MEASUREMENT OF DEFORMATION HARDENING OF HEAT- RESISTANT STEEL OF THE WWER-TYPE REACTOR

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1 8th Interntionl DAAAM Bltic Conference "INDUSTRIAL ENGINEERING April 2012, Tllinn, Estoni MEASUREMENT OF DEFORMATION HARDENING OF HEAT- RESISTANT STEEL OF THE WWER-TYPE REACTOR Okipnyi, I.; Mruschk, P.; Sorochk, A. & Sergejev, F. Astrct: Presented investigtions llowed reveling the correltion etween microhrdness nd the chrcteristics of strength of het-resistnt steel 15Kh2MFA(II). The sensitivity of the indenttion methods is shown to e dependent on the degree of involvement of structurl levels in the process of deformtion. At the initil stge, this is the intrgrnulr sliding, t the next stge sher of grins nd grin conglomertes, nd their self-coordinted propgtion in the steel. On the one hnd, this llows ttining greter mcrodeformtion, nd, on the other hnd, does not cuse the ultimte exhustion of the mteril plsticity t the mcrolevel. Key words: frcture, microhrdness, structure, dmge 1. INTRODUCTION Deformtion processes in het resistnt steels cn e descried using the phenomenologicl pproches, which llow estlishing the reltionship etween filure mechnisms, mteril structure, nd its mechnicl properties [ 1 ]. Additionl informtion out the mteril hrdening kinetics cn e otined y the indenttion method. This method llows reveling the fundmentl regulrities in the colescence, ccumultion, nd selforgniztion of the disloction sustructure of the mteril [ 2 ]. However, there re numer of works, in which the regulrities in plstic deformtion of the mteril during microindenttion contrdicted the min regulrities of the theory of plsticity [ 3,4 ]. This confirms the need in considering the scle fctor during investigtions of mterils, in prticulr, the geometricl correltion etween the indenttion size nd structurl elements of the het resistnt steel. It is known tht lrge indenttions hve significnt grdient of the implementtion strin, which is preconditioned y dispersion of the disloction ccumultion kinetics in different structurl elements of the mteril. In this cse, oundry vlues re verged within the limits of the re nlyzed. Microindenttion, on the contrry, provides for locliztion of the disloction ccumultion in structurl components of the mteril [ 5 ]. In previous works, numer of regulrities were found, in prticulr, close correltion etween microhrdness nd true strin of the mteril deformed under high temperture conditions [ 6,7 ]. In ddition, reltionship ws found etween the density of disloctions within the low-ngled oundries nd hrdness (microhrdness) of numer of het resistnt steels. These results llow for the engineering ssessment of the structurl condition of the mteril sed on the indenttion results. They re lso n ttempt to ring together the pproches of the theory of disloctions nd mechnics of the deformle solid ody, which requires further development nd generliztion. The purpose of this work is to evlute hrdening of het resistnt steel 15Kh2MFA(II) fter high-temperture deformtion. 694

2 2. MATERIALS AND RESEARCH TECHNIQUE In this work, 15Kh2MFA(II) (Cr-Mo-V) steel ws investigted, which is used for producing of the pressure vessels of WWER-440 nd WWER-1000 rectors. The mechnicl chrcteristics nd chemicl composition of steel fter therml tretment, which simultes the mteril emrittlement in the middle of service life of the WWER-440 rector: tempering t 1273 K for 6 hours in oil followed y nneling, re given in tle 1. Tle 1 Mechnicl chrcteristics of tested steel 15Kh2MFA(II) Steel 15Kh2MF A(II) Yield Trensile Elongtioing Neck- strength strength σ 0,2, MP σ m, MP, %, % Chemicl composition Elements C Mo Si V Mn Content, wt.% Elements Cr S P Ni Ti Content, wt.