Improvement in mechanical properties of 13Cr Martensitic Stainless Steels using modified heat treatments

Transcription

1 Proceedings of the 28th ASM Het Treting Society Conference Octoer 20 22, 2015, Detroit, Michign, USA Copyright 2015 ASM Interntionl All rights reserved sterntionl.org Improvement in mechnicl properties of 13Cr Mrtensitic Stinless Steels using modified het tretments Mr. Srivts Kulkrni*, Mr. Perl Srinivs, Mr. P. K. Biswl, Dr. G Blchndrn, Mr. V Blsurmnin Klyni Crpenter Specil Steels Pvt. Ltd, Pune, Mhrshtr, Indi *Correpsonding uthor mil id: srivts.kulkrni@kcssl.com which gve good corrosion resistnce compred to cron nd low lloy steels ut expensive compred to cron, low lloy steels nd 13%Cr steels [6]. The 13%Cr steel in OCTG is however, susceptile to corrosion t high temperture nd stress corrosion crcking in moderte H2S environment nd crevice corrosion. Another importnt criteri is the sulphide stress corrosion crcking resistnce which is defined y NACE MR0175/ ISO stndrd where the given steel functions t threshold prtil pressure of H2S of 1.5 psi nd the locl ph vlue > 3.5 up to 150 C [7]. These steels hve further requirement of meeting specified su-zero impct properties. This is property tht gets impired on het treting in thicker cross sections. Hence people tend to micro lloy with Ni nd Mo to improve toughness nd chieve microstructurl stility. Novel het tretment techniques which significntly improve su-zero temperture impct toughness hs een reported for hrdened nd tempered CrMo lloy steel nd few mrtensitic stinless steels contining Ni [8]. In the present study, the effect of het tretment on the mechnicl properties nd microstructure hs een studied on the 13%Cr mrtensitic stinless steel. A modified doule quenching het tretment is shown to enhnce significntly the su-zero impct properties. Astrct The 13% Cr 0.2%C steel is extensively used in OCTG ppliction. This steel ws exed in the doule hrdened condition t 1040oC/OQ followed y 980 C/OQ s ginst conventionl hrdening tretment t 980 C/OQ. It ws oserved tht the doule hrdening het tretment resulted in higher cron dissolution in the mtrix without ny residul grin oundry necklce crides while single hrdening t 980 C shows remnnt grin oundry cride. Doule hrdening het tretment ws found to refine the grin size y recrystlliztion of the defects introduced in first hrdening. During doule tempering het tretment, the crides in the 980 C tretment were corse nd found to nuclete long grin oundry. The doule hrdened smple on tempering showed n even distriution of cride throughout without grin oundry cride. The doule hrdened smple show improved strength nd toughness compred to the single hrdened smple t similr tempering conditions. The microstructurl nlyses t vrious stges of processing hve een correlted to the mechnicl properties otined. Introduction Experimentl Methods The demnd for corrosion resistnt steels in oil country tuulr gs pplictions (OCTG) hs een growing. These lloys re used in deep se where corrosive environment of H2S, CO2, rine is encountered. In ddition, the condition of pressure, temperture nd the ph chnge s one goes deep inside the se [1][2]. So selection of correct lloy which cn meet the requirements of oil nd gs industry for such pplictions is essentil. NACE MR0175/ISO [3] gives the lloy steels which cn meet the service conditions in OCTG pplictions. The development of steels used for OCTG pplictions [4] strted with cron steels which re lest expensive ut suffered loclized corrosion even with corrosion inhiitors. Mrtensitic stinless ws developed to overcome the limit of cron nd low lloy steels. The 13%Cr steel grde elongs to mrtensitic stinless steel clss which is widely used in OCTG ppliction. These steels re through hrdenle to chieve dequte strength long with corrosion resistnce. The grde is lloyed with 0.2% C content which imprts good resistnce to sulphide stress corrosion crcking promoted y uniform mrtensitic microstructure free from delt ferrite. The high cron content however ffects the weldility of the steel nd hence super mrtensitic grde with lower cron, long with Ni nd Mo dditions hs een introduced [5]. Duplex stinless steels were lso developed The steel ws melted t Klyni Crpenter Specil Steels Pvt. Ltd. in 35 MT Electric rc furnce followed y ldle refining nd vcuum