Bele kromove litine za valje, legirane z molibdenom White Chromium Čast Irons for Rolls, Alloyed vvith Molybdenum D. Kmetic*, F. Mlakar**, V. Tucič**, J. Žvokelj*, UDK: 669.15'26-194:669.14.018.255 F. Vodopivec*, M. Jakupovič*, B. Ralič* ASM/SLA: M28, N8b, TSk, 5, Cr, W23k Kromove bele litine, legirane z molibdenom in še nekaterimi drugimi elementi, se zaradi dobre obrabne obstojnosti, trdote in zadovoljivih mehanskih lastnosti vedno več uporabljajo za d vos lojno lite valje. Litine imajo tudi dobro korozijsko obstojnost. Delo obravnava mikrostrukturne značilnosti zlitin v litem stanju in po toplotni obdelavi. Narejena sta izotermna transformacijska diagrama za destabilizacijo avstenita in destabiliziran avstenit in kontinuirni transformacijski diagram za destabiliziran avstenit. UVOD V valjarnah je poleg ustrezne kvalitete valjanih proizvodov zelo pomembna ekonomičnost proizvodnje. Določena jekla se vroče valjajo v nizkih temperaturnih C v °/o SI. 1: Kemična sestava belih kromovih litin in litin za druge vrste valjev v faznem diagramu (KV — kovani valji, AD — adamitni valji, IND — indefinitni valji, KGR — nodularni valji, Cr — bele kromove litine, 8) Fig. 1 Chemical composition of vvhite chromium čast irons, and čast irons for other types of roils in the phase diagram (KV — forged rolls, AD — adamite rolls, IND — indefinite chill rolls, KGR — spheroidal-graphite rolls, Cr — vvhite chromium čast irons, 8) * SŽ — Metalurški inštitut Ljubljana ** SŽ — Železarna Štore * Institute of Metallurgy, Ljubljana ** Store Ironworks Chromium vvhite čast irons alloyed vvith molybdenum and some other elements are more and more applied for compound čast rolls due to good vvear resistance, hardness, and satisfactory mechanical properties. The čast irons have also good corrosion properties. Paper treats the microstructural characteristics of čast irons as čast, and after the heat treatment. Isother-mal transformation diagrams for the destabilization of austenite, and for the destabilized austenite vvere con-structed next to the continuous transformation diagram for the destabilized austenite. INTRODUCTION In rolling plants, the economy of manufacturing is very important next to the suitable quality of rolled pro-ducts. Some steel is hot rolled in low-temperature re-gions vvith high partial reductions, and low permissible dimensional tolerances. Narovver and narovver toler-ances are demanded also for the cold rolled strips. These are the reasons that rolls of vvhite čast iron vvith high chromium cpntent, and alloyed vvith Mo, Ni (Cu), V, Ti, and W are more and more used in hot and cold rolling plants. The rolls are čast by a compound centrifugal casting. Data on chemical composition of rolls of vvhite chromium čast irons are in references given in wide in-tervals. Data on manufacturing rolls, and on their heat treatment are scarce. The phase diagram in Fig. 1 pres- Ogljik v utežnih procentih Weight percentage of carbon SI. 2: Fazni diagram Fe-Cr-C za 17 % Cr Fig. 2 Fe-Cr-C phase diagram at 17 % Cr SI. 3: Likvidus površine in lega zlitin glede na razmerje Cr/C v faznem diagramu Fe-Cr-C po R. S. Jacksonu (1) Fig. 3 Liquidus surfaces and the position of alloys related to the Cr/C ratio in the Fe-Cr-C phase diagram, according to R. S. Jackson (1) področjih z velikimi parcialnimi redukcijami, pri čemer se zahtevajo ozke dimenzijske tolerance. Vedno bolj ozke tolerance se zahtevajo tudi pri hladno valjanih trakovih. To so razlogi, da se v vročih in hladnih valjarnah vedno bolj uporabljajo valji iz bele litine z visoko vsebnostjo Cr, legirane še z Mo, Ni (Cu), V, Ti in W. Valji se izdelujejo po postopku dvoslojnega centrifugalnega litja. Literaturni podatki o kemični sestavi valjev iz bele kromove litine so podani v širokih mejah. Podatki o izdelavi valjev in toplotni obdelavi so zelo skopi. V faznem diagramu na sliki 1 je za primerjavo navedeno področje kemične sestave valjev iz bele kromove litine in drugih vrst valjev. Zlitine Fe-Cr-C so že dolgo poznane in so v literaturi opisane številne raziskave. Fazni diagram Fe-Cr-C za 17 % Cr je prikazan na sliki 2 (3). Za razvoj teh zlitin so poleg začetnih raziskav F. Osmonda, ki je v mikrostruk-turi omenjenih litin že leta 1892 opazil kompleksne karbide, najpomembnejše raziskave R. S. Jacksona, ki je v faznem diagramu Fe-Cr-C opredelil likvidus površine (si. 3) in sistematične raziskave vpliva Mo na mikro-strukturne značilnosti, ki sta jih naredila F. Maratray in R. Usseglio-Nanot (1,2, 4). Mikrostruktura belih kromovih litin sestoji iz primarnih in evtektičnih karbidov in avstenitne matice, oziroma njenih transformacijskih produktov (sekundarni karbidi, perlit, bainit, martenzit). Za mikrostrukturne značilnosti je zelo pomembno razmerje Cr/C in vsebnost legiranih elementov, predvsem Mo, Mn, Ni (Cu) in W. EKSPERIMENTALNO DELO Na osnovi literaturnih podatkov, ki smo jih imeli na voljo, smo v železarni Štore izdelali preizkusne taline z različno vsebnostjo legirnih elementov, in sicer z 2,5 do 3,8 % C, 11,3 do 19,4 % Cr, 0,39 do 0,66 % Mo, 0,59 do 1,37 % Si, 0,68 do 0,93 % Mn, 0,56 do 0,78 % Ni, 0,023 do 0,11 % Ti in z 0,06 do 0,11 % V. Štiri zlitine smo legi-rali z 0,80 do 0,93 % W. Zlitine legirane z W so trše in se uporabljajo za valje za hladno valjanje trakov. Vsebnost P mora biti pod 0,08 % in S pod 0,05 %. Preizkusne zlitine imajo razmerje Cr/C od 3,62 do 7,76. Vzorce, preizkusne valjčke, premera 100 in višine 150 mm, smo ulili tako, da je bila polovica valjčka ulita v kokilo in polovica v pesek. Tako smo dobili na enem vzorcu dve različni hitrosti strjevanja. Pogoji litja bistveno vplivajo na izoblikovanje mikrostrukture in s tem na mehanske lastnosti litine. Zato ents the regions of chemical compositions of rolls of vvhite chromium čast iron, and of some other types of rolls (8), Fe-Cr-C alloys are already for a long tirne known, and numerous investigations are cited in references. The Fe-Cr-C phase diagram for 17 % Cr is shovvn in Fig. 2 (3). For development of these alloys, the most essential are the investigations by R. S. Jackson who determined the liquidus surfaces in the Fe-Cr-C phase diagram (Fig. 3), and the systematic investigations on the influence of Mo on the microstructural characteristics done by F. Mara-tray, and R. Usseglio — Nanot, beside the initial investigations by F. Osmond who already in 1892 observed complex carbides in the microstructure of the men-tioned čast irons (1, 2, 4). Microstructure of vvhite chromium čast irons con-sists of primary and eutectic carbides, and austenitic matrix, or of its transformation products (secondary carbides, pearlite, bainite, martensite). Essential for the microstructural characteristics are the Cr/C ratio and the content of alloying elements, mainly Mo, Mn, Ni (Cu), and W. EXPERIMENTAL WORK Based on the data in references, being available, test melts vvith various contents of alloying elements, i. e. vvith 2.5 to 3.8 % C, 11.3 to 19.4 % Cr, 0.39 to 0.66 % Mo, 0.59 to 1.37 % Si, 0.68 to 0.93 % Mn, 0.56 to 0.78 % Ni, 0.023 to 0.11 % Ti, and 0.06 to 0.11 % V vvere made in the Štore Ironvvorks. Alloys vvith added W are harder and they are used for rolls for cold rolling of strips. Phospho-rus content must be belovv 0.08 %, and sulphur belovv 0.05 %. The test melts had the Cr/C ratio between 3.62 and 7.76. The samples as testing cylinders vvith diameter 100 mm and 150 mm high vvere čast so that one half of the cylinder vvas čast into mould, another one into sand. Thus two various solidification rates vvere obtained on the same specimen. Casting conditions have essential influence on the formation of microstructure, and thus on the mechanical properties. Therefore melting points and solidification in-tervals vvere determined for some alloys. Microstructural characteristics of as čast alloys, and after the heat treatment vvere determined by investigations vvith optical microscope, scanning electron micro-scope (SEM), and electron microanalyzer. To reveal the microstructural characteristics various etching agents (nital, Villela's, ferric chloride, alkaline picrate, Muraka-mi's, and 4% sodium hydroxide saturated