M. Pouyamanesh et al, Journal of Advanced Materials and Processing, Vol. 1, No. 2, 2012,

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1 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, Influence of Initil Microstructure on Hot Deformtion Behvior of Duplex Stinless Steels M. Pouymnesh*, B. Eghli, Gh. R. Erhimi c nd M. Sdti Deprtment of Mterils Engineering, Esfryen University, Esfryen, Irn School of Mterils Engineering, Shnd University of Technology, Triz, Irn c Deprtment of Mteril Engineering, Ferdowsi University of Mshhd, Mshhd, Irn ARTICLE INFO Article history: Received 1 Sep 2012 Accepted 25 jn 2013 Aville online 20 Jnury 2013 Keywords: Duplex stinless steel Hot compression Flow curves Dynmic recovery Dynmic recrystlliztion 1. Introduction ABSTRACT Duplex stinless steels re well developed in mrine, chemicl nd petrochemicl industries due to good corrosion resistnce properties. Proper chemicl composition, mechnicl properties nd corrosion resistnce, re the min resons for widespred ppliction of these steels [1, 2]. These steels show higher strength nd higher toughness thn common ustenitic nd ferritic steels, respectively. Stress corrosion crcking, hydrogen emrittlement nd inter-grnulr corrosion resistnce in ddition with good In this reserch the effect of initil microstructure on hot deformtion ehvior in terms of Ferrite-to-Austenite rtios is studied. Two types of stinless steels C1 nd C2 were homogenizing het-treted nd deformed under hot compression exmintions t tempertures 900ºC nd 1100ºC t strin rte of 0.1s -1. The results showed tht the flow stress levels of specimens re strongly relted to deformtion prmeters nd initil microstructures of steels. Moreover, during cooling from 1350ºC to the deformtion temperture, flow stress level incresed for oth smples ecuse of increment in ustenite content. weldility re some other enefits of them [3, 4]. At the initil steps of hot deformtion process, the strin concentrtes on ferrite phse nd there is lnce etween work hrdening nd dynmic recovery. While the strin is incresed, deformtion concentrtion shifts on ustenite phse nd work hrdening ecomes the only dominnt mechnism. In this sitution y incresing the strin, recrystlliztion strts in ustenite phse nd decreses the flow stress level simultneously. Restortion mechnism in ferritic-ustenitic duplex stinless steels consists of dynmic recovery nd dynmic recrystlliztion phenomen [5-8]. * Corresponding uthor Tel.: Emil ddress:m_estiri2000@yhoo.com(m. Pouymnesh).

2 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, There re few reserches on the reltionship etween microstructure nd hot deformtion ehvior, when deformtion condition nd chemicl composition re chnging. In this study, the effect of initil microstructure on hot deformtion ehvior is investigted. Where v, ε, l, l nd l0 denote velocity of mchine's hydrulic rm, strin rte nd vrition of specimen's height during the deformtion, finl height nd primry height of specimens, respectively. 2. Experimentl Procedure In this study, two types of c-cst duplex stinless steels coded C1 nd C2 re investigted. Chemicl compositions of them re listed in Tle 1. Tle1 Chemicl Compositions of Mterils. Steel C1 C2 Cylindricl compression specimens of 10mm in initil dimeter nd 15mm in height were mchined from csted sls. In order to homogenize the microstructures, ll the specimens re held t temperture of out 1250ºC for 2 h efore hot deformtion tests. To evlute the ustenite volume frction effects on hot deformtion ehvior of duplex stinless steels, two different het tretment cycles, on-heting nd on-cooling, re exerted on specimens (Fig. 1). A Z250 Zwick/Roe unixil hot compression mchine equipped with resistnce furnce is used for pplying the tests. In on-heting cycle, specimens were preheted over the temperture rnge of 900ºC to 1100ºC for 5 min nd then hot compression test ws crried out t constnt strin rte of 0.1s -1. In on-cooling cycle, specimens were held t 1350ºC for 20min t first nd then cooled down to the deformtion temperture nd held 1min, then hot compression test ws performed t strin rte of 0.1s -1. In order to exert strin rte of 0.1s -1 the constnt velocity of mchine's hydrulic rm ws clculted 63mm/min ccording to eqution 1. v %C %Cr %Ni %Mo %Si %Mn (1) On-heting On-cooling Fig.1. Schemtic of het tretment cycles. Moreover, when hot deformtion ws finished, specimens were wter quenched for less thn 5 sec nd were cut longitudinlly for microstructurl investigtions. After preprtion nd etching of specimens in HCl nd potssium metisulfite, K 2 S 2 O 5, microstructures were studied y employing CLEMEX softwre. 3. Results nd Discussion Microstructurl investigtions of specimens fter homogenizing het tretment showed different volume frctions of two ustenite nd ferrite phses (Fig. 2). Austenite volume frction of steel C2 t room temperture is higher thn steel C1, s shown in Fig. 2. It should e noted tht ferrite-to-ustenite rtio vries with vrition of temperture [6].

3 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, Austenite volume frction vritions for different tempertures fter pplying onheting nd on-cooling het tretment cycles re presented in Fig. 3. According to some other investigtions done on these two types of steels [9], strin rte sensitivity, m, for oth steels re clculted y utilizing hot deformtion test results of on-cooling cycle t strin rtes of 0.1s -1 nd 0.001s -1. The mount of m depends on disloctions density. The higher the disloction motion, the lrger will e the mount of m [2]. Annihiltion nd rerrngement of disloctions t high tempertures i.e. dynmic recovery, DRV, is more convenient in ferrite phse. In steel C1, the mgnitude of m declines with increse of temperture which is in ccordnce with oncooling curve in Fig.3 science volume frction of ustenite increses s the temperture rises. In on-cooling cycle for steel C2, s temperture went up, volume frction of ustenite decresed (Fig. 3) nd the mgnitude of m incresed. The mgnitude of m for steel C1 is more thn steel C2 t ll tempertures; ecuse ustenite volume frction of steel C1 is less thn steel C2. Fig.2. Microstructure of homogenized smples () C2 (The light phse is ustenite nd the mtrix is ferrite) () C1 (Essentilly ferritic microstructure). Fig.3. Dependence of ustenite volume frction on deformtion temperture for cycles 1 nd 2. True stress-strin curves of steels C1 nd C2 which re resumed from hot deformtion tests t 900ºC nd 1100ºC re presented in Fig. 4. As illustrted in Fig. 4 nd 4, flow stress level increses with plummet of hot deformtion temperture. Moreover, s strin is incresing, flow stress increses nd reches mximum vlue nd then tkes stedy stte which shows tht dynmic recovery phenomenon, DRV, (Fig. 4 nd 4) is occurring. It should e noted tht flow stress level of deformed on-cooling specimens t 900ºC is higher thn their on-heting cycles (compre Fig. 4 nd 4). In other words, for smples which re cooled from 1350ºC to hot deformtion temperture (900ºC), flow stress level is higher thn those held t hot deformtion temperture. It is ttriuted to vritions in volume frction of ustenite (Fig.3).True stress-strin curves yielded from hot deformtion of steel C2 t different tempertures re illustrted in Fig. 4c nd 4d. As shown in Fig. 4d, flow stress increses with increse of strin nd reches mximum vlue; then tkes stedy stte which revels the occurrence of dynmic

4 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, recovery phenomenon. Also, flow stress level hs decresed with increse of temperture. Chnging the het tretment process chnges the microstructure nd consequently chnges hot deformtion ehvior. In hot deformtion t 900ºC, increse of strin increses the flow stress to rech mximum vlue nd then decreses smoothly s represented in Fig. 4d. Decrese in flow stress fter the pek stress is ttriuted to the dynmic recrystlliztion phenomenon, DRX.It cn e seen tht increse in temperture decreses flow stress level nd so clerly elucidtes pek chrcterizing recrystlliztion on the grph nd revels occurrence of dynmic recrystlliztion phenomenon. It should e stted tht flow stress levels for C2 specimens re higher thn C1 specimens under the sme condition, nmely on-heting cycle. It is ttriuted to higher volume frction of ustenite in steel C2 (compre Fig. 4 nd 4d).Therefore it is expected tht dynmic recrystlliztion could e oserved in grphs when considerle mixture of ferrite nd ustenite phses exist in microstructure.according to grphs of Fig. 4, it cn e interpreted tht flow stress for steel C2 is higher thn C1 nywhere. High ductility of steel C1 is cused y two fctors: ) ese of disloction nnihiltion nd su-oundry formtion s reson for rther low flowstress nd ) movement of high-ngle oundries which restrint crck formtion in min oundries. These crcks usully re creted y grin oundry slip in triple point of ustenite grin oundries [10-12].Fig. 5 exhiits mximum stress content vs. temperture for steels C1 nd C2. It is perceived tht mximum stress nd thus flow stress decreses drmticlly with increse of tempertures. Flow stress level for steel C2 is higher thn steel C1 which is consequence of higher volume frction of ustenite in steel C2 thn C1. On-cooling process offers higher strength thn on-heting process for steel C1. As mentioned efore, this stress vrition could hve een resulted from the presence of ustenite prticles in on-cooling cycle microstructures. On-heting On-cooling On-heting On-cooling Fig.4. True stress-strin curves for steels C1 nd C2 under different deformtion condition withstrin rte of 0.1 sec -1. c d

