TEMPERING RESPONSE OF SUB-ZERO PROCESSED Cr-V LEDEBURITIC STEEL VANADIS 6

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1 , Brno, Czech Repulic, EU TEMPERING RESPONSE OF SUB-ZERO PROCESSED Cr-V LEDEBURITIC STEEL VANADIS 6 Peter JURČI, Mrtin KUSÝ, Mári Dománková, Luomír ČAPLOVIČ, Jn SOBOTOVÁ, Petr SALABOVÁ c, Otkr PRIKNER c Drj JENKO d Fculty of Mterils nd Technology of the STU, Pulínská 16, Trnv, Slovk Repulic, EU, p.jurci@seznm.cz CTU in Prgue, Fculty of Mechnicl Engineering, Krlovo nám. 13, Prgue 2, Czech Repulic, EU c Prikner - tepelné zprcování kovů, Mrtínkovice 279, , Czech Repulic, EU d Institute of Metls nd Technology, Lepi pot 11, Ljuljn, Sloveni, EU Astrct The ledeuritic steel Vndis 6 hs een ustenitized t the tempertures o C, nitrogen gs quenched nd 2x tempered t the tempertures o C. Ech tempering cycle ws 2 h. Su-zero tretment crried out vi vrious regimes hs een inserted in-etween quenching nd tempering, except one set of specimens. The effect of su-zero tretment on the tempering chrcteristics of the mteril hs een investigted. It hs een found tht the mrtensite fter su-zero processing differs from tht of conventionlly trnsformed in terms of sustructure. The su-zero tretment induced multiple reduction of the volume frction of retined ustenite. This mkes difference etween s-tempered microstructure of su-zero treted nd no-su-zero treted steel, which results in different s-tempered hrdness. 1. INTRODUCTION It hs een estlished recently tht the su-zero tretment (SZT) of Cr- nd Cr-V ledeuritic tool steels cn improve some mechnicl nd technologicl properties like hrdness [1], toughness it is worth noticing tht some uthors hve reported no effect of the SZT or little worsening [2] while others hve estlished slight improvement [3, 4], dimensionl stility [5] nd wer resistnce [6-11]. These improvements were ttriuted to vrious microstructurl chnges due to SZT, nmely: reduction of the volume frction of retined ustenite ( R ) [12-15], low-temperture conditioning of the mrtensite ( ) [14, 16], more complete mrtensitic trnsformtion connected with more uniform precipittion of crides during tempering nd their refinement [3, 13, 15, 17]. However, n exct explntion of the phenomen occurring in the Cr- nd Cr-V ledeuritic steels is lcking until now ecuse of: i) The use of different mterils for the investigtions, hence, there is no comprehensive study on the effect of SZT, excepting the D2-steel, where Ds et l. relted the improvement in wer resistnce to oth the reduction of R content nd the modifiction of precipittion mechnism nd kinetic of smll crides during tempering [13], ii) The choice of different het tretment regimes some uthors used tempertures from the rnge -60 to -80 o C for SZT while others performed the SZT t oiling temperture of liquid nitrogen (-196 o C), iii) The use of different prmeters for the wer testing counterprts mteril, loding, sliding speed, sliding distnce etc. The im of the pper is to present current stte of knowledge on the structurl chnges induced y SZT, in widely used Cr-V ledeuritic steel Vndis EXPERIMENTAL PM ledeuritic steel Vndis 6 (2.1 %C, 1.0 %Si, 0.4 %Mn, 6.8 %Cr, 1.5 %Mo, 5.4 %V nd Fe s lnce) hs een used for the investigtions. The smples were vcuum ustenitized t tempertures from the rnge o C, nitrogen gs quenched nd 2x tempered, ech tempering cycle ws 2h. Tempering

2 , Brno, Czech Repulic, EU tempertures were chosen from the rnge o C, with step of 10 o C. SZTs hve een inserted inetween quenching nd tempering nd their prmeters were s follows: -90 o C/4 h, -196 o C/4 h nd -196 o C/10 h. The microstructure hs een recorded using the light microscopy (LM), trnsmission electron microscopy (TEM) nd high resolution TEM. Hrdness hs een mesured y Rockwell C method. Five mesurements were done on ech smple nd the men vlues hve een clculted. 3. RESULTS AND DISCUSSION Light microgrphs, Fig. 1 show the microstructure of the Vndis 6 steel fter quenching from different ustenitizing tempertures (T A ) nd with/without the SZT. The microstructure is composed of the mtrix nd undissolved crides. The mtrix contins the mrtensite, lue/drk colored nd R (rown-colored). It is ovious t the first sight tht the portion of R decreses with the ppliction of the SZT. This oservtion is well consistent with the X-ry diffrction mesurements where triple lowering of the R-content hs een estlished for the sme mteril due to the SZT [3, 15]. crides R c d Fig. 1 Light microgrphs showing the microstructure of the Vndis 6 ledeuritic steel: - fter quenching from T A = 1000 o C, fter susequent SZT t 196 o C/4 h, c - fter quenching from 1075 o C, d fter susequent SZT t 196 o C/4h. Colour etching using the Berh-mrtensite gent Figure 2 shows the microstructure of no SZT () nd SZT (-196 o C/4 h) () steel ustenitized t 1000 o C. The min structurl fetures of no-szt Vndis 6 steel re the mrtensitic domins with high disloction density inside the domins. The microstructure of SZT mteril does not differ from tht of no SZT steel, however, the mrtensitic domins re rther smller in terms of their length nd width. Moreover, it seems tht the mrtensite of SZT Vndis 6 steel exhiits higher disloction density thn tht of the no SZT mteril. Figure 3 depicts HRTEM microgrphs of the microstructure of the Vndis 6 steel fter quenching to room temperture () nd fter susequent SZT t -196 o C/4h (). On the left hnd side microgrph, there re lot of smll prticles visile (1, 2) their length rnges etween 10 nd 20 nm nd hve few nnometers in thickness. The prticles re locted minly in the vicinity of originl grin oundry (GB). Point EDX-nlysis reveled tht (it should e noted tht the results do not represent the prticle only ut lso it s close vicinity) the res with the prticles contin of round 70 t.% of C. The microstructure of SZT mteril contins the mtrix (3) with low cron content, in which severl importnt fetures re emedded. Firstly, the res with

3 Hrdness HRC , Brno, Czech Repulic, EU extremely enhnced C-content should e mentioned (4, 5). They re not visile on the microgrph ut only fter providing the EDS-mesurements. The second importnt fetures re the Moiré fringes (5). Fig. 2 Bright-field TEM microgrphs showing the microstructure of the Vndis 6 ledeuritic steel: - fter quenching from T A = 1000 o C, fter susequent SZT t 196 o C/4 h. 4 3 GB nm 10 nm Fig. 3 HRTEM microgrphs showing the microstructure of the Vndis 6 ledeuritic steel: - fter quenching from T A = 1000 o C, fter susequent SZT t 196 o C/4 h ustenitized t 1025 C Tempering temperture C ustenitized t 1025 C + SZT -90 C/4 h Fig. 4 Prt of tempering curves of no-szt nd SZT Vndis 6 steel. Our previous investigtions fixed higher s-quenched hrdness of SZT mteril Vndis 6 y 2-3 HRC thn tht of conventionlly het treted [18]. Figure 4 demonstrtes segment of tempering curves otined y twice tempering of oth the SZT nd the no-szt steel ustenitized t 1025 o C. The SZT steel hs higher hrdness up to tempering temperture of 500 o C. Beyond tht, however, the hrdness of no-szt steel

4 , Brno, Czech Repulic, EU ecme higher y HRC. Further importnt fct is tht there is no clssicl secondry hrdness pek on the tempering curve of SZT Vndis 6 steel. c d Fig. 5 Microstructure of no-szt Vndis 6 steel fter ustenitizing t 1000 o C nd tempering 530 o C/2x2h., c - right-field TEM microgrphs,, d corresponding drk-field imges + diffrction ptterns of precipittes c Fig. 6 Microstructure of Vndis 6 steel fter ustenitizing t 1000 o C, SZT -196 o C/4h nd tempering 530 o C/2x2h., c - right-field TEM microgrphs,, d corresponding drk-field imges + diffrction ptterns of precipittes Figure 5 depicts the microstructure of no-szt Vndis 6 steel fter ustenitizing t 1000 o C, quenching nd doule tempering. Bright-field TEM microgrphs, Figs. 5, c show tht the mteril contins lot of smll precipittes inside the mrtensitic domins. Corresponding drk-field microgrphs nd diffrction ptterns, Figs. 5, d, document tht the precipittes re the M 3 C- nd M 7 C 3 -crides, respectively.

