Vpliv segregacij oligoelementov Influence of interfacial po mejah na lastnosti železovih segregation of residuals on the zlitin properties of iron base alloys M. Guttmann* A. UVOD Segregacije na kristalnih mejah so pojav, ki je pogosto vzrok poslabšanju lastnosti na kristalnih mejah različnih gradiv in v raznolikih delovnih pogojih. Vpliv tega pojava na poškodbe strojnih delov ni toliko znan, kot bi zaradi razširjenosti zaslužil. Namen prispevka je pokazati različne oblike tega pojava. Raziskovalni in tehnološki pomen ter zanimanje za segregacije so zasnovani na dejstvu, da je veliko število nečistoč ali namerno dodanih legirnih elementov nagnjeno k segregi-ranju in da so mnoge lastnosti medfaznih površin (kemične, mehanske, fizikalne) zelo odvisne od njihove kemične sestave. Obravnavani so samo primeri, v katerih so bile segregacije zanesljivo identificirane in/ali izmerjene, ali primeri, v katerih ni bilo nobenega dvoma o vrsti segregacije, ki je povzročila spremembo določene lastnosti. Reverzibilna popustna krhkost (RTE), ki se kaže v mnogih v tem članku obravnavanih pojavih, je le omenjena, saj je predmet drugih prispevkov te konference. B. VISOKOTEMPERATURNA KRHKOST PRI LEZENJU Kaže, da velja splošno pravilo, da se duktilnost pri porušitvi zaradi lezenja zmanjšuje z dalj-šanjem časa do porušitve. Zmanjšanje duktilnosti spremlja povečanje deleža interkristalnega preloma. Ta oblika poškodb je zelo pogosta pri lezenju. Vpliva segregacij v tem procesu pa ni bilo vedno mogoče ugotoviti. V številnih novejših študijah so z različnim uspehom poskušali prikazati zvezo med nečistočami, ki segregirajo po kristalnih mejah, in zmanjšanjem duktilnosti. Problem je veliko bolj zamotan, kot je zapleteno razumevanje krhkosti pri nizkih temperaturah, zavoljo tega, ker se lahko sproži več mehanizmov delovanja zaradi segregacij teh elementov. a) Do nukleacije por in razpok lahko pride na tri načine: — segregacije na mejah kristalnih zrn ali mejah med delci sekundarne faze in matice povzroče * Centre des Materiaux ENSMP, B. P. 87, 91003 EVRY, CEDEX France A. INTRODUCTION Intergranular segregation has recently emerged as a major cause of impairment of grain boundary properties in various materials and service condi-tions. Hovvever, the importance of such processes occuring at the atomic scale in the macroscopic failure of engineering components has not been as widely recognized as its ubiquity deserves, and it is the aim of the present paper to illustrate this variety. The scientific and technological importance of intergranual segregations is based on the fact that a large number of elements present in materials as umvanted impurities or as intentionally added alloying species are susceptible to segregation, but also that many properties (mechanical, physical, chemical, ete.) of interfaces are critically sensitive to their chemical composition. As a rule, only cases where segregations have effectively been identified and/or measured or cases where it is knovvn for sure which segregation is involved, will be discussed. Reversible temper embrittlement (RTE), although it is aetu-ally involved in most of the phenomena discussed in this paper, will be omitted because it is the sub-ject of another article in this Conference. B. HIGH TEMPERATURE BRITTLENESS IN CREEP CONDITIONS It appears to be generally true that the creep-rupture ductility of metals decreases with inerea-ing time to rupture, and this decrease tends to be accompanied by inereasing amounts of intergranular fraetures. Since this fialure mode is so frequent in creep conditions, it has not always been realized that impurity segregations could take a part in this process. Recently, a number of studies have attempted, with varying degrees of succes, to correlate the effect of impurities which segregate to the grain boundaries with this trend tovvards ductility loss. Hovvever, the problem is much more complicated than that of low temperature brittleness since much large number of mechanisms can produce an effect of segregated or segregating elements: a) Nucleation of cavities and cracks can be affected in three ways: — By altering the cohesion of particle/matrix interfaces and of grain boundaries, segregation at nukleacijo por in razpok; ta proces poteka verjetno z lokalnimi krhkimi porušitvami mej1'5, povečini tako kot pri nizkotemperaturni krhkosti; — zdrs po kristalnih mejah (interkristalni zdrs) je eden od procesov, ki poveča deformacijo, potrebno za to dekohezijo (si. 22); njegova hitrost pa je zanesljivo odvisna od kemične sestave kristalnih mej; — segregacije na površinah novo nastalih por zmanjšujejo površinsko napetost por (ys) in s tem njihovo kritično velikost (rc = 2ys/a), nad katero lahko rastejo pri lokalni natezni napetosti (cr); segregacije tako povečujejo število rasti sposobnih por, oziroma navidezno hitrost nukleacije por. b) Rasti por in razpok sta odvisni tudi od segregacij na mejah in prostih površinah, zato ker vplivajo na zdrs po kristalnih mejah in transportne pojave. Hitrost rasti je namreč odvisna od difuzije vzdolž kristalnih mej in po površinah novo nastalih por in razpok3'4'5. Ta pojav pa je prav tako tesno povezan s sestavo teh površin6'7. 1. Porušitev pri lezenju Pri iskanju povezave med segregacijami nečistoč in porušitvijo pri lezenju smo se močno opirali na izkušnje s popustno krhkostjo. Večina raziskav obravnava vpliv elementov IVa, Va in Via skupin iz periodnega sistema elementov8. Rezultati so protislovni, -ker vpliv mikrostrukture pri tem pojavu lahko presega vpliv segregacij. To je često primer pri bainitnem jeklu z 2,25 % Cr in 1 % Mo, ki je mnogo manj občutljivo na popustno krhkost od večine malolegiranih jekel. Slika 1 Vpliv različnih kombinacij dodatkov Mn, Si in P v jeklu z 2,25 % Cr in 1 % Mo po stopenjskem ohlajanju na poruš-no trdnost (cv) in kontrakcijo (RA) po različnem času lezenja. (HP — zelo čista jekla, COM — tehniška jekla). Fig. 1 Influence of Mn, Si, P additions in various combinations and of a step-cooling treatment prior to the creep tests, on the rupture stress (ov), ductility (RA), and creep life (tO of 2 1/4 Cr-lMo steels (after ref. 8). these interfaces can affect the nucleation of cavi-ties and wedge-cracks respectively, since this proces is novv believed to occur through localized brittle failure of these interfaces1'5 in very much the same way as lovv temperature brittleness. — Intergranular sliding is one of the proceses giving rise to the strain concentration necessary to produce these decohesions, fig. 22, and its rate is certainly dependent on grain boundarly composition. — Segregation at the surface of a newly for-med cavity decreases its surface tension ys and therefore the critical size rc = 2 xs/a above vvhich it can grovv under a local tensile stress c: segregation vvill thus increase the number of cavities able to grovv, i.e. the apparent nucleation rate. b) Cavity and crack grovvth also depend on segregations at internal interfaces and at free surfaces, through their effect on: — grain boundary sliding; — transport phenomena, since the grovvth rate is also controlled by the diffusivities along grain boundaries and on the surfaces of the newly for-med cavities or cracks3'4'5 vvhich are obviously dependent on the composition of these interfaces 6'7. 1. Creep rupture In the attempts to correlate the effect of segregation impurities vvith creep, rupture properties the experiences vvith temper embrittlement have been heavily relied upon and most of the in-vestigations have dealt vvith the effects of Group IVA, V A and VIA of the periodic table8. The evidence is conflicting because, in addition to the above mentioned reasons, the creep rupture properties are extremely sensitive to microstructure vvhose effects can largely over ride those of the segregations. This is often the čase, for instance of the bainitic 21/4Cr-lMo steels (vvhich are, incidentall much less sensitive to temper embrittlement than the majority of lovv alloy steels). Lonsdale and Flevvitt2 shovved that, in these steels, impurities can induce a definite decrease in creep life, vvhich they attributed to the enhancing effect of impurities on both the nucleation and grovvth rate of cavities. Hovvever, Visvvanathan9 vvas unable to find any evidence of an impurity effect on their strength and ductility. Pope et al.8 even found a beneficial effect of Mn, P and Si additions, and especially of their association, on the ductility of such steels, fig. 1. They suggest that this improve-ment can take plače essentially because the segregations decrease the grovvth rate of cavities, pro-bably by hindering intergranular diffusivity and in spite of their enhancing influence on cavity nucleation due to the embrittlement of particle/ /matrix interfaces. Conversely, vvhen the steel is step-cooled prior to creep testing, the ductility is Lonsdale in Flewitt2 sta ugotovila, da lahko nečistoče v teh jeklih skrajšajo trajanje lezenja. To pripisujeta segregacijam zaradi vpliva na hitrost nukleacije, pa tudi na hitrost rasti por. Viswanathanu9 ni uspelo ugotoviti vpliva nečistoč na trdnost in duktilnost teh jekel. Pope in sodelavci8 pa navajajo celo primere ugodnega vpliva Mn, P in Si na duktilnost tovrstnih jekel; posebej ugodna so bila jekla, iki so imela legirane kombinacije teh treh elementov (si. 1). Po njihovem mnenju je bistvo izboljšanja duktilnosti v tem, ker naj bi te segregacije zmanjšale hitrost rasti por zaradi oviranja difuzije po mejah, kljub dejstvu, da segregacije sicer povečujejo nukleacijo por zaradi pojava krhkosti na medfazni površini segre-gacija/matica. Če je jeklo postopno ohlajeno pred lezenjem, pa je nasprotno njegova duktilnost obratno sorazmerna deležu nečistoč (si. 1). Zato velike segregacije, ki so bile prisotne že na začetku lezenja, stimulirajo porušitev po mejah in večajo hitrost nukleacije por do takega obsega, da se duktilnost poslabša. Ta mehanizem je lahko primerna razlaga tudi za opazovano naraščanje hitrosti drsenja po kristalnih mejah. Ponašanje Cr-Mo-V jekel je popolnoma drugačno, vplivi nečistoč na njihove lastnosti so različni. Vzrok tem razlikam naj bi bili vanadij evi karbidi, ki bolj utrjujejo matico kot kristalne meje. Tipler in Hopkins10''! sta dognala, da so vse značilne karakteristike porušitve pri lezenju: trdnost, čas lezenja in duktilnost pri 550° C, v industrijskih talinah (CP) jekel z 0,5 % Cr, 0,5 % Mo in 0,25 % V in jeklih z 1 % Cr, 1 % Mo in 0,25 °/o V znatno manjše kot v ustreznih zelo čistih laboratorijskih talinah (HP), v katerih so bile koncentracije P, Sn, Sb, As in Cu na nižjem nivoju (si. 2). Pogostost por na kristalnih mejah pri porušitvi se je v tali- Cas lezenja do porušitve (ure). Rupture iife fhrs) Slika 2 Primerjava »časa lezenja«, trdnosti (a), raztezka (b) in deleža por na mejah kristalnih zrn (c) pili porušitvi v tehniških (CP) in zelo čistih talinah (HP) jekla % 0,5 % Cr, 0,5 °/o Mo, ini 0,5 % V, preizkušenih na lezenje pri 550 »C (po ref. 10). Fig. 2 Comparison of creep lives, rupture stresses (a), ductilities (b) and densities of integranular cavities at rupture (c) in a commercial purity CP and high purity HP heat of 1/2 Cr — 1/2 Mo — 1/2 V steel, creep tested at 550 °C (after ref. 10). adversely affected by the impurities, fig. 1. In this čase, the large segregations which already exist at the onset of primary creep, during which highly localized strains are produced, favors interfacial fracture and therefore increase the nucleation rate to such an extent that ductility is impaired. This mechanism might also be responsible for the ob-served increase in grain boundary sliding rate. The behavior of Cr-Mo-V steels is quite diffe-rent: they are much more adversely affected by impurities. This difference may be due to the pre-sence of vanadium carbides which preferentially harden the matrix vvith respect to the grain bound-aries. Tipler and Hopkins10-11 have clearly establi-shed that ali the creepjrupture icharacteristics, strength life and ductility at 550 °C are consider-ably lower in commercial purity heats (CP) of 1 /2 Cr -1/2 Mo — 1/4 V and 1 Cr — 1 Mo — 1/4 V steels than in the corresponding laboratory heats of hig-her purity (HP),i.e. in which the P, Sn, 'Sb, As and Cu content had been kept at a lovver level, fig. 2. The eavity density on the grain boundaries at rupture is increased by up to three orders of magnitu-de by the presence of impurities10, fig. 2, and this is already true throughout the creep test11, fig. 3. The authors found that cavity density correlates better vvith the product e. t of creep deformation e and time t than vvith either of them separatelv", which indicates that cavitation is not controlled solely by plastic deformation but also by a diffus- Creep duration Ihrsl x elongation.1%) Slika 3 Vpliv čistosti jekla na nastajanje poškodb med lezenjem v odvisnosti od produkta trajanja lezenja (t) in raztezka (deformacije) (e) (po ref. 11) (AC-hlajeno na zraku, T-popuščeno) Fig. 3 Influence of purity on creep damage evolution throughout creep life plotted versus the product of creep duration t times elongation e (after ref. 11). nah s primesmi povečala za tri rede velikosti10 (si. 2), kar so potrdili tudi poskusi z lezenjem11 (si. 3). Ugotovili so, da je povezava med pogostostjo por in produktom deformacije in časa lezenja mnogo bolj izrazita od povezave s samo deformacijo ali časom lezenja11. To kaže, da pojav por ni odvisen zgolj od plastične deformacije, temveč tudi od difuzij skih procesov. Ločenega vpliva vsake nečistoče v tej študiji niso ugotavljali. Znano pa je da vse te nečistoče pri temperaturah poskusov lezenja segregirajo na kristalnih mejah ali prostih površinah12. Rezultati (si. 3) jasno povedo, da je povečana nukleacija por zaradi segregiranih nečistoč osnovni vzrok znižanju lastnosti pri porušitvi zaradi lezenja. Razen porušitve pri lezenju bomo analizirali še dve drugi lastnosti jekel, ki imata elemente mehanizma lezenja. 2. Pojav razpok pri žarjenju za odpravo napetosti (SRC) Interkristalne razpoke, ki nastanejo pri žarjenju za odpravo napetosti v coni toplotnega vpliva ali v samih varih, so povezane z mehanizmom lezenja. Razpoke nastanejo pod vplivom lokalnih napetosti, oz. pod vplivom gradienta napetosti, ki ga povzroči naglo ogrevanje in ohlajanje med varjenjem. Razpokanje je torej v neposredni zvezi z lokalno duktilnostjo gradiva. V znanju o škodljivem vplivu nečistoč je še mnogo nejasnega. To znanje obsega le del spektra zlitin in možnih eksperimentalnih pogojev. Večina strokovne literature skuša povezati razpoke z nominalno koncentracijo nečistoč v jeklih, katerih sestava, mi-krostruktura in mehanske lastnosti se spreminjajo v standardno predpisanih mejah in v pogojih preizkušanja, katerih ponovljivost ni bila ravno velika zavoljo različic, ki se pojavljajo pri varjenju ali varilnih poskusih8. Tovrsten vpliv nečistoč, posebej še v povezavi s popustno krhkostjo, sta jasno pokazala Brear in King13 pri posodah z notranjim tlakom, izdelanih iz Mn-Mo-V jekel. Ugotovila sta, da zelo čista jekla niso občutljiva na pojav razpok, medtem ko je občutljivost jekel, ki so jim bile namenoma dodane nečistoče, rasla s koncentracijo le-teh. Nasprotno kot pri pojavu popustne krhkosti pri žarjenju za odpravo napetosti Sb, Sn in As bolj škodljivo vplivajo kot P. Prav tako je bil ugotovljen tudi škodljiv vpliv bakra14. Z Augerjevo spektroskopijo so na intergranu-larnem prelomu jekla z 0,5 % Cr, 0,5 % Mo in 0,25 % V po žarjenju za odpravo napetosti pri temperaturi 700 °C ugotovili precejšnje segregacije Sn, Sb, B, N in Cu, segregacij P pa zanemarljivo malo15. Izmerjene segregacije Sn in Sb se skladajo z opazovanji segregacij teh dveh kovin na prostih površinah v čistem železu17'18. Novejša raziskava Popa in sodelavcev8 je nadaljni prispevek k poznavanju pojava razpok pri žarjenju za odpravo napetosti pri jeklih za posode z notranjim ion process. Although the specific influence of each impurity vvas not isolated in this study, it is knovvn that ali of them segregate to the boundaries and to the free surfaces12 at the temperature of the creep tests, and the results in fig. 3 strongly suggest that the enhancement of cavity nucleation by segregating impurities is a major factor res-ponsible for the loss of creep rupture characte-ristics. Besides creep rupture itself tvvo other properties of steels vvhich involve creep mechanism s should be quoted. 2. Stress relief-cracking (SRC) (or »Reheat cracking«) The formation of intergranular cracks in the heat affected zone or occasionally the vveld metal in a vvelded assembly vvhen it is reheated to relieve residual stresses is novv recognized to be associa-ted vvith a creep (or relaxation) mechanism under the local stress, more precisely the stress gradient, induced by the rapid heating and cooling of the vvelding cycle. The cracking is therefore directly related to the local creep ductility of the material. Here again, the deleterious effect of impurity elements is far from being widely recognized. This is partly due to the variety of alloys and experi-mental conditions. In the majority of the publish-ed studies, it has been attempted to correlate the cracking behavior vvith the nominal impurity content in commercial steels vvhose compositions, microstructures and mechanical properties varied vvithin the range of accepted standards and under test conditions vvhose reproducibility may not have been accurate enough, considering the seve-rity of the vvelding or vvelding simulation test8. Hovvever such an effect of some impurities, in particular those associated vvith temper embrittlement, has unambiguously been demonstrated by Brear and King13 for Mn-Mo-Ni pressure vessel steels (A 533-B). They found that a high purity heat vvas not susceptible vvhile the sensitivity of impurity-doped heats increased vvith addition of various impurities. In opposition to temper embrittlement, SRC appears rather more sensitive to Sb, Sn, As than to P; also- some sensitivity to Cu vvas observed vvhich vvas not the the čase vvith RTE14. Auger electron spectroscopy performed on the intergranular fracture urface of a 1/2 Cr—l/2Mo — 1/4 V steel vvhich had endured extensive cavi-tation during a stress relaxation test at 700 °C demonstrated that considerable segregation of Sn, Sb, B, N and Cu occured at the free surface of the cavities vvhile that of P vvas negligible15. The observed segregations of Sn and Sb vvere in agree-ment vvith the free surface segregation measure-ments on pure iron 12>16. Recent vvork by Pope et al.8 has provided further insight into the SRC behaviour of pressure 0,35 "i 0.30 s ro 0.25 * 0,20 SO 8 0.15 0,10 0.05 0. m V 1 l • SA533-B o 5 A 508 -2 • • • • e • ^ \ o o \ o \ (a) (b) \ 0,1 0,2 0,3 0.4 0 Parameter CERL za primesi CERL parameter:impurity content Q2[Cu]+0MlSb [Ph 1.81Asi* 1,9[Sn]+2,7[Sbl (ut%) twt.%) 0,1 0.2 0.3 0,U 0.5 Parameter CERL*[Crl (ut. %) CERL parameter+[Crl(wt%] Slika 4 Zveza med krhkostjo zaradi žarenja za odpravo napetosti, merjeno pri 15 %-nem širjenju zareze (NOD) in nominalno sestavo jekla A533-B/A 508-2, (a). Ce se spreminja količina Cr parameter nečistoč CERL ni zadosten za oceno krhkosti zaradi (SRC) (b). Ce se v parameter vključi še količina Cr se dobi zelo dobra povezava (po ref. 8). Fig. 4 Correlation betvveen stress relief embrittlement (as mea-sured in terms of notch opening displacement NOD for 15 % cracking) and nominal composition of A533-B/A508-2 steels (a). When the Cr content is varied, the CERL impu-rity parameter is insufficient to account for SRC behavior (b). VVhen the Cr content is uncluded in the parameter an excellent correlation is obtained (after ref. 8). tlakom. Potrdili so, da se občutljivost jekel A 508—C 1.3 in A 533—gr. B na pojav razpok pri žarjenju za odpravo napetosti (SRC) spreminja z vsebnostjo nečistoč skladno s parametrom sestave, ki so ga že poprej definirali drugi avtorji13 ob upoštevanju vpliva legirnih elementov. V simuliranem ciklu varjenja je bil krom bolj škodljiv dodatek (si. 4). To razloži, zakaj je jeklo A 508 — 3 (0,6 % Mn in 0,35 % Cr) bolj občutljivo na pojav razpok kot jeklo A 533 — B (1,4 % Mn in 0,11% Cr). Škodljiv vpliv kroma pri žarjenju za odpravo napetosti je posledica utrjevanja trdne raztopine in zadrževanja poprave8 zaradi ogljika, ki zavira gibanje dislokacij. Vpliv legirnih elementov, npr. kroma moramo skrbno ločiti od vpliva oligoelementov na pojav razpok pri žarjenju za odpravo napetosti, saj krom vpliva preko masnega efekta, oligoelementi pa s segregacijami na novo nastalih površinah por. Pospešeno segregiranje je pogojeno z difuzijo po mejah. Segregacije na mejah ferita s karbidi in na kristalnih mejah so prav tako pomembne za nukleacijo por. 3. Popustna krhkost v pogojih lezenja in poprave Vijaki parnih turbin so iz jekla odpornega proti lezenju (npr. 28 CD V 5—0,8, z 0,28 % C, 1,25 % Cr, 0,1 % Mo in 0,04 % V), ki je komaj podvrženo re- vessel steels. They confirmed that the suscepti-bility of A 508-CI. 3 and A 533-gr. B steels to SRC varies vvith impurity content according to the Composition parameter defined by the previous authors,13 provided that the effect of alloying elements is also taken into account. For the weld simulation cycle chosen, Cr vvas the more dele-terious addition, fig. 4, vvhich explains that A 508-3 (0.6 % Mn, 0.35 % Cr) is in general more susceptible than A 533J3 (1.4 % Mn, 0.11 % Cr). This deleterious influence of Cr on SRC is essent-ially due to its strenghtening effect in solid solution vvhich retards relaxation by interaction vvith carbon to impede dislocation motion8. The influence of alloying elements such as Cr and residual impurities on SRC must therefore be care-fully distinguished, the former acting essentially via a bulk effect vvhile the latter segregate to the surface of newly formed cavities. This very rapid segregation is certainly controlled by intergranular diffusion and is therefore accelerated vvhen these impurities are segregated in the grain boundaries. Segregation at ferrite/carbide interfaces and grain boundaries is also important in that it controls cavity nucleation as already discussed above. 3. Temper embrittlement under creep or relaxation conditions Steam turbine bolts are made of creep resistant steels (e.g. 28 CD V 5-08, i. e. 0.28 C — 1.25 Cr — 0.8 Mo — 0.4 V) vvhich are hardly susceptible to reversible temper embrittlement17. Hovvever, these bolts may fracture intergranularly vvhen cooling dovvn after being refastened follovving a service period of several thousand hours. This phenomenon has been shovvn17 to be a form of stress-en-hanced temper brittleness: the embrittlement increases vvith applied stress and can become partly or totally irreversible, fig. 5, especially after ageing under stress at higher temperatures (e. g. 550 °C), the degree of irreversibility increasing vvith creep strain17-1«, fig. 6. Intergranular failure results from the cumulative embrittling action of both the segregated impurities (P, Sn Sb) and creep cavities at the grain boundaries18. Irreversi-bility of embrittlement takes plače vvhen the latter is sufficient to promote intergranular decohesion at a smaller segregation level obtained after a given »de-embrittling« treatment (e.g. 650°C, 1 h). This explains18 that a threshold strain et exists for the appearance of irreversibility, fig. 16. Belovv this value, the grain boundaries are not a preferred fracture path compared to cleavage in the »de-embrittled« condition (i. e. no irreversibility is ob-served), but in the embrittled condition vvhere grain boundaries are already vveakened by a larger segregation the embrittling effect of the same density cavities can take plače because it only needs to add itself up to that of the impurities. This explains18 that the enhancement of temper embrittlement by stress-induced cavitation is not neces- verzibilni popustni krhkosti17. Ti deli se lahko porušijo z intergranularnim prelomom potem, ko se po nekaj tisoč urah dela ohlade. Ta pojav je oblika napetostne inducirane popustne krhkosti17. Krhkost narašča s prisotnimi napetostmi, postane pa lahko delno ali popolnoma ireverzibilna (si. 5). V pogojih staranja pod obremenitvijo pri višjih temperaturah (npr. 550 °C) raste stopnja nepovračljivo-sti z deformacijo pri lezenju17- ^,'sl. 6). Intergranu-larna porušitev je posledica skupnega vpliva na krhkost segregacij nečistoč (P, Sn, Sb) in tudi por, nastalih pri lezenju po kristalnih mejah18. Če število por preseže določeno množino, pride do ire-verzibilne krhkosti tudi po ustreznih toplotnih obdelavah za odpravo krhkosti (npr. žarjenja lh na 650 °C). To govori za to, da ireverzibilnost krhkosti nastopi potem, ko presežemo mejno deformacijo st (sl. 6)18. Pod to deformacijo kristalne meje niso prednostna področja porušitve v primerjavi s cepljenjem v pogojih »nekrhkosti«. V pogojih krhkosti, kjer so meje zrn že oslabljene zaradi večjih segregacij, se lahko pojavi vpliv enake pogostosti por, saj se oba efekta seštevata. To kaže18, da pospeševanje popustne krhkosti z napetostno inducirano poroznostjo ni nujno povezano z ire-verzibilno krhkostjo, kot je to npr. po staranju Slika 5 Vpliv napetosti na popustno krhkost. Razlike v prehodni temperaturi (ATTX) po staranju brez (00) ali pod vplivom napetosti (□□). Vidi se tudi preostala krhkost v »obrnjenih« pogojih (A) tj. potem, ko je bilo jeklo starano 1 h pri 650 °C in pod vplivom obremenitve. ATT, je razlika v prehodni temperaturi jekla po določenem tretmanu (X) in »nekrhkim« zelo čistim jeklom, pri čemer so bile vse epruvete obdelane tako, da so imele enako trdoto (po ref. 18). Fig. 5 Temper embrittlement under stress. Comparison of em-brittlement ATTX after ageing without (00) and with (□□) an applied stress. The residual embrittlement in the »reverted« condition (A), i. e. tempered 1 h at 650 "C after embrittlement under stress, is also shovvn. ATTX is the transition temperature shift betvveen each condition X and the non-embrittled (NE) condition of the purest heat after correcting for constant hardness (after ref. 18). Slika 6 Nepovračljivost popustne krhkosti zaradi staranja pod vplivom napetosti; ATTR kot funkcija deformacije pri lezenju (e); ATTr je razlika med prehodno temperaturo v, obrnjenih pogojih in tisto za nekrhko najčistejše jeklo (po ref. 18). Fig. 6 Irreversibility of temper embrittlement induced by ageing under stress. ATTr, as a function of the creep strain z. ATTe is the difference between the transition temperature of the reverted condition and that of the NE condition of the purest heat (after ref. 18). sarily accompanied by an irreversibilty of this embrittlement, as is the čase e. g. after ageing under stress at 500 °C, fig. 5. Temper embrittlement and cavitation-induced embrittlement are therefore not simply additive, especially if it is remembered that cavitation itself is strongly influenced by the segregating impu-rities. This phenomenon therefore provides us vvith a striking example of the co-operative and synergistic action of two different phenomena, in-volving interfacial segregation effects, on the impairment of mechanicial properties in service condition. C. BEHAVIOR OF GRAIN BOUNDARIES IN AGGRESSIVE ENVIRONMENTS 1. Hydrogen-assisted cracking in temper brittle steels It has been suspected for a long time that RTE and hydrogen-assisted cracking (HAC) must be connected or interact with each other. Cabral, and Constant19 gave the first evidence for such an interrelation by shovving that a temper embrittling treatment at 500 °C reduced the resistance of smooth specimens of a 0.3 C — 2.75 Ni — 0.7 Cr (30 NC 11) steel to HAC in H2S04 solution and changed the fracture mode from transgranular to intergranular. More recent work has taken advantage of the development of Auger Electron Spectroscopy and of fracture mechanics techniques. Systematic investigations on the behavior of several high pod obremenitvijo pri temperaturi 500 °C (si. 5). Popustna krhkost in s porami inducirana krhkost zato nista enostavno aditivni, posebej če vemo da na nastanek por močno vplivajo segregacije nečistoč. Ta fenomen je tipičen primer kooperativnega in poudarjenega delovanja dveh različnih pojavov na poslabšanje mehanskih lastnosti v delovnih pogojih. C. OBNAŠANJE KRISTALNIH MEJ V AGRESIVNIH OKOLJIH 1. Vodikova krhkost Že dlje časa je veljal sum, da sta reverzibilna popustna krhkost (RTE) in vodikova krhkost (HAC) povezani, ali pa da vplivata druga na drugo. Cabral, Hacke in Constant19 so prvi dokazali to povezavo s poskusi na poliranih epruvetah jekla z 0,3 °/o C, 2,75 % Ni in 0,7 °/o Cr (30 NC11). Zaradi popustne krhkosti, nastale z žarjenjem pri 500 °C, se je zmanjšala tudi odpornost proti HAC ( v raztopini H2S04) in se je spremenil tudi mehanizem porušitve iz transkristalnega v interkristal-nega. Novejše raziskave izkoriščajo možnosti Auger-jeve spektroskopije in lomne mehanike za sistematične raziskave visokotrdnih jekel v različnih medijih (npr. H2S04, H2S, vodiku ipd.). Joshino in Mc Mahon20 sta primerjala poškodbe jekla HY 130, ki je bilo v dveh stanjih; po toplotni obdelavi, ko ni bilo krhko, in po stopenjskem ohlajanju, ki je povzročilo precejšne segregacije Si in P ter tudi N in Ni, zaradi česar je bilo podvrženo krhkosti. V zvezi s »statično« utrujenostjo sta opazila tri posebne oblike delovanja (si. 7): — segregacije zmanjšajo parameter Klc (na zraku) in spremene prelom v interkristalnega (si. 7a), — vodik inducira porušitev v obeh primerih, le obliki preloma sta različni: pri postopno ohla- Š s Čas do porušitve (Kamini. Slika 7 Vpliv predhodne toplotne obdelave na nastanek krhkosti jekla. Čas do porušitve v odvisnosti od razmerja začetnih faktorjev intenzivnosti napetosti Kn. Jeklo HY130 je preizkušano v 0,1 n H2SO4 (po ref. 20) in na zraku. Fig. 7 Influence of a prior temper embrittling treatment on tirne to failure plotten as a function of the initial stress inten-sity factor K„ for a HY130 steel tested in 0,1 n H2SO«, (after ref. 20). strength steels in various environments (H2S04, H2S, gaseous H, ete.) were undertaken. Yoshino and Mc Mahon20 compared the delayed failure behaviors in a H2S04 solution of a 5 Ni-Cr-Mo-V steel (HY 130) in unembrittled condition and after a step-cooling treatment vvhich induces conside-rable segregations of Si and P but also of N and Ni. Three essential features can be seen on the static fatigue plots, fig. 7: — segregation causes the toughness in air Klc to decrease and the fraeture mode to become inter-granular, as already mentioned, fig. 7-a; — hydrogen induces delayed fraeture in both conditions but the failure modes are different: it is always intergranular decohesion in the step-cooled samples vvhile it remains transgranular cleavage in the unembrittled condition at ali failure times; — the segregated (step-cooled) condition is much more sensitive to hydrogen than the unembrittled condition as shovvn by fig. 7-b vvhere the initial Kn for failure at time t in H2S04 have been normalized vvith respect to their value in air for each heat treatment. In unembrittled state, this ratio decreases slovvly by 30 % in 100 h, vvhile in the presence of segregation, it rapidlv decreases by 60 % vvithin the first hour of the test. Briant, Mc Mahon and Feng21 have measured the crack propagation velocities of the same steel in gaseous H2 after varying the time of the segregation ageing at 480 °C. Fig. 8 shovvs that this rate inereases drastically vvith ageing time at ali values of the applied stress intensity K. Moreover, the threshold value of K, KTH, i. e. that corresponding to the deteetable crack grovvth rate of 10—7 m/s, was very high in non-embrittled condition (~ 185 MN m—2) vvhile HAC remained essentially transgranular. Hovvever, KTH decreases catastro-phically and intergranular decohesion appeared vvhen the ageing time vvas inereased. This behavior is illustrated in fig. 9, vvhere it appears that very rapid initial decrease in KTH is apparently associ-ated vvith rapid although limited segregation of P vvhile the later steady decrease is apparently asso-ciated vvith a larger but more steady increase in Si segregation. If the effect of both elements is additive, P is apparently much more harmful than Si vvith regard to the resistance to HAC, but the effect of each individual segregant is stili far from bein accurately characterized. Hydrogen is much more deleterious in high strength steels sensitive to one-step temper embrittlement (OSTE, cf Dumoulin, this Confe-rence). This is in particular the čase of 0.4 C — 1.75Ni-Cr-Mo steels (4340) and of silicon-modified 4340 steels (300 M i. e. 4340 + 1.6 % Si) tempered at low temperatures22. In these steels, fraeture is always intergranular in the presence of hydrogen, as it already is in a non-agressive medium, and the idan 2dm 3dni i, dni 1day 2 days 3days 1 Time to fraeture jenih epruvetah je prelom vedno interkristalen; v nasprotnem primeru pa ostaja značilna oblika transkristalnega cepljenja, — jeklo s segregacijami nečistoč po kristalnih mejah (stopenjsko ohlajeno) je mnogo bolj občutljivo na porušitev ob prisotnosti vodika, kot jeklo brez segregacij. To ugotovitev ilustrira si. 7b, kjer je prikazana odvisnost med razmerjem faktorjev koncentracije napetosti KIf za preizku-šance v HzS04 ter na zraku in časom do porušitve, in sicer za »krhko«, t.j. postopno ohlajeno in »ne-krhko« jeklo. O i p- ■ N O) o $ 10* ', 104 i I 105 81 P *0-6 10 CJi 10' i f? Starano 100Č ur / p t i i 'taran OOhrs >200ur rv v' • / : t r S r» * Starar 50 hr o 501 J t i i J> r« H • •• • -i j i Ni * arano No aging l i 20 UO 60 Jeklo HY130 Hitrost rasti razpok v vodiku Temperatura: 22 °C Pritisk: 16 psig — Vzorci W0L • Vzorci CT 80 100 120 140 160 180 200 K fMNm'3/2) HY 130 Steel Crack growth rates in gaseous hydrogen Temperature ■• 22 "C Pressure : 16 psig — W0L specimens • CT specimens effect of a temper embrittling treatment is much less conspicious. However in a high purity labo-ratory heat of 4340 steel, intergranular failure in H2 disappears and the threshold KTH value is five-fold that of a commercial heat of similar base composition22, compare B 7 and B 2 in fig. 10. The threshold is again lowered by an addition of Mn + Si, as shown for heat B 6 in fig. 10. As in the čase of one-step temper embrittlement of the same steels, the effect of impurities (here Si) is evident even at the very low segregation levels produced by austenitizing treatment owing to very high strength levels and probably the pre-sence of plate-like carbides in grain boundaries. The behaviour of steels in the presence of hy-drogen therefore reflects the characteristics of PCT Fe 703eV Višina pika.Peak height P 0,8 i6 5A „ 0.5 3 —i— 6,1 i i 16 20 22 i-1-r— 24 A N Si 200 Slika 8 Vpliv staranja pri 480 "C na hitrost razpokanja jekla HY 130 v vodiku (po ref. 21). Fig. 8 Influence of ageing at 480 °C on crack ve!ocity of HY 130 steel in gaseous hydrogen (after ref. 21). jBriaut, Mc Mahon in Feng21 so na vzorcih istega jekla, staranih pri temperaturi 480 °C, merili hitrosti napredovanja razpok v vodiku v odvisnosti od časa staranja. SI. 8 kaže, da se hitrost rasti razpok povečuje zelo hitro s časom staranja, ne glede na vrednosti faktorja intenzivnosti napetosti K. Mejna vrednost faktorja K, KTH, ki ustreza merljivi hitrosti napredovanja razpoke (10—7 m/s), je pri »nekrhkem« jeklu zelo velika (pribl. 185 MN m-2). Porušitev zaradi vodika • -UCr-UNi °-16Cr-UNrCL02C &-19Cr-UNI-O07C Ost. Bal. Fe Ni IK , I K No IGA . ga ~10 102 103 W Vsebnost fosforja Vsebnost silicija Phosphorus concentration (ppm) Silicon concentration (ppm) Slika 13 Vpliv P (a) in Si (b) na interkristalno korozijo nerjavnega avstenitnega jekla v raztopini HN03 in ionov Cr«+ (po ref. 33). Fig. 13 Influence of P (a) and of Si (b) on the intergranular corrosion behaviour of austenitlc stainless steels in HNO3 + Cr6+ solution (after ref. 33). which have been tentatively attributed to some kind of intergranular segregation are those which could not be associated with a Cr-depleted zone such as that induced by the so-called sensitization treatment or those whose magnitude did not correlate in a sirnple way vvith that of Cr-deple-tion.31 In particular, corrosion in highly oxidizing acid media such as boiling nitric acid containing potassium dichromate (Cr+6 ions) can be drasti-cally localized at the grain boundaries even in unsensitized alloys (i. e. quenched from the solution temperature). Chaudron32 first showed that a high purity alloy was not susceptible. Systematic research shovved that this type of intergranular corrosion was rvery sensitive to the P an Si content. Armijo33 demonstrated that the corrosion rate of an austenitic stainless steel increases steeply vvith phosphorus content above 100 wt. ppm this increase being associated vvith the onset of prefe-rential intergranural dissolution, fig. 13-a. A mar-ked rnaximum is also observed vvith respect to Si content around lwt%, the peak region correspond-ing to intergranular corrosion33, fig. 13-b. In 'the ni-ckelbase alloy 600 Vermilyea et al34, shovved that intergranular penetration measured as the decrease in fracture strength of a smooth specimen after the corrosion test also increases vvith nominal P content, fig. 14-a, but that the effect of Si is much less critical in this alloy, fig. 14-b. Several studies have demonstrated that phosphorus actually segregates at the grain boundaries of austenitic stainless steels24 and alloy 60035,36-37. In particular it has been shovvn that segregation in alloy 600 obeys the lavvs of equilibrium segregation,37 fig. 15. It is therefore qualitatively similar in Ni-base alloys and in temper brittle steels, although it occurs at higher temperatures and vvith a higher enrichment ratio in the former alloy 1000 at 700 °C), indicating that the segregation energy is larger. Another quantitative difference Slika 14 Vpliv P in Si na interkristalno korozijo zlitine Inconel 600 merjen z zmanjšanjem porušne trdnosti po korozijskem testu (po ref. 34). Fig. 14 Influence of P and Si on the intergranular corrosion of Inconel 600 as measured by the decrease in fracture strength after corrosion tests (after ref. 34). 800 1000 12000 Temp. (°C) i Čas staranp (ure). VAgeing ti me thrs) Slika 15 Odvisnost ravnotežnih segregacij (a) od temperature in kinetike segregiranja fosforja na kristalnih mejah v zlitini Inconel 600 (po ref. 37). Fig. 15 (a) Temperature dependence of equilibrium segregation and (b) segregation ldnetics of P in the grain boundaries of alloy 600 (after ref. 37). na s kalijevim dikromatom (Ci^+J, je lahko izrazito omejena na kristalne meje celo pri zlitinah, ki so npr. gašene s temperature raztopnega žarjenja. Chaudron32 je prvi dognal, da zelo čiste zlitine niso podvržene temu pojavu. Sistematične raziskave so pokazale, da je ta način korozije tesno povezan z vsebnostjo P in Si. Armijo33 je ugotovil, da raste hitrost korozije avstenitnega jekla s koncentracijo fosforja, če le-ta preseže 100 ppm. Krivulja raste zaradi prednostnega raztapljanja jekla na kristalnih mejah (si. 13a). Podoben jasen maksimum je povezan s koncentracijo silicija (pribl. 1 %), pri čemer je maksimum posledica in terkris talne korozije jekla33 (si. 13b). Vermilyea in sodelavci34 so ugotovili, da z nominalno koncentracijo fosforja raste hitrost interkri-stalne korozije v zlitini Inconel 600 (si. 14a). Vpliv silicija na pojav interkristalne korozije je v tej zlitini mnogo manj pomemben. Stopnjo korozije 90 60 <0.02Si-i 0.10 Si 0,46 Si 1.01 Si- '0 5 10 15 20" 0 5 10 15 20 Čas v raztopiniHN03/Cr6(ure). Time in HNOs/Cr'6 solution-hrs so ocenjevali iz zmanjšanja porušnih trdnosti po-liranih epruvet po korozijskem preizkusu (si. 14b). V različnih študijah je pokazano, da se kristalne meje avstenitnih jekel24 in v Inconelu 60 0 35-36-37 obogate s fosforjem. Za zlitino Inconel 600 je ugotovljeno, da količina segregacij sledi zakonu o ravnotežnih segregacij ah37 (si. 15). Podobno velja za druge nikljeve zlitine in popustno krhka jekla. Dejstvo, da se v Inconelu 600 to pojavlja pri višjih temperaturah in pri večjih obogatitvah (npr. obogatitev 1000 x pri 700 °C), kaže da je energija se-gregiranja večja. Druge kvantitativne razlike pa so zato, ker je kinetika difuzije v ploskovno centrirani rešetki počasnejša kot v matici s prostorsko centrirano rešetko. Analize s pomočjo Augerjeve spektroskopije dokazujejo torej interkristalen mehanizem korozije, kar je ilustrirano s si. 14a. c) Elektrokemična korozija na kristalnih mejah. Pojav interkristalne korozije je lahko razložiti tudi s pomočjo elektrokemičnih poskusov. Korozijski potencial raztopine kromovih (Cr6-1") ionov v solitrni kislini ustreza t.i. transpasivacij-skemu področju, in kot je opisano v referenci38, je precejšnje število zlitin, pri katerih pride do prednostnega interkristalnega korozijskega napada, brž ko potencial raztopine seže do začetka transpasivacijskega vrha. Raztopina solitrne kisline in kromovih (Cr6+) ionov ni specifična za s kromom osiromašena področja, saj v transpasiva-cijskem področju pada kar je popolnoma nasprotno obnašanju v aktivacijskem in pasivacijskem področju. Mnogo avtorjev meni, da je prednostna korozija po kristalnih mejah v tej raztopim jasen znak segregacij po kristalnih mejah30-39. 3. Interkristalna napetostna korozija (IGSCC) V nekaterih posebnih primerih je lahko nedvoumno pokazati škodljiv vpliv segregacij nečistoč na odpornost proti interkristalni napetostni koroziji (IGSCC). Tak primer je zlitina železa s 3 % niklja in z dodatki P in Sn40, ki je bila preizkušana v 5-molarni raztopini NaOH in obremenjena do 90 % meje tečenja. Za primerjavo s popustno krhkostjo je bila dolžina razpok primerjana v referenčnih pogojih in pogojih, ko so nastale segrega-cije nečistoč zaradi staranja po llh pri 550°C. Rezultati kažejo, da po določeni inkubacijski dobi (105 s) raste razpoka v starani zlitini približno 3 krat hitreje kot v primerjalnih vzorcih (si. 16). Avtorji 40 menijo, da ima ta pojav svoj vzrok v pospeševanju anodnega raztapljanja materiala po kristalnih mejah, obogatenih z nečistočami. Lea je študiral pojav IGSCC v Cr-Mo jeklih (2,25 % Cr, 1 % Mo) v raztopinah amonijevega nitrata41. Primerjali so razmerje R (R = Kmin/.KmaI) med povprečnim časom do porušitve v korozijskem mediju in inertni sredini (vroč parafin) za jekla z različno koncentracijo P. Primerjali so rezultate talin, v katerih so bile segregacije fosforja is that the diffusion kinetics is slower in the f.c.c. than in the b.c.c. matrix. The Auger results therefore substantiate the intergranular corrosion behaviour illustrated by fig. 14-a. c — Electrochemical corrosion of grain boundaries The intergranular corrosion results can be in-terpreted in the light of electrochemical experi-ments. The corrosion potential of nitric acid + Cr6+ solution is actually situated in the trans-passive region, and it has been shown38 that a variety of alloys exhibited preferential intergranular dissolution when the potential vvas chosen in the initial rise of the transpassive peak. It can ibe understood novv that the HN03 + Cr<>+ solution is not specific to Cr-depleted zones because in the transpassive region the corrosion rate decreases vvith decreasing Cr content, in opposition to the vvell knovvn behaviour in the active and passive regions. Many authors novv agree that preferential attack of the grain boundaries in this solution is essentially indicative of intergranular segrega-tion30'39 . 3. Intergranular stress corrosion cracking (IGSCC) In some cases deleterious effect of a segrega-ted impurity on resistance to IGSCC can be unam-biguously demonstrated. This is the čase40 of a Fe-3 % Ni alloy doped vvith P and Sn and tested in 5 m NaOH under a stress equal to 90 % of its yield strength. In reference to temper embrittle-ment, the crack lengths vvere compared in a reference condition and in a segregated condition Sn ? Krhko ■ * Embrittled D o žarJeno Anne aled Začetek Initiation— - širjenje —— propagation-- / r i / T / ■ 1 t i > s X I L_ 0\-1--L 0 12 3 Čas. Time; s x10s Slika 16 Vpliv popustne krhkosti na dolžino razpok vsled napetostne korozije v zlitini Fe — 3 % Ni v NaOH (po ref. 40). Fig. 16 Influence of temper embrittlement on the length of stress corrosion cracks in Fe — 3'% Ni tested in 5 M NaOH (after ref. 40). 1 •S s (t P t <3 M HP(D\ ~ CP(D). - L X HPP(D) iT - HP(zn pp \č>__ hppIe) - 1 Slika 17 Vpliv količine P na kristalnih mejah (Xp) na občutljivost za napetostno korozijo (SCC) jekla z 2,25 °/o Cr in 1 % Mo. (CP) — tehniško jeklo, (HP) — zelo čisto jeklo, HPP = HP zelo čisto jeklo z dodatki P (po ref. 41). Fig. 17 Influence of P content at the grain boundaries X, on the SCC susceptibility of 2 1/4 Cr-1 Mo steel. CP: commercial purity heat; HP: high purity heat; HPP = HP doped vvith P (after ref. 41). na kristalnih mejah, in tiste, kjer tega pojava ni bilo (si. 17). Ta slika pove, da se nevarnost porušitve zaradi napetostne korozije povečuje s koncentracijo fosforja na kristalnih mejah do približno 20 % P in da je nevarnost napetostne korozije že pri segregacij ah, ko je približno 5 % P v mejnem sloju. Z dodatkom lantana, ki veže fosfor v matici, se lahko znatno izboljša odpornost proti napetostni koroziji. 4. Krhkost zaradi staljenih kovin (LME) Podobno vodikovi krhkosti lahko povzroči LME transkristalen ali interkristalen prelom. Ta vrsta krhkosti je vendarle bolj poudarjena na kristalnih mejah, kar je tudi posledica kemične sestave tega dela kovine. Dinda in Warke42 sta ugotovila, da kositer in antimon povečujeta LME jekel 3340 (3,5 % Ni, 1,7 % Cr) v talinah svinca in kositra. Nasproten vpliv ima fosfor, ki naj bi očitno povečal odpornost proti LME v talinah teh dveh kovin. D. UTRUJENOST V martenzitnih jeklih, ki so bila podvržena po-pustni krhkosti, je bil tudi ugotovljen vpliv segregacij nečistoč na pojav utrujenosti. Pri napredovanju utrujenostne razpoke so bile ugotovljene tri značilnosti43 (si. 18), ki jih lahko označimo kot parametre mikrostrukture. Pri majhnih hitrostih rasti razpoke (režim A) in ko sprememba faktorja koncentracije napetosti AK doseže svojo mejno vrednost z(K0, je zelo velik vpliv mikrostrukture in povprečne napetosti na hitrost rasti razpok. Na to vpliva tudi predhodno spreminjanje napetosti in okolica. Pri večjih hitrostih napredovanja razpok (režim C), ko vrednost Kmax doseže KIc, sta vpliva mikrostrukture kovine in povprečnih napetosti obtained by ageing 11 h at 550 °C. The results indi-cate, fig. 16, that after the same incubation period of 105 s, the crack grovvth rate in aged (segregated) specimens vvere increased by a factor of 3 vvith respect to that in reference samples. The authors suggested40 that this effect vvas actually due to the acceleration of anodic reaction of dissolution of the grain boundary material by segregated impurites. Lea has studied the IGSCC behaviour of 2 1/4 Cr-1 Mo steels in ammonium nitrate solution41. The ratios R of the average times to failure dn the corrosive solution and in an inert medium (hot paraffin) vvere compared for heats of various P content in both the embrittled E (step-cooled) and de-embrittled D (quenched from 700 °C) conditions, fig. 17. This figure shovvs that SCC properties deteriorate vvith increasing phosphorus grain boundary level up to about 20 % and that SCC suscepti'bility is affected even at very lovv P segre-gation levels (5 % monolayer). An addition of La vvhich ties up P in the matrix produces a marked improvement in SCC properties. C 4. Liquid metal embrittlement (LME) Like hydroden-induced cracking LME can be either intergranular or transgranular. Hovvever it is often more severe at grain boundaries and here again an influence of grain boundary composition can be expected. Dinda and Warke42 have observed that the se-gregation of Sn and Sb enhanced the susceptibility of temper embrittled 3.5 Ni — 1.7 Cr steels <3340) to LME by Pb or Sn. Hovvever, a reverse effect vvas noted for P vvhich apparently improves the resistance to LME in these two environments. D. FATIGUE It is again in martensitic steels susceptible to reversible temper embrittlement that the effect of segregated impurities on fatigue properties has been clearly established. In the fatigue crack propagation curve, three ranges of behaviours should be distinguished43, fig. 18, in relation to the effects of vvhat could be called the »microstructural« parameters vvhich include intergranular segregation, on primary fracture mechanisms. At lovv grovvth rates (regi-me A) vvhen the alternating stress intensity A K approaches its threshold value A K, there is a strong influence of microstructure and mean stress (characterized by the load ratio R = Kmin/ /Kmax) on grovvth rates, together vvith an increased sensitivity to stress history and environmental effects. At higher grovvth rates, (regime C), vvhen Kmax approaches KIc the influence of microstructure and mean stress is also prominent. Only in the Režim A Režim B Režim C Nezvezni Kontinuirni Statične oblike mehanizem mehanizem mehanizmov velik vpliv: (rast brazd) | (cepljenje, interkrist. majhen vpliv: I in vlaknat) 1. mikrostrukture J. mikrostrukture velik vpliv: 2. poprečnih 2.popreč. napetosti I 1. mikrostrukture napetosti 3. blage okolice j 2. pop reč. nape tos ti 3. okolice 4. debeline jekla J 3. debeline jekla majhen vpliv: I 4. okolice Regime A Regime B v2 NDn-continium Cont. mechanism 1 i |nC \Zlom 1J Final mechanisms (striation growth) large influence of: little influence of: J failure 1 microstructure 1. microstructure 1 C / 2. mean stress 2. mean stress -S! k - 3. environment 3 dilute environment ! / 4. thickness i / / y s ! jfda/dNzClAKf" l- - I s i Regime C •g \m i, Static mode"mechanisms —--' i (cleavage, intergranular ana fibrous) _ 1 large influence of: j /. microstructure Z-' / 1 2 mean stress ! 3 thickness 106 _ ' /A Prag \ / thresholdAKo J j little influence of: i 4.environment 1 i log AK Slika 18 Osnovni mehanizmi porušitve v povezavi s tremi področji na krivulji spreminjanja hitrosti rasti utrujenostne razpoke (da/dN) s koncentracijo napetosti (AK) (po ref. 43). Fig. 18 Primary fraeture mechanisms associated with the three regions in the sigmoidal variation of fatigue crack propa-gation rate (da/dN) vvith alternating stress intensity (AK) (after ref. 43). prav tako odločilna. Le pri vmesnih hitrostih napredovanja razpoke (režim B), kjer v splošnem velja Parisov zakon, sta vpliva povprečnih napetosti in mikrostrukture manjša. Ritchie43 je primerjal širjenje razpoke v jeklu 4340 z dodatkom silicija (300 M). Za poskuse, ki jih je opravil na vlažnem zraku, je izbral v olju ohlajene (s 650 °C) in popustne (postopno ohlajene) preizkušance, ki so imeli bistveno večje segregacije P in Si na kristalnih mejah. a) Pri srednjih hitrostih rasti razpoke (10-3 mm/nihaj > da/dN > 10-5 mm/nihaj) (si. 19) so bile za prelom po obeh toplotnih obdelavah značilne brazde (»letnice«), ki so nastale s transkristalnim širjenjem razpoke. Hitrost rasti razpoke ne kaže posebne odvisnosti od segregacij in sprememb obremenitvenega razmerja R. b) Pri manjših hitrostih rasti razpoke (da/dN < 10—5 mm/nihaj) (si. 19) se s segregacijami nečistoč povečuje hitrost rasti razpoke celo za velikostni red. Sočasno pa se znižuje prag JK,, za približno 30 % in se na prelomu pojavijo interkristal-na področja. Delež teh področij na prelomljeni površini i se spreminja z zlK; npr pri R = 0.05 je i = 5 %, ko je AK tik nad AK^-, pri AK. = 10 MPa m1/2 doseže i = 20 % in izgine pri AK. = 15 MPa m1/2. Nasprotno pa pri jeklu, ohlajenem v olju, ni bilo interkristalnih prelomnih površin, ne glede na velikost AK in R. midrange of grovvth rates (regime B), where Pariš' law is in general obeyed is there a little influence of mean stress and microstructure. Ritchie43 has compared the crack propagation behavior in humid air of a Si-modified 4340 steel (300 M) in two quenched and tempered conditions the so-colled non-embrittled condition (oil quen-ched from 650 °C) and embrittled condition (step-cooled from 650 °C), which differ essentially by a high segregation level of P and Si in grain boundaries of the latter. a — In the intermediate range of growth rates (10-3 mm/cycle > da/dN > 10-5 mm/cycle) fig. 19, the failure mode remains transgranular stria-tion in both conditions and the growth rates are not very much affected by the segregation treatment and variations of the load ratio R. b — At smaller grovvth rates (da/dN < 10—5 mm/cycle) fig. 19, the segregation treatment inere-ases the grovvth rate by up to one order of magni-tude and lovvers the threshold zlK0 by 30 % vvhile intergranular facets appear on fraeture surface. Their fraetional area i varies vvith AK: vvhen R = = 0,05 for instance, i = 5 % for AK just above AKol then reaches 20% for AK =lMPam"2 and drops to 0 again for zlK > 15 MPa m1/2. Con-versely, no intergranular facet is observed for any value of AK and R in non-embrittled