UDK 669.15-194.2:543.428.2:669.779 ISSN 1318-0010 Izvirni znanstveni članek KZLTET 33(6)423(1999) A. VÝROSTKOVÁ ET AL.: SOME ASPECTS OF CARBIDE PRECIPITATION AND PHOSPHORUS… SOME AS PECTS OF CAR BIDE PRE CIP I TA TION AND PHOSPHORUS GRAIN BOUND ARY SEG RE GA TION IN Cr-V LOW ALLOY STEELS NEKATERE ZNAČILNOSTI TVORBE KARBIDOV IN SEGREGACIJ NA KRISTALNIH MEJAH PRI MALOLEGIRANIH CrV JEKLIH Anna Výrostková 1, Jana Perháčová 2, Viera Homolová 2, Peter Ševc 1, Jozef Janovec 1, Hans Jürgen Grabke 3 1Slovak Academy of Sciences, Institute of Materials Research, Watsonova 47, 043 53 Košice, Slovakia 2University of P. J. Šafárik, Department of Experimental Physics, Park Angelinum 9, 041 54 Košice, S lovakia 3Max-Planck Institut für Eisenforschung, Max-Plack-Strasse 1, D-40 237, Düsseldorf, Germany Prejem rokopisa - received: 1999-11-10; sprejem za objavo - accepted for publication: 1999-11-22 Phos pho rus grain bound ary seg re ga tion was in ves ti gated at 773 and 853 K for three Cr-V low al loy steels with dif fer ent va na dium con tents. AES and TEM meth ods were used for de ter mi na tion of the seg re gat ing e l e ments en rich ment on grain bound aries and phase iden ti fi ca tion, re spec tively. Phos pho rus seg re ga tion enthalpy and en tropy, as well as the phase equi lib ria for both tem per a tures, were de ter mined by means of ther mo dy namic cal cu la tions. Aging up to 1000 hours was suf fi cient to achieve the equi lib rium state of phos pho rus seg re ga tion at the ag ing tem per a tures. In ag ree ment with Langmuir-McLean the ory, the phos pho rus grain bound ary equi lib rium con cen tra tion de creases with the tem per a ture. The ag ing con di tions, how ever, were not suf fi cient to achieve the phase equi lib rium. Ac cord ing to ther mo dy namic cal cu la tion s, the steels should con sist of fer rite, M7C3, and MC phases while only the fer rite, M 3C and MC (M 7C3 only in one state at the higher ag ing tem per a ture) were found ex per i men tally. Chem i cal com po si tion changes of an a lyzed car bides also con firmed the n on-equilibrium state of pre cip i ta tion. The changes ac com pa ny ing the non-equilibrium de vel op ment of phases show a di rect in flu enc e on phos pho rus seg re ga tion, es pe cially at 773 K. As a con se quence, the sup port ing ef fect of va na dium on phos pho rus s eg re ga tion, ob served at 853 K is not clear at this lower tem per a ture. Key words: low al loy steels, car bide pre cip i ta tion, carbide com po si tion, grain bound aries , phos pho rus seg re ga tion Raziskana je bila segregacija fosforja na kristalnih mejah pri 773 in 853 K pri treh malolegiranih jeklih z nizko vsebnostjo vanadija. Uporabljeni sta bili AES in TEM metodi. Entalpija in entropija segregacije ter fazna ravn otežja so bila termodinamično izračunana. Staranje 1000 ur je bilo zadostno za doseganje ravnotežne segregacije fo sforja. V soglasju s teorijo Langmuir-McLean je ravnotežna segregacija fosforja manjša pri nižji temperaturi. Staranje v trajanj u 1000 ur pa ni bilo zadostno za doseganje faznega ravnotežja. Na osnovi termodinamskih izračunov je pričakovana fazna s estava jekla ferit, M 7C 3 in MC, eksperimentalno pa so najdeni ferit, M 3C in MC ter M 7C3 samo po žarjenju pri višji temperaturi. Tudi kemična sestava izločenih karbidov je dokazala neravnotežno izločanje. Na sta nek neravnotežnih konstituent mikrost rukture kaže na neposreden vpliv segregacije fosforja, posebno pri 773 K. Kot posledica se pri nižji temperaturi ni jasno poka zal pospeševalen vpliv vanadija na segregacijo fosforja. Ključne besede: malo legirano jeklo, precipitacija karbidov, sestava karbidov, kristalne meje, segr egacija fosforja 1 IN TRO DUC TION Low alloy steels are widely used in various industrial areas in conditions of high temperature and stress. However, in these circumstances the steels are relatively sensitive to intercrystalline embrittlement as a consequence of grain boundary impurity segregation 1 ,2 . This work deals with the influence of vanadium on the phosphorus grain boundary (GB) segregation and the influence of carbide reactions during aging. The thermodynamic description of segregation is based on the Langmuir-McLean isotherm. There are many approaches 3 -7 modifying the isotherm. Individual models and approaches have been discussed in detail e.g. by Lejček 8. Thermodynamic aspects of phosphorus GB segregation in low alloy CrMoV steels have recently been studied by Janovec et al. 9. One of the main conclusions was that the above mentioned Langmuir-McLean equation for a binary ideal solid solution can be KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 6 successfully used in routine thermodynamic calculations for multicomponent steels. In accordance with this finding, the following form of the Langmuir-McLean equation8 was used in this paper: -P S AHP — ASP T = — RT ln —---------— (1) XP (1 — XP ) where: AH0p and AS0p are the enthalpy and entropy of phosphorus segregation, respectively, R is the gas constant, T is the aging temperature, and XPS and XPB are the grain boundary and bulk phosphorus concentrations, respectively. 