Effect of Boron on the Microstructure Low-Carbon Steel Resistance Seam Welds

Transcription

1 WELDING RESEARCH SUPPLEMENT I 249-s f Effect f Brn n the Micrstructure Lw-Carbn Steel Resistance Seam Welds Small levels f brn in steel lead t hard weld metal micrstructure and therefre must be cnsidered in a weld specificatin BY S. S. BABU, G. M. GOODWIN, R. J. ROHDE AND B. SIEIEN ABSTRACT. Small levels f brn (20-40 wt ppm) in steel were fund t have a large influence n the micrstructure and hardness f resistance seam welds. The decmpsitin kinetics f austenite t ferrite were retarded and resulted in the frmatin f bainitic and martensitic micrstructures in the weld metal. The welds with lw and high brn cncentratins shwed large differences in hardness. This micrstructure develpment is in agreement with theretical calculatins f time-temperature-transfrmatin diagrams. This wrk stresses the imprtance f cnsidering brn levels in steels n the weld prperties. Intrductin The resistance seam welding prcess fr steel sheets is well established. It is pssible t prduce high-quality welds with prper welding parameters such as electrde gemetry, wheel frce, weld time, welding current and cntact verlap (Ref. 1). This prcess is being extensively used t fabricate heat exchanger panels used in electrical transfrmers. The welding parameters are designed t prduce defect-free liquid-tight jints with the desired weld gemetry. Besides the physical attributes, the strength f these welds needs t be maintained at cnstant level. The strength f the welds is related t the micrstructural evlutin during welding. In turn, the micrstructural evlutin in these welds depends n the steel cmpsitin and the weld S. S. BABU and G. M. GOODWIN are with Metals and Ceramics Divisin, Oak Ridge Natinal Labratry, Oak Ridge, Tenn. R. J. ROHDE and B. SIELEN are with Waukesha Electric Systems, Waukesha, Wis. thermal cycle. Fr example, fr a given steel cmpsitin, faster weld cling will lead t harder micrstructural cnstituents such as bainite and martensite. In cntrast, slw weld cling will lead t sft ferritic micrstructures (Ref. 2). Therefre, fr a given steel cmpsitin and welding prcedure specificatin, the micrstructure and prperties in weld metal and the heat-affected-zne (HAZ) are expected t be similar. Hwever, in rutine weldability evaluatins f tw similar steels frm different suppliers, anmalus weld micrstructural evlutin was bserved. Fr similar welding cnditins, ne f the steels exhibited sft ferritic micrstructure and the ther steel exhibited hard bainitic and martensitic micrstructure. This wrk evaluates the reasns fr the abve effect. Experimental Tw ht-rlled steel sheet materials (A and B), prcured t the same specificatin (Table 1) and used fr fabricatin f heat exchanger weld panels, were as- KEY WORDS Resistance Seam Welds Lw-Carbn Steel Decmpsitin Kinetics Austenite Ferrite Brn Time-Temperature- Transfrmatin Diagrams sessed. The actual cmpsitin f these steels is given in Table 1. The cmpsitin f the steels was measured by standard spectrscpic techniques. Brn chemical analyses were validated by cmparing with NIST standards. The steel sheets were welded in a resistance seam welding machine after standard industry cleaning practice. The welding parameters were as fllws: material thickness, 1.5 mm; wheel frce, N ( Ib); cntact area f wheels, 19 x 7.62 mm (0.75 x 0.3 in.); welding current, 18,000-20,000 A; welding speed, 3.2 m/min -1 (126 in./min-0. Transverse sectins f the welds were characterized with ptical micrscpy. The samples were etched with 2% Nital slutin. The hardness (Vickers pyramid hardness) variatins acrss the weld and the base metal were measured using a cmmercial autmatic hardness tester. The hardness indents were made at equal intervals f 150 lam ver an area f 6000 x 600 pm with 200-g frce. T evaluate austenite-t-ferrite decmpsitin in a cntrlled thermal cycle, Gleeble thermal simulatin was perfrmed. Small rectangular specimens were cut frm the steel sheets and subjected t thermal simulatin. The samples were heated t an austenitizing temperature f 00 C and were allwed t cl at a rate f -70 C s-1. Hwever, n artificial cling methd was used t maintain the specified cling rate. Due t the evlutin f the heat f transfrmatin, a sudden change in the actual cling rate f the sample was bserved. T mnitr such change, the actual temperature f the sample was mnitred cntinuusly during cling. The temperature at which this change ccurs is taken as the austenite-t-ferrite decmpsitin start temper-