% The tests were performed on the STM-100 servo-hydrulic mchine. In cse of the comined loding, the sttic tensioning ws superimposed y the cyclic sinusoidl loding with the frequency of 25 Hz nd mplitude of = MP. Specimens were deformed under unixil nd comined loding t tempertures of 150 о С nd 350 о С. Schemes of loding re given in Fig. 1. t Δσ Fig. 1. Scheme of deformtion of het resistnt steel 15Kh2MFA(ІІ) under sttic () nd comined loding (). t During the experiment, the longitudinl strin of the specimen nd the loding force were mesured. The true current strin of the specimen ws determined from formul [ 7 ]: e ln 1, (1) where l k l 0 / l0, l 0 is the initil specimen length, l k is the current specimen length. In order to study the effect of strin on microhrdness of steel 15Kh2MFA(II), the true trnsverse necking ~ ws clculted from formul [7]: ~ ln( F / F ), (2) 0 k where F 0 nd F k re the initil nd current res of the specimen cross-section, respectively. The quntittive evlution of the mteril hrdening fter plstic deformtion ws mde t different vlues of ~ sed on the microhrdness mesurement dt. Microhrdness ws mesured using the PTM-3 device with n lod of 1 N nd hold time of 15 s. 3. RELATIONSHIP BETWEEN STRUCTURE AND STEEL HARDENING PROCESSES Steel 15Kh2MFA elongs to het resistnt steels of the perlitic clss. During therml tretment, which simultes rdition emrittlement of the mteril under the ction of neutron irrdition in the middle of service life of the WWER- 440 rector pressure vessel, steel 15Kh2MFA hs initic-mrtensitic structure (Fig. 2). Bsed on the results of erlier investigtions, nded disloction microstructure in the form of system of prllel disloction suoundries is found in the initic component of steel. Needleshped cride protrusions re oriented in one direction t n ngle of pproximtely 60 о to the disloction suoundries (Fig. 2, ). The presence of cride protrusions 695

3 oriented t n ngle of 60 о to the crystl xis llows identifying the structure s inite, in contrst to the tempered mrtensite, in which lmellr protrusions re oserved in three orienttions simultneously [ 8 ]. Aprt from the needleshped cride protrusions, the initicferritic structure contins finely dispersed protrusions nd cride protrusions on oundries of structurl elements (Fig. 2, ). mteril hrdening. Lter on, microhrdness ecomes stle (it ttins sturtion), Fig. 3а. The stiliztion of microhrdness is ccompnied y n increse in the non-uniformity of deformtion of surfce lyers, which mnifests itself in the formtion of corrugtions. Thus, it is possile to identify severl stges of development of deformtion processes in the mteril: - stge of deformtion hrdening (I) this stge is ccompnied y n intensive disloction reorgniztion nd ccumultion of disloctions on oundries of grins nd sugrins of the mteril; 3250 H μ, MP Fig. 2. Microstructure of steel 15Kh2MFA(ІІ):, initic-mrtensitic structure Mrtensitic sections of the structure represent dislocted lth mrtensite. Prllel lths of mrtensite form mrtensite pockets (Fig. 2). Plstic deformtion cuses n increse in the numer of disloctions in the mteril, the resistnce to their displcement increses ccordingly, which is depicted y the incresed microhrdness tht lso chrcterizes deformtion properties of the mteril. Microhrdness mesurements of steel 15Kh2MFA(II) fter plstic deformtion in cse of different vlues of the true trnsverse necking of specimens re presented in (Fig. 3). An increse in the vlues of plstic strin leds to n intensive H μ(mx), MP ~ ~ Fig. 3. Dependence of microhrdness () nd hrdness () of steel 15Kh2MFA(ІІ) on the true trnsverse necking of the specimen t tempertures of 150 о С (1,4) nd 350 о С (2,3) fter sttic (1,2) nd comined (2,3) deformtion - stge of sturtion (II) this stge is chrcterized y the stiliztion of the deformtion process with the formtion of plstic shers in the mteril structure, the disloction density increses long with the 696