5 , Brno, Czech Repulic, EU Figure 6 shows the microstructure of Vndis 6 steel fter ustenitizing t 1000 o C, quenching, SZT t 196 o C/4h nd doule tempering. Bright-field TEM microgrphs, Figs. 6, c show tht the smple contins reltively gret numer of smll precipittes, with size well elow 50 nm for the phse M 3 C nd elow 100 nm for the M 7 C 3, respectively. The fct tht SZT Vndis 6 steel hs higher hrdness thn no-su-zero processed mteril cn e considered s nturl nd well expected since: Higher volume frction of the ustenite ws converted to the mrtensite due to the SZT nd the mrtensite of SZT steel exhiits higher degree of tetrgonlity s found previously [4, 18]. Here, it should e noted tht the Tyshchenko s finding on lowered tetrgonlity of the in X220CrVMo 13-4 steel hs not een confirmed. The sitution of tempered mteril without/with SZT is more complex. It is worth noticing tht the secondry hrdness of Cr-V ledeuritic steel occurs due to superposition of severl processes, nmely: i) trnsformtion of R to (hrdness increse), ii) tempering of the (hrdness decrese) nd iii) precipittion of smll, coherent crides from oth the R nd the mrtensite (hrdness increse). The role these processes cn e descried s follows: i) SZT mteril contins less R thn tht no-szt, hence, the contriution of it s conversion into the mrtensite to the finl hrdness is unmiguously lower for SZT steel. ii) The mrtensite of SZT mteril is more supersturted nd hs higher degree of tetrgonlity thn tht of no-szt mteril. However, it undergoes softening due to the tempering nd, one cn expect tht the softening will e prcticlly in the sme extent thn tht of conventionlly produced mrtensite when tempered t tempertures round the secondry hrdness pek. iii) The contriution of the third process to the finl hrdness of no-szt mteril is commonly given y the precipittion of specil chromium crides ove 450 o C. The role of intermedite phses, like -cride or -cride cn e estimted only roughly. SZT of the mteril produces the mrtensite with higher disloction density nd more supersturted with cron nd lloying elements. Enhnced disloction density, compred to no-szt steel, is result of extensive plstic deformtion of virgin mrtensite t very low temperture due to the volume effect of the to trnsformtion. No minority phses were found in the SZT-steel Vndis 6, which is nturl since the precipittion of crides is impossile due to immoility of cron t low tempertures. On the other hnd, the disloctions re moile t these tempertures nd they cn cpture cron toms during the deformtion of freshly formed mrtensite [14]. In this wy, cron clouds cn e formed t disloctions nd serve s nucletion sites for precipittion of crides t susequent tempering. There is neither relevnt dt on the composition of cron clouds nor on their size. Anywy, current investigtions indicted the presence of cron clustering in SZT Vndis 6 steel, with locl cron content of more thn 50 t%. Precipittion of crides during tempering of SZT Cr- nd Cr-V ledeuritic steels differs from tht of no-szt mterils in following spects: It produces finer prticles uniformly distriuted throughout the mtrix [13, 17]. Even t reltively low tempering tempertures, M 23 C 6 -crides nd not cementite (or other intermedite precipittes) hve een formed in D2-steel, for instnce [9, 13], e.g. the precipittion process ws estlished to e ccelerted due to SZT. The popultion density of the crides is higher in the cse of SZT steels [6-11]. Our experimentl efforts did not revel ny differences in the nture of precipittes formed t tempering. Wht only seems tht the popultion density of the prticles might e different for SZT nd no- SZT steel, ut the verifiction of this fct needs further investigtions. Hence, possile ccelertion of precipittion connected with lower R content re suggested to e responsile for oth the lower s-tempered hrdness of Vndis 6 steel nd the loss of secondry hrdness pek.