2 EX PER I MEN TAL Three experimental low alloy steels with different vanadium contents were used for the investigation, see table 1. The experimental material was heat treated as follows: 423 A. VÝROSTKOVÁ ET AL.: SOME ASPECTS OF CARBIDE PRECIPITATION AND PHOSPHORUS… 1 . austenitization 1323 K/0.5h —» quenching in 10% water solution of KOH (average grain size 100-150 |J.m), 2. tempering 953 K/2h —» water cooling, 3. aging 773 and 853 K for 0.1, 0.5, 1, 5, 10, 100, and 1000 h —» water cooling. The grain boundary concentration of segregated elements was evaluated from Auger spectra achieved by means of an AES microprobe operating at UHV 10-8 Pa, with energy 5 keV, primary beam current 3 |jA, and primary beam size 50 |j,m. Carbon extraction replicas were used for phase analysis by TEM and EDX. Ta ble 1: Chem i cal com po si tion of in ves ti gated steels Šmass %] Steel C Cr Mn P Si S V 11 0.12 0.89 0.33 0.045 0.23 0.004 0 33 0.13 0.88 0.34 0.047 0.24 0.004 0.26 37 0.13 0.88 0.33 0.040 0.23 0.004 0.51 *5 773 K 13 n A 10 -5 A ¦ --------studil * --------sted3,7 0 B 'A A* I V »I iE4 iE a o,(n 1PE> 1CCC lf>PE>P Aging ti ma Šh] Fig ure 1: GB con cen tra tion of P,V, Cr in steel 11 (0% V), steel 33 (0.26% V) and steel 37 (0.51% V) at 773 K Slika 1: Koncentracija P, V in Cr na mejah v jeklu 11 (0%V), jeklu 33 (0.26%V) in jeklu 37 (0.51%V) po staranju pri 773 K 3 RE SULTS Auger peak heights P120eV, C272eV, Cr529eV, V473eV, and Fe703eV were used for calculation of the GB concentration of appropriate elements according to Eq. 2 Š111: X K „ K \ j S \ (2) where Ki and Kj are the Auger peak heights of the elements i and j, respectively, and Si and Sj are the relative Auger sensitivities of the corresponding elements. In Figs. 1 and 2, the concentrations of P, V and Cr are plotted as a function of aging time. The phosphorus segregation kinetics in ideal solid solution can be described by the McLean equation Š101: S S ( 0) X P (t) — X P X (0) X = 1 — exp Peq 4DPk č. 2 2 a d erfc 2-jDPt Ě ad (3) where the term XSP(t) represents the GB concentration of phosphorus at time t, Xsp(0) is the initial GB concentration of phosphorus (after tempering and cooling), d is the GB thickness, the term Dp is the coefficient of phosphorus bulk diffusion in a-iron. The enrichment factor a represents the ratio of XPeq S /XPB. In the calculations the value d=5.5*10-10 m was used. By fitting Eq. 3 to the measured GB phosphorus concentration in figs. 1a and 2a, the values of XPeqS (tab. 2) and DP were determined Šin m2/s]: DP = 3.4*10 10 *exp(-137 kJmol-1/RT) for steel 11, DP = 5.7*10-9 *exp(-141 kJmol-1/RT) for steel 33, and DP = 1.7*10-8 *exp(-148 kJmol-1/RT) for steel 37. On the basis of Eq. 1, the values of the Gibbs free energy for phosphorus GB segregation in the form AG = AH — AS* T were achieved by linear p P P regression: steel 11: AG0p = -24041-18.8*T ŠkJ/mol] steel 33: AG0p = -13778-31.2*T ŠkJ/mol] steel 37: AG0p = -10212-37.5*T ŠkJ/mol]. Phase analyses of the experimental states showed the presence of ferrite, MC and M3C phases. In steel 11, without vanadium, MC carbide was absent, however MX particles, most probably Cr 2N, were found. In steel 37, with 0.5 mass % V, M3C carbide was not found at all. Moreover, traces of M7C3 carbide were found in steel 33, aged at 853 K for 1000 h. According to the thermodynamic phase equilibria calculations provided by the PD-pp software package12, the systems in equilibrium should contain the carbide M7C3, instead of M3C. Experimentally measured ratios 853 K ?5 \\ 424 1E-4 1E-3 t>,01 0,1 1 10 100 10«« 140 Aging time Šh] Fig ure 2: GB con cen tra tion of P,V, Cr in steel 11 (0% V), steel 33 (0.26% V) and steel 37 (0.51% V) at 853 K Slika 2: Koncentracija P, V in Cr na mejah v jeklu 11 (0%V), v jeklu 33 (0.26%V) in v jeklu 37 (0.51%V) po staranju pri 853 K KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 6 S A. VÝROSTKOVÁ ET AL.: SOME ASPECTS OF CARBIDE PRECIPITATION AND PHOSPHORUS… of V/Cr in MC, and calculated volume fractions of M 7C3 and MC are given in Tab. 2 . Ta ble 2 : Some ex per i men tal and cal cu lated data for in ves ti gated steels Steel T Š K] XPeq S Šat. % ] Volume portion (calc.) Švol.% ] V/Cr Šat.%/at.