degssing. It ws cst in ottom poured up-hill ingot csting process in to 4 Metric ton steel ingot. The ingot ws directly hot rolled through loog mill nd finish rolling mill to 170 mm dimeter r. The overll reduction rtio of the rs ws mintined t Cut rs of 170mm length ws chosen for het tretment in lortory scle muffle furnce. The cut r ws sujected to the following het tretment. One set ws hrdened y ustenizing t 980 C followed y oil quenching. Another set of rs ws ustenitised t 1040 C nd oil quench followed y second hrdening tretment y ustenizing t 980oC followed y oil quenching. The heting rte mintined during hrdening nd tempering ws 100 C/h. All the smples post hrdening were sujected to doule tempering. First tempering ws done t 710 C followed y second tempering t 680OC. Post tempering the steels were cooled in oil. The steel chemistry ws detered using ARL 3460 opticl emission spectrometer nd the steel gs nlysis ws otined 335

2 using LECO gs nlyzers. The hrdness ws mesured using Brinell hrdness tester. The s-quenched hrdness ws mesured y slicing 170 mm di from n s quenched r. The mechnicl testing ws crried out t mid rdius loction using Shimdzu mke tensile testing mchine nd the test smple conformed to ASTM- E8 round smple of guge length 50 mm. The impct properties were evluted t suzero tempertures (-10 C) for het treted conditions using stndrd Chrpy V-notch method using n FIE mke testing mchine. Smples for tensile nd impct test were prepred s per ASTM -E23. The microstructure ws evluted with Villell etchnt t vrious conditions using opticl microscope nd select smples were exed in the Crl Zeiss mke Scnning Electron Microscope. The SEM microstructure for cride distriution ws quntified using Imge J softwre. The prior ustenite grin size ws detered y oxidizing the grin oundries y heting the smples in furnce s per ASTM -E112. The recommended het tretment procedure for OCTG ppliction involves hrdening t temperture rnge etween 980 to 1000oC followed y doule tempering [First tempering: 700oC & Second tempering: 640oC]. The lower hrdening temperture is preferred to chieve n optimum comintion of strength, toughness nd corrosion resistnce. Ms( C)=540-(497%C+6.3%Mn+36.3%Ni+10.8%Cr+46.6%Mo [1] 13%Cr Theoreticl Behviour of 13Cr Steels The physicl metllurgy of the 13%Cr steel with 0.2%C content cn e understood from the pseudo inry phse digrm [9] shown in Fig. 1. The 0.2%C in the steel long with the residul ustenite stilizers Mn, Ni, Cu nd N expnd the ustenite field which is necessry to hrden the mtrix y mrtensite formtion on cooling. A temperture ove 950oC is required for ustenistion nd it should not exceed 1150oC, to prevent the formtion of high temperture δ-ferrite phse. Once formed, the δ-ferrite phse gets retined when cooled to room temperture due to the sluggishness in its trnsformtion. The δ-ferrite in the mtrix ffects the workility nd the mechnicl properties specificlly the impct toughness of the steel. The equilirium digrm further indictes tht 0.2%C steel forms K1 (Cr23C6) nd K2 crides (Cr7C3). In the cse of super mrtensitic steel, the 13%Cr steel hs lower C content to void formtion of detrimentl K1 nd K2 crides tht ffect weldility. The ustenite field is then expnded with 5%Ni content. Further ddition of the ferrite stiliser, Mo enhnces corrosion resistnce nd improves the stility of the crides during the tempering het tretment. The effect of lloying dditions on 13Cr steels [10] is summrized in the tle 1 elow. The CCT digrm of 13%Cr-0.2%C [11] is shown elow in Fig. 2. It is seen tht irrespective of the quenchnt used for 170 di, the microstructure t the center of the r is 100% mrtensite. The Ms Temperture clculted y using the elow formul ws estimted to e ~ 300 C [12]. It is cler tht no retined ustenite is formed s the steel is devoid of Ni nd Mo. K1 =Cr23C6 ; K2= Cr7C3; Kc =(FeCr)3C Figure 1: Phse digrm of Fe-13% Cr lloy [9] Tle 1: Role of lloying dditions in 13%Cr steels [10] Alloy elements C Mn Si Cr 336 Purpose Strong ustenite stilizer nd enhnces strength Austenite stilizer nd solid solution strengthener Solid solution strengthener nd ferrite stilizer Promotes pssivtion nd stronger cride former. Increses the wer resistnce, hrdenility nd resist tempering y forg secondry chromium crides like Cr23C6 or Cr7C3. Also leds to intergrnulr corrosion.

3 The cride volume frction in the hrdened conditions decreses with incresing ustenitising temperture nd the volume frction ws lowest in the doule hrdened condition. Tle 3: Effect of ustenizing temperture on s quenched hrdness nd microstructure Figure 2: CCT digrm of 13Cr-0.2C steel for vrying dimeter [11] Results nd Discussion The chemicl composition of the 13%Cr steel mde for OCTG ppliction in this study shows very low S nd P content s shown in Tle 2. The steel is lso devoid of Ni nd Mo which plys vitl role for su zero impct toughness. Tle 2: Chemicl composition of the steel Austenizing Temperture 980 C/ 3h C/ 3 h C/ 3h 15 OQ+ 980 C/ 3 h 15 As quenched Remrks hrdness 430 BHN Presence of grin oundry cride chin. A remnnt of lth mrtensite structure is oserved; Cride Volume= 4.2%; ASTM grin size no= 6.5; 465 BHN Crides in the mtrix is very fine no cride chins. Mrtensite lths corsening seen; Cride Volume= 1.2%; ASTM grin size no = BHN Microstructure ws completely free of K 1 crides nd fine lth mrtensite ws uniformly present throughout the mtrix; Cride Volume= 0.45%; ASTM Grin size no = 7.5. Alloy C Mn Si S P Cr Actul Aprt from the recommended ustenising het tretment of 980 o C, the microstructure ws exed t higher ustenising tempertures of 1040 o C nd doule hrdening t 1040 o C o C. The hrdness, microstructure nd cride volume frction in the vrious smples in the s-quenched condition ws mesured nd compiled in Tle 3. The s-quenched hrdness of the vrious smples ustenitised t vrying conditions shows significnt vrition in hrdness from 430 BHN to 500 BHN in Tle 3. The smple hrdened t 980 o C shows the lest hrdness of 430 BHN, while hrdening t 1040 o C incresed the hrdness to 465 BHN. The sme mteril on doule hrdening [1040 C OQ C OQ] showed further improved hrdness of 500 BHN. The microstructures of the s-hrdened smples exed using opticl microscope show crides in lth mrtensitic mtrix s shown in Fig. 3 [-c]. The microstructure corresponding to smple het tretment nd hrdness is rought out in Tle 3. It is seen in Tle 3 tht t the low temperture hrdening t 980 o C, there re residul crides within the mtrix nd there is lso grin oundry necklce crides long the grin oundries in the mtrix. The results show tht t low temperture hrdening t 980 o C, the grin oundry cride previls long with some residul cride in the mtrix, while t 1040 o C there is good dissolution of crides. Doule hrdening gve lest cride content s it enles complete dissolution of the grin oundry cride. The decrese in cride volume frction indictes tht ustenitising ensures cride dissolution tht ultimtely results in mrtensite richer in cron content. Although the mrtensite morphology remins s lths, the dissolved cron in the mrtensite differs resulting in hrdness vrition. The prior ustenite ASTM grin size no in the vrious hrdening conditions is lso shown in Tle 3 nd Fig. 4 [-c].it is seen tht the grin size is higher t 1040 o C while it is lower for 980 o C nd lest in the cse of doule hrdening. The corsening of grin size occurs t 1040 o C long with cride dissolution. The finer grins t 980 o C re due to grin oundries getting pinned y the crides. The doule hrdening shows much finer grin size. This my e due to the high defect density introduced y first stge hrdening y lth mrtensite formtion which gets recrystllized to finer grin size during the second lower temperture hrdening. Thus, the doule hrdening my e one wy to refine the grin size. Similr results hve een reported y Li et l. [8]. The hrdening of the s-quenched structure my lso contriute to the fineness of grin size due to Hll Petch 337