6 , Brno, Czech Repulic, EU CONCLUSIONS The min findings of the investigtions cn e summrized s follows: i) the volume frction of R decreses nd the mrtensitic trnsformtion is more completed due to the SZT, ii) the SZT produces the mrtensite with finer su-structure thn the conventionl het tretment, iii) the microstructure of the mteril quenched to room temperture contins lot of smll prticles, whose length rnges etween 10 nd 20 nm nd hve few nnometers in thickness. The prticles re locted minly in the vicinity of originl grin oundries, iv) the microstructure of SZT Vndis 6 steel is free of nno-prticles ut it contins res with extremely enhnced C-content nd the sites showing the Moiré fringes, v) the nture of precipittes in SZT nd no-szt mteril is the sme one of the phses is the M 7 C 3 -cride nd the second one, rther surprisingly, is the M 3 C, vi) the s-tempered hrdness of SZT mteril is higher up to temperture of 500 o C nd, eyond this temperture it is lower thn tht of no-szt steel, vii) one cn suggest tht the precipittion of crides is ccelerted due to the SZT, which tends towrds lowering of stempered hrdness t higher tempering tempertures s well s towrds loss of secondry hrdness pek. ACKNOWLEDGEMENTS This pper is result of the projects: VEGA 1/1035/12 nd TIP FR-TI1/003 supported y the Ministry of Industry nd Trde of the Czech Repulic. REFERENCES [1] BERNS, H.: HTM 29 (1974) 4, 236. [2] DAS, D., SARKAR, R., DUTTA, A.K., RAY, K.K.: Mter. Sci. Engng. A528 (2010) 589. [3] JURČI, P., ŠUŠTARŠIČ, B., LESKOVŠEK, V.: Mterili in tehnologije/mterils nd Technology, 44 (2010) 2, 77. [4] SOBOTOVÁ, J., JURČI, P., SALABOVÁ, P., PRIKNER, O.: In: Proceedings of the 20th Int. Conference METAL 2011, Brno, My 2011, TANGER, s.r.o., CD-ROM. [5] SURBERG, C.H., STRATTON, P., LINGENHOELE, K.: Cryogenics 48 (2008) 42. [6] STRATTON, P.F.: In.: Proc. of the 1st Int. Conf. on Het Tretment nd Surf. Eng. of Tools nd Dies, Pul, Croti, , 11. [7] DAS, D., RAY, K.K., DUTTA, A.K.: Wer 267 (2009) [8] DAS, D., DUTTA, A.K., RAY, K.K.: Wer 266 (2009) 297. [9] DAS, D., DUTTA, A.K., RAY, K.K.: Mter. Sci. Engng. A527 (2010) [10] DAS, D., DUTTA, A.K., RAY, K.K.: Cryogenics 49 (2009) 176. [11] MOHAN LAL, D., RENGANARAYANAN, S., KALANIHIDI, A.: Cryogenics 41 (2001) 149. [12] JURČI, P.: Mteriálové inžinierstvo/mterils Engineering, 17 (2010) 1, 1. [13] DAS, D., RAY, K.K.: Mter. Sci. Engng. A541 (2012) 45. [14] TYSHCHENKO, A.I., THEISEN, W., OPPENKOWSKI, A., SIEBERT, S., RAZUMOV, O.N., SKOBLIK, A.P., PETROV, Y.N., GAVRILJUK, V.G.: Mter. Sci. Engng. A527 (2010) [15] SOBOTOVÁ, J., JURČI, P., ADÁMEK, J., SALABOVÁ, P., PRIKNER, O., ŠUŠTARŠIČ, B., JENKO, D.: Mterili in tehnologije/mterils nd Technology 47 (2013) 1, 93. [16] COLLINS, D.N.: Adv. Mter. Proc. 12 (1998) 24. [17] d SILVA FARINA, P.F., Frin, A.B., Bros, C.A., Goldenstein, H.: In: Proc. of the 9th Int. Tooling Conf., Montnuniversitt Leoen, Leoen, Austri, Septemer 11 14, 2012, 42. [18] JURČI, P., SOBOTOVÁ, J., SALABOVÁ, P., PRIKNER, O., ŠUŠTARŠIČ, B., JENKO, D.: In: Proc. of the 9th Int. Tooling Conf., Montnuniversitt Leoen, Leoen, Austri, Septemer 11 14, 2012, 89.