vvith pota- smo za nekatere zlitine določili temperature tališča in intervale strjevanja. Mikrostrukturne značilnosti zlitin v litem stanju in po toplotni obdelavi smo opredelili s preiskavami z optičnim mikroskopom, v raster elektronskem mikroskopu (SEM) in v elektronskem mikroanalizatorju. Za odkrivanje mikrostrukturnih značilnosti smo uporabili različna jedkala (nital, Villela, feriklorid, alkalijski pikrat, Murakami in 4 % natrijev hidroksid, nasičen s kalijevim permanganatom). Sekundarne karbide in faze, nastale pri transformaciji avstenita, smo lahko dobro opredelili v SEM. V elektronskem mikroanalizatorju smo določili sestavo primarnih in evtektičnih karbidov in koncentracije nekaterih legirnih elementov v matici. Za eno od zlitin z najustreznejšo kemično sestavo in mikrostrukturo smo naredili izotermna transformacijska diagrama za nedestabilizirano in destabilizirano av-stenitno matico in kontinuirni transformacijski diagram za destabiliziran avstenit. Od mehanskih lastnosti smo merili le trdoto zlitin in posameznih mikrostrukturnih faz. V literaturi smo zasledili raziskave, ki obravnavajo upogibno trdnost in žilavost teh zlitin (13). Za valje je poznavanje teh parametrov zelo pomembno, vendar smo zaradi težavne priprave mehanskih preizkušancev te preiskave odložili na kasnejši čas. REZULTATI PREISKAV Tališča in interval strjevanja zlitin Žilavost in obrabna obstojnost litine je tem boljša, čim bolj drobni so evtektični karbidi in čim enakomer-neje so porazdeljeni po matici. (19) Zato mora potekati strjevanje belih kromovih litin hitro. Pri previsokem pregretju in počasnem strjevanju lahko nastanejo poleg grobih evtektičnih klarbidov še veliki primarni karbidi. V talilnem mikroskopu smo določili tališča in intervale taljenja nekaterih zlitin, izbranih tako, da smo pokrili ves interval razmerij Cr/C (tabela 1). Zaradi reka-lescence je razlika med talilnim in strjevalnim intervalom majhna. Zlitine so močno izcejane in se rezultati paralelk in vrednosti, izmerjene večkrat na istem vzorcu, med seboj precej razlikujejo. Tabela 1: Tališča in intervali taljenja Zlitina %C % Cr Cr/C Nastanek kapljic °C Začetek talj. Staljeno Interval talj. "C °C •c 1 2,49 19,31 7,76 1200 1245 1345 100 2 2,63 19,43 7,39 1170 1220 1350 130 (legira-noz W) 3 2,72 14,90 5,48 1200 1250 1305 55 4 2,76 19,21 6,96 1210 1250 1325 75 5 3,20 17,95 5,61 1210 1250 1285 35 7 3,31 11,97 3,62 1190 1225 1300 75 8 3,48 16,23 4,66 1170 1215 1250 35 (legira-noz W) Zlitine imajo tališča med 1350 in 1250 °C. Čim bolj se sestava zlitine približuje evtektični sestavi, ožji je interval strjevanja. Na sliki 2 se vidi, da ima zlitina s 17 % Cr evtektično sestavo pri 3,4 % Č. Na tališče in in- ssium permanganate) were applied. Secondary car-bides, and phases formed during the transformation of austenite were well determined by SEM. Electron micro-analyzer helped us to determine the composition of pri-mary and eutectic carbides, and the concentrations of some alloying elements in the matrix. For one of the alloys, vvith the most suitable chemical composition and the microstructure, the isothermal transformation diagrams for undestabilized and des-tabilized austenitic matrix, and the continuous transformation diagram for destabilized austenite were con-structed. Of mechanical properties only hardness of alloys and of single microstructural phases vvas measured. In refer-ences, investigations treating the bending strength, and the toughness of these alloys vvere found (13). Though the knovvledge of these properties is very important for the behaviour of rolls, these investigations vvere post-poned for later due to difficult preparation of testing specimens. RESULTS OF INVESTIGATIONS Melting Points and Soliditication Interval of Alloys Toughness and vvear resistance of the alloy are the better the smaller are eutectic carbides, and the more uniformly they are distributed in the matrix (19). There-fore the solidification of vvhite chromium čast irons mUs.t be fast. At a too high superheating and low solidification rate big primary carbides next to coarse eutectic carbides can be formed. Melting points and solidification intervals of some al-loys vvere determined by fusion microscope. The alloys vvere chosen in such a way that he vvhole interval of the Cr/C ratios vvas covered (Table 1). Due to recalescence the difference betvveen the melting and the solidification interval is small. The alloys exhibit intensive segregating, thus the results of parallel tests, and the values measured more times on the same sample differ a great deal. Table 1 Melting Points and Solidification Intervals Alloy %C % Cr Cr/C Formation of Begin. of Melted Melting interval "C drops 'C melting "C •c 1 2,49 19,31 7,76 1200 1245 1345 100 2 2,63 19,43 7,39 1170 1220 1350 130 (alloyed with W) 3 2,72 14,90 5,48 1200 1250 1305 55 4 2,76 19,21 6,96 1210 1250 1325 75 5 3,20 17,95 5,61 1210 1250 1285 35 7 3,31 11,97 3,62 1190 1225 1300 75 8 3,48 16,23 4,66 1170 1215 1250 35 (alloyed vvith W) The alloys have the melting points betvveen 1350 and 1250° C. The closer is the alloy composition to the eutectic composition the narrovver is the solidification interval. It is evident from the Fig. 2 that the alloy vvith 17 % Cr has eutectic composition at 3.4 % C. The melting points and the solidification intervals are mainly influenced by the carbon content, to a lesser extent by the Cr/C ratio, terval strjevanja vpliva predvsem vsebnost ogljika, manj pa razmerje Cr/C in koncentracije ostalih legirnih elementov. Od vsebnosti ogljika, ki sicer znižuje temperaturo tališča, in razmerja Cr/C je odvisen delež karbidne faze v mikrostrukturi, kar tudi vpliva na tališče in interval strjevanja zlitin. Iz faznih diagramov Fe-Cr-C se vidi, da se z naraščajočo vsebnostjo Cr evtektična točka pomika v levo in k višjim temperaturam. Mikrostruktura zlitin v litem stanju Mikrostruktura zlitin je odvisna od kemične sestave, razmerja Cr/C in pogojev strjevanja. Vse zlitine smo le-girali z Mo, zato imajo v mikrostrukturi poleg primarnih in evtektičnih karbidov M7C3 tudi karbide Mo2C. Mikrostruktura evtektika je odvisna od deleža avste-nitne faze, ki nastaja med procesom strjevanja. Če nastane med strjevanjem veliko avstenita in je majhen delež preostale taline, ki se strdi kot evtektik, imajo evtek-tični karbidi tendenco, da segregirajo vzdolž kristalnih mej avstenitnih zrn. Take mikrostrukture, ki je značilna za zlitine z do 20 % karbidne faze, pri naših zlitinah, ki imajo od 25 do 35 % karbidne faze, nismo opazili. V nekaterih zlitinah smo opazili v evtektiku bolj ali manj lamelarno izoblikovane karbide, ki rastejo iz sredine meddendritskih prostorov (si. 4) Pri drugih zlitinah, pri katerih je avstenitne faze zelo malo in ta praktično ni omejevala strjevanja evtektika, imajo karbidi popolnoma lamelarno obliko (si. 5). Čeprav so veliki primarni karbidi heksagonalne oblike značilni za litine z nad 35 % karbidne faze, smo te opazili tudi pri nekaterih naših zlitinah, in to predvsem na sredini preizkusnih valjčkov, kjer so bili za njihov nastanek ustreznejši pogoji (si. 6.). Deleže karbidne faze v mikrostrukturi smo za nekatere zlitind izračunali po enačbi (1): % K = 12,33 (%C) + 0,55 (% Cr) - 15,2 Izračunane vrednosti se dobro ujemajo z vrednostmi, ki smo jih dobili z meritvami po linearni intercepcij-ski metodi v optičnem mikroskopu (tabela 2). Vsebnosti Mo in W sta majhni in ne vplivata bistveno na delež karbidne faze. Tabela 2: Delež karbidne faze (% K) v mikrostrukturi Zlitina Cr/C %C % Cr % K izračunan % K izmerje 7 3,62 3,31 11,97 32,2 30 6 4,80 3,21 15,42 32,9 29 3 5,48 2,72 14,90 26,5 26 5 5,61 3,20 17,95 34,1 35 2 7,39 2,62 19,43 27,9 28 1 7,76 2,49 19,31 26,1 25 Mikrostruktura matice je odvisna od razmerja Cr/C, vsebnosti Mo in pogojev ohlajevanja. Matica ima v litem stanju avstenitno mikrostrukturo, oz. je med ohlajanjem potekla delna ali popolna transformacija avstenita v perlit. Pri litju v kokilo potekata strjevanje in ohlajanje hitreje, kot pri litju v pesek, in perlitna transformacija je zavrta. Pri zlitinah brez Mo lahko pričakujemo popolnoma avstenitno matico pri razmerju Cr/C večjem od 7,2 (1,2). Z legiranjem z Mo se razmerje Cr/C, pri katerem dobimo popolnoma avstenitno matico, pomika proti nižjim vrednostim. To pomeni, da ima lahko litina pri isti vsebnosti Cr več C in zato v mikrostrukturi večji delež karbidne faze in avstenitno matico. and the concentration of the other alloying elements. The carbon content vvhich namely reduces the melting point, and the Cr/C ratio determine the amount of car-bide phase in the microstructure vvhich has also influence on the melting point and the solidification interval of alloys. The Fe-Cr-C phase diagrams shovv that the in-creasing Cr content shifts the eutectic point tovvards the left and to higher temperatures. Microstructure of As Čast Alloys Microstructure of alloys depends on the chemical composition, the Cr/C ratio, and the conditions of solidification. Ali the alloys were alloyed vvith Mo, thus the microstructure contains also Mo2C carbides next to the primary and eutectic M7C3 carbides. Microstructure of eutectic depends on the amount of austenitic phase vvhich is formed during the solidification. If a high amount of austenite is formed during the solidification, and the portion of the remaining melt vvhich solidifies eutectically is small, the eutectic carbides exhi-bit tendency to segregate along the boundaries of austenitic grains. Such a microstructure being charac-teristic for the alloys vvith up to 20 % of carbide phase was not observed in our alloys which contained 25 to 35 % of carbide phase. In some alloys more or less lamellar carbides were observed vvhich grovv from the centre of interdendritic spaces (Fig. 4). In other alloys vvith a very lovv amount of austenitic phase vvhich did not hinder the solidification of eutectic, the carbides exhibited fully lamellar shape (Fig. 5). Though big primary carbides of hexagonal shape are characteristic for the čast irons vvith over 35 % of carbide phase, they vvere obseved also in some of our alloys, but mainly in the centre of the testing cylinders vvhere the conditions for their formation vvere the most suitable (Fig. 6). The portions of carbide phase in the microstructure vvas for some alloys evaluated by the equation (1): % K = 12.33 (% C) +0.55 (% Cr) - 15.2 The obtained values are in a good agreement vvith the values vvhich vvere obtained by the measurements in optical microscope by the intercept method (Table 2). Contents of Mo and W are lovv and they do not influence essentially the portion of carbide phase. SI. 4: Eutektični karbidi rastejo iz sredine meddendritskih prostorov, lito stanje (3,21 % C, 15,42 % Cr, 0,53 % Mo, Cr/C 4,80). Pov. 100 x Fig. 4 Eutectic carbides grovv from the centre of interdendritic spaces, as čast (3.21 % C, 15.42% Cr, 0.53% Mo, Cr/C 4.80). Magn. 100x Table 2 Portion of Carbide Phase (% K) in the Microstructure Zlitina Cr/C %C % Cr Karbidi primarni eutek. Matica perlit austenit 7 3.62 3.31 11.97 0.58 38.3Cr 32.9 Cr 6.7 Cr 0.55 Mo 0.45 Mo 0.26 Mo _ 0.8 Mn 0.7 Mn 0.6 Mn — 6 4.80 3.21 15.42 0.53 42.4Cr 37.0Cr 7.7 Cr 7.0 Cr 0.44Mo 0.46 Mo 0.26 Mo 0.34 Mo 3 5.48 2.72 14.90 0.56 44.8Cr 39.9 Cr — 10.5Cr 0.97 Mo 0.92 Mo - 0.13 Mo 0.9 Mn 0.8 Mn _ 0.7 Mn 5 5.61 3.20 17.95 0.63 48.4Cr 43.8 Cr — IO,2Cr 0.42 Mo 0.42 Mo — 0.23 Mo 0.95 Mn 0.9 Mn — 0.8 Mn 2 7.39 2.62 19.43 0.52 50.7 Cr 49.0 Cr _ 10.8Cr 0.83 0.49 Mo 0.46 Mo _ 0.3 Mo W 0.59 W 0.61 W 0.44 W 0.9 Mn 0.9 Mn _ 0.7 Mn 1 7.76 2.49 19.31 0.54 51.2Cr 49.0 Cr _ 10.5 Cr 0.41 Mo 0.44 Mo — 0.31 Mo Alloy Cr/C %C % Cr % K calculated measured 7 3.62 3.31 11.97 32.2 30 6 4.80 3.21 15.42 32.9 29 3 5.48 2.72 14.90 26.5 26 5 5.61 3.20 17.95 34.1 35 2 7.39 2.62 19.43 27.9 28 1 7.76 2.49 19.31 26.1 25 ■MM MHHBP* "»PSMfi^tess«. "'SfiSdK. HB SI. 5: Lamelami eutektični karbidi, lito stanje (3,20 % C, 17,95 % Cr, 0,63 % Mo, Cr/C 5,61). Pov. 100 x Fig. 5 Lamellar eutectic carbides, as čast (3.20% C, 17.95% Cr, 0.63% Mo, Cr/C 5.61). Magn. 100 x Preiskovane zlitine so legirane z Mo in preizkusni valjčki imajo na presekih, ulitih v kokilo pri razmerjih Cr/C nad 5,5, popolnoma avstenitno matico (si. 5). Le v večji oddaljenosti od površine smo pri nekaterih vzorcih opazili v mikrostrukturi manjša perlitna zrna. S padajočo vrednostjo razmerja Cr/C narašča v matici delež perlitne faze. Avstenitno perlitna mikrostruktura matice je prikazana na sliki 4. Perlitno matico lahko reavstenitiziramo in tako zagotovimo, da ima litina po destabilizaciji in transformaciji s stališča mehanskih lastnosti ustreznejšo mikrostrukturo matice (martenzit). Menimo, da z ogrevanjem avstenitno perlitnih litin 50 °C pod solidus temperaturo dobimo avstenitno mikrostrukturo matice. Za natančnejše pogoje reavstenitizacije so v literaturi podani diagrami (1, 2). Vsekakor pa je ugodneje, da z razmerjem Cr/C, legiranjem z Mo in pogoji strjevanja že v litem stanju zagotovimo litini avstenitno mikrostrukturo matice (9). Kemična sestava karbidov in matice Koncentracije Cr, Mo, Mn in W v primarnih in ev-tektičnih karbidih in v matici, izmerjene v elektronskem mikroanalizatorju, so podane v tabeli 3. Meritve smo naredili na vzorcih ulitih v kokilo. Tabela 3: Vsebnosti Cr, Mo, Mn in W v karbidih in matici Microstructure of matrix depends on the Cr/C ratio, amount of Mo, and conditions of solidification. Matrix as čast has austenitic microstructure, or a partial or com-plete transformation of austenite into pearlite occured during the solidification. Solidification and cooling are faster in casting into moulds than in casting into sand, and pearlitic transformation is retarded. In alloys vvithout Mo fully austenitic matrix can be expected at the Cr/C ratios higher than 7.2 (1, 2). Alloying vvith Mo shifts the Cr/C ratio at vvhich fully austenitic matrix is obtained tovvards lovver values. This means that čast iron can con-tain at the same Cr more C, and thus a greater portion of carbide phase can be in the microstructure. Investigated alloys were alloyed vvith Mo, and the testing cylinders exhibit on the cross sections čast into mould a fully austenitic matrix if Cr/C ratio vvas over 5.5 (Fig. 5). Only at a greater distance from the surface smaller pearlitic grains vvere observed in the microstructure of some samples. The reduced Cr/C ratio causes an increased amount of pearlitic phase in the matrix. The austenitic-pearlitic microstructure of the matrix is shovvn in Fig. 4. Pearlitic matrix can be reaustenitized, and thus it is ensured that the čast iron has a more suitable microstructure of matrix (martensite) from the vievvpoint of mechanical properties after the destabilization and the transformation. It is supposed that heating austenitic-pearlitic čast iron at 50° C belovv the solidus temperature gives austenitic microstructure of the matrix. More de-tailed conditions of reaustenitization are given in graphs in references (1,2). Anyhow, it is more favourable to en-sure the austenitic microstructure of the matrix in the as čast alloy by the Cr/C ratio, alloying vvith Mo, and the conditions of solidification. Chemical Composition of Carbides and of Matrix Concentrations of Cr, Mo, Mn, and W in the primary and the eutectic carbides, and in the matrix, measured by the electron microanalyzer are presented in Table 3. Table 3 Contents of Cr, Mo, Mn, and W in Carbides and in the Matrix V diagramu na sliki 7 je prikazana odvisnost med razmerjem Cr/C v zlitinah in razmerjem Fe/Cr v karbidih Alloy Cr/C % C % Cr M Carbides Primary Eutec. Matrix Pearlite Austen. 7 3.62 3.31 11.97 0.58 38.3 Cr 32.9 Cr 6.7 Cr _ 0.55 Mo 0.45 Mo 0.26 Mo — 0.8 Mn 0.7 Mn 0.6 Mn — 6 4.80 321 15.42 0.53 42.4 Cr 37.0Cr 7.7 Cr 7.0 Cr 0.44 Mo 0.46 Mo 0.26 Mo 0.34 Mo 3 5.48 2.72 14.90 0.56 44 8Cr 39.9 Cr — 10.5 Cr 0.97 Mo 0.92 Mo — 0.13MO 0.9 Mn 0.8 Mn 0.7 Mn 5 5.61 3.20 17.95 0.63 48 4Cr 43.8Cr — 10.2 Cr 0.42 Mo 0.42 Mo — 0.23 Mo 0.95 M n 0.9 Mn — 08 Mn 2 7.39 262 19.43 0.52 50 7Cr 49.0 Cr — 10.8 Cr 0.83 0.49 Mo 0.46 Mo — 03 Mo W 0.59 W 0.61 W — 0.44W 0.9 Mn 0.9 Mn — 0.7 Mn 1 7.76 2.49 19.31 0.54 51.2Cr 49.0Cr — 10.5Cr 0.41 Mo 0.44 Mo — 0.31 Mo SI. 6: Primarni karbidi heksagonalne oblike. Pov. 100x Fig. 6 Primary carbides of hexagonal shape. Magn. 100x M7C3. Z razmerjem Fe/Cr je podana sestava karbidov M7C3, ki se sicer lahko spreminja od (Cr2Fe5) C3 do (Cr5Fe2)C3. Primarni in evtektični karbidi imajo v naših zlitinah sestavo od malo nad stehiometričnim razmerjem (Cr3Fe4) Cr3 do (Cr4Fe3) C3. Krivulja za evtektične karbide leži nad krivuljo za primarne karbide, ker imajo evtektični karbidi pri istih vrednostih Cr/C manjšo vsebnost Cr. Razlika v vsebnosti Cr med primarnimi in evtektičnimi karbidi je največja pri najnižjem razmerju Cr/C. Z naraščajočo vsebnostjo tega razmerja proti 8 se vsebnost Cr v primarnih in evtektičnih karbidih približuje isti vrednosti. Poleg Cr, ki v kristalni mreži karbidov nadomešča atome Fe, smo v karbidih izmerili tudi določene koncentracije Mo, Mn in W. Vsebnost Mo v zlitinah je majhna, zato je v mikrostrukturi malo karbidov Mo2C. Ti karbidi so drobni, vendar smo jih lahko določili v elektronskem mikroanalizatorju, kot tudi karbide W v zlitinah, legiranih s tem elementom. Vsebnost Cr v matici narašča z vrednostjo razmerja Cr/C. Meritve koncentracij Cr in Mo v matici so pokazale, da je ta zelo nehomogena (5). Odstopanja od povprečnih vrednosti so pri Cr v mejah ±20%. Bistveno večje je izcejanje Mo, in sicer večinoma v mejah ± 50 %. V nekaterih primerih pa smo v izcejah izmerili tudi do 2 % Mo. Podobno smo ugotovili, da tudi karbidi nimajo homogene sestave. Pri večjih, predvsem primarnih karbidih, je koncentracija Cr največja v sredini in se zmanjšuje proti robu karbidnega zrna. V karbidih smo merili koncentracije Cr, Mo in W tudi na vzorcih, žarjenih 2, 4 in 8 ur na temperaturi 1050 °C. Pri tej temperaturi poteka izločanje sekundarnih karbidov in s tem destabilizacija avstenitne matice. Izmerjene razlike v koncentraciji omenjenih elementov med litim in žarjenim stanjem niso sistematične in so odstopanja v mejah merilnih napak. Diagram izotermne destabilizacije avstenita Sistematične preiskave destabilizacije avstenita, izo-termna transformacijska diagrama za nedestabiliziran in destabiliziran avstenit (TTT) in kontinuirni transformacijski diagram za destabiliziran avstenit (CTT) smo naredili za zlitino 5 z naslednjo kemično sestavo: 3,2% C, 1,22% Si, 0,86% Mn, 0,035 % S, 0,030% P, 17,95% Cr, 0,63% Mo, 0,69% Ni, 0,08% Ti in 0,095% V. Zlitina ima razmerje Cr/C 5,61 in v litem stanju avstenitno matico (si. 5). Izotermna transformacijska diagrama smo naredili na osnovi metalografskih Measurements vvere made on the samples čast into mould. Plot in Fig. 7 gives the relationship betvveen the Cr/C ratio in alloys and the Fe/Cr ratio in M7C3 carbides. The Fe/Cr ratio defines the composition of M7C3 carbides vvhich varies betvveen (Cr2Fe5)C3 and (Cr5Fe2)C3. Primary and eutectic carbides in our alloys have the composition betvveen the composition vvhich is slightly above the stoichiometric one of (Cr3Fe4)C3, and the composition of (Cr4Fe3)C3. The curve for eutectic carbides is above the curve for primary carbides since eutectic carbides at equal Cr/C ratios have smaller contents of Cr. The differ-ence in Cr content betvveen the primary and the eutectic carbides vvas the highest at the lovvest Cr/C ratio. If this ratio goes tovvards 8, the Cr content in primary and eutectic carbides approaches to the same value. Beside Cr vvhich in crystal lattice of carbides substi-tutes Fe atoms, certain concentrations of Mo, Mn, and W vvere found in carbides. Mo content in alloys is small therefore the microstructure contains small amount of Mo2C. These carbides are fine but they were determined by the electron microanalizer, as well as the tungsten carbides in the alloys alloyed vvith that element. Cr content in matrix is increased vvith the increased Cr/C ratio. Measurements of Cr and Mo concentrations in the matrix shovved that matrix is very unhomogeneous (5). Deviations from the mean values are for Cr in the li-mits ±20 %. Essentially greater are segregations of Mo, mainly in limits ±50%. In some cases in segregations, even up to 2 % Mo vvas found. Similarly, it vvas found that also carbides do not have a homogeneous composition. In bigger, mainly primary carbides the concentration of Cr is the greatest in the centre and it is reduced tovvards the edge of the carbide grain. In carbides, the concentrations of Cr, Mo, and W vvere measured also in the samples annealed 2, 4 and 8 hours at 1050° C. At this temperatures secondary carbides are precipitated and thus the austenitic matrix is (Cr3Fe4)C3 (Cr4Fe3)C3 -(Cr2Fe5)C3 "234 5 6 7 8 Razmerje - Ratio : Cr/C SI. 7: Odvisnost med razmerjem Cr/C in razmerjem Fe/Cr v karbidih M7C3 Fig. 7 Relation betvveen the Cr/C ratio and Fe/Cr ratio in M7C3 carbides -(Cr5Fe2)C3 eutektični - eutectic karbidi carbides primarni karbidi -primary carbides preiskav. Mikrostrukturne spremembe smo opredelili v optičnem mikroskopu in v SEM. V nekaterih primerih, ko je bilo težko določiti mikrostrukturne komponente, smo si pomagali še z meritvami mikrotrdot in selektivnim jedkanjem (15, 16). Za izdelavo diagrama izotermne destabilizacije av-stenita smo vzorce izotermno žarili različno dolgo časa v temperaturnem področju med 500 in 1150° C. Razpad avstenita poteka v dveh temperaturnih področjih, ki se v ozkem področju prekrivata (si. 8). Za toplotno obdelavo zlitin je pomembna destabilizacija avstenita z izločanjem sekundarnih karbidov (Ks), ki poteka v višjem temperaturnem področju. Izločanje sekundarnih karbidov je najhitrejše med 940 in 990 °C. Nad temperaturo Ac3 poteka transformacija y = y + Ks in pod to temperaturo y = y + a + K,. Nad temperaturo Ac3 se iz avstenita izločajo karbidi M7C3. V temperaturnem področju med Ac3 in Ac, pa se iz avstenita izločajo tudi karbidi M23C6 (i0). Izločanje sekundarnih karbidov je za nadaljnjo toplotno obdelavo bistvenega pomena. Brez predhodne destabilizacije, pri kateri se zaradi izločanja sekundarnih karbidov v avstenitu zmanjša vsebnost Cr in C, transformacija avstenita v martenzit, kot tudi v bainit, niti ni mogoča. Perlitna transformacija pa poteka v destabiliziranem avstenitu počasneje. V nižjem temperaturnem področju razpada poteka transformacija avstenitne matice v perlit. Transformacija poteka najhitreje med 670 in 710 °C. Sekundarni karbidi se začnejo izločati iz avstenita po določeni inkubacijski dobi, in to ob kristalnih mejah med avstenitnimi zrni in na meji avstenitnih zrn z evtek-tičnimi karbidi. Proti sredini avstenitnih zrn poteka izločanje hitreje po določenih kristalografskih ravninah (si. 9, 10, 11). V začetni fazi izločanja so karbidi drobni, destabilized. The measured differences of concentra-tions of the mentioned elements betvveen the čast and annealed state are not systematic, and the deviations are in the limits of measuring errors. Diagram of Isothermal Destabilization of Austenite Systematic investigations of the destabilization of austenite, isothermal transformation diagrams for un-destabilized and destabilized austenite (TTT), and the continuous transformation diagram for destabilized austenite (CTT) were constructed for the alloy 5 with the follovving composition: 3.2% C, 1.22% Si, 0.86% Mn, 0.035 % S, 0.030 % P, 17.95 % Cr, 0.63 % Mo, 0.69 % Ni, 0.08 % Ti, and 0.095 % V. The Cr/C ratio of the alloy was 5.61, and the as čast alloy exhibits austenitic matrix (Fig. 5). The isothermal transformation diagrams were constructed from data of metallographic investigations. Microstructural variations were determined in optical microscope and by SEM. In some cases when the microstructural components were not easy to be determined, measurements of microhardnesses, and selec-tive etching were applied (15, 16). To construct the diagram of isothermal destabilization of austenite, the samples were isothermally annealed for various times in the temperature interval 500 to 1150° C. Decomposition of austenite occurs in two temperature intervals vvhich overlap in a narrow region (Fig. 8). The destabilization of austenite vvith precipita-tion of secondary carbides (Ks) occuring in the higher temperature interval is important for the heat treatment of alloys. Precipitation of secondary carbides is the fas-test betvveen 940 and 990° C. Above Ac3 transformation y = Y + Ks takes plače, and belovv that point y = y + + a + Ks. Above Ac3 M7C3 carbides are precipitated from austenite. In the temperature interval betvveen Ac3 and Ac, also M23C(5 carbides are precipitated from austenite (10). Precipitation of secondary carbides is essential for fur-ther heat treatment. VVithout the predestabilization vvhen due to the precipitation of secondary carbides the con-centrations of Cr and C are reduced, the transformation of austenite into martensite as well as into bainite is not possible. Pearlitic transformation is slovver in the destabilized austenite. In the lovver temperature interval of the decomposition, the transformation of austenitic matrix into pearlite takes plače. It is the fastest betvveen 670 and 710° C. SI. 9: Izločanje sekundarnih karbidov v avstenitu, 80 s žarjeno na 1050°C. Pov. 500x Fig. 9 Precipitation of secondary carbides in austenite, annealed 80 s at 1050° C. Magn. 500 x Kemična sestava: Chemical composition Sekunde 1_1 1 1 nun_1 1 ..........111111 Seconds 1 10 100 1000 CQS *" Minute 1_I 1 1 111111_l_ Time Minutes 1 10 Ure Hours SI. 8: Diagram izotermne destabilizacije avstenita (A — austenit, F — ferit, P — perlit, Ks — sekundarni karbidi) Fig. 8 Diagram of isothermal destabilization of austenite (A — austenite, F — ferrite, P — pearlite, Ks — secondary carbides) 800 SI. 10: Morfologija izločanja sekundarnih karbidov iz avstenita, 40 s žarjeno na 1050° C (matica je iz avstenita in martenzita) Fig. 10 Morphology of precipitation of secondary carbides from austenite, annealed 40 s at 1050" C (Matrix is of austenite and mart-ensite) s časom izotermnega žarjenja pa rastejo. Največji vpliv na ,rast sekundarnih karbidov ima temperatura in nad 1050 °C je njihova rast že zelo hitra. Inkubacijski čas za potek premene v perlitnem področju je daljši. Morfologija izločanja cementita je podobna kot pri izločanju sekundarnih karbidov, le izločanje cementita po prednostnih kristalografskih ravninah je manj izrazito. Sam potek transformacije je hitrejši kot proces destabilizacije. Na nekaterih mestih se vidi, da je transformacija potekla hitro po celem zrnu avstenita (si. 12). Oblika cementitnih lamel in medlame-larna razdalja v perlitu sta odvisni od temperature transformacije. Pri 750 °C je cementit grob in globula-ren, le na sredini večjih zrn je nakazana lamelama oblika. S padajočo temperaturo transformacije ima cementit vedno bolj lamelarno obliko (si. 13, 14). Najmanjšo SI. 12: Perlitna transformacija, 5 min žarjeno na 690° C. Pov. 500 x Fig. 12 Pearlitic transformation, annealed 5 min. at 690° C, Magn. 500 x SI. 11: Morfologija izločanja sekundarnih karbidov iz avstenita, 5 min žarjeno na 950° C (matica je iz avstenita in martenzita) Fig. 11 Morphology of precipitation of secondary carbides from austenite, annealed 5 min. at 950° C (Matrix is of austenite and martensite) Secondary carbides start to precipitate from austenite after a certain induction period, and this occurs on the grain boundaries betvveen the austenite grains, and on the boundaries of austenite grains vvith eutectic carbides. Tovvards the centre of austenite grains the precipitation is faster on certain crystallographic planeš (Figs. 9,10, and 11). In the initial phase of precipitation, the carbides are fine, but they grovv vvith the time of isothermal annealing. The greatest influence on the grovvth of secondary carbides has the temperature, and above 1050° C their grovvth is already very fast. Induction period for the transformation in the pearlitic region is longer. Morphology of cementite precipitation is similar to that of the secondary carbides, only precipitation of cementite on the preferred crystalogra-phic planeš is less pronounced. Transformation itself is faster than the process of destabilization. On same spots it is evident that the transformation vvas fast through the vvhole austenite grain (Fig. 12). The shape of cementite lamellae and the interlamellar spacing in per-lite depend on the transformation temperature. At 750° C cementite is coarse and globular, only in the centre of bigger grains lamellar formation is indicated. With de-creasing temperature of transformation the shape of cementite is becoming more lamellar (Figs. 13 and 14). The smallest interlamellar spacing in pearlite is found in the alloys at the transformation temperature around 650° C. At lovver temperatures pearlitic transformation is slovver, and lamellae can be observed only after longer annealing times. The curves of the initial precipitation of secondary carbides, and of pearlitic transformation were metallo-graphically exactly determined by optical microscopy and by SEM. Bigger problem vvas to determined the time vvhen both processes are completed. Precipitation of carbides and the pearlitic transformation move from the grain boundaries into the interior of the grains. The time SI. 13: Morfologija perlitne transformacije, 10 min žarjeno na 750°C Fig. 13 Morphology of pearlitic transformation, annealed 10 min. at 750" C SI. 14: Morfologija perlitne transformacije, 120 min žarjeno na 650° C Fig. 14 Morphology of pearlitic transformation, annealed 120 min. at 650° C medlamelarno razdaljo v perlitu ima zlitina v temperaturnem področju transformacije okoli 650 "C. Pri nižjih temperaturah poteka perlitna transformacija počasneje in lamele opazimo le pri daljših časih žarjenja. Krivulji začetka izločanja sekundarnih karbidov in perlitne transformacije smo lahko metalografsko točno določili z optičnim mikroskopom in v SEM. Večji problem je določiti čas, v katerem sta oba procesa končana. Izločanje karbidov in perlitna transformacija potekata s kristalnih mej v notranjost zrn. Čas, v katerem je proces končan, je zato odvisen od velikosti kristalnih zrn. V nekaterih primerih tudi sicer težko točno opredelimo konec procesa izločanja sekundarnih karbidov, ker v martenzitni osnovi težko ločimo karbidna zrna. Z meritvami trdote si prav tako težko pomagamo, saj se trdota, ko je izločenih že več kot 80 % sekundarnih karbidov, ali se perlitna transformacija približuje koncu, bistveno ne spremeni in so odstopanja v mejah merilnih napak. Na potek destabilizacije in perlitne transformacije pa vpliva tudi izcejanje legirnih elementov. Iz teh razlogov sta krivulji, ki označujeta konec obeh procesov, opredeljeni le približno. of the process termination thus depends on the size of crystal grains. In some cases the exact determination of the end of the precipitation of secondary carbides is dif-ficult since carbide grains can hardly be distinguished in the martensitic matrix. Measurements of hardness can also not help since the hardness changes very little when more than 80 % of secondary carbides are precipi-tated or the pearlitic transformation approaches to its end, and the deviations are in the limits of measuring er-rors. The destabilization process and the pearlitic transformation are influenced also by the segregations of al-loying elements. Therefore the curves determining the completion of both processes are approximate. In some samples being destabilized belovv 900° C also retained austenite was observed in the microstructure. Due to fast cooling (microstructure vvas stabilized by quenching) beside the stable austenite also residual austenite is present in the microstructure of the matrix in partial destabilization. The both austenites differ in the content of alloying elements (Cr, Mo, and C) (Fig. 15). The Ac3 point vvas determined dilatometrically. SI. 15: Delno destabiliziran avstenit (850° C, 2 min). V sredini zrn je stabilni avstenit. Ob kristalnih mejah, kjer so se izločili sekundami karbidi, je med martenzitnimi iglami zaostali avstenit. Pov. 500 x Fig. 15 Partially destabilized austenite (850° C, 2 min.). In the centre of grains there is stable austenite. On grain boundaries vvhere secondary carbides are precipitated there is residual austenite betvveen the martensitic needles. Magn. 500 x Pri nekaterih vzorcih, destabiliziranih pri temperaturah pod 900 °C, smo opazili v nikrostrukturi tudi zaostali avstenit. Zaradi hitrega ohlajanja (mikrostruktura smo stabilizirali z gašenjem) je v mikrostrukturi matice pri delni destabilizaciji prisoten poleg stabilnega avstenita še zaostali avstenit. Avstenita se razlikujeta po vsebnosti legirnih elementov Cr, Mo in C (si. 15). Temperaturo Ac3 smo določili z dilatometrom. Izotermni transformacijski diagram za destabiliziran avstenit. Temperatura destabilizacije 970 °C je istočasno izhodna temperatura nadaljnje toplotne obdelave. Omenili smo že, da višje razmerje Cr/C in legiranje z Mo zavirata transformacijo avstenita v perlit. Pri de-stabiliziranem avstenitu moramo upoštevati še vpliv sekundarnih karbidov. V primerjavi s TTT diagramom za nedestabiliziran avstenit je pri destabiliziranem avstenitu področje nastajanja perlita pomaknjeno močno v desno, v temperaturah pa ni nobene razlike. Več je v avstenitu izločenih sekundarnih karbidov, daljša je inkubacijska doba. Za praktično uporabo diagrama je seveda pomembna le popolna destabilizacija avstenita (si. 16). Določena razlika je v morfologiji nastajanja perlita. V nedestabiliziranem avstenitu poteka transformacija predvsem s kristalnih mej proti sredini avstenitnih zrn. Pri destabiliziranem avstenitu poteče premena hitro po celem, oziroma delu avstenitnega zrna. S časom žarje-nja narašča število transformiranih kristalnih zrn. (si. 17). Sekundarni karbidi delujejo kot kali in v destabiliziranem avstenitu poteka kontinuirna transformacija (si. 18). Tudi pri teh pogojih transformacije je iz že omenjenih razlogov nemogoče točno opredeliti konec premene. Destabilizacija: 970°C , 60min -Oestabilization 800,-1 ; MIHI-1 I ' "Hll- 1 ....... I 1 llllll-rn^p 1 10 Sekunde Seconds Čas —• Time 10* 103 10 10= ' i 'i.....i........"_i i i iiiiii 1 10 100 1000 Minute i i i...... i Minutes 1 10 Ure Hours Isothermal Transtormation Diagram tor Destabilized Austenite The destabilization temperature of 970° C is simul-taneously the starting temperature for further heat-treat-ment processes. It was already mentioned that higher Cr/C ratio and alloying vvith Mo retard the transtormation of austenite into pearlite. In destabilized austenite also the influence of secondary carbides must be taken in account. Compared to the TTT diagram for understabilized austenite the region of formation of pearlite in destabilized austenite is shifted significantly to the right while there are no differences related to the temperatures. Amount of secondary carbides precipitated in austenite is greater, longer is also the induction period. Only the complete destabilization of austenite (Fig. 16) is certain-ly important for practical application of the diagram. There is a certain difference in the morphology of pearlite formation. In undestabilized austenite the transtormation goes mainly from crystal boundaries tovvards the centre of austenite grains. In destabilized austenite the transtormation is fast over the vvhole or over a part of austenite grain. Longer annealing tirne increases the number of transformed crystal grains (Fig. 17). Secon-dary carbides are nuclei, and continuous transtormation takes plače in destabilized austenite (Fig. 18). Also in these conditions of transtormation it is not possible to determine exactly the termination of the transtormation due to the reasons already mentioned. Formation of cementite is influenced also by secon-dary carbides beside the temperature of isothermal transtormation. They are bigger in the bigger austenite grains, but their density is lovver. Thus the grovvth of cementite in the bigger grains is less hindered than in the smaller ones. Bainitic transtormation is possible only after the destabilization of austenite and at Cr/C ratios smaller than 5.2. By alloying vvith Mo, bainite can be obtained also at higher concentrations of Cr (1, 6). Induction period for the bainitic transtormation is long. Significant portion of bainitic phase in the microstructure is obtained only after longer times of isothermal annealing. A completely bainitic matrix can be expected only after a very long times of isothermal annealing when the alloy has a suit-able Cr/C ratio and is alloyed vvith Mo. SI. 16: Izotermni transformacijski diagram za destabiliziran avstenit (A — avstenit, K, — sekundarni karbidi, P — perlit, B — bai- nit) Fig. 16 Isothermal transtormation diagram for destabilized austenite (A — austenite, K — secondary carbides, P — pearlite, B — bainite) SI. 17: Perlitna transformacija destabilizirane avstenitne matice, 60 min žarjeno na 650° C. Pov. 500 x Fig. 17 Pearlitic transtormation of destabilized austenitic matrix, an-nealed 60 min. at 650° C. Magn. 500 x SI. 18: Meja med perlitnim zrnom (temnejše) in netransformiranim avstenitom (svetlejše) v destabilizirani matici Fig. 18 Boundary betvveen the pearlite grain (darker) and not trans-formed austenite (brighter) in the destabilized matrix Na izoblikovanje cementita vplivajo poleg temperature izotermne transformacije še sekundarni karbidi. Ti so v večjih avstenitnih zrnih večji, njihova gostota pa je manjša. Zato je v večjih zrnih rast cementita manj ovirana, kot v manjših. Bainitna transformacija je možna le po destabilizaciji avstenita in razmerjih Cr/C manjših od 5,2. Z legi-ranjem zlitin z Mo lahko dobimo bainit tudi pri višjih koncentracijah Cr (1,6). Inkubacijska doba za potek bainitne premene je dolga. Pomemben delež bainitne faze v mikrostrukturi dobimo le pri daljših časih izo-termnega žarjenja. Popolnoma bainitno matico pa lahko pričakujemo po zelo dolgih časih žarjenja zlitin z ustreznim razmerjem Cr/C in legiranih z Mo. Bainitno in martenzitno mikrostrukturo lahko ločimo le v SEM (17). Bainit stabilizira avstenit, zato je v mikrostrukturi matice še precej netransformiranega avstenita (si. 19). To so potrdile tudi meritve mikrotrdot. Začetek martenzitne transformacije smo določili z dilatometrom. Ms temperatura za popolnoma destabili-zirano zlitino (970 °C, 60 min) je 180 °C. Potek martenzitne transformacije ni odvisen le od delne ali popolne destabilizacije, temveč tudi od temperature destabiliza-cijskega žarjenja. Ms temperatura se znižuje z naraščajočo temperaturo destabilizacije (1, 6, 7). Pri kaljenju delno destabiliziranega avstenita je v matici poleg marten-zita tudi avstenit. Kontinuirni transformacijski diagram za destabiliziran avstenit Diagram, prikazan na sliki 20 velja za popolnoma destabilizirano zlitino (970 °C, 30 min). Preizkuse smo naredili tudi pri drugih temperaturah destabilizacije in na delno destabiliziranih vzorcih, da smo dobili čim več podatkov o vplivu različnih pogojev destabilizacije na mikrostrukturne značilnosti pri icontinuirnem ohlajanju. SI. 19: Morfologija bainitne transformacije v destabiliziranem avste-nitu (300° C, 4 ure). Mikrostruktura matice je iz avstenita, bai-nita, martenzita in sekundarnih karbidov Fig. 19 Morphology of bainitic transformation in destabilized austenite (300° C, 4 hours). Micostructure of matrix is of austenite, bai-nite, martensite, and secondary carbides Bainitic and martensitic microstructure can be distin-guished only by SEM (17). Bainite stabilizes austenite therefore a good deal of not transformed austenite can be found in the microstructure of the matrix (Fig. 19). This was confirmed also by the microhardness measure-ments. The beginning of the martensitic transformation was determined by dilatometer. Ms point for a completely destabilized alloy (970° C, 60 min.) is at 180° C. Course of martensitic transformation does not depend only on the partial or complete destabilization but also on the temperature of the destablization annealing. Ms temperature is lovvered vvith the increasing temperature of destabilization (1, 6, 7). After hardening the partially destabilized austenite, also austenite next to the martensite is found in the matrix. Continuous Transformation Diagram for Destabilized Austenite Diagram is presented in Fig. 20 and it is valid for a completely destabilized alloy (970° C, 30 min.). Experi-ments vvere made also at other temperatures of destabilization, and vvith partially destabilized samples in order to obtain the most possible data about the influences of various conditions of destabilization on the microstruc-tural characteristics in continuous cooling. In slovv cooling at 300° C/h, the transformation oc-curs at first in pearlitic stage, then also in the bainitic stage. The extent of transformation is in both stages ap-proximately equal. Martensitic transformation is namely observed, but amount of formed martensitic phase is small. At shorter times of destabilization the extent of transformation in pearlitic stage is smaller and in the bainitic one greater, but at temperatures around 200° C also martensitic transformation is observed. Destabilizacija 970°C, 30min - Destabilization 10 Sekunde Seconds Čas-► Time 1 Minute Minutes ' 600 0 1 500 8. 1400 300 100......... 1000 1 ......io Ure Hours SI. 20: Kontinuirni transformacijski diagram za destabiliziran avstenit (A — avstenit, K, — sekundarni karbidi, P — perlit, B — bainit) Fig. 20 Continuous transformation diagram for destabilized austenite (A — austenite, Ks — secondary carbides, P — pearlite, B — bainite) Pri počasnem ohlajevanju 300 °C/h pride do premene najprej v perlitni, nato pa v bainitni stopnji. Obseg transformacije je v obeh stopnjah približno enak. Mar-tenzitna premena se sicer opazi, martenzitne faze pa je nastalo malo. Pri krajših časih destabilizacije je obseg tranformacije v perlitni stopnji manjši, v bainitni pa večji, s tem da se pri temperaturah okrog 200 °C opazi tudi martenzitna premena. Pri večjih hitrostih ohlajanja (merjeno v sekundah ohlajanja med 800 in 500° C, oznaka t8/5) dobimo pri hitrosti t8/5 = 165 s že popolnoma martenzitno premeno. V začetku martenzitne premene smo opazili anomalijo (črtkana krivulja Ms). Podobni rezultati iz literature to anomalijo omenjajo, vendar brez ustrezne razlage. Ugotavljamo pa, daje anomalija pri krajših časih destabilizacije bolj izrazita. Naša preizkušanja so bila izvedena v omejenem obsegu, vendar se vidi, da ima čas destabilizacije, ki vpliva na obseg izločanja sekundarnih karbidov, bistveno vlogo na transformacijo avstenita pri kasnejšem ohlajevanju. Večja stopnja destabilizacije avstenita pospešuje obseg transformacije v perlitni stopnji. Anomalija pri martenzitni premeni, ki je večja pri manjši destabilizaciji avstenita, je verjetno povezana z nehomogenostjo avstenita. Ta je vsekakor večja pri nepopolni destabilizaciji. Premenske točke, ki so vrisane na diagramu, smo določili na podlagi dilatometrskih krivulj pri kontinuir-nem ogrevanju s hitrostjo 300 °C/h. Mikrostruktura matice je odvisna od hitrosti ohlajanja. Pri delni destabilizaciji je v sredini kristalnih zrn še avstenit. At higher cooling rates (measured in seconds for cooling from 800 to 500° C, marked by t8/5) a complete martensitic transformation is obtained at the rate t8/5 = 165 s. In the beginning of the martensitic transformation an anomaly vvas observed (dashed curve Ms). Similar results in references mention this anomaly but vvithout any explanation. It vvas found that the anomaly is more pronounced at shorter times of destabilization. Our testing vvas limited but it is evident that the tirne of destabilization vvhich influences the extent of secon-dary-carbide precipitation has an essential role in the transformation of austenite at further cooling. Higher stage of austenite destabilization accelerates the extent of transformation in the pearlitic stage. The anomaly in the martensitic transformation vvhich is greater at small-er destabilization of austenite is probably connected vvith the unhomogeneity of austenite. This is anyhow greater at uncomplete destabilization. Transformation points being plotted into the diagram vvere determined from dilatometric curves in continuous heating at the rate 300° C/h. The microstructure of the matrix depends on the cooling rate. In partial destabilization stili austenite is found in the centre of crystal grains. Residual austenite can be obtained at harsher condi-tions of hardening and at higher contents of Mn and Ni. Morphology of pearlite depends on the degree of destabilization as it vvas already explained at the isother-mal conditions of the transformation. Hardness of Alloys Corresponding applicable properties of vvhite chrom-ium čast irons depend on the amount of eutectic carbides in the microstructure, and on the hardness of ma-trix (6, 7). The best vvear resistance and the hardness possess the alloys vvith martensitic matrix. Alloys vvith martensitic-pearlitic matrix are softer, their hardness depends on the portion of pearlitic phase (11, 12). The alloy for vvhich the transformation diagrams vvere constructed exhibited as čast on the cross section čast in mould the average hardness 650 HV, and on the cross section čast into sand 635 HV respectively. Destabilized samples vvith martensitic matrix had hardnesses betvveen 760 and 800 HV. The highest hardness of alloys vvith 15 to 18 % Cr is obtained by hardening from the temperature betvveen 940 and 970° C. For the alloys vvith higher contents of Cr (over 20 %) the temperatures of hardening are higher, up to 1010° C. Martensite is harder if more carbon is dis-solved in austenite. Solubility of carbon increases vvith the increased temperature of hardening (18). By hardening from higher temperatures, also residual austenite can be obtained in the microstructure, especially if al-loys contain over 1 % Mn. Also Ni (Cu) acts like Mn vvhich on the other hand improves the through-hardena-bility. In our alloys the residual austenite vvas not detect-ed by dilatometric investigations. The residual austenite vvas obtained in the matrix only under harsher conditions of hardening from higher temperatures. The samples vvere hardened in air from the destabilization temperature of 970° C. Martensite is stable and the hardness starts to drop in tempering above 400° C (Fig. 21). The hardness of alloys vvith sufficient portion of residual austenite increases in tempering betvveen 450 and 550° C due to the decomposition of residual austenite into martensite. The transformation is connected vvith volume changes, and it is undesired be-cause of internal stresses. Microstructural characteris-tics of tempered čast iron are shovvn in Figs. 22 to 26. SI. 21: Vpliv temperature popuščanja na trdoto destabilizirane litine z različno mikrostrukturo matice (M — martenzit, B — bainit, P — perlit, Az — zaostali avstenit) Fi9' 21 intiuence of temperature of tempering on the hardness of de-stabilized čast iron vvith various microstructures of matrix (M — martensite, B — bainite, P — pearlite, Az — residual austenite) SI. 24: Mikrostruktura kaljene litine 2 uri popuščane na 600" C. Pov. 500 x Fig. 24 Microstructure of hardened čast iron, tempered 2 hours at 600° C. Magn. 500 x Zaostali avstenit lahko dobimo pri ostrejših pogojih kaljenja in pri višjih vsebnostih Mn in Ni. Morfologija perlita je odvisna od stopnje destabilizacije, kot smo to že razložili pri izotermnih pogojih transformacije. Trdota zlitin Ustrezne uporabne lastnosti belih kromovih litin so odvisne od deleža evtektičnih karbidov v mikrostrukturi in trdote matice (6, 7). Najboljšo obrabno obstojnost in trdoto imajo zlitine z martenzitno matico. Zlitine z martenzitno perlitno matico so mehkejše, njihova trdota pa je odvisna od deleža perlitne faze. (11, 12). Zlitina, za katero so narejeni transformacijski diagrami, ima v litem stanju na preseku ulitem v kokilo, povprečno trdoto 650 HV in na preseku ulitem v pesek 635 HV. Destabilizirani vzorci z martenzitno matico imajo trdoto med 760 in 800 HV. Največjo trdoto zlitin s 15 do 18% Cr dobimo pri kaljenju s temperature med 940 in 970 °C. Za zlitine z višjo vsebnostjo Cr (nad 20 %) so temperature kaljenja višje, do 1010 °C. Martenzit je trši, čim več C je raztopljenega v avstenitu. Topnost C narašča z naraščajočo temperaturo kaljenja (18). Pri kaljenju z višjih temperatur pa lahko dobimo v mikrostrukturi še zaostali avstenit, zlasti še, če vsebujejo zlitine nad 1 % Mn. Podobno kot Mn učinkuje tudi Ni (Cu), ki sicer izboljša prekalji-vost. V naših zlitinah zaostalega avstenita z dilatometr-skimi preiskavami nismo zasledili. Zaostali avstenit smo dobili v matici le pri ostrejših pogojih kaljenja z višjih temperatur. Vzorce smo kalili na zraku s temperature destabilizacije 970 °C. Martenzit je stabilen