Microstructure, Deformation and Fracture Behavior of Cr-Mo-V Steels

Transcription

1 Internatinal Jurnal f Metallurgical Engineering 2013, 2(2): DOI: /j.ijmee Micrstructure, Defrmatin and Fracture Behavir f Cr-M-V Steels C. Gupta 2,*, J. K. Chakravartty 1, S. Bane rjee 1 1 Materials Grup, Bhabha Atmic Research Centre, Trmbay, Mumbai, , India 2 On Sabbatical as JSPS Pst-Dctral Fellw t 3D/4D Material Science Lab Tyhashi University f Technlgy, Japan Abstract The paper reprts the uniaxial defrmatin and fracture behavir f tw Cr-M-V steels used fr nuclear structural cmpnents. The tw steels are a new member f lw ally steel in the 3Cr-1M class and a plain tempered martensitic stainless steel cntaining 12% Cr. The micrstructure investigatins were carried ut n the 3Cr-1M steel by establishing the cntinuus cling transfrmatin diagram using dilatmetry cmplemented by ptical micrscpy studies. The nature f cntinuus cling transfrmatin diagram shwed typical characteristics f bainitic transfrmatin ver wide range f cling rates fr the 3Cr-1M steel. The defrmatin behavir f the tw steels were characterized using a series f tensile tests carried ut in the temperature range C and ver a wide range f strain rate. The characteristic anmalies bserved frm the temperature and strain rate dependence f strength and ductility parameters btained frm the tensile tests prvided indicatin t the presence f Dynamic strain aging (DSA) phenmenn in bth steels. It was fund that the embrittling features f DSA phenmenn was rather inncuusly manifested in 3Cr-1M steel with bainitic structure as cmpared with the 12%Cr steel with tempered martensitic steel. The extent f reductin in fracture resistance f the tempered martensitic steel in the DSA regime was characterized in the J IC frmat using high temperature fracture tests. The studies revealed that a reductin in fracture resistance in the DSA regime ccurred primarily due t the presence f cracking instabilities during the fracture tests. The difference in the manifestatins f DSA in the tw steels was ratinalized n the effectiveness f the interstitial impurities t interact with dislcatins in the martensitic and bainitic micrstructure. Keywrds Cr-M-V Steels, Dynamic Strain Aging, Bain ite, Martensitic Steels Defrmatin, Fracture Tughness 1. Intrductin Ferritic steels are well knwn materials f cnstructin f varius structural cmpnents in nuclear reactrs[1,2]. Reactr pressure vessels and reactr supprt structures in the PWR s and BWR s, heat transprt piping and end fitting in PHWR s are sme f the imprtant examples f applicatins where ferritic steels have been emplyed as cmpnent materials. There exists a great cncern ver the influence f embrittlement phenmena n the ability f the material t discharge their functinal duties during the lifetime f the critical cmpnents in use in reactrs. This has grwn a fertile area f research dealing with the influence f embrittlement due t thermal, irradiatin and envirnmental surces n the cmpnents[1,2]. The presence f plastic instabilities is knwn t influence the perfrmance f structural materials adversely[1,3]. These steels pssess nan/ultrafine grained micrstructures with excellent cmbinatins f strength and ductility[3,4]. A related but distinctly different phenmenn is the Prtevin * Crrespnding authr: jy_gupta71@yah.c.in (C. Gupta) Published nline at Cpyright 2013 Scientific & Academic Publishing. All Rights Reserved Le Chatelier (PLC) effect, which als has been knwn t ccur in structural steels. It was first reprted by Le Chatelier in 1909 and subsequently by Prtevin and Le Chatelier in 1924[3]. Estrin and Kubin[3,4] have classified the varius types f plastic instabilit ies int thse that are caused when the wrk hardening has been suppressed and thse that result frm strain rate sftening. The Lüders band frmatin during the ccurrence f yield pint phenmena belngs t the frmer while the PLC phenmena t the latter. The PLC effect has been regarded as a particularly insidius manifestatin f the dynamic strain aging (DSA) phenmena [3]. The physical rigin f DSA phenmena is due t the dynamic interactin f diffusing slute atms with mving dislcatins[4,5]. The manifestatin f DSA in several ally systems such as Fe, Ti, Al, Au, Zr allys has been studied[4-10]. These studies cnvey that despite the early knwledge and investigatins int the ccurrence f DSA, the manifestatins f PLC depend n the ally system underging DSA. As a result there is a sustained need fr characterizing the manifestatin f DSA in the either newly develped allys destined fr structural applicatins r in thse materials being used structural cmpnents, where it has nt been studied as yet. Further, the influence f the micrstructure n the manifestatins f DSA in ferritic steels has nt received serius attentin. Hence the scpe f

2 Internatinal Jurnal f Metallurgical Engineering 2013, 2(2): investigatins includes the study f relative manifestatins f DSA in tw different structural steels with nearly similar interstitial impurity cntent but with significant differences in ally cntent and micrstructure prduced frm γ α shear transfrmatin in steels. Such a cmparisn is expected t elucidate the influence f micrstructure n the n the extent f degradatin f tensile and fracture prperties due t presence f DSA in structural steels. 2. Experimental The materials used fr the investigatins are a new member f 3Cr-1M class f lw ally steel and a plain 12Cr tempered martensitic steel. This 3Cr-1M type steel is a new generatin nuclear reactr pressure vessel material with ptimized cmpsitin t mitigate temper and irradiatin embrittlement[1]. The AISI 403 steel is used fr end fitting applicatins in pressurized heavy water reactr. The respective chemical cmpsitins f the tw steels are given in table 1. A defrmatin dilatmeter Bahr DIL 805 was perated in the quenching mde fr the dilatmetry studies. Hllw tubular samples f utside diameter (O.D) 4.0 mm, 250μm th ickness and 10 mm length were used fr the tests. The sample was placed between tw alumina rds, ne f which was cnnected t a LVDT with less than 1μm reslutin fr measuring the change in length during the phase transfrmatins. A thermcuple was spt welded t the O.D f the sample and it was inductively heated t the austenitic temperature. The heating rate f 100 C/min was emplyed fr all tests fr carrying the austenitising f the sample at 1100 C fr 10 minutes. Fr the purpse dilatmetry studies a series f samples were cled ver the cling rate range 1075 C/min t 2.2 C/min. The sample change in length (ΔL) and temperature (T) were acquired nline by a 16 bit DAQ during the entire heat-hld-cl cycle. Ta ble 1. Chemical cmpsitin f the steels in %wt element C Cr M V Mn Ni AISI Cr-1M type The tensile tests were carried ut using a flr mdel f instrn make universal testing machine. Uni-axial tensile tests cmprised f mntnic pull as well strain rate change tests ver the temperature range C. The tensile parameters f strength and ductility were derived frm the digital data acquired during the tests. A three zne furnace was used t heat the sample, the temperature f which was mnitred using a K type thermcuple attached t sample guage. The high temperature fracture tests were carried ut in J IC frmat using 1 inch thick fatigue pre-cracked cmpact tensile samples with a/w rati f 0.6. The unlading cmpliance technique as well as DCPD technique was used fr characterizing the fracture tughness as a functin temperature. Fr the high temperature tests the crack length was mnitred using bth COD guage as well as using the direct current ptential technique. The cracklength cnversin frm the measured ptential drps was estimated frm the Jhnsn s frmula. A custm made furnace was made t accmmdate the COD guage arms and DCPD current and ptential leads inside the furnace. The electrnics f the COD guage was air cled. 3. Results and Discussin The dilatmetry studies n 3Cr-1M type steel yielded the temperature range f austenite transfrmatin as a functin f the prescribed cling rates frm the nn-linear reg ins in the change in length with temperature plts. The transfrmatin start and fininsh pints pltted n the lci f the cling curve in a semi-lg temperature time axis prvided the the CCT diagram f the 3Cr-1M steel. The CCT diagram f the 3Cr-1M steel is shwn in figure 1. It is clearly seen that a single bay in the transfrmatin is seen t ccur ver a wide range f cling rates. At slwer cling rates the austenite decmpsitin ccurs at much higher tempertaure int tw prducts with clear demarcated regins in the temperature time space. Figure 1. CCT diagram f the 3Cr-1M type steel The ptical micrstructures f the samples expsed t selected cling rates are shwn in figure 2. At the faster cling rates abve 35 C/min the micrstructure appears similar t the bainitic micrstructure typically fund in lw ally steels. Further the CCT diagram shwing a single bay with transfrmatin start temperatures t be insensitive t the cling rates is typical f nn-isthermal transfrmatin characteristics f bainitic transfrmatin seen in many steels[11]. At the slwer cling rates the ptical micrstructure shws that frmatin f pr-eutectid ferrite (typical C- curve characteristics in the CCT diagram) fllwed by the frmatin f granular bainite. Within this regin, the high reslutin f the dilatmeter detected mu ltip le transitins belw the bainite transfrmatin start temperatures. Further investigatins using interrupted quench studies need t be carried ut establish the rigin f these transitins. In this tw phase regin when the primary ferrite frmatin precedes the bainitic transfrmatin, the

3 144 C. Gupta et al.: Micrstructure, Defrmatin and Fracture Behavir f Cr-M-V Steels transfrmatin start temperatures fr the granular bainite decreases with slwer cling rate. Figure 2. Optical micrscpy f the dilatmetry sample cled at (a) 1080 C/min (b) 108 C/min (c) 43 C/min Figure 3. Optical micrscpy f the dilatmetry sample cled at (a) 12 C/min (b) 6.6 C/min (c) A magnified picture granular bainite (500x) Figure 4. Variatin f (a) UT S and (b) elngatin (ttal, unifrm and nn-unifrm) with test temperature fr AISI 403. (c) Variatin f,, ttal and unifrm elngatin with temperature fr 3Cr-1M type steel

4 Internatinal Jurnal f Metallurgical Engineering 2013, 2(2): Thus the verall characteristics f the CCT diagram and ptical micrscpy studies clearly establish that the dminant micrstructure ver a wide cling rate is predminantly bainitic. This view is cnsistent with ther micrstructural studies n lw ally steels with similar ally cntent fr reactr pressure vessel applicatins[11]. The Bainite micrstructure is a mu lti-cmpnent system with unique mrphlgy f the bainitic ferrite having typical spatial rientatin relatinships with carbides, and austenite. TEM studies are envisaged t reveal these imprtant micrstructural characteristics in the bainitic structure f the new steel. The tensile results fr the tw steels carried ut are summarized in the figure 4. The temperature variatin f strength and ductility parameters fr the respective steels has been pltted. It is seen that fr the AISI 403 there is a sharp increase in the tensile strength values accmpanied by a decrease in ductility and stasis in the temperature dependence f yield strength in the range C. In cmparisn it can be seen that the 3Cr-1M type steel displays n such increases in the tensile strength while underging similar reductins in the ductility parameters as AISI 403. It is t be nted hwever that, within the temperature ranges C the unifrm ductility fr the 3Cr-1M type steel imprves befre sharply reducing at higher temperatures. The ttal ductility cntrastingly shws significant upswing at temperatures beynd 400 C fr bth steels. The ccurrence f serrated flw ccurred in bth steels a typical examp le is shwn in figure 5 fr AISI 403. It needs t be mentined hwever that the in-hmgenus flw was quite damped in case f 3Cr-1M steel, with n clear delineatin f crit ical strain. / lnε (MPa) AISI 403 CrMVNi Temperature ( 0 C) Figure 6. Plt cmparing the strain rate sensitivity variatin with temperature fr the tw steels It is t be nted that the SRS values sharply decline t lwer values fr the AISI 403 steel than the 3Cr-1M type steel. The cmparisn f the tensile results between the tw steels can summarized by the temperature dependence f the rati, which gives an idea f the extent f wrk hardening with the respect t that at the ambient cnditins. The ductility parameters has been cmpared in ε ε terms f the rati 2 1 Uni Uni, which gives an idea f the relative magnitudes f unifrm ductility btained frm tensile tests carried ut n AISI 403 and Cr-M-V steels. The p lt is shwn in the figure 7. It can be seen frm the figure that within the DSA temperature regime, while there is dramatic increase in rati t levels even greater than that at rm temperature fr AISI 403, the rati fr 3Cr-1M type steel shws a gradual decrease with increase in temperature. Figure 5. Stress-strain plt f AISI 403 shwing serrated flw The strain rate change tests carried ut n bth the steels enables characterizatin f the strain rate sensitivity as a functin f temperature. The results are shwn in figure 6. It can be clearly seen that bth the steels shw a reductin in the strain rate sensitivity twards negative values within the range C. Figure 7. Cmparing the temperature variatin f the ratis pertaining t strength and ductility fr the tw steels