5 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, process ffects ustenite volume frction in specimen's microstructures. On-cooling, 900ºC Fig.5. Dependence of the pek stress on deformtion temperture for steels C1 nd C2. In order to evlute the effect of thermo mechnicl prmeters of microstructure, specimens re immeditely quenched fter hot deformtion. Fig. 6 revels microstructures of hot-deformed C1 specimens. According to Fig. 6, microstructure (t deformtion temperture of 900ºC) consists of dispersed ustenite prticles in ferrite mtrix. Increse of deformtion temperture to 1100ºC, leds to corsening nd increse in volume frction of ustenite phse. Finer ferrite grins re produced t this temperture thn 900ºC (see Fig. 6 nd 6). Moreover, in higher deformtion tempertures, cride precipittes re formed in microstructure. Microstructures of specimens which re cooled from 1350ºC to the deformtion temperture revel n increse in ustenite prticles (compre Fig. 6 nd 6c). Therefore, it cn e inferred tht het tretment process s well s deformtion temperture influence the ustenite formtion. In other words, ustenite phse percentge rises y increse of deformtion temperture. Also, increse in ustenite prticles content hs incresed flow stress level (Fig.5). Fig. 7 shows microstructure of hot deformed C2 specimens. It cn e seen tht microstructures re composed of two phses, ustenite nd ferrite, which hve more ustenite content thn C1 specimens. Microstructurl imges of these specimens cooled from 1350ºC to the deformtion temperture illustrte thicker lyers of ustenite phse nd more pcked microstructure, s well (compre Fig. 7 nd 7 with 7c nd 7d). It is concluded tht the type of het tretment On-cooling, 1100ºC On-heting, 900ºC On-heting, 1100ºC Fig.6. Microstructures of hot deformed steel C1 t 900 C nd 1100 C.The drk phse is ferrite nd the light one is ustenite c d

6 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, On-cooling, 900ºC In ddition, increse of deformtion temperture (1100ºC) leds to negligile increment of ustenite phse content. It cn e seen from Fig. 5 tht differences in flow stress levels for oth on-heting nd on-cooling cycles re smller for C2 specimens thn C1. On-cooling,1100ºC On-heting, 900ºC On-heting,1100ºC c d 4. Conclusions Increse in ustenite phse content, rises flow stress level significntly.flow stress level for specimens cooled from 1350ºC to deformtion temperture is higher ecuse of their higher ustenite volume frction. In fct, flow stress levels for on-cooling cycles re higher thn on-heting cycles. Furthermore, it cn e deduced tht het tretment process s well s deformtion temperture influence the ustenite formtion. In other words, the use of on-cooling cycle results in more ustenite content thn onheting cycle. It cn e used to choose either the suitle temperture or suitle type of het tretment for chievement of certin ustenite volume frction in microstructure. References [1] J.M. Crer, "Hot deformtion of duplex stinless steels",journl of Mterils Processing Technology, Vol , 2003, pp [2] H. Frnoush, "Hot deformtion chrcteristics of 2205 duplex stinless steel sed on the ehvior of constituent phses",mterils & Design,Vol. 31, 2010, pp [3] L.Chen, "Processing mp for hot working chrcteristics of wrought 2205 duplex stinless steel",mterils & Design, In Press, Corrected Proof. Fig.7. Microstructures of hot deformed steel C2 t 900 C nd 1100 C.The drk phse is ferrite nd the light one is ustenite. [4] E. Evngelist, "Hot workility of 2304 nd 2205 duplex stinless steels",cndin Metllurgicl Qurterly, Vol. 43, 2004, pp

7 M. Pouymnesh et l, Journl of Advnced Mterils nd Processing, Vol. 1, No. 2, 2012, [5] O. Blncin, W.A.M. Hoffmnn, J.J. Jons, "Influence of microstructure on the flow ehvior of duplex stinless steels t high tempertures",metllurgicl nd Mterils Trnsctions A, Vol. 31, 2000, pp [6] L.Duprez, B.C. De Coomn, N. Akdut, "Flow stress nd ductility of duplex stinless steel during high-temperture torsion deformtion",metllurgicl nd Mterils Trnsction A,Vol. 33, 2002,pp [7] M. Sdti, M. pouymnesh, B. Eghli, Gh. R. Erhimi, "An evlution on hot deformtion ehvior of 2205 duplex stinless steel", Steel Symposium 90, AnnulConference of Iron nd Steel society of Irn, Esfhn's Morkeh Steel Compny, Irn. Fe. 2012, pp [8] H. Keshmiri, "Effect of temperture nd Strin Rte on Secondry Phse Formtion in 2205 Duplex Stinless Steel under Hot Working Condition", 2008, Tehrn. [9] M. pouymnesh, B. Eghli, Gh. R. Erhimi, M. Sdti, "A comprison etween hot deformtion ehvior of two s-cst duplex stinless steel." Steel Symposium 89,Annul Conference o f Iron nd Steel society of Irn, Isfhn steel compny, Irn, Mr. 2011, pp [10]G.W.Fn, "Hot ductility nd microstructure in csted 2205 duplex stinless steels",mterils Science nd Engineering: A, Vol. 515, 2009, pp [11]M. Mrtins, L.C. Csteletti, "Sigm phse morphologies in cst nd ged super duplex stinless steel",mterils Chrcteriztion, Vol. 60, 2009, pp [12] M. Mrtins, L.C. Csteletti, "Microstructurl chrcteristics nd corrosion ehvior of super duplex stinless steel csting",mterils Chrcteriztion, Vol. 60, 2009, pp