4 vlue of microdistortions of the mteril lttice, surfce corrugtion is formed on the specimen surfce; - stge of prefilure (III) the nucletion nd colescence of pores nd structurl defects tke plce, which re then ccompnied y the ppernce of mcrocrck. Deformtion of specimens from steel 15Kh2MFA(ІІ) y sttic nd comined tensioning ~ increses the rel plstic strin nd microhrdness of the mteril due to the ccumultion of disloctions [ 2 ]. Irrespective of the temperture conditions during testing, microhrdness of the deformed steel increses in ccordnce with similr lws. Imposition of the cyclic component cuses intensifiction of the mteril deformtion, Fig. 3. It is found tht microhrdness of steel 15Kh2MFA is y MP greter thn tht under sttic deformtion t the sme vlues of the rel necking. Moreover, the mteril hrdening t 150 о С is more intensive t the mcrolevel s compred to deformtion t 350 о С. Oviously, plstic deformtion t higher temperture (350 о С) is ccompnied y the processes of therml turn, which consist in certin liquidtion of the off-lnced residue of point defects nd reorgniztion of the disloction structure. In erlier works it ws found tht in cse of the voluntry structurl reorgniztion of steel 15Kh2MFA the free crystl energy decreses. Therefore, the disloction density decreses s result of the therml turn, nd the remining disloctions form stle configurtions in the form of low-ngled oundries [8]. Thus, t the microstructurl level the effect of deformtion is depicted y n increse in microhrdness, nd, ccordingly, disloction density in lowngled oundries, n increse in the disorienttion of sugrins of the initic structure, frgmenttion of mrtensitic lths, decrese in the distnce etween low-ngled oundries (decrese in the size of structurl elements), wheres t 350 о С it consists in the formtion of stle configurtions in the form of low-ngled oundries resulting from the therml turn. The sid fctors cuse n increse in the stress necessry for the propgtion of microcrcks through the oundries of structurl elements, which increses the seprtion stress of steel 15Kh2MFA(ІІ). A dependence of the mximum microhrdness vlues on the rel necking vlue reveled during filure of specimens y different deformtion schemes is shown in Fig. 3. The results otined cn e descried y single dependence, ecuse in ll the cses investigted n increse in the ultimte strin ws ccompnied y n increse in microhrdness. Bsed on the results of erlier investigtions it ws found tht strength of the het resistnt steel increses fter plstic deformtion [ 8,9 ]. For steel 15Kh2MFA(ІІ) investigted, n increse in the test temperture leds to chnge in the regulrities of the mteril filure. Loclistion of the deformtion processes moves from the specimen surfce to its centre, Fig. 4, d, which preconditions trnsition from the mixed micromechnism of filure to the intrgrnulr one y the scheme sher+seprtion, Fig. 4, c, f. A chnge in the orienttion of the frcture mcrosurfce is cused y trnsition from the loclised filure to the multicentric one. Moreover, these chnges cn e connected with the test temperture effect on the mteril nisotropy t the mcro- nd microlevel. The frcture surfce of the mterils investigted t 150 о С nd 350 о С is perpendiculr to the specimen xis, Fig. 4, d. The source of filure (stress concentrtor) is locted on the specimen surfce nd hs shpe of ded spot whose size increses with n increse in the test temperture. The deformtion relief in the form of longitudinl nds is formed on the specimen surfce. Figure 4 e shows frgment of the deformtion relief on the specimen lterl surfce. As is seen, the non-uniformity of strin distriution 697

5 revels itself most clerly ner the specimen surfce. Mcrolocliztion of the plstic yielding develops round these zones, which then leds to the mteril filure [ 10 ]. Two chrcteristic zones re found on the frcture surfce of the specimens investigted, Fig. 4, d: - ner surfce zone, in which filure took plce y the scheme sher + seprtion. This zone is formed y concentric scly lyers perpendiculr to the outer surfce. At the mcrolevel, the relief contins flt sections with smooth structureless surfce typicl of the intergrnulr filure. Elongted dimple-like formtions with trces of sher re found within individul sections. c А B d А B e f Fig. 4. Filure frctogrms of the specimen from steel 15Kh2MFA(II) t 150 о С (-c) nd 350 о С (d-f), respectively: A is the ner surfce zone of filure, В is the internl zone of filure 698

6 - internl lyer, in which the mteril filed ccording to the intrgrnulr mechnism y the norml seprtion scheme. The frcture surfce is locted t n ngle of о to the specimen xis. Development of sher strins in the vicinity of the stress concentrtor leds to the qusi-rittle filure of the specimen y the sher mechnism. The min crck is formed in one of the mcronds of the loclized deformtion. CONCLUSIONS We investigted the effect of deformtion y sttic tensioning nd tensioning with the imposition of the cyclic low mplitude component (comined tensioning) on the microstructurl chnges in steel 15Kh2MFA fter therml tretment, which simultes rdition emrittlement of the mteril under the ction of neutron irrdition in the middle of service life of the rector pressure vessel. It is found tht with n increse in the rel mteril necking the microhrdness of steel 15Kh2MFA increses irrespective of the loding type nd temperture conditions during testing. An increse in the rel strin fter deformtion y tensioning nd comined tensioning leds to n increse in microhrdness, which is more pronounced fter deformtion t 150 o С thn t 350 о С. This confirms the previously otined results, which link the high temperture deformtion to the formtion of such disloction structures tht provide for the mteril hrdening nd preserve its plsticity. REFERENCES 1. Poklud, J.; Šnder, P. Micromechnisms of Frcture nd Ftigue: In Multi-scle Context, 2010, Springer, 293 p. 2. Ysnii, P.; Mrushchk, P.; Hldo, V.; Brn D. Correltion of the microdisloction prmeters with the hrdness of plsticlly deformed het-resistnt steels. Mterils Science. 2008, 44, Elmustf, A.A., Stone, D.S. Nnoindenttion nd the indenttion size e6ect: kinetics of deformtion nd strin hrdening. J. Mech. Phys. Solids. 2003, 51, Fleck, N.A.; Hutchinson, J.W. A phenomenologicl theory for strin grdient effects in plsticity. J. Mech. Phys. Solids 1993, 41, Achry, A.; Beudoin, A.J. Grin-size effect in visco-plstic polycrystls t moderte strins. J. Mech. Phys.Solids, 2000, 48, Ysniy, P.; Hldo, V.; Mruschk, P.; Brn, D. Evlution of hrdening of plsticlly deformed steels, Proc. of the 13- th Interntionl Conference «Experimentl Anlysis of Nno nd Engineering Mterils nd Structures», Alexndroupolis, Greece, (July 1-6), 2007, Ysniy, P.V.; Mruschk, P.O.; Pnin, S.V.; Bishchk R.T. Scle levels of deformtion nd frcture nd mechnicl properties of 25Cr1Mo1V steel efore nd fter nonisotherml loding. Physicl Mesomechnics. 2011, 14, Ysnii, P.V.; Hld o, V.B.; Okipnyi, I.B.; Tsyrul nyk, O.T. Microstructure nd frcture stresses of plsticlly deformed nd hydrogented het-resistnt 15Kh2MFA steel. Mterils Science, 2008, 44, Ysniy, P.V.; Okipnyi, I.B.; Mruschk P.O.; Bishchk R.T.; Sorochk A.P. Toughness nd filure of het resistnt steel efore nd fter hydrogention. Theoreticl nd Applied Frcture Mechnics. 2011, 56, Swdener, J.G.; George, E.P.; Phrr, G.M. The correltion of the indenttion size effect mesured with indenters of vrious shpes. J. Mech. Phys. Solids. 2002, 50,