5 146 C. Gupta et al.: Micrstructure, Defrmatin and Fracture Behavir f Cr-M-V Steels These trends imply that wrk hardening fr AISI 403 with martensitic structure is significantly higher than that f 3Cr-1M type steel with baintic structure. A cmparisn f ductility rati shws that it is less than unity at all temperatures, signifying that unifrm elngatin is smaller fr the AISI 403 as cmpared with the 3Cr-1M type steel. It is t be nted that in the DSA temperature regime the ductility rati decreases and reaches minimum at the temperature where the rati fr AISI 403 is the maximum. Thus the maximum difference in the ductility between the tw steels ccurs at the temperature where the wrk hardening fr AISI 403 is the highest. As the unifrm ductility fr 3Cr-1M type steel remains relatively cnstant in the temperature range C (Fig. 4(c)) it clearly implies that there has been greater embrittlement f AISI 403 as cmpared with 3Cr-1M type steel. These results bring ut the fact that the AISI 403 steel have been mre susceptible t extreme manifestatins f DSA as cmpared with 3Cr-1M type steel. This has resulted in the greater deteriratin f ductility in the temperature ranges where DSA was bserved t ccur fr AISI 403 as cmpared with that f 3Cr-1M type steel. The tensile results als shw that the unifrm elngatin and ttal elngatin curves diverge sharply beynd the DSA temperature range (figure 4). At the temperatures, where the SRS was fund t be psitive the cntributin f nn-unifrm elngatin t the ttal ductility f the steels is bserved t be higher. This has been reprted earlier in steels[12] and culd be ratinalized in the fllwing way. It is well knwn that the attainment f tensile strength is cntrlled by bth wrk hardening rate and strain rate hardening. Hwever, nly the latter remains active beynd. As SRS cntrls the extent f strain rate hardening. The cntributin f nn-unifrm elngatin t the ttal elngatin is directly related t its magnitude and sign f SRS. When it is negative, (i.e. serrated flw behavir) lcalizatin f defrmatin in the frm prpagative bands (eg. Lüders bands) and increase in wrk hardening prmte develpment f lcal znes f damage, leading t decrease in bth the ductility parameters. Beynd the DSA temperatures, a psitive SRS wuld increase the strain rate hardening ver that f wrk hardening rate. This is further exacerbated by prcesses such as dynamic recvery that prmtes wrk sftening[12]. While, these factrs results in reducing the unifrm elngatin by inducing premature necking, they wuld be beneficial fr nn-unifrm elngatin, leading t the bserved divergence between ttal and unifrm elngatin parameters as bserved in figure 4. In rder t explre the extent f embrittlement f AISI 403 high temperature fracture tughness tests were carried ut in the temperature range where serratins were bserved in the steel. The strain rate - temperature dmain f the ccurrence f serratins in AISI 403 are shwn in the figure 8, superimpsed in which are the test cnditins where the fracture tughness tests were carried ut. The calculatin f the effective crack tip strain rate was carried ut frm frmula derived cnsidering fu lly plastic slutins at crack tip[13] and was estimated as 3.8x10-3 s -1 fr the applied pull rate shwn in figure 8. Figure 8. Plt shwing the temperature strain rate cmbinatins where serrated flw (marked as filled circles) has been fund in the tensile tests in AISI 403. The crrespnding pull rates have als been shwn Figure 9. An enlarged prtin f the lad displacement plt btained frm the DCPD tests at 350 C and pull rate f 3.33 mirn/s and 33.3 micrn/s. The crrespnding DCPD signal is als shwn fr each test The lad displacement curve and the crrespnding DCPD signal is shwn in figure 9 fr tw pull rates applied during the fracture tughness tests. It is clearly seen that the crack grwth stage f the test (which ccurs after the maximum lad ) shws perturbatins in lad and DCPD signal fr the lwer pull rate. This pull rate was within the DSA temperature strain regime f AISI 403 where serrated flw was bserved. On increasing the pull rates by a factr f 10, such perturbatins in the lad and DCPD signal was nt bserved. The estimatin f the elastic plastic fracture

6 Internatinal Jurnal f Metallurgical Engineering 2013, 2(2): tughness parameters frm the fracture test frm the unlading cmpliance methd and DCPD technique enabled the temperature variatin f J IC and dj/da t characterized fr AISI 403. Figure 10 shws the plt f J IC and dj/da with temperature. It is clearly seen that bth the fracture tughness parameters decrease quite significantly in the temperature range C. Figure 10. The evlutin f fracture tughness in J IC frmat and crack grwth resist ance in t he dj/da frmat ver the t emperat ure range 200 C t 425 C carried ut using pull rate f 3.33 micrn/s. Inset shws the SEM fractgraph picture f the sample displaying the lwest fracture tughness value The inset