% ] in MC (exp.) M7C3 MC 100h 1000h 11 773 24.5 1.77 - - - - 853 18.6 1.75 - - - - 33 773 23.3 1.07 0.61 4.7 - 4.4 853 19.9 1.21 0.54 4.7 6.1 2.98 37 773 25.2 0.38 1.05 9.8 - 7.3 853 24.0 0.38 1.05 9.8 26.3 6.02 4 DIS CUS SION The kinetics of phosphorus GB segregation for the lower aging temperature is slower than at the higher temperature in all the investigated steels, Figs. 1a and 2a . Grain boundaries are approximately saturated after 100 hours in the former case, while this state is achieved after one hour at the higher temperature. The phosphorus grain boundary concentrations at 773 K are higher (24.5, 23.3, 25.2 at.%) than at 853 K (18.6, 19.9, 24.0 at.%) which is in agreement with the Langmuir-McLean theory. The experimental results also show a positive correlation between bulk vanadium concentration and phosphorus GB concentration at 853 K. This behaviour can be explained by the indirect influence of vanadium through the P-C site competition effect 13,14 . Strong V-C affinity and precipitation of the V-rich carbides leads to a decrease of carbon activity and thus enables P atoms to segregate at the GB. At the lower aging temperature, differences between phosphorus GB concentrations are smaller. Also, in contrast to 853 K, the experimental values in steel 33 with a higher bulk content are lower than the values in steel 11, without vanadium. This could be ascribed to the precipitation processes that are discussed later. The GB concentration of vanadium can be said to be practically constant during the aging treatment, Figs. 1b and 2b . In the case of Cr, especially at the lower temperature, a slight tendency for a concentration increase (steel 11) and concentration decrease (steel 37) can be seen, Figs.1c and 2c , respectively. Comparison of the experimentally determined phases and the calculated equilibrium phase composition shows clearly that after 1000 hours of aging the phase equilibrium has not yet been achieved for both temperatures. The applied aging conditions are not sufficient for precipitating the M 7C3 carbide 15,16 , the amount of which, according thermodynamic calculations, decreases from 1.77 to 0.38 vol.% in the investigated steels with increasing vanadium bulk KOVINE, ZLITINE, TEHNOLOGIJE 33 (1999) 6 content. The conditions were suitable for the precipitation of M 3C in which the Fe/Cr atomic ratio decreases with increasing time from 16.6 to 2.6 in steel 11 and from 4 to 0.81 in steel 33, which means enrichment of the carbide by Cr. This, together with Cr 2N precipitation at the GB 17 leads to the increase in Cr concentration at the GB in steel 11. Precipitation of Cr 2N particles results in a decrease of Cr activity which can also be a cause of M 7C3 carbide precipitation delay. No M 3C was found in steel 37, but there is a large quantity of MC, in particular (V,Cr)C, which precipitates mainly in the matrix. During aging, this carbide is enriched by Cr as can be seen from the decreasing V/Cr ratio, Tab. 2 . Simultaneously, the amount of (V,Cr)C carbide increases, the carbon activity reduces which worsens the conditions for precipitating both M 3C and M7C3 carbides. Such behaviour is known in steels with a high V content (similar to steel 37) without any other strong carbide-forming element and a Cr content of about 2 mass 15 . This could explain the decrease in Cr concentration at the GB in steel 37. It can be stated that the phosphorus GB concentration values will become constant only after the systems achieve equilibrium from the point of view of phase composition. 5 CON CLU SIONS By investigating GB segregation and precipitation processes in three Cr-V low alloy steels it was found that after aging at 773 K and 853 K, equilibrium values of phosphorus GB segregation were achieved. Concerning the phase composition, 1000 h aging at both temperatures was not sufficient to achieve phase equilibria. The following further conclusions can be drawn: 1. at the lower aging temperature, the phosphorus GB segregation is slower and achieves higher equilibrium values than at the higher temperature; 2. vanadium indirectly influences the phosphorus GB segregation, the higher the bulk vanadium content the higher the GB concentration of phosphorus; 3. precipitation of MX (Cr 2N) at GBs leads to a delay in M7C3 carbide formation and to the slight increase in GB chromium concentration in the steel without vanadium; 4. precipitation of (V,Cr)C in the matrix results in the delay of M 7C3 carbide formation and a slight decrease of GB chromium concentration in the steel with the highest vanadium bulk content. AC KNOWL EDGE MENT The present work was carried out in the frame of the German-Slovak Scientific and Technological Cooperation under project No. X262.51. It was also 425 A. 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