in trdota prične padati pri popuščanju nad 400° C (si. 21). Trdota zlitin z zadostnim deležem zaostalega avstenita pri popuščanju med 450 in 550 °C naraste zaradi razpada zaostalega avstenita v martenzit. Premena je povezana z volumskimi spremembami in je zaradi notranjih napetosti nezaže-ljena. Mikrostrukturne značilnosti popuščene litine so prikazane na slikah 22, 23, 24, 25 in 26. Fig. 23 Microstructure of hardened čast iron, tempered 2 hours at 400° C. Magn. 500 x SI. 22: Mikrostruktura destabilizirane litine kaljene na zraku. Matica je iz sekundarnih karbidov in martenzita. Pov. 500 x Fig. 22 Microstructure of destabilized čast iron hardened in air. Matrix is of seconday carbides and martensite. Mag. 500 x Si. 23: Mikrostruktura kaljene litine 2 uri popuščane na 400° C. Pov. 500 x 200 300 400 500 600 700 800 Temperatura popuščanja v °C Tempering temperature . °C I oM XP+B»M(20%P •P*M(907.P; □ P + M+Az- SI. 25: SEM posnetek na zraku kaljene litine. Mikrostruktura matice je iz sekundarnih karbidov in martenzita. Fig. 25 SEM picture of air-hardened čast iron. Microstructure of matrix is of secondary carbides and martensite. S kontinuirnim ohlajanjem smo pripravili vzorce z mešano perlitno-bainitno-martenzitno mikrostrukturo. Potek trdote v odvisnosti od temperature popuščanja je podoben kot pri vzorcih z martenzitno matico, le izhodna trdota je nižja. Trdota vzorcev s perlitno matico se pri popuščanju ne spremeni. Trdote ostalih preiskovanih zlitin z avstenitno matico v litem stanju so podobne, tiste z avstenitno-perlitno ali popolnoma perlitno matico pa so mehkejše in je njihova trdota med 480 in 550 HV. Temu ustrezne so tudi trdote po toplotni obdelavi. Trdota je pri zlitinah z avstenitno matico v litem stanju enaka po celem preseku preizkusnih valjčkov. Mikrotrdote posameznih mikrostrukturnih faz so podane v tabeli 4. Pri karbidih M7C3 moramo upoštevati, da je njihova trdota odvisna od kristalografske smeri, v kateri jo merimo (6). Tabela 4: Mikrotrdote mikrostrukturnih faz HV Primarni in evtektički karbidi M7C3 900-1300 Martenzit, sekundarni karbidi 650—700 Avstenit 400-520 Perlit, sekundarni karbidi 360—420 SI. 26: SEM posnetek kaljene litine 2 uri popuščane na 300° C Fig. 26 SEM picture of hardened čast iron, tempered 2 hours at 300° C By continuous cooling the samples vvith mixed pearl-itic-bainitic-martensitic microstructure vvere prepared. The variation of hardness depending on the tempering temperature is similar to that in the samples vvith mart-ensitic matrix, only the initial hardness is lovver. Hardness of samples vvith pearlitic matrix does not change in tempering. Hardnesses of the other investigated as čast alloys vvith austenitic matrix are similar to that of alloys vvith austenitic-pearlitic matrix, but the alloys vvith fully pearlitic matrix are softer and their hardnesses varied betvveen 480 and 550 HV. Similar relationship remains also after the heat treatment. Hardness of the as čast alloys vvith austenitic matrix is practically equal through the vvhole cross section of testing cylinders. Microhardnesses of single microstructural phases are given in Table 4. It is necessary to take into account that the hardness of M7C3 carbides depends on the crys-tallographic direction in vvhich it is measured (6). Table 4 Microhardnesses of Microstructural Phases Phase HV M7C3 primary and eutectic carbides 900 .. . 1300 Martensite, secondary carbides 650 .. . 700 Austenite 400 .. . 520 Pearlite, secondary carbides 360 .. . 420 ZAKLJUČEK Opisane so nekatere mikrostrukturne značilnosti in pogoji toplotne obdelave belih kromovih litin, legiranih z Mo in z Ni, Si, Mn, V, Ti, in W, namenjenih za centrifugalno dvoslojno lite valje. Iz preiskav se vidi, da so mehanske lastnosti odvisne od mikrostrukturnih značilnosti in s tem od kemične sestave, pogojev strjevanja in ohlajanja ter toplotne obdelave. Z ustreznim razmerjem Cr/C, legiranjem z Mo in hitrim strjevanjem dobimo v litem stanju drobne evtek- CONCLUSIONS Some microstructural characteristics and the condi-tions for heat treatment of vvhite chromium čast irons al-loyed vvith Mo, and vvith Ni, Si, Mn, V, Ti, and W vvhich are intended for centrifugal compound casting of rolls, are presented in the paper. The investigations show that mechanical properties depend on the microstructural characteristics, and thus on the chemical composition, conditions of solidification and cooling, and on the heat treatment. tične karbide in avstenitno matico, ki je s stališča nadaljnje toplotne obdelave najustreznejša. Sicer pa je matica lahko tudi avstenitno-perlitna ali perlitna. Delež karbidne faze je odvisen predvsem od vsebnosti C. Primarni heksagonalni karbidi, ki nastajajo pri počasnem ohlajanju ali pri deležu karbidne faze, večjem od 35 %, so nezaželjeni, ker bistveno poslabšajo žilavost litin. Po kemični sestavi ustrezajo karbidi ste-hiometričnemu razmerju od (Cr3Fe4) C3 do (Cr4Fe3) C3. Z naraščajočo vrednostjo razmerja Cr/C narašča vsebnost Cr v karbidih. Karbidi M7C3 z več Cr so trši. V ev-tektičnih karbidih je pri istem razmerju Cr/C manj Cr kot v primarnih karbidih. Trdota litin je odvisna od deleža evtektičnih karbidov in trdote mikrostrukture, ki jo dobimo po toplotni obdelavi. Osnova za toplotno obdelavo litin sta TTT diagrama za destabilizacijo avstenita in za destabiliziran avstenit in CTT diagram za destabiliziran avstenit. Morfologija izločanja sekundarnih karbidov (destabilizacija avstenita) in perlitne transformacije nedesta-biliziranega avstenita je podobna. Oba procesa potekata s kristalnih mej proti sredini kristalnih zrn, in to hitreje po določenih kristalografskih ravninah. V destabi-liziranem avstenitu poteka kontinuirna perlitna transformacija. Izločanje sekundarnih karbidov, ki poteka najhitreje med 940 in 990° C, je bistvenega pomena za nadaljnjo toplotno obdelavo litin. Le v destabilizirani matici je mogoča martenzitna in tudi bainitna transformacija. Bainitna transformacija poteka počasi in je za prakso manj pomembna. Najtrše so litine z martenzitno matico. Z ustrezno toplotno obdelavo pa lahko dobimo zlitine z martenzitno-perlitno ali perlitno matico, ki so mehkejše. Pri popuščanju litin z martenzitno ali martenzitno-perlitno matico prične trdota padati pri temperaturah popuščanja nad 400 °C. Suitable Cr/C ratio, alloying vvith Mo, and fast solidification enable the formation of fine eutectic carbides in the austenitic matrix in čast state vvhich is the most de-sired from the vievvpoint of further heat treatment. Matrix can also be austenitic-pearlitic or only pearlitic. Portion of carbide phase depends mainly on the car-bon content. Primary hexagonal carbides formed during slovv cooling or in alloys containing more than 35 % of carbide phase are undesired since they essentially re-duce the toughness of alloys. According to the chemical compositions the carbides correspond to stoichiometric ratios from (Cr3Fe4)C3 to (Cr4Fe3)C3. The increasing value of the Cr/C ratio causes the increased content of Cr in carbides. M7C3 vvith higher Cr content are harder. Eutectic carbides at the same Cr/C ratio contain less Cr than the primary ones. Hardness of čast irons depend on the amount of eutectic carbides and on the hardness of the microstructure obtained by the heat treatment. Basis for the heat treatment of čast irons are the TTT diagrams for the de-stabilization of austenite, and for destabilized austenite, beside the CTT diagram for the destabilized austenite. Morphologies of precipitation of secondary carbides (destabilization of austenite), and of pearlitic transformation of undestabilized austenite are similar. Both pro-cesses start on the crystal boundaries and proceed tovv-ards the centre of grains, and the process is faster along certain crystallographic planeš. In destabilized austenite continuous pearlitic transformation takes plače. Precipitation of secondary carbides vvhich is the fas-test betvveen 940 and 990° C is essential for further heat treatment of čast irons. Only in destabilized matrix the martensitic and also bainitic transformation is possible. 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