in the figure 10 shws the SEM picture f the fracture surface f the sample tested at the temperature crrespnding t the lwest fracture tughness value. The SEM fractgraph clearly shws the arrangement f under develped dimples interspersed between well develped dimpled areas. The width f these relatively flat areas in the SEM phtgraph is abut 40µm. It is therefre seen frm the results btained frm the fracture tughness tests as well as fractgraphic examinatin that effect f DSA n fracture behaviur in AISI 403 results in embrittlement by way f reductin in the fracture parameters in terms f bth initiatin and prpagatin tughness. The fractgraphic bservatin clearly shw evidences f flat featureless areas crrbrating the jerky lad-displacement recrd btained in the crack grwth tests. The feature-less areas cntaining few ductile dimples are signatures f the ccurrence f the crack jumps, because as pinted ut by Mhan and Marschall[14], they result in rapid separatin f the crack faces leaving less time fr large number f vids t nucleate during the ductile crack grwth. The presence f these ductile cracking instabilities can als be assciated with excessive machine cmpliance[14]. Hwever in the multi pull rate crack grwth tests the jerky lad-displacement recrd appeared nly in the lwer pull rate that gave an average crack tip strain rate within the DSA favrable strain rates. This rules ut the influence n machine cmpliance. The secnd reasn fr these cracking has been attributed in ealrier studies[15,16], primarily carried ut n lw carbn steels fr piping applicatins, t DSA althugh the exact mechanism leading t peridic crack jumps has remained unclear. A pssible ratinalizatin f the tensile test results, the appearance f cracking instability and its implicatin n the fracture parameters is nw presented. Earlier investigatins carried ut by Mukherjee and Sellars[17] n Fe-Cr-C steels with tempered martensitic structure n a range f Cr cntents (0.87% %) attributed the dynamic strain aging t ccur as a cnsequence f frmatin f Cr-C cmplexes, which enabled interstitial lcking f dislcatins upt 400 C. Thus the trends seen in AISI 403 are cnsistent with earlier investigatins where the rle f Cr has been unequivcally established. In the case f lw ally steels, t which the 3Cr-1M type steel belngs, studies have shwn that they are susceptible t DSA due t interactin f dislcatins with C and N[15-17]. The chemical nature f carbides in the tempered bainitic structure shws that they are Cr rich carbides 32. As a result the elements such as Mn and Ni are free t frm X-N r X-C cmplexes (X = Mn/Ni), thereby curtailing their migratin t the dislcatins. The frmatin f cmplexes f C and N with Mn are knwn t reduce the mbility f these interstitial atms (i.e. C and N)[18]. Thus, as a cnsequence f the frmatin f Cr rich carbides, elements such as Ni/Mn culd frm cmplexes with C/N atms, which may lead t a reductin in the effectiveness f dislcatins pinning by these slutes. The macrscpic effect f slutes pinning dislcatins is t reduce the strain rate sensitivity. Luchet and Brechet[19,20] have studied theretically the pssible influences f strain rate sensitivity n tughness. They have argued that the strain rate sensitivity influences the actin f crack tip relaxatin prcess such as shielding and blunting[19]. Cnditins that result in small values f strain rate sensitivity prmte flw lcalizatin and retard the crack tip relaxatin prcess, which can act as a driving frce fr crack extensin by increasing the rate f stred elastic energy. Further, they have shwn that in-hmgenus defrmatin, such as the release f PLC bands at the crack tip lwers the stresses during their scanning time at the tip[20]. Cnsidering the inferences frm these studies[19,20] it may be prpsed that in the fracture tughness tests carried ut n AISI 403, with the release f PLC bands during DSA, the extent f plastificatin at the crack tip may be affected. With repeated release f these bands perturbatins in the plastic zne size ahead f the tip may appear, which will impair the relaxatin f lcal crack tip stresses. As a result an increase f the elastic strain energy rate wuld ccur and prvide the driving frce fr the crack extensin in the frm f jumps. The relatinship between the plastic zne size and ability fr crack tip shielding has been shwn analytically by Chen et. al. where frmatin f reduced plastic zne size resulted in lwer ability t relax the crack tip stresses[21]. As a result, the embrittling manifestatins f DSA n the mechanical prperties f AISI 403 steel are significantly increased.