condition. In this range zfKo and da/dN become increasingly sensitive to R. This behaviour has been attributed43 to the effect of environment, the embrittlement in fatigue conditions being attributed to the adsor-ption of hydrogen at the crack tip. Segregated impurities vvould play the role of recombination pois-on for H into gaseous H, there, favoring the propagation of crack along grain boundaries. At higher grovvth rates those processes vvould not have eno- •»■9! i«? II II If I* 10' 162 103 10■* : PofH&čeno Hlqeno - temper cooling ■9i u |8 = § S.g> 10 v 300-M legirano jeklo alloy steel R MPa m 2 x 650 "C olje. oil 0.05 40 50 They have studied in further details the rela-tionships betvveen intergranular decohesion and fatigue grovvth in this range of higher grovvth rates. Besides the increase of da/dN vvith R in embrittled (1 h at 540 °C) condition, fig. 21, they notice that at 120° C the crack grovvth rate is lovver than at room temperature and insensitive to the value of R vvhile intergranular decohesion disap-pears: this is typically a transition temperature effect analogous and akin to that encountered vvith brittle failure. The fractional area i of intergranular decohesion increases vvith /jK and R, but the more interesting result is that it is approxi-mately a unique function of Kmax and independent of R values, fig. 22. This confirms that intergranular fracture here is a tensile stress-controlled static mode of propagation vvhich is simply super-posed to the transgranular striation mode. There-fore there exist a unique relationship betvveen i and da/dN at 25 °C, fig. 23. On this figure, da/dN at 25 °C has been normalized vvith respect to its value at 120 °C vvhich can be taken as an internal reference for the embrittled condition, since at this higher temperature propagation is essentially transgranular and independent of R. \ Z12 C13 AK 17,5 20 22,5 a 5 30 35 40 + o • □ * X © Slika 22 Spreminjanje deleža intekristalnega preloma (i) s Kmax v jeklu z 12% Cr (po ref. 45). Fig. 22 Variation of the percentage of intergranular decohesion i vvith K„„„ in a 12 % Cr steel (after ref. 45). Slika 23 Zveza med »normalizirano« hitrostjo rasti razpoke pri sobni temperaturi in (deležem interkristalnega preloma v v jeklu z 12 «/o Cr (po ref. 45). Fig. 23 Correlation betvveen normalized crack propagation rate at room temperature and fraction of intergranular decohesion in a 12 % Cr steel (after ref. 45). Summarizing, reversible temper embrittlement, i. e. the intergranular segregation of impurities in martensitic steels promotes preferential crack Segregacije nečistoč na mejah kristalnih zrn martenzitnih jekel pospešujejo prednostno rast razpok vzdolž kristalnih mej pri zelo majhnih, kot tudi pri velikih hitrostih širjenja razpoke, toda iz povsem različnih vzrokov: — pri večjih vrednostih AK, t.j. pri večjih Kmax je statični tip krhkosti zelo soroden navadni popustni krhkosti, — v okolici praga za /fK^ je vpliv okolice soroden krhkosti, ki je posledica interakcije vodika in segregacij (HAC). Primeri utrujenosti zgovorno ilustrirajo različne kombinacije škodljivih vplivov s segregacijo nečistoč na uporabne lastnosti gradiv. F. ZAKLJUČKI V tem preglednem članku je pokazano, da lahko v tehnološko pomembnih železovih zlitinah se-gregira na kristalnih mejah precejšnje število nečistoč, kar močno vpliva na njihove lastnosti. Le v malo primerih je bil ugotovljen ugoden vpliv teh nečistoč. S primerom, ki ga označujemo z reverzibilno popustno krhkostjo jekla, je pokazano, kako je na ta ali drugačen način možno vplivati na normalne lastnosti jekla. Razen empiričnih podatkov je zelo malo znanega o mehanizmih medsebojnega delovanja segregacij in drugih vrst poškodb na kristalnih mejah. V večini primerov ni jasno niti to, ali se njihovi vplivi enostavno seštevajo ali pa drug drugega izrazito poudarjajo. Popolnoma jasno je tudi, da na to vprašanje ni splošnega odgovora. Znano je, da se vpliv različnih vrst nečistoč v različnih jeklih različno odraža, saj so tudi posamezne lastnosti na specifičen način odvisne od mikrostrukture kovine. Znani primeri kažejo, da se isti efekti pokažejo v različnih materialih. Avtorjevo mnenje je, da spektakularen razvoj metod in naprav za površinsko analizo ne bo le širil seznama škodljivih pojavov zaradi primesi, ampak bo pomagal odkriti tudi primere sestavljenih (»množičnih«) segregacij in posebej še tiste legirne elemente, katerih prisotnost koristno vpliva na uporabne lastnosti kovin. Zahvala Ta članek je izsek iz predavanja na Journees d'Automne de le Societe Francaise de Metallurgie, ki je bil 23. X. 1979 v Parizu in objavljen v »Advan-ces in the Mechanics and Physics of Surfaces«, vol. 1 in sta ga pod naslovom »The influence of Inter-facial Segregation in Embritllement Phenomena« uredila R. M Latanision in R. J. Courtel. growth along grain boundaries at both ends of the range of grovvth rates and alternating stress inten-sities, but for entirely diferent reasons: — at larger JK, i.e. at larger Km„ a static type of brittleness closely akin to plain temper embrittlement is involved; — in the vicinity of the threshold zfKo it vvould rather be an environment-controlled effect more akin to segregation-enhanced hydrogen embrittlement. The example of fatigue strikingly illustrates the variety of combinations of harmful effects by vvhich impurity segregation can affect a given engineering property. E. CONCLUSION Although selective this revievv has shovvn that a vvide variety of elements can segregate in grain boundaries of iron-base alloys of current technolo-gical interest and severely affect their properties. Only in a minority of cases have beneficial effects been observed. The example of vvhat has been cal-led reversible temper embrittlement of steels has revealed itself particularly instructive since vve have been able to shovv throughout this paper that ali their usual properties are critically affected in one or another way. However, besides the empi-rical evidence available very little is knovvn on the mechanisms of interaction betvveen segregations and other types of damage of grain boundaries: it is not even clear yet in most cases vvhether these effects are additive or synergistic. It is quite cer-tain that there is no universal ansvver vvhich is already evident in the fact that different types of steel exhibit widely varying response vvith respect to each type of sollicitation since the correspond-ing properties are also controlled by the material's microstructure in a specific manner. Nevertheless, the available examples have shovvn that effects of the same type can be expec-ted in different materials and it is the author's opinion that the immediate consequence of the spectacular development of modern surface analy-sis techniques vvill be not only to lengthen the list of phenomena associated vvith the segregation of detrimental impurities but also to reveal the im-portance of multiple segregations and especially that of beneficial alloying elements the majoritv of vvhich remains to be found. Acknowledgements This paper is an excerpt of a talk delivered at the »Journees d'Automne de la Societi Frangaise de Metallurgie«, Pariš, October 23,1979, and publi-shed in »Advances in the Mechanics and Physics of Surfaces«, vol. 1, R. M. Latanision and R. J. Courtel editors: »The influence of Interfacial Segregation in Embrittlement Phenomena«. Literatura - References 1. A. J. Perry: J. Mater. Sci., 9 (1974) 1016. 2. D. Lonsdale and P. E. J. Flewitt, Mat. 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