložka v potisni peči..........KZT 26 (1992) 1-2. 085-088 Leš P., F. Dover: Tehnologija valjanja radialno orebrenih cevi za prenos toplote.............KZT 26 (1992) 1-2, 089-093 Krajcar Josip, V. Ferketič, D. Vukovič, A. Ivančan, J. Bu-torac, A. Iharoš: Površinske greške čeličanskog izvora na bešavnim cijevima............KZT 26 (1992) 1-2, 094-096 Iharoš B.: Izrada normativno-tehnološke karte hladnog vučenja Celičnih cijevi........KZT 26 (1992) 1-2, 097-099 Torkar Matjaž, B. Šuštaršič: Preiskava vodno atomiziranega prahu iz zlitine Nimonic 80A .. KZT 26 (1992) 1-2, 100-103 Šuštaršič Borivoj, Z. Lengar, S. Tašner, J. Holc, S. Be-seničar: Izdelava AlNiCo trajnih magnetov iz vodno atom-iziranih prahov...............KZT 26 (1992) 1-2. 104-109 Rihar G.: Obdelava kovin z žarkovnimi izvori energije..... .............................KZT 26 (1992) 1-2, 110-113 Božič Antonija: Problematika sežigalnih naprav........... .............................KZT 26 (1992) 1-2, 114-117 Renko M., A. Osojnik: Razvoj metod za analizo redkih zemelj v zlitinah s posebnimi lastnostmiKZT 26 (1992) 1-2, 118-122 Lovrečič Saražin Marko: Hamiltonski indeks grafa........ .............................KZT 26 (1992) 1-2, 123-124 Tehovnik F., B. Koroušič, V. Prešern: Optimizacija modifikacije nekovinskih vključkov v jeklih obdelanih s Ca...... .............................KZT 26 (1992) 1-2, 125-130 Kurbos Mojca, J. Lamut, T. Kolenko, M. Debelak: Vpliv pogojev konti litja na lastnosti slabov...................... .............................KZT 26 (1992) 1-2, 131-133 Lesjak D., J. Lamut, V. Gontarev, A. Purkat, M. Debelak: Modelne raziskave v jeklarstvu. KZT 26 (1992) 1-2, 134-139 Kanalec Slavko, M. Tolar, J. Lamut: Racionalizacije porabe apna v jeklarni Bela..........KZT 26 (1992) 1-2. 140-142 Kert A., J. Apat, J. Lamut: Pretaljevanje sekundarnih surovin .............................KZT 26 (1992) 1-2, 143-146 Hajnže D., F. Vodopivec, M. Jenko: Rast rekristaliziranih zrn v zlitini Fe in Si..............KZT 26 (1992) 1-2, 147-150 Drofenik B., F. Vodopivec, M. Jenko: Rast rekristaliziranih zrn v zlitini železa in silicija mikrolegiram s selenom in kositrom.....................KZT 26 (1992) 1-2, 151-155 Vehovar Leopold, M. Mavhar: Korozijska odpornost ner-javnega superferitnega jekla Acrom 1 super v primerjavi z avstenitnim nerjavnim jeklom Acroni 11 Ti................ .............................KZT 26 (1992) 1-2, 156-164 Vehovar Leopold, M. Pečnik: Korozijska odpornost superzlitine Ravloy 4................KZT 26 (1992) 1-2. 165-168 Vehovar Leopold, T. Pavlin: Prepustnost mikrolegiranih jekel za vodik.....................KZT 26 (1992) 1-2, 169-171 Smolej A.: Razvoj sodobnih zlitin aluminija............... .............................KZT 26 (1992) 1-2, 172-177 Kostajnšek J.: Čiščenje talin z vpihovalnim rotorjem za uvajanje čistilnih plinov v talino. . .KZT 26 (1992) 1-2, 178-180 Kristan T.: Aluminij in avtomobilska industrija............ .............................KZT 26 (1992) 1-2, 181-182 Doberšek M., I. Kosovinc: Vplivi indija na lastnosti dentalnih zlitin Ag-Pd-Aul-Cu-Zn5-In5 .. KZT 26 (1992) 1-2, 183-187 Segel Jože: Računalniška podpora krmiljenju proizvodnje ... .............................KZT 26 (1992) 1-2, 188-193 Koroušič Blaženko: Pomen matematičnega modeliranja pri študiju jeklarskih procesov.....KZT 26 (1992) 1-2, 194-196 Štok B., N. Mole: Numerična simulacija procesa izdelave ingotov po EPŽ postopku.......KZT 26 (1992) 1-2, 197-200 Jenko Monika, F. Vodopivec, B. Praček: Raziskave segre- gacij na površini Fe-Si-C-Sb zlitin z metodo AES.......... .............................KZT 26 (1992) 1-2, 201-204 Smajič Nijaz: Dinamični model vakuumske redukcije VOD žlinder......................KZT 26 (1992) 1-2, 205-208 Šuštaršič Borivoj, M. Torkar, M. Jenko, B. Breskvar, F.Vodopivec: Procesi atomizacije kovinskih gradiv in konsolidacije kovinskih prahov — II. del........................ .............................KZT 26 (1992) 1-2, 209-214 Rodič Jože, K. Habijan, M. Strohmaier, J. Dolenc, A. Jagodic, D. Sikošek, A. Rodič, A. Osojnik, J. Klofutar: Razvoj domače proizvodnje stelitnih zlitin................. .............................KZT 26 (1992) 1-2, 215-218 Breskvar Bojan, B. Hertl, A. Osojnik, I. Banič-Kranjčevič: Frakcionirana kristalizacija aluminija...................... .............................KZT 26 (1992) 1-2, 219-222 Smajič Nijaz: Razvoj superferitnega nerjavnega jekla....... .............................KZT 26 (1992) 1-2, 223-225 Paulin Andrej, V. Gontarev, D. Dretnik: Pridobivanje plemenitih kovin iz sekundarnih surovin z majhnim deležem teh kovin........................KZT 26 (1992) 1-2, 226-229 Obal M., S. Rozman, G. Todorovič, S. Fajmut-Štrucelj: Pridobivanje Ge-preeipitata iz ZnS-koncentrata flotacije Rudnika Mežica......................KZT 26 (1992) 1-2, 230-233 Obal M., S. Rozman, R. Jager, M. Kolenc, A. Osojnik: Naravni zeoliti v procesih čiščenja odpadnih voda s povečano vsebnostjo ionov kovin........KZT 26 (1992) 1-2, 234-239 Kaker Henrik: Monte Carlo simulacija v raster elektronski mikroskopiji.................KZT 26 (1992) 1-2, 240-241 Ferlež R.: Uporabne lastnosti superzlitine Ravloy 4........ .............................KZT 26 (1992) 1-2. 242-243 Uršič Viktor, M. Tonkovič-Prijanovič, R. Jud: Volumske spremembe med strjevanjem nodularne litine............... .............................KZT 26 (1992) 1-2, 244-249 Serdarevič M., F. Mujezinovič, H. Babahmetovič: Poboljšanje kvaliteta kovačkih ingota izradom djelomično de- zoksidiranog čelika u peči i dezoksidacijom u vakuumu..... .............................KZT 26 (1992) 1-2, 250-253 Pihura D.: Utjecaj RH-postupka na smanjenje rezidual-nih i oligo elemenata u visokokvalitetnim visokougljeničnim čelicima i unaprijedjenje kvaliteta......................... .............................KZT 26 (1992) 1-2, 254-254 Kert J., J. Kunstelj, M. Podgornik, J. Lamut: Razvoj ekološkega inženiringa v slovenskih železarnah............. .............................KZT 26 (1992) 1-2, 254-254 Kejžar Rajko, U. Kejžar, M. Hrženjak, L. Kosec: Produktivno navarjanje visoko legiranih jekel na konstrukcijska jekla .............................KZT 26 (1992) 1-2, 254-255 Kejžar Rajko, B. Kejžar, M. Hrženjak, J. Lamut, J. Sa-vanovič: Sintetični minerali v oblogi bazičnih elektrod in varilnih praškov..................KZT 26 (1992) 1-2, 255-256 Šarler B., A. Košir, A. Križman, D. Križman: Posodobitve procesa kontinuiranega ulivanja na podlagi eksperimentalno potrjenega matematičnega modeliranja..................... .............................KZT 26 (1992) 1-2, 257-257 Kejžar Rajko, M. Hrženjak: Izdelava orodij z navarjanjem v kokilo.....................KZT 26 (1992) 1-2. 257-258 Kejžar Rajko, B. Kejžar: Spremembe mletih ferozlitin pri proizvodnji dodajnih materialov KZT 26 (1992) 1-2. 258-259 Risteski Ice B.: Dimenzioniranje meniska tokom kontinuira-nog livenja čelika...............KZT 26 (1992) 3, 271-274 Bratina Janez: Električni lok v obločni peči............... ...............................KZT 26 (1992) 3. 275-282 Bolčina Marjan: Uporaba PC preglednic s poudarkom na reševanju temperaturnih polj in polj mešanja taline.......... ...............................KZT 26 (1992) 3. 283-288 Smajič Nijaz: Computer Simulation and Optimization of VOD Treatment......................KZT 26 (1992) 4, 313-317 Vodopivec Franc: Microalloying of Steel................. ...............................KZT 26 (1992) 4, 319-328 Črnko Josip: The Dependence of the Heat Energy Consumption upon the Working Intensity and the Frequency of the Iso- lation Maintenance of a Pusher-type Furnace............... ...............................KZT 26 (1992) 4, 329-331 Arzcnšek Boris, B. Šuštaršič, I. Kos, K. Zalesnik, F. Marolt, G. Velikajnc: Tool-Steel Wire Drawing at Ele- vated Temperatures......................KZT 26 (1992) 4, 333-335 Odesskij P. 1)., V. A. Kučerenko, N. Kudajbergenov, I. Kosec, F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation......KZT 26 (1992) 4. 337-342 Gojič M., M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina: Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement..................KZT 26 (1992) 4, 343-349 Jenko Monika, A. Kveder, S. Spruk, L. Koller: Chromizing of Iron.........................KZT 26 (1992) 4, 351-355 2. Avtorsko kazalo Arzcnšek Boris, B. Šuštaršič, I. Kos, K. Zalesnik, F. Marolt, G. Velikajne: Tool-Steel Wire Drawing at Ele- vated Temperatures......................KZT 26 (1992) 4, 333-335 Ažman Slavko: Razvoj in problematika mikrolegiranih jekel za petrokemično industrijo.....KZT 26 (1992) 1-2. 015-022 Bolčina Marjan: Uporaba PC preglednic s poudarkom na reševanju temperaturnih polj in polj mešanja taline......... ............ ..................KZT 26 (1992) 3, 283-288 Božič Antonija: Problematika sežigalnih naprav........... .............................KZT 26 (1992) 1-2. 114-117 Bratina Janez: Električni lok v obločni peči............... ...............................KZT 26 (1992) 3, 275-282 Breskvar Bojan, B. Hertl, A. Osojnik, I. Banič-Kranjčevič: Frakcionirana kristalizacija aluminija...................... .............................KZT 26 (1992) 1-2, 219-222 Črnko Josip: The Dependence of the Heat Energy Consumption upon the Working Intensity and the Frequency of the Iso- lation Maintenance of a Pusher-type Furnace............... ...............................KZT 26 (1992) 4, 329-331 Doberšek M., L Kosovinc: Vplivi indija na lastnosti dentalnih zlitin Ag-Pd-Au 1 -Cu-Zn5-In5 .. KZT 26 (1992) 1-2. 183-187 Dobi D.: Uporaba instrumentiranega Charpyja pri razvoju jekel .............................KZT 26 (1992) 1-2. 063-073 Drofenik B., F. Vodopivec, M. Jenko: Rast rekristaliziranih zm v zlitini železa in silicija mikrolegirani s selenom in kositrom.....................KZT 26 (1992) 1-2, 151-155 Ferlež R.: Uporabne lastnosti superzlitine Ravloy 4........ .............................KZT 26 (1992) 1-2, 242-243 Gojič M., M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina: Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement..................KZT 26 (1992) 4. 343-349 Hajnže D., F. Vodopivec, M. Jenko: Rast rekristaliziranih zrn v zlitini Fe in Si..............KZT 26 (1992) 1-2, 147-150 Iharoš B.: Izrada normativno-tehnološke karte hladnog vučenja čeličnih cijevi........KZT 26 (1992) 1-2, 097-099 Jenko Monika, F. Vodopivec, B. Praček: Raziskave segre- gacij na površini Fe-Si-C-Sb zlitin z metodo AES......... .............................KZT 26 (1992) 1-2, 201-204 Jenko Monika, A. Kveder, S. Spruk, L,. Koller: Chromizing of Iron.........................KZT 26 (1992) 4, 351-355 Kaker Henrik: Monte Carlo simulacija v raster elektronski mikroskopiji.................KZT 26 (1992) 1-2. 240-241 Kanalcc Slavko, M. Tolar, J. Lamut: Racionalizacije porabe apna v jeklarni Bela..........KZT 26 (1992) 1-2. 140-142 Kejžar Rajko: Oplemenitenje površin z navarjanjem in metalizacijo..................KZT 26 (1992) 1-2. 079-084 Kejžar Rajko. U. Kejžar, M. Hrženjak, L. Kosec: Produktivno navarjanje visoko legiranih jekel na konstrukcijska jekla .............................KZT 26 (1992) 1-2. 254-255 Kejžar Rajko, B. Kejžar, M. Hrženjak, J. Lamut, J. Sa-vanovič: Sintetični minerali v oblogi bazičnih elektrod in varilnih praškov..................KZT 26 (1992) 1-2. 255-256 Kejžar Rajko, M. Hrženjak: Izdelava orodij z navarjanjem v kokilo.....................KZT 26 (1992) 1-2. 257-258 Kejžar Rajko, B. Kejžar: Spremembe mletih ferozlitin pri proizvodnji dodajnih materialov KZT 26 (1992) 1-2. 258-259 Kcrt A., J. Apat, J. Lamut: Pretaljevanje sekundarnih surovin .............................KZT 26 (1992) 1-2. 143-146 Kert J., J. Kunstelj, M. Podgornik, J. Lamut: Razvoj ekološkega inženiringa v slovenskih železarnah............. .............................KZT 26 (1992) 1-2. 254-254 Kolenko Tomaž, B. Glogovac, D. Novak, D. Žagar, B. Omejc: Konceptualna rešitev procesnega vodenja ogrevanja vložka v potisni peči..........KZT 26 (1992) 1-2, 085-088 Koroušič Blaženko: Pomen matematičnega modeliranja pri študiju jeklarskih procesov.....KZT 26 (1992) 1-2, 194-196 Kosec Ladislav, N. Igerc, B. Kosec, B. Godec, B. Urnaut: Temperaturna utrujenost jekel.. KZT 26 (1992) 1-2, 074-078 Kostajnšek J.: Čiščenje talin z vpihovalnim rotorjem za uvajanje čistilnih plinov v talino... KZT 26 (1992) 1-2, 178-180 Krajcar Josip, V. Ferketič, D. Vukovič, A. Ivančan, J. Bu-torac, A. Iharoš: Površinske greške čeličanskog izvora na bešavnim cijevima............KZT 26 (1992) 1-2, 094-096 Kristan T.: Aluminij in avtomobilska industrija............ .............................KZT 26 (1992) 1-2, 181-182 Kurbos Mojca, J. Lamut, T. Kolenko, M. Debelak: Vpliv pogojev konti litja na lastnosti slabov...................... .............................KZT 26 (1992) 1-2, 131-133 Lamut Jakob, F. Pavlin, A. Poklukar: Taljenje vložka s plinom v jaškasti kupolni peči . KZT 26 (1992) 1-2, 038-040 Legat Franc: Toplotna obdelava verig s poudarkom na indukciji.....................KZT 26 (1992) 1-2, 049-052 Lesjak D., J. Lamut, V. Gontarcv, A. Purkat, M. Debelak: Modelne raziskave v jeklarstvu.KZT 26 (1992) 1-2, 134-139 Leš P., F. Dover: Tehnologija valjanja radialno orebrenih cevi za prenos toplote.............KZT 26 (1992) 1-2, 089-093 Lovrečič Saražin Marko: Hamiltonski indeks grafa........ .............................KZT 26 (1992) 1-2, 123-124 Obal M., S. Rozman, G. Todorovič, S. Fajmut-Štrucclj: Pridobivanje Ge-precipitata iz ZnS-koncentrata llotacije Rudnika Mežica......................KZT 26 (1992) 1-2, 230-233 Obal M., S. Rozman, R. Jager, M. Kolenc, A. Osojnik: Naravni zeoliti v procesih čiščenja odpadnih voda s povečano vsebnostjo ionov kovin........KZT 26 (1992) 1-2, 234-239 Odesskij P. D., V. A. Kučerenko, N. Kudajbergenov, I. Kosec, F. Kržič: Resistance of Structural Steel to Crack Formation and Propagation......KZT 26 (1992) 4, 337-342 Paulin Andrej, V. Gontarev, D. Dretnik: Pridobivanje plemenitih kovin iz sekundarnih surovin z majhnim deležem teh kovin........................KZT 26 (1992) 1-2, 226-229 Pihura D.: Utjecaj RH-postupka na smanjenje rezidual-nih i oligo elemenata u visokokvalitetnim visokougljeničnim čelicima i unaprijedjenje kvaliteta......................... .............................KZT 26 (1992) 1-2, 254-254 PloStajner Henrik, V. Prešern, G. Todorovič: Optimizacija tehnologije izdelave in odlivanja jekla v Železarni Store..... .............................KZT 26 (1992) 1-2, 034-037 Rcnko M., A. Osojnik: Razvoj metod za analizo redkih zemelj v zlitinah s posebnimi lastnostmi.......................... .............................KZT 26 (1992) 1-2, 118-122 Rihar G.: Obdelava kovin z žarkovnimi izvori energije..... .............................KZT 26 (1992) 1-2, 110-113 Risteski Ice B.: Dimenzioniranje meniska tokom kontinuira-nog livenja čelika...............KZT 26 (1992) 3, 271-274 Rodič Alenka, J. Žvokelj, F. Legat, S. Krivec: Vpliv kemijske sestave na lastnosti jekel za verige po toplotni obdelavi. .............................KZT 26 (1992) 1-2, 053-057 Rodič Jože, K. Habijan, M. Strohmaicr, J. Dolenc, A. Jagodic, D. Sikošek, A. Rodič, A. Osojnik, J. Klofutar: Razvoj domače proizvodnje stelitnih zlitin................. .............................KZT 26 (1992) 1-2, 215-218 Serdarevič M., F. Mujezinovič, H. Babahmetovič: Poboljšanje kvaliteta kovačkih ingota izradom djelomično de- zoksidiranog čelika u peči i dezoksidacijom u vakuumu..... .............................KZT 26 (1992) 1-2, 250-253 Smajič Nijaz: Dinamični model vakuumske redukcije VOD žlinder......................KZT 26 (1992) 1-2, 205-208 Smajič Nijaz: Razvoj superferitnega nerjavnega jekla....... .............................KZT 26 (1992) 1-2, 223-225 Smajič Nijaz: Computer Simulation and Optimization of VOD Treatment......................KZT 26 (1992) 4, 313-317 Smolej A.: Razvoj sodobnih zlitin aluminija............... .............................KZT 26 (1992) 1-2. 172-177 Šarler B., A. Košir, A. Križman, D. Križman: Posodobitve procesa kontinuiranega ulivanja na podlagi eksperimentalno potrjenega matematičnega modeliranja..................... .............................KZT 26 (1992) 1-2, 257-257 Segel Jože: Računalniška podpora krmiljenju proizvodnje... .............................KZT 26 (1992) 1-2, 188-193 Stok B., N. Mole: Numerična simulacija procesa izdelave ingotov po EPŽ postopku.......KZT 26 (1992) 1-2, 197-200 Šuštaršič Borivoj, Z. Lengar, S. Tašner, J. Holc, S. Be-seničar: Izdelava AlNiCo trajnih magnetov iz vodno atom-iziranih prahov...............KZT 26 (1992) 1-2, 104-109 Šuštaršič Borivoj, M. Torkar, M. Jenko, B. Breskvar, F.Vodopivec: Procesi atomizacije kovinskih gradiv in konsolidacije kovinskih prahov — II. del........................ .............................KZT 26 (1992) 1-2, 209-214 Tehovnik F., B. Koroušič, V. Prešern: Optimizacija modifikacije nekovinskih vključkov v jeklih obdelanih s Ca...... .............................KZT 26 (1992) 1-2, 125-130 Tolar Miha, J. Lamut: Racionalizacija in optimiranje proizvodnje v jeklarni Bela .... KZT 26 (1992) 1-2, 023-029 Torkar Matjaž, B. Šuštaršič: Preiskava vodno atomiziranega prahu iz zlitine Nimonic 80A .. KZT 26 (1992) 1-2, 100-103 Triplat Jože: Razvoj tehnologije izdelave nerjavnih jekel v Železarni Jesenice od leta 1984 naprej..................... .............................KZT 26 (1992) 1-2, 030-033 Uršič Viktor, M. Tonkovič-Prijanovič, R. Jud: Volumske spremembe med strjevanjem nodularne litine............... .............................KZT 26 (1992) 1-2, 244-249 Vehovar Leopold, M. Mavhar: Korozijska odpornost nerjavnega superferitnega jekla Acrom 1 super v primerjavi z avstenitnim nerjavnim jeklom Acroni 11 Ti................ .............................KZT 26 (1992) 1-2, 156-164 Vehovar Leopold, M. Pečnik: Korozijska odpornost superzl-iline Ravloy 4................KZT 26 (1992) 1-2, 165-168 Vehovar Leopold, T. Pavlin: Prepustnost mikrolegiranih jekel za vodik.....................KZT 26 (1992) 1-2, 169-171 Vižintin J.: Razvoj in pomen tribologije doma in v svetu ... .............................KZT 26 (1992) 1-2, 041-048 Vodopivec Franc: Microalloying of Steel................. ...............................KZT 26 (1992) 4, 319-328 Vojvodič-Gvardjančič Jelena: Nizkotemperatuma meja uporabnosti mikrolegiranih jekel s stališča lomne mehanike . . .............................KZT 26 (1992) 1-2, 058-062 Contents N. Smajič Computer Simulation and Optimization of VOD Treatment KZT, 26 (1992) 4, p 313-317 Mathematical model and the software CAPSS (Computer Aided Produc tion of Stainless Steel) developed as a part of URP-C2-2566 research program were used for computer simulation of EAF-VOD-CC stainless steelmaking technology line. Basic aim of model testing was to optimise VOD treatment vvith emphasis on obtaining the maximum productivity at the lowest possible thermal load of VOD ladle and EAF tap temperature. It vvas concluded that only computer controlled oxygen blowing can secure maximum productivity at the acceptable thermal load of VOD ladle and lowest EA furnace tap temperature. Author's Abstract Microalloying, microstructure, mechanical properties, controlled rolling, precipitation F. Vodopivec Microalloying of Steel KZT, 26 (1992) 4,'p 319-328 The article is a revievv on the effects of microalloying on steel properties. The follovving topics are deseribed: austenite grain size and homogenetic, precipitation hardening, precipitation processes during the hot vvorking and the inhibition of static recrystallisation of austenite, yield stress, noteh toughness and transition temperature brittle-ductile fracture, interaetion of elements in precipitates, controlled rolling and forging, rolling vvith controlled recrystalli-sation and the economy of microalloying of structural steels vvith aluminium, niobium, vanadium and titanium. Author's Abstract J. Čmko The Dependence of the Heat Energy Consumption upon the Working Intensitv and the Frequency of the Isolation Maintenance of a Pusher-type Furnace KZT, 26 (1992) 4, p 329-331 The paper presents results of the analysis of the influence of the vvorking intensity and isolation maintenance frequency increase on the example of a pusher-type furnace in a strip and billet rolling mili. The results obtained analytically show that the specific heat energy consumption decreases for about 18% if the intensity of the pusher-type furnace inereases 1.81 times. The results obtained analysing operating data of the furnace work do not show significant deviations from the results obtained analytically. The same way, the increase of the isolation maintenance frequency and that of the layers removal from the floor to the half of the period (6 mths, instead of 12 mths) would decrease the average specific heat energy consumption for about 10%. The increase of the pusher-type furnace maintenance frequency can be realized successfully by better month and vveek planning of rolling, vvhereas bringing of the coefficient of capacity utilization of the furnace to normal limits (0.85-0.95) is not possible only by interventions in planning. Author's Abstract Steel workability, tool steels, vvire dravving at elevated temperatures B. Arzenšek, B. Šuštaršič, G. Velikajne, I. Kos, K. Zalesnik, F. Marolt Tool-steel VVire Dravving at Elevated Temperatures KZT, 26 (1992) 4, p 333-335 Tool steels transfer mostly tvvo dravvs at eold dravving, therefore the dravving technology at elevated temperatures vvas developed. The dravving abilities of BRM2 tool steel at temperatures up to 700° C and dravving device for dravving at elevated temperatures, developed at our institute in cooperation vvith Steel Plant Ravne, vvas deseribed in this vvork. The aim at development of the technology vvas to use eold vvire dravving devices as much as possible to cheapen the technology and enable the technology transfer in industrial produetion. Author's Abstract P.D. Odesskij, N. Kudajbergenov, L. Kosec, F. Kržič Resistance of Structural Steel to Crack Formation and Propagation KZT, 26 (1992) 4, p 337-342 Parameters of LEFM can be a measure for seleetion of steel with various strengths and yield strengths. They are applicable only if they are measured in conditions of constrained plastic deformation. This can be achieved by an influence of corrosion medium or vvith impact tests. These parameters are closely interrelated vvith the steel micro strueture and purity. They are suitable for designing struetures resistant to brittle fracture if they succeed to enclose the operational conditions of the strueture. Author's Abstract M. Gojič, M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement KZT, 26 (1992) 4, p 343-349 Various types of damages on pipes used in oil and gas produetion are deseribed. The pipes are made of low-alloyed manganese- vanadium steel. Not heat-treated steel pipes are very sensitive to hydrogen embrittlement (embrittlement index 86%). Hardening and tempering highly improves the resistivity of pipes to hydro gen embrittlement (embrittlement index 25%) vvhich is also inter related to the fracture mechanism in steel. Author's Abstract Contents Chromium diffusion, chromizing of iron, vacuum chro mizing of iron, miniature hermetic relays, Ie-Cr layers resis tive to corrosion media, coercitivity M. Jenko, A. Kveder, S. Spruk, L. Koller Chromizing of Iron KZT, 26 (1992) 4, p 351 -355 The theoretical aspects of CVD processes of iron chromizing and the comparison with PVD process are presented. It is shown that nonc of CVD processes is suitable for the proteetion of iron parts for magnetic circuit in miniature hermetic relays. For this purpose PVD process of vacuum chromizing was developed. With this procedure the maximal chromium content of 15% Cr at the surface was obtained. Such chromium layer assures the optimal magnetic properties, a very low coercitivity and a good weldability. Author's Abstract Vsebina N. Smajič Računalniška simulacija in optiniiranje VOD obdelave KZT, 26 (1992) 4, s 313-317 V okviru petletnega raziskovalnega programa URP-C2-2566 izdelani model in računalniški program CAPSS (Computer Aided Production of Stainless Steel) je bil uporabljen za računalniško simulacijo EOP-VOD-KL tehnologije za izdelavo neijavnih jekel. Osnovni namen modelnih poskusov je bil optimiranje VOD obdelave s posebnim poudarkom na zagotavljanju maksimalne produktivnosti VOD naprave ob najmanjši možni toplotni obremenitvi VOD ponovce in minimalni temperaturi preboda. Ugotovili smo, da le računalniško programirano pihanje zagotavlja maksimalno produktivnost ob še sprejemljivi toplotni obremenitvi VOD ponovce in minimalni temperaturi preboda. Avtorski izvleček F. Vodopivec Mikrolegiranje jekla KZT, 26 (1992) 4, s 319-328 Mehanizmi vpliva mikrolegiranja na trdnostne lastnosti in žilavost jekla. Vpliv na velikost zrn, izločilno utrditev, procesi vroče deformacije in gospodarnost mikrolegiranja z Al, Nb, V in Ti. Avtorski izvleček J. Črnko Ovisnost utroška toplinske energije od inteziteta rada i učestalosti održavanja izolacije potisne peči KZT, 26 (1992)4, s 329-331 U radu su prikazani rezultati analize utjecaja povečanja intenziteta rada i učestalosti održavanja izolacije na primjeru potisne peči u valjaonici traka i gredica. Rezultati dobiveni analitičkim putem pokazuju da se specifični utrošak toplinske energije smanji za oko 18% ako se poveča intenzitet rada potisne peči 1.81 puta. Značajna odstupanja od rezultata dobivenih analitičkim putem ne pokazuju ni rezultati dobiveni analizom pogonskih podataka o radu potisne peči. Isto tako, povečanje učestalosti održavanja izolacije i čiščenja poda od nastalih naslaga na polovicu vremena (6 mjeseci) od dosadašnjeg (12 mjeseci) smanjilo bi prosječni specifični utrošak toplinske energije za oko 10%. Povečanje učestalosti održavanja potisne peči moguče je uspješno ostvariti boljim mejsečnim i tjednim planovima valjanja, dok dovo—enjdco-eficijenta iskorištenja kapaciteta potisne peči u normalne granice (0.85-0.95) nije moguče samo zahvatima u planove valjanja ostvariti. Avtorski izvleček Preoblikovanje jekel, orodna jekla, vlečenje žice pri povišanih temperaturah