7 148 C. Gupta et al.: Micrstructure, Defrmatin and Fracture Behavir f Cr-M-V Steels 4. Summary The results in this wrk cmpares the manifestatins f DSA in tw steels with tempered martensitic and bainitic structures and attempts t shw that phase transfrmatin prducts als exert a nn-trivial influence n the anmalies in tensile prperties caused by the peratin f the phenmena. Earlier studies where the austenite transfrmatin prducts were ferrite, pearlite, martensite r a cmbinatin f them, chemistry cntrl was fund t be the cntrlling factr t reduce the influence f DSA n the tensile prperties f the material. The results being reprted in this wrk n the new generatin lw ally steel shw that chemistry cntrl alng with the presence f bainitic phase can be used t limit the full extent f manifestatins f DSA n tensile prperties. Further, as the ptential fr DSA in the material is pushed at higher temperatures, the ascendant influence f recvery prcesses (eg dynamic recvery) wuld suppress the embrittling features f the phenmena. ACKNOWLEDGEMENTS CG gratefully acknwledges Japan Sciety fr Prmtin f Science (JSPS) fr the JSPS fellwship award and grant-in-aid fr scientific research (N ). REFERENCES [1] C. Gupta, Defrmatin and Fracture f Advanced Structural Steels: Effect f Dynamic Strain Aging, Ratcheting and Creep. Lambert Academic Publishers, Germany, 2012 pp [2] K. L. Murty and C. S. Sek, JOM, Vl p. 23. [3] Y. Estrin and L. P. Kubin in: Cntinuum Mdels fr Materials with Micrstructure ed. H. B. Mühlhaus, Jhn Wiley and Sns, Germany 1995 pp [4] Y. Estrin and L. P. Kubin, Mat. Sci. Eng. Vl. A p [5] K. L. Murty J. Metals Vl p.34. [6] A. Gysler, G. Lutjering, and V. Gerld Acta. Metall. Vl p.901. [7] S. Banerjee and U. Naik, Acta. Mater. Vl 44, 1996 p [8] P. Rdriguez, Bull. Mater. Sci. Vl p.653. [9] E. Pink and A. Grinberg Mater. Sci. Eng Vl p.1. [10] A. Van Den Beukel, Phys. Stat. Sl. (a) Vl 30, p.197. [11] H. K. D. Bhadesia Bainite in Steels 2nd ed. IOM Lndn [12] W.P. Lng and R. E. Reed Hill, Scripta. Mat. Vl p.833. [13] J. H. Yn B. S. Lee, Y. J. h and J. H. Hng, Int. Jur. Press. Vess. Pip Vl p.150. [14] R. Mhan and C. Marschall Acta. Mater. Vl p [15] B. Mukherjee, Nucl. Eng. Des. Vl p. 63. [16] J. W. Kim and I. S. Kim, Nucl. Eng. Des. Vl 172 p.49. [17] T. Mukherjee and C. M. Sellars, Met. Trans Vl p [18] C. C. Li and W. C. Leslie Met. TransA Vl p [19] Y. Brechet and F. Luchet, Acta. Metall. Mater. Vl p [20] Y. Brechet and F. Luchet, Acta. Metall. Mater. Vl p [21] J. Chen and S. Kitaka, Int. Jur. Fract Vl p.307.