Sensitization behaviour of 11-12% Cr AISI 409 stainless steel during low heat input welding

Transcription

1 Univesity of Wollongong Resech Online Fculty of Engineeing nd Infomtion Sciences - Ppes: Pt A Fculty of Engineeing nd Infomtion Sciences 2011 Sensitiztion behviou of 11-12% C AISI 409 stinless steel duing low het input welding Conelis Jnise Vn Niekek Univesity of Petoi Mdeleine du Toit Univesity of Wollongong, mdt@uow.edu.u Publiction Detils Vn Niekek, C. J. & Du Toit, M. (2011). Sensitiztion behviou of 11-12% C AISI 409 stinless steel duing low het input welding. Jounl of the South Aficn Institute of Mining nd Metllugy, 111 (4), Resech Online is the open ccess institutionl epositoy fo the Univesity of Wollongong. Fo futhe infomtion contct the UOW Liby: esech-pubs@uow.edu.u

2 Sensitiztion behviou of 11-12% C AISI 409 stinless steel duing low het input welding Abstct The study compised of the investigtion into the sensitiztion chcteistics of AISI 409 titnium stbilized feitic stinless steel duing low het input welding. AISI 409 is fully feitic stinless steel used in ctlytic convetes nd tubing fo utomotive exhust systems due to the supposition tht sensitiztion does not occu duing low het input welding. Two pltes of 2 nd 4 mm wee tested fo sensitiztion duing low het input welding. The study confimed tht chomium cbide (M 23 C 6 ) pecipittion occus on the gin boundies in the het-ffected zone with consequent depletion of C djcent to the gin boundies duing welding. In the 2 mm plte sensitiztion occued in the het input nge of 0.1 kj/mm to bove 0.25 kj/mm nd in the 4 mm plte sensitiztion occued in the het input nge of 0.2 to 0.9 kj/mm. Ti stbiliztion is ineffective t the het inputs used in this investigtion due to the pid cooling te though the tempetue egion whee TiC pecipittes. The pesence of N ws found to be detimentl since it consumes Ti on cooling, foming TiN, effectively loweing the mount of Ti vilble fo TiC fomtion duing pid cooling fte welding. Anneling fo 5 minutes t 725 C does not impove the sensitiztion chcteistics of the welded plte. Thee is line incese in gin size s het input inceses. The esistnce to pitting coosion deceses in the sensitized, welded plte in the het-ffected zone djcent to the fusion line of welding. Keywods sensitiztion, duing, behviou, het, 409, stinless, isi, steel, c, welding, low, 12, input, 11 Disciplines Engineeing Science nd Technology Studies Publiction Detils Vn Niekek, C. J. & Du Toit, M. (2011). Sensitiztion behviou of 11-12% C AISI 409 stinless steel duing low het input welding. Jounl of the South Aficn Institute of Mining nd Metllugy, 111 (4), This jounl ticle is vilble t Resech Online:

3 Sensitiztion behviou of 11 12% C AISI 409 stinless steel duing low het input welding by C.J. vn Niekek* nd M. du Toit* Ppe witten on poject wok cied out in ptil fulfilment of BEng Metllugicl Engineeing J o u n l Synopsis The study compised of the investigtion into the sensitiztion chcteistics of AISI 409 titnium stbilized feitic stinless steel duing low het input welding. AISI 409 is fully feitic stinless steel used in ctlytic convetes nd tubing fo utomotive exhust systems due to the supposition tht sensitiztion does not occu duing low het input welding. Two pltes of 2 nd 4 mm wee tested fo sensitiztion duing low het input welding. The study confimed tht chomium cbide (M 23 C 6 ) pecipittion occus on the gin boundies in the het-ffected zone with consequent depletion of C djcent to the gin boundies duing welding. In the 2 mm plte sensitiztion occued in the het input nge of 0.1 kj/mm to bove 0.25 kj/mm nd in the 4 mm plte sensitiztion occued in the het input nge of 0.2 to 0.9 kj/mm. Ti stbiliztion is ineffective t the het inputs used in this investigtion due to the pid cooling te though the tempetue egion whee TiC pecipittes. The pesence of N ws found to be detimentl since it consumes Ti on cooling, foming TiN, effectively loweing the mount of Ti vilble fo TiC fomtion duing pid cooling fte welding. Anneling fo 5 minutes t 725 C does not impove the sensitiztion chcteistics of the welded plte. Thee is line incese in gin size s het input inceses. The esistnce to pitting coosion deceses in the sensitized, welded plte in the het-ffected zone djcent to the fusion line of welding. Keywods AISI 409 feitic stinless steel, sensitiztion, gin gowth, esistnce to pitting coosion, low het input welding. Intoduction The weldbility of AISI 409 is consideed fi nd in this ppe its sensitiztion behviou fte gs tungsten c welding (GTAW) ws investigted. AISI 409 is low cbon, 10.5 to 12 pe cent C, Ti stbilized, feitic stinless steel. It is used in pplictions such silences, ctlytic convetes nd tubing fo utomotive exhust systems. It hs the specified chemicl composition limits s shown in Tble I. Its melting tempetue is in the nge of C1. AISI 409 is utilized in ctlytic convetes in utomotive exhust systems in fvou of its good coosion esistnce, modete pice nd the supposition tht it hs good esistnce to sensitiztion duing theml cycles, thus eplcing luminized cbon steel. Gs metl c welding (GMAW) is used in the mnufctuing pocess. Due to the coosive envionment of the ppliction of AISI 409 integnul coosion (IGC) hs been detected in the het-ffected zone (HAZ) of weldments. The coosive tmosphee in n exhust system is minly due to condensed wte nd exhust gs contining Cl -, SO 4 2-, SO 3-, CH 3 COOH, CO 3 2- nd HCO 3-2. Figue 1 is coss-section of the teny Fe-C-C phse digm. The steel consideed hee is designed to pss, on cooling, though the digm to the ight of the (δ + λ) loop, i.e. it emins fully feitic fom its melting tempetue down to oom tempetue. Although AISI 409 hs mximum cbon content of 0.03 pe cent, Figue 1 is illusttive of the shpe of the ustenite nd (ustenite + feite) phse fields of the lloy. An incese in cbon content will enlge these phses t the expense of feite, yet is compensted fo by excess titnium (n excellent feite fome when in solid solution), shifting the λ nd (δ + λ) phse boundies to the left of the digm, which esults in the fully feitic (δ) mtix. As esult of the bsence of gin boundy ustenite/mtensite, AISI 409 undegoes gin gowth in the het-ffected zones (HAZ) of weldments. As consequence in the swelded stte it hs poo toughness, ftigue nd tensile popeties1,3. The cbon nd nitogen contents e kept to minimum to impove toughness, minimize sensitiztion of the steel nd to mintin feitic stuctue. Consequently, feitic stinless steels e not high-stength steels. In the nneled condition these steels hve yield stengths in the ode of MP4. AISI 409 is typiclly supplied in the nneled condition. Anneling is cied out between 700 C nd 750 C1. Duing the nneling of titnium stbilized stinless * Univesity of Petoi. The Southen Aficn Institute of Mining nd Metllugy, SA ISSN X/ Ppe eceived M. 2011; evised ppe eceived M P p e The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL

4 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Tble I Specified chemicl composition limits fo gde AISI 409 (% by mss, blnce Fe, single vlues e mximum levels) 1 C Si Mn P S C Ni N Ti Figue 1 Veticl section of the teny Fe-C-C phse digm fo steels with Cbon content less thn 0.01% 3 steels in this tempetue nge the mjoity of cbon nd titnium pecipitte homogeneously thoughout the mtix s highly stble titnium cbides (TiC), titnium nitides (TiN), o cbo-nitides (TiCN). This inhibits the pecipittion of sensitizing chomium-ich cbides due to cbon s highe ffinity fo titnium when comped to chomium, i.e. titnium cbide pecipittion equies lowe fee enegy (ΔG) to nuclete thn chomium cbides. This effectively stbilizes the steel ginst integnul coosion cused by sensitiztion3. Austenitic consumbles such s 308L, 309L nd 316L e ecommended s fille metls to impove the toughness of welds. In ode to limit gin gowth, it is genelly consideed beneficil to use minimum het inputs duing welding. Edge welds should be voided. When welding e.g. lp joints, mximum combined thickness of 3 mm should be mintined, i.e mm thickness on ech plte being joined by welding1,5. Litetue eview Accoding to Kim et l.2 sensitiztion to IGC cn occu in AISI 409 specimens, ged in the tempetue nge of C, which epesents the sensitiztion nose of the timetempetue-sensitiztion (TTC) digm t 600 C, especilly fte welding. Kim et l.6 lso found tht highe chomium contents decese the extent of IGC but does not pevent it. Vious models hve been poposed fo the sensitiztion of stinless steels. Sensitiztion by mens of chomium depletion due to the pecipittion of chomium-ich cbides is the most widely geed upon2,7. Accoding to this theoy cbides of the type M 23 C 6 o the (Fe,C) 23 C 6 pecipitte integnully whee sufce is vilble which will lowe the enegy equied fo nucletion of the pecipitte. This esults in chomium depletion of the mtix diectly djcent to the gin boundies on which the pecipittes fom. The steel becomes susceptible to integnul coosion when the chomium content in the ffected mtix djcent to the gin boundy e flls below tht equied fo pssivtion (in the ode of 11.5 pe cent C). It educes the coosion esistnce of the steel in the vicinity of the gin boundies nd the steel my suffe pefeentil ttck when exposed to coosive envionment s foementioned. In genel, chomium-ich M 23 C 6 pecipittes fom in feite in the tempetue nge of C8. Cbon solubility is lowe in feite thn in ustenite due to much moe densely pcked body-cented cubic (BCC) cystl stuctue which hs less intestitil spces thn the ustenitic fce-cented cubic (FCC) lttice. A consequence of this stuctue is tht cbon hs much fste diffusivity in feite thn in ustenite. This llows sensitiztion to occu moe esily nd pidly thn in ny othe stinless steel even when subjected to pid quench fte welding3,8. Due to the high diffusion tes in feite, cbides fom pidly duing nneling. In the s-supplied condition (fully nneled) these steels theefoe contin cbides, but e in the fully de-sensitized condition due to chomium diffusion into ny C-depleted zones duing het tetment. These steels cn only esensitize by being heted to tempetue bove the cbide dissolution tempetue (± 950 C) followed by pid cooling to pevent cbon bck-diffusion. A loweing of the cbon content to below even 0.01 pe cent does not pevent sensitiztion in feitic stinless steels; theefoe, stbilizing gents e dded such s Ti nd Nb. These stbilizes e dded in sufficient quntities to combine with most of the cbon in the steel. These elements minimize the fomtion of M 23 C 6 cbides but they lso educe the tempetue t which these cbides undego dissolution9. Duing nneling, Ti stbilized feitic stinless steel such s AISI 409 foms titnium cbides (TiC), which is stble t high tempetues. TiC pecipittion equies lowe diving foce thn M 23 C 6 nd theefoe Ti cts s stbilizsing gent ginst sensitiztion, esting ny chomium cbides tht would subsequently fom on cooling in n othewise nonstbilized stinless steel. Excess cbon is consumed in the supestuted feite mtix by pecipitting TiC t tempetues highe thn the M 23 C 6 pecipittion nge, hence no cbon is vilble to fom cbides with chomium t lowe tempetues APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy

5 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Equtions [1] nd [2] hve been estblished in litetue fo the detemintion of the ppopite Ti content fo successful stbiliztion of 12 pe cent C Steels, s follows11. %Ti > 4 * (%C + %N) [1] %Ti > 0.08% + 8 * (%C + %N) [2] These equtions pomote ove-stbiliztion. Duing high het input welding chomium bck-diffusion in feite gins plys n impotnt ole in mitigting sensitiztion. The sensitized es e given time to hel when chomium fom the gin inteios diffuse into the depleted nd sensitised zones until the pe cent C is gin bove the necessy level fo pssivtion, estoing the coosion esistnce which is due to the chomium oxide film C 2 O Fou modes of HAZ sensitiztion hve been identified in 12 pe cent C steel, most of them involving dul phse (γ + δ) stinless steel whee the ustenite in the steel tnsfom to mtensite duing the pid cooling tes ssocited with welding. Mode 3 sensitiztion coesponds to the sensitiztion found in titnium stbilized stinless steels with high Kltenhuse feite fcto (KFF) o fully feitic stinless steel such s AISI 40911,12,13. Mode 3 sensitiztion occus in the cose-gined egion diectly djcent to the fusion line. In dul phse feitic stinless steels this egion consists mostly of feite-feite gin boundies nd in the cse of AISI 409, fully feitic gins. The bsence of ustenite (which cts s cbon sink) in this egion nd the low solubility of cbon in feite, cuses the feite mtix to be supestuted with cbon. Chomium-ich cbides pecipitte extensively long gin boundies in the initil stges of cooling fte welding. Mode 3 cn theefoe occu long the gin boundies of the high tempetue het-ffected zone (HTHAZ) of weldments duing low het input, single-pss weld with fst subsequent cooling te, shllow weld toe cusps nd c stikes. The pid theml cycle of low het input welding does not llow fo chomium bck-diffusion to ffected es fom the gin inteios nd the HAZ contins continuous sensitized netwok of feite-feite gin boundies down to oom tempetue12,13,14. The chomium depleted o sensitized sites will undego nodic dissolution in coosive envionment, which esults in integnul coosion (IGC). In the pesence of consideble opeting nd esidul stesses this kind of coosive ttck is usully initited s pitting coosion. Individul pits eventully link up, esulting in integnul stess coosion ccking (IGSCC)11. Expeimentl pocedue Alloy composition Two pltes of AISI 409 of diffeent thickness (2 nd 4 mm) wee used in this investigtion, s the sensitiztion behviou is both function of plte thickness nd chemicl composition. Tbles II nd III gives the specific chemicl compositions of these pltes. Welding pocedue Autogenous (bed on plte welds without fille metl) welding ws pefomed on both pltes using gs tungsten c welding (GTAW) with negtive electode polity. Pue welding-gde gon shielding gs t flow te of 15 l/min ws used. The het input of the welds on the 2 nd 4 mm pltes wee nge of kj/mm nd kj/mm espectively. Eqution [3] ws used to clculte the het input (HI) of welding fo ech espective weld tht ws investigted. It incopotes the welding pmetes used whee V is the c voltge, I is the welding cuent (A), ν is the tvel speed of welding (mm/s) nd q is the powe o het flux (W). [3] The vege c efficiency (η) ws estimted t 47.5 pe cent which lies close to the uppe limit nomlly stipulted fo GTAW which is in the nge of 22 to 48 pe cent15. Tbles IV nd V lists the welding pmetes selected to poduce the desied het inputs of welding. Sensitiztion testing To test fo sensitiztion the ASTM Stndd Pctices fo Detecting Susceptibility to Integnul Attck in Feitic Stinless Steels (A ) wee used on both the 2 nd 4 mm thick pltes fte welding16. Pctice W equies 10% oxlic cid etch followed by investigtion using metllugicl micoscope. Etching ws cied out t 1 A/cm2 fo 1.5 minutes. It evels the pesence J o u n l P p e Tble II Chemicl composition of the 2 mm thick plte of AISI 409 (% by mss, blnce Fe) C Si Mn P S C Mo Ni Al Ti Nb Cu V < < Tble III Chemicl composition of the 4 mm thick plte of AISI 409 (% by mss, blnce Fe) C S P Mn Si Cu Ti Mo C Ni Al Nb V N B The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL

6 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Tble IV Het input nge of weldments on the 2 mm thick plte Weld Welding cuent (A) Ac potentil (V) Welding speed (mm/s) Powe (W) Het input (kj/mm) Tble V Het input nge of weldments on the 4 mm thick plte Weld Welding cuent (A) Ac potentil (V) Welding speed (mm/s) Powe (W) Het input (kj/mm) of chomium cbides by emoving ny cbide pesent on the sufce e of the micostuctue exposed to the oxlic cid solution duing etching. Theefoe, ny continuous nd dkened netwok of gin boundies is n indiction of the pesence of chomium-ich cbides. Pctice Z equies the smples to be boiled fo 24 hous in coppe-coppe sulphte-16% sulphuic cid solution. The smples e to be in contct with coppe clippings to induce glvnic effect between the steel nd the coppe to enhnce the coosion effect of the cid on the steel. Pevious wok cited the pmetes of the test too sevee8. Fo this eson the totl time of boiling ws educed to 4 hous nd the sulphuic cid ws used t concenttion of 6% H 2 SO 4. The smples wee then nlysed nd scnning electon micoscope (SEM) imges wee tken. Pctice Z evels the chomium depleted zones djcent to gin boundies. Enegy dispesive spectoscopy (EDS) ws done fte Pctice Z on titnium cbides, titnium nitides nd chomium cbides. Anneling A smple fom the 4 mm thick plte, welded t het input of 0.5 kj/mm ws nneled t tempetue of 725 C with holding time of 5 minutes. The smple ws then subjected to ASTM pctice Z to detemine the effect of nneling on the extent of chomium bck diffusion into C-depleted gin boundies. Gin size estimtion The vege gin dimete of the HAZ ws detemined fo ech smple of the 4 mm thick plte, welded t the het inputs stted in Tble V, with the Heyn Linel Intecept Pocedue specified in the ASTM Stndd Test Methods fo Detemining Avege Gin Size (E )17. Coosion tests The coosion, pitting nd potection potentils wee estblished fo the unwelded (s-eceived) plte nd smple welded t het input of 0.4 kj/mm using the Stndd ASTM pctice G The coosion test ws cied out in n Avest Cell (n electochemicl cell which limits the occuence of cevice coosion) with 3.5% NCl solution, deeted fo 30 minutes with nitogen gs pio to testing. A Ag/AgCl efeence electode ws used. The esults obtined fom the coosion tests wee comped to estblish the influence of sensitiztion nd welding on the coosion chcteistics of the steel. Het flow modelling In ode to quntify the effect of het input on the theml cycle, Rosenthl s het flow equtions of moving point het souce wee used to model conduction diven het flow in the het-ffected zones of the 4 mm welded smples 100 μm fom the fusion line19. Phse stbility egions simulted in FctSge TM softwe The theml cycle of welding is not n equilibium pocess but equilibium clcultions give good indiction of wht phses e stble t specific tempetues. The specific chemicl composition of the 4 mm steel ws used to pedict pseudo equilibium phse stbility digm using FctSgeTM softwe. The sme ws done with the titnium content hlved. Tempetues fom these digms wee used to pedict the time spent within ech phse stbility egion duing cooling fte welding, using the Rosenthl equtions. FctSgeTM ws lso used to pedict, t equilibium, the weight pecentge of TiN nd TiC tht would pecipitte duing cooling of the plte fom the molten stte. 246 APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy

7 Sensitiztion behviou of 11 12% C AISI 409 stinless steel XRD In ode to nlyse the pecipittes, five welding beds, welded in the egion whee sensitiztion ws found to be most polific, wee dissolved in solution of 35% HCl. The solution ws then filteed using glss fibe filte ppe with n vege petue dimete of 0.7 μm. The filteed poduct ws insed with distilled wte nd cetone to chieve gglometion of the filtte nd i died befoe XRD nlysis. Results nd discussion Figues 2 nd 3 shows the diffeence in the micostuctues (4 mm smples) of the s-eceived nd welded pltes. The smple of figue 3 ws welded t het input of 0.4 kj/mm nd both wee etched with 50 ml HNO 3, 50 ml HCl, nd 50 ml wte by submesion fo 90 seconds. 2 mm thick plte fte GTAW Pctice W Figues 4 7 e opticl micogphs tken with n opticl micoscope of the 2 mm thick 409 steel smples, welded t the het inputs listed in Tble IV, subjected to ASTM Pctice W. J o u n l P p e Figue 4 2 mm thick AISI 409 welded t het input of kj/mm (Pctice W) Figue 2 Typicl AISI 409 micostuctue in the s-eceived condition (etched with 50 ml HNO3, 50 HCL nd 50 ml wte fo 90 seconds). TiC pecipittes e lso visible Figue 5 2 mm thick AISI 409 welded t het input of 0.11 kj/mm (Pctice W) Figue 3 Fou distinct phses pesent on the micostuctue fte welding (4 mm plte welded t het input of 0.4 kj/mm nd etched with 50 ml HNO3, 50 ml HCL nd 50 ml wte) Figue 6 2 mm thick AISI 409 welded t het input of 0.14 kj/mm (Pctice W) The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL

8 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Figue 7 2 mm thick AISI 409 welded t het input of 0.15 kj/mm (Pctice W) Figue 4 (welded t het input of kj/mm) shows some gin boundies outlined by Pctice W, yet no gin boundies evel the pesence of chomium cbide pecipittion (M 23 C 6 ) in the HAZ. Figues 5 7 evel the pesence of integnul chomium cbide pecipittes in the high tempetue het-ffected zone (HTHAZ) close to the weld bed. Chomium cbides e indicted by dkened gin boundies whee the cbides wee ttcked (nd dissolved) by Pctice W. Pctice Z Figues 8 12 e photomicogphs tken on scnning electon micoscope (SEM) of the HAZ of the welded 2 mm smples tht wee subjected to the modified ASTM Pctice Z. Figue 8 is n unwelded nd unsensitised contol smple dded to the test fo comptive puposes. Figue 8 Unsensitized micostuctue of contol smple subjected to modified Pctice Z Figue 10 HAZ of the 2 mm thick AISI 409 welded t het input of 0.11 kj/mm (Modified Pctice Z) Figue 9 HAZ of the 2 mm thick AISI 409 welded t het input of kj/mm (Modified Pctice Z) Figue 11 HAZ of the 2 mm thick AISI 409 welded t het input of 0.14 kj/mm (Modified Pctice Z) 248 APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy

9 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Figue 9 compes well with Figue 8 in tht no gin boundy ditching is evident. None of the es djcent to the gin boundies expeienced nodic dissolution duing Pctice Z. This is n indiction of the bsence of chomium depletion in these es, coesponding to Figue 4. Figues 9 12 show extensive gin boundy ditching. It is evident tht the es djcent to the gin boundies wee depleted of chomium whee chomium cbides pecipitted integnully. Whee the C content dopped below the concenttion equied fo pssivtion (bout 11.5 pe cent) these es undewent nodic dissolution duing the implementtion of the modified Pctice Z. Cooling though the tempetue nge whee C toms e mobile in the feite mtix is too pid fo C fom within the gin inteios to diffuse bck to those es depleted of C. 4 mm thick plte fte GTAW Pctice W Figue 13 4 mm thick AISI 409 welded t het input of 0.1 kj/mm (Pctice W) Figue 15 4 mm thick AISI 409 welded t het input of 0.50 kj/mm (Pctice W) Figue 14 4 mm thick AISI 409 welded t het input of 0.20 kj/mm t highe mgnifiction (Pctice W) Figue 16 4 mm thick AISI 409 welded t het input of 0.60 kj/mm t highe mgnifiction (Pctice W) The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL Figue 12 HAZ of the 2 mm thick AISI 409 welded t het input of 0.25 kj/mm (Modified Pctice Z) Figues e opticl micogphs tken with n opticl micoscope of the 4 mm thick 409 plte. These smples wee welded t the het inputs given in Tble V, subjected to ASTM Pctice W. J o u n l P p e

10 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Evident fom Figue 13 no chomium cbides pecipitted t het input of 0.1 kj/mm. In the het input nge of kj/mm, chomium cbides pecipitted integnully in the het-ffected zones. At het input of 1.0 kj/mm thee is gin n bsence of integnul chomium cbides. The bsence of cbides t het input of 0.1 nd 1.0 kj/mm is scibed to the cooling te. At het input of 0.1 kj/mm (Figue 18) the cooling te is too pid fo cbide pecipittion nd t 1.0 kj/mm the cooling te is such tht enough time is llocted in the theml cycle of welding to llow fo titnium stbiliztion to be effective. Modified pctice Z Figues e micogphs tken on the SEM of the HAZ of the welded smples of the 4 mm thick plte fte the ppliction of the modified Pctice Z, welded t the espective het inputs stted in Tble V. Figues 19 (het input of 0.1 kj/mm) nd 23 (het input of 1.0 kj/mm) shows no ditching of the gin boundies, which indictes tht no chomium depletion djcent to gin boundies took plce which is in tun indictive of no cbides (M 23 C 6 ) pecipitted on the gin boundies. This coesponds to Figues 13 nd 18 of Pctice W of smples Figue 19 4 mm thick AISI 409 welded t het input of 0.10 kj/mm (Modified Pctice Z) Figue 20 HAZ of 4 mm thick AISI 409 welded t het input of 0.20 kj/mm t lge mgnifiction (Modified Pctice Z) Figue 17 4 mm thick AISI 409 welded t het input of 0.70 kj/mm (Pctice W) Figue 18 4 mm thick AISI 409 welded t het input of 1. 0 kj/mm (Pctice W) Figue 21 4 mm thick AISI 409 welded t het input of 0.32 kj/mm (Modified Pctice Z) 250 APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy

11 Sensitiztion behviou of 11 12% C AISI 409 stinless steel welded t the sme het input levels. Also t highe het input levels nd consequent slowe cooling tes, chomium bck diffusion plys ole in heling the es tht did indeed undego chomium depletion due to cbide pecipittion. Figues shows extensive gin boundy ditching whee the e djcent to the gin boundies wee depleted of chomium below the level necessy fo pssivtion. Anneling fte welding of 4 mm thick AISI 409 plte A smple, welded t 0.5 kj/mm tht eveled cbide pecipittion though Pctice W nd chomium depletion of the es djcent to the gin boundies though the modified Pctice Z, ws nneled fte welding t tempetue of 725 C fo 5 minutes. The smple ws i cooled nd subjected to the modified Pctice Z, to investigte the extent of chomium bck diffusion in the nneling tempetue nge. Figues 24 nd 25 wee tken fte the modified Pctice Z on the SEM. Figue 24 is n electon bcksctteed imge of the HAZ. Fom Figues 24 nd 25 it is evident tht nneling t these conditions is not sufficient to induce chomium bckdiffusion successfully s sensitiztion occus in the welding bed nd chomium depletion is still pevlent within the HAZ. A double nneling tetment, i.e. two nneling cycles t 725 C fo 5 minutes ech with cooling in between, my be beneficil. Chemicl nlyses on TiC, TiN nd M23C6 Figues 26 nd 27 show titnium cbides (TiC) nd titnium nitides (TiN) tht wee obseved on the micostuctues of the welded 4 mm thick plte fte the ppliction of Pctice W nd the modified Pctice Z espectively. J o u n l P p e Figue 22 HAZ of 4 mm thick AISI 409 welded t het input of 0.50 kj/mm t lge mgnifiction (Modified Pctice Z) Figue 24 Electon bck-sctteed micogph of the HAZ of 4 mm thick AISI 409 plte welded t het input of 0.50 kj/mm nd nneled t 725 C fo 5 minutes (Modified Pctice Z) Figue 23 4 mm thick AISI 409 welded t het input of 1.0 kj/mm (Modified Pctice Z) Figue 25 4 mm thick AISI 409 welded t het input of 0.50 kj/mm nd nneled t 725 C fo 5 minutes showing sensitiztion of the welding bed (Modified Pctice Z) The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL

12 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Titnium cbides ppe s cubic pecipittes vied in size nd t ndom positions though the feite gins nd on some gin boundies. TiC lso nucletes on ledy existing titnium contining pecipittes like TiN, which nucletes nd eches stbility befoe TiC duing cooling. This is evident in Figues 28 nd 29 whee n EDS ws pefomed on two diffeent points on such pecipitte. In Figue 29 it is ppent tht the dk egion in the cente of the cubic pecipitte contins nitogen nd the light oute egion contins cbon. XRD confimed the pesence of TiN nd TiC in the weld bed nd het-ffected zones of the welded smples. An EDS scn ws pefomed on chomium cbide found long gin boundy in the HAZ of smple tht ws subjected to the modified Pctice Z. The pticle is shown in Figue 30 nd the EDS in Figue 31. Phse stbility egions simulted in FctSge TM softwe A phse stbility digm, Figue 32, showing the egions whee the phses of inteest to this study e stble duing equilibium cooling t vying chomium contents ws simulted using FctSgeTM softwe. The digm is bsed on the specific chemicl composition s given in Tble III. Figue 28 TiC nd TiN in feite gins fte Pctice Z (weld het input 0.8 kj/mm) Figue 26 TiC nd TiN in feite gins fte Pctice W (weld het input 0.6 kj/mm) Figue 29 TiC nd TiN EDS on SEM fte Pctice Z (weld het input 0.8 kj/mm) Figue 27 TiC nd TiN in feite gins fte modified Pctice Z (weld het input 0.4 kj/mm) Figue 30 M23C6 on ditched gin boundy fte Pctice Z (weld het input 0.8 kj/mm) 252 APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy

13 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Accoding to Figue 32, AISI 409 with C content of 11.4 pe cent will solidify fom the liquidus tempetue s feite nd TiN t tempetue just bove C followed by the fomtion of TiC t tempetues below C. This stuctue emins stble down to oom tempetue, nd suggests tht, unde equilibium cooling conditions, the steel contins enough Ti to pevent sensitiztion, hence no chomium cbide pecipittion is pedicted. At the non-equilibium cooling conditions of the welding theml cycle, the wok piece cools too pidly fo Ti stbiliztion to be effective. This ws simulted in FctSgeTM by hlving the Ti content of the oiginl chemicl composition in the equilibium digm of Figue 33. The fomtion of chomium cbides is evident in Figue 33 with ineffective titnium stbiliztion. Pdo et l.20 found tht the genel equtions used to pedict the wt% Ti equied fo successful stbiliztion (Equtions [2] nd [3]) is indequte t non-equilibium cooling to pevent the fomtion of chomium ich M 23 C 6. This is due to TiN fomtion nd pid cooling t the elevted tempetues whee TiC becomes stble in the mtix nd stt to pecipitte. On the phse digm of Figue 32 it is shown tht, on cooling, TiN fomtion occus befoe TiC fomtion which coesponds well to the micogphs of Figues 26 nd 28 whee TiC ws obseved to hve nucleted on existing TiN pecipittes. The fomtion of TiN consumes Ti, which esults in moe C vilble fo the fomtion of M 23 C 6, lso evident in Figue 33. Figue 34 is simultion (using FctsgeTM) of the mounts of TiN nd TiC tht would fom unde equilibium cooling conditions fo the chemicl composition of the 4 mm thick plte stted in Tble III. Het flow modelling fo non-equilibium cooling fte welding Using Rosenthl s theoy on moving point het souces, it ws possible to model the het flow of the weld theml cycle of the 4 mm plte t the vious het inputs. A point ws selected t 0.1 mm wy fom the fusion line t which to model the het flow, the point flls within the high tempetue het-ffected zone of ech welded smple. Figue 35 shows the diffeence in cooling te esulting fom the mximum nd minimum het inputs used in this investigtion. Evident fom Figue 35, s the het input inceses, the cooling te deceses. This llows moe cooling time though the tempetue nge whee TiC pecipittes, thus less M 23 C 6 J o u n l P p e Figue 31 M23C6 EDS on SEM fte Pctice Z (weld het input 0.4 kj/mm) Figue 33 Phse stbility digm fo AISI 409 simulted using FctSge TM softwe t hlf the titnium content Figue 32 Phse stbility digm fo AISI 409 simulted using FctSge TM softwe Figue 34 Wt% TiN nd TiC t equilibium cooling s function of cooling tempetue simulted using FctSge TM softwe The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL

14 Sensitiztion behviou of 11 12% C AISI 409 stinless steel fom on cooling. The slowe cooling te lso llows moe time fo C bck-diffusion to the es djcent to gin boundies tht hve been depleted of C nd pssivity is estoed whee the C content hve dopped to such n extent tht pssivity ws lost. By using Rosenthl s theoy in conjunction with the phse stbility digms simulted in FctSgeTM the mount of time spent on cooling in ech phse stbility egion could be estimted fo ech espective het input weld. The tempetue nge in which TiC is stble on cooling ws tken to be C nd tht in which M 23 C 6 is stble ws tken to be C. This esulted in the two espective cuves of Figue 36. Figue 36 shows tht the time spent in the egion whee M 23 C 6 pecipittes is vitully double the time spent in the egion whee TiC pecipittes. The holding times in the egion whee TiC becomes stble is evidently not enough fo Ti to successfully combine with ll of the fee cbon in the mtix t fst cooling tes. Due to cbon s pid diffusion te though the mtix t elevted tempetues enough C toms pile up in the gin boundies whee C is emoved fom the djcent es to fom stble chomium cbides. Gin size estimtion The vege gin size of the HTHAZ of ech welded smple of the 4 mm plte ws detemined ove the nge of het inputs using the Heyn Linel Intecept Pocedue. The smples peped fo the mesuement of gin size wee etched by submesion in 50 ml HNO 3, 50 ml HCL nd 50 ml wte solution fo 2 minutes. Figue 37 shows the men linel intecept, o vege gin dimete, s function of the het input of welding. It cn be seen tht s the het input inceses the vege gin dimte inceses s line function of the het input. Indicted on Figue 37 is the pe cent eltive ccucy. On welding AISI 409 t low het inputs compomise needs to be eched between sensitiztion nd the mount of gin gowth tht cn be toleted. Figue 38 shows the vege gin boundy e within volume of 1 mm3 of the HTHAZ tht ws detemined using Eqution [4]. l being the men linel intecept. Figue 38 indictes how, s het input of welding inceses, the e occupied by sensitized gin boundies deceses. The gin boundy e is diect function of the vege gin dimete nd het input. It lso incopotes the gin boundy e of the s-eceived nd nneled smples. Thee is mked diffeence between the gin [4] Figue 35 Clculted tempetue cycles expeienced by the high tempetue het-ffected zone t point locted 100 µm fom the fusion line fo het inputs of 0.1 kj/mm nd 1.0 kj/mm nd plte of thickness 4 mm Figue 37 Men linel intecept vs. het input of welding Figue 36 Clculted mount of time spent in TiC nd M23C6 stbility nge on cooling fte welding fo smples 4 mm thick plte welded t het inputs nging fom 0.1 kj/mm to 1.0 kj/mm Figue 38 Gin boundy sufce e Sv (mm 2 ) pe unit volume (1 mm 3 ) of the HTHAZ s function of the het input of welding 254 APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy

15 Sensitiztion behviou of 11 12% C AISI 409 stinless steel Tble VI Diffeence in coosion, pitting, potection potentils nd esistnce to pitting coosion between the s-eceived nd the welded plte in tems of the Ag/AgCl efeence electode ΔEco (VAg/AgCl) ΔEpit (VAg/AgCl) ΔEpot (VAg/AgCl) Δ(Epit - Epot) (VAg/AgCl) J o u n l boundy e of the unwelded (s-eceived) smple nd the gin boundy e of the smple welded t het input of 0.1 kj/mm. Effect of sensitiztion on coosion chcteistics Coosion tests wee cied out on the s-eceived plte nd on smple of the plte welded t 0.4 kj/mm. Tble VI shows the diffeence in coosion, pitting nd potection potentils between the s-eceived nd welded smples in tems of the Ag/AgCl efeence electode. Fo the welded plte, pitting coosion will stt t lowe potentil thn the unwelded plte, indicting the esistnce to pitting coosion ws educed duing welding with V. This gives n indiction tht the welded smple, which ws welded in the het input nge whee sensitiztion ppeed polificlly, hs comptively poo coosion chcteistics. This cn diectly be scibed to the sensitiztion of the HAZ of the weld. Conclusions AISI 409 sensitizes duing low het input welding. Fo plte thickness of 2 mm, sensitiztion occus fo het inputs of 0.1 to bove 0.25 kj/mm, nd fo 4 mm plte thickness in the 0.2 to 0.9 kj/mm het input nge. Due to the fste cooling te of thicke sections the 4 mm steel expeienced sensitiztion ove much lge het input nge thn the 2 mm steel. TiN is stble t tempetues below C, theefoe thee is decese in the vilbility of Ti to fom TiC nd incesed M 23 C 6 pecipittion. Rpid cooling though the TiC stbility egion limits the effectiveness of Ti stbiliztion. HAZ gin size is diectly popotionl to the het input. Anneling t 725 C fo 5 minutes does not necessily impove the sensitiztion chcteistics of the micostuctue. Afte nneling, the weld bed ws sensitised nd gin boundy ditching nd gin dopping ws still evident in the het-ffected zone. Due to sensitiztion pitting coosion initites t lowe potentil in the welded smple thn in the unwelded plte nd the esistnce to pitting coosion is educed duing welding. Refeences 1. CS409 Technicl Dt. Technicl Mnul published by Columbus Stinless KIM, J.K., KIM, Y.H., UHMB, S.H., LEE, J.S., nd KIM, K.Y. Integnul coosion of Ti-stbilized 11 wt% C feitic stinless steel fo utomotive exhust systems. Gdute Institute of Feous Technology, Pohng Univesity of Science nd Technology, Pohng, Koe, FOLKHARD, E. Welding Metllugy of Stinless Steels. Spinge-Velg, Vienn, vol. 3, no. 11, 1988, pp LESLIE. The Physicl Metllugy of Steels. McGw-HILL, 1983, p. 326, GROBLER, C. Weldbility studies on 12% nd 14% chomium steels. PhD dissettion, Univesity of Petoi, South Afic, KIM, J.K., KIM, Y.H., LEE, J.S., nd KIM, K.Y. Effect of chomium content on integnul coosion nd pecipittion of Ti-stbilized feitic stinless steel. Gdute Institute of Feous Technology, Pohng Univesity of Sciences nd Technology, Pohng, Koe, TULING, A. EELS study of sensitiztion in 12% chomium steel. Poceedings of the Micoscopy Society of Southen Afic 31, 2001, p GREEFF, M.L. The influence of welding pmetes on the sensitiztion behviou of 3CR12.MSc thesis. Univesity of Petoi, South Afic, LO, K.H., SHEK, C.H., nd LAI, J.K.L. Recent developments in stinless steels. Univesity of Hong Kong, Hong Kong, Chin, MESSLER, R.W. J. Pinciples of Welding. Wiley-VCH, 2004, pp. 550, , WILLIAMS, J.G., nd BARBARO, F.J. Susceptibility nd Pevention of Weld HAZ Sensitiztion nd Integnul Stess Coosion Ccking in Vious 12% C steels. Austlsin Welding Jounl vol. 50(4th qute), 2005, pp DU TOIT, M., VAN ROOYEN, G.T., nd SMITH, D. An oveview of the hetffected zone sensitiztion nd stess coosion ccking behviou of 12% chomium type feitic stinless steel. Coosion, vol. 63, no. 5, VAN WARMELO, M., NOLAN, D., nd NORRISH, J. Mitigtion of sensitiztion effects in ustbilised 12%C feitic stinless steel welds. Fculty of Engineeing, Univesity of Wollongong, Austli, GREEFF, M.L. nd DU TOIT, M. Looking t the sensitiztion of 11-12% chomium EN stinless steels duing welding. Welding Jounl, vol. 85, no. 11, 2006, pp. 243s 251s. 15. EASTERLING, K. Intoduction to the Physicl Metllugy of Welding. 2nd Edition, Buttewoth-Heinemnn, 1992, p ASTM A Stndd Pctices fo Detecting Susceptibility to Integnul Attck in Feitic Stinless Steels. ASTM Intentionl, ASTM E Stndd Test Methods fo Detemining Avege Gin Size. ASTM Intentionl, ASTM G Stndd Test Method fo Electochemicl Citicl Pitting Tempetue Testing of Stinless Steels. ASTM Intentionl, ROSENTHAL, D. The theoy of moving souces of het nd its ppliction to metl tetments. Tnsctions of the AIME 68, 1946, pp PARDO, A., MERINO, M.C., COY, A.E., VIEJO, F., CARBONERAS, M. nd ARRABAL, R. Influence of Ti, C nd N concenttion on the integnul coosion behviou of AISI 316Ti nd 321 stinless steels. Deptmento de Cienci de Mteiles, Fcultd de Ciencis Quı mics, Univesidd Complutense de Mdid, Mdid, Spin, P p e The Jounl of The Southen Aficn Institute of Mining nd Metllugy VOLUME 111 APRIL

16 Sensitiztion behviou of 11 12% C AISI 409 stinless steel 256 APRIL 2011 VOLUME 111 The Jounl of The Southen Aficn Institute of Mining nd Metllugy