B. Arzenšek, B. Šuštaršič, G. Velikajne, I. Kos, K. Zalesnik, F. Marolt Vlečenje žice iz orodnih jekel pri povišanih temperaturah KZT, 26 (1992) 4, s 333-335 Orodna jekla prenesejo pri vlečenju v hladnem stanju največ dva vleka, zato smo razvili tehnologijo vlečenja jekel pri povišanih temperaturah. V delu smo opisali vlečne sposobnosti jekla BRM2 pri temperaturah do 700° C in linijo za vlečenje žice, ki smo jo razvili na Inštitutu za kovinske materiale in tehnologije v Ljubljani v sodelovanju s sodelavci iz Železarne Ravne. Cilj pri razvoju tehnologije je bil uporabiti čim več opreme, ki jo uporabljamo tudi pri hladnem vlečenju, kar tehnologijo poceni in omogoča njen hitrejši prenos v proizvodnjo. Avtorski izvleček P.D. Odesskij, N. Kudajbergenov, L. Kosec, F. Kržič Odpornost gradbenih jekel proti nastanku in širjenju razpoke KZT, 26 (1992) 4, s 337-342 Parametri LEFM so lahko merilo za selekcijo jekel z različno trdnostjo ali napetostjo tečenja. Uporabni so le, če so izmerjeni v pogojih zelo omejene plastične deformacije. To se lahko doseže z vplivom korozijskega medija ali pri udarnih preizkusih. Ti parametri so tesno povezani z mikrostrukturo jekla in njegovo čistostjo. Primeri so za izračun konstrukcij odpornih proti krhkemu lomu, če uspejo zapopasti pogoje pri uporabi objektov. Avtorski izvleček M. Gojič, M. Balenovič, L. Kosec, L. Vehovar, L.J. Malina Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti KZT, 26 (1992) 4, s 343-349 Opisane so različne oblike poškodb cevi za pridobivanje nafte in zemeljskega plina iz malolegiranega mangan-vanadijevega jekla. Cevi iz toplotno neobdelanega jekla so zelo občutljive na vodiko vo krhkost (indeks krhkosti 86%), s poboljšanjem jekla pa se odpornost cevi proti vodikovi krhkosti zelo poveča (indeks krhkosti 25%), kar je povezano tudi z mehanizmom prelomov jekla. Avtorski izvleček Vsebina Difuzija kroma, difuzijsko kromanje, vakuumsko kromanje, miniaturni her-metični releji, Fe-Cr plasti odporne proti koroziji, koercitivnost M. Jenko, A. Kveder, S. Spruk, L. Koller Difuzijsko kromanje železa KZT, 26 (1992) 4, s 351-355 Podani so teoretični vidiki CVD postopkov difnzijskega kromanja železa v primerjavi s PVD postopkom. Raziskava CVD postopkov je pokazala, da za zaščito sestavnih delov železnega magnetnega kroga miniaturnih her-metičnih relejev ni primeren nobeden izmed le-teh. Zato so avtorji razvili PVD postopek - tehnologijo vaku umskega kromanja. S tem postopkom je dosežena maksimalna končen tracija kroma na povriSini 15% Cr, kar je dovolj, da so vzorci korozijsko obstojni v medijih, ki jih predpisujejo standardi. S PVD postopkom dosežemo optimalne magnetne lastnosti (nizka koercitivnost) in dobro varivost. Avtorski izvleček .nO debelo, srednjo in tanko pločevino hladno valjane trakove in pločevino dinamo trakove in pločevino nerjavne trakove in pločevino vlečeno, brušeno in luščeno jeklo valjano in vlečeno žico patentirano žico pleteno patentirano žico za prednapeti beton hladno oblikovane profile kovinske podboje za vrata dodajni material za varjenje žičnike tehnične pline STORITVE prevaljanja, vlečenja, iztiskanja in toplotne obdelave pločevin in žice tehnične dejavnosti: elektro, strojne, konstrukcijske, obrtne in tehnične IZDELUJE □ MIKROLEGIRANA JEKLA □ NERJAVNA JEKLA □ ELEKTRO PLOČEVINE IN TRAKOVE □ vroče valjane trakove in pločevine □ hladno valjane trakove in pločevine □ dinamo trakove in pločevine □ nerjavne trakove in pločevine □ hladno oblikovane profile □ kovinske podboje za vrata □ NUDIMO TUDI STORITVE □ prevaljanja, iztiskanja, krojenja in toplotne obdelave pločevin 64270 JESENICE, Cesta železarjev 8 - telefon: (064) 83-561,84-261,81-341 - telefax: (064) 83-395 telex: 37219,37212 zeljsn - telegram: Železarna Jesenice - Slovenija