4 reltion. Finer grin structure lso introduces higher disloction density due to fineness of lths otined during the hrdening het tretment. tretment [710 C/ 5h/oil cool +680 C/5h/ oil cool for 170 mm cross section smple]. Doule tempering is reported to temper the retined ustenite, if ny tht previls fter the first tempering. The mechnicl property in the tempered condition my e compred with the specifiction vlues in typicl OCTG ppliction. The results re shown in Tle 4. c Figure 3: Opticl microgrph of s quenched smples ustenizitized t () 980 C () 1040 (c) C [Villell Regent] c It is of interest to note the morphology nd distriution of the residul cride on hrdening. For this steel, the recommended het tretment cycle is 980oC hrdening. It is seen tht in this condition there is the presence of grin oundry necklce crides. The prevlence of this cride cn hve serious effect on the impct properties of the mteril. The SEM nlysis shows the cride s in Fig. 5. The mechnicl property of the steels ustenitised t vrious conditions of hrdening, were sujected to the sme tempering cycle throughout this study. The recommended tempering cycle for the steel is doule tempering het Figure 4: Grin size s per ASTM E112 oxidtion method comprison t () 980 C () 1040 C (c) C The hrdening temperture nd sequence ffect the steel properties. The yield strength vlue in the single hrdening t 980oC nd 1040oC respectively were 570 nd 640 MP which re t two extremes of specifictions [Tle 4]. The toughness vlues in the single hrdening were short of the specifiction requirement. Doule hrdening tretment gve strength t 338

5 ove 680oC, led to the formtion of the equilirium K1 crides [13]. The tempering temperture is high enough for cron diffusion which forms corsening of crides. When cride corsens, depletion of C from mtrix mkes the ferrite very tough. The finer grin size nd high defect density in doule hrdened steel enle finer cride nucletion during the doule tempering, which improves strength nd toughness. mid-rnge of specifiction nd toughness 45% more thn the specifiction requirement. Tle 4: Mechnicl properties in doule tempered condition[710oc+ 680oC] for vrying hrdening tretment Mechnicl properties Austenizing Temp o C 980 to 1040oC Hrd ness BHN YS MP UTS MP %E %RA CVN t -10oC (J) Elements CK CrK FeK Atomic% Figure 5: SEM imge showing Cr rich crides in smples ustenitized t 980 followed y oil quenching [Villell Regent] The property vrition in Tle 4, ws studied s function of microstructure in the tempered condition. The opticl microstructures of steel condition re shown in Fig. 6[-]. In oth the cse, single hrdening t 980oC nd doule hrdening (1040oC+980oC) there is no delt ferrite in the tempered mrtensitic microstructure. The single het tretment microstructure shows the presence of grin oundry crides nd very fine ferrite distriution. The ferrite lths re corser in the cse of doule hrdening. While the structure ppers more spherodized in Fig.6 () for single hrdening t 980 oc, the lths re still visile in the cse of doule hrdened smple [Fig.6 ()]. There is no necklce crides in doule hrdening s shown in Fig. 6(). Tempering of such high Cr mrtensitic stinless steels enles crides to nuclete nd grow in the high ngle grin oundries, while intrgrnulr crides form t the mrtensite lth oundries. Tempering t the temperture Figure 6: Tempered microstructure of () Single het tretment () Doule het tretment [Villell Regent The single hrdening t 980OC followed y tempering enles cride corsening long with pre-existing grin oundry necklce crides nd this leds to lower strength nd toughness. During the first stge of hrdening t 1040oC, the 339

6 crides effectively dissolve in the mtrix long with mrtensite lths during ustenizing s in Fig.2(c). When this steel with initil mrtensitic microstructure chieved in first quenching is sujected to second ustenitising, the grin structure is recrystllized from the defect mtrix of the mrtensite lths otined in the first quenching. Conclusions The conclusions drwn from the studies re s follows 1. The 13%Cr steel ws sujected to single hrdening tretment t 980oC cused residul grin oundry crides tht tend to impir impct toughness. 2. Modified het tretment involving doule quenching t 1040 oc oc give finer grin size long with higher dissolution of cron in ustenite mtrix. This results in improved strength nd superior toughness compred to single hrdening t 980 C. Acknowledgments The uthors wish to thnk the constnt encourgement nd support provided y Dr B Klyni, Chirmn & Mnging Director nd Mr R K Goyl, Mnging Director, Klyni Crpenter Specil Steels Ltd., Pune. References [1] Crig, B. D. Sour Gs Design considertions, SPE Monogrph No. 15, 1993 [2] Treseder, R.S. Tuttle, R.N. Corrosion Updte No. 1 CRAs in oil nd gs production, NACE, Houston.TX [3] ISO 15156/MR0175, Petroleum nd Nturl Gs industries- Mterils for use in H2S contining in oil nd gs production- Prt two crck resistnt CRAs nd other lloys. [4] Peterson, C.W. Requirements for corrosion resistnt lloy production tuings SPE Pper presented t 64th nnul fll technicl conference, 8-11 Octoer, 1992, Texs. [5] Rmirez, J. E., Weldility evlution of supermrtnesitic stinless steels pipes, Welding Journl, Vol. 86, 2007, pp 125s 134s. [6] Huizing, S. et.l Offshore 22Cr duplex stinless steel crcking- Filure nd prevention, NACE 2005, pper [7] Mrcheois, H. et.l Sour service limits of 13%Cr nd super 13%Cr stinless steel for OCTG: effect of environmentl fctor, NACE 09084, 2009 [8] Li, J.K. et l. Stinless steels nd introduction nd their recent development (Benthm EBooks, 2012), pp [9] Bhdeshi, H Steels: Microstructure nd properties, Butterworth Heinemnn [10] Brlow, L. D., Effect of ustenizing het tretment on microstructure nd hrdness of mrtensitic stinless steel AISI 420, Journl of mterils engineering nd performnce, Vol. 21(7), 2011 [11] Nehni, N. C. et.l, Filure of stinless steel pipe welds in fossil fuel power plnt, Eng. Filure nlysis, Vol 9(3): 2002, Figure 7: SEM imge of temper microstructure () showing necklce cride for single het tretment () sprsely distriuted cride in doule het tretment [Villell Regent] During tempering, very fine crides occur in low ngle nd high ngle oundries. The SEM microgrph in the doule hrdened nd doule tempered steel is shown in Fig. 7[]. The cride content is extremely less nd the grin oundry crides re extremely smll compred to single het tretment process shown in Fig. 7[]. This is one of the resons for significnt improvement in the impct toughness in the doule hrdened condition. Studying the influence of cride distriution s function of corrosion resistnce, Cndleri et.l [14], record tht cride dissolves t higher ustenitising temperture. Higher Cr in mtrix improves mtrix corrosion resistnce. During tempering fine crides produced my resist steel from corroding. In order to meet the su-zero impct toughness compnies tend to do micro lloying of the steel with expensive Ni nd Mo content which cn e voided y pproprite het tretment. 340

7 [12] Mintz, B. Arrowsmith, J. M., Impct properties of Fe- 13Cr thick pltes, Toughness of ferritic stinless steels, ASTM STP 706, 1980, pp [13] Brlow, L.D., Effect of ustenizing het tretment on microstructure nd hrdness of mrtensitic stinless steels AISI 420, Journl of Mterils Engineering nd performnce, Vol. 21(7), [14] Cndelri, A. F. et.l, Influence of het tretment on the corrosion resistnce of the mrtensitic stinless steel type AISI 420, Journl of mterils Science letters 22, 2003,