2 250-s I IUNE 1998 Table 1--Steel Cmpsitins (wt-%) Element Specificatin C Mn P maximum S Si maximum Ni Cr AI V B <0.0005(a) (5 wt. ppm) (b) (20-40 wt.% ppm) N Fe (a) Belw detectable level, that is 5 wt. ppm. (b) Brn levels are validated by cmparing with NIST standards. The range was encuntered in different lcatins in. Fig Cmparisn f base metal micrstructure. A -- ; B-. ature; the lwer the transfrmatin start temperature, the higher the pssibility fr the frmatin f hard bainitic and martensitic micrstructures. T understand and delineate the effect f steel cmpsitin n austenite-tferrite decmpsitin kinetics, the timetemperature-transfrmatin (TTT) diagrams were calculated using a published mdel (Ref. 3). It is imprtant t nte that the diagrams are nt specific t heataffected zne r fusin zne. These dia- 1. The calculatins can be accessed via the Wrld Wide Web at grams give an verview f austenite-t-ferrite decmpsitin behavir. The mdel allws ne t calculate the time (t) fr the initiatin f ferrite transfrmatin frm austenite. This time is related t the chemical driving frce fr nucleatin f ferrite, which is a functin f weld metal cmpsitin (C, Si, Mn, Ni, Cr and M) and temperature. The reader is directed t Ref. 3 fr mre details 1. Thermdynamic calculatins with ThermCalc sftware (Ref. 4) were perfrmed t estimate the slid sluble cncentratin f brn. The TTT diagrams are then used t calculate the cntinuus cling transfrmatin (CCT) diagrams using the additivity rule (Ref. 5). Results and Discussin Micrstructural Characterizatin The base metal micrstructures f Steels A and B are cmpared in Fig. 1. There is n distinct difference either in ferrite grain size r ferrite mrphlgy. This is expected because bth steels were manufactured by a standard ht rlling peratin. The hardness f was fund t be VPH and that f was fund t be VPH. This shws that bth the steels have essentially identical strength levels. Since the cmpsitins f Steels A and B (Table 1) are similar, the micrstructure and prperties f the welds might be expected t be the same. Hwever, the macr- and micrstructure f seam welds in Steels A and B shwed majr differences -- Fig. 2. The weld nugget size f Weld B was fund (based n many transverse sectins) t be larger than Weld A. The reasn fr the large weld zne is nt clear. It is speculated that the cntact resistance f Steel B may be higher than and might have resulted in larger nuggets. Further wrk is necessary t understand the nugget frmatin in these tw steels. This difference in nugget size is expected t change the micrstructural evlutin by altering the thermal cycle. Since the weld nugget size f Weld B is larger than Weld A, the cling rate in Weld B is expected t be lwer than in Weld A. This difference in cling rate is expected t prduce sfter ferritic micrstructure in Weld B. Hwever, cntradictry micrstructural results were bserved. Careful bservatin f Weld B at high magnificatin (Fig. 2D) revealed predminantly bainitic and martensitic micrstructure in the weld metal regin. In cntrast, the micrstructure f Weld A (Fig. 2C) cntained predminantly al- Itrimrphic ferrite and small amunts f bainite and martensite in the weld metal regin. Optical micrscpy revealed n significant difference in prir austenite grain size between the welds. In the fusin zne regin, in all the lcatins, there was a negligible amunt f alltrimrphic ferrite alng the austenite grain bundary. Besides the micrstructural differences, the hardness measurements als shwed differences between Welds A and B. The hardness values acrss the weld regin are cmpared in Fig. 3. The measured area extended ver the base metal, the heataffected zne and the weld zne. The plt shws that the weld metal regin in Weld B is harder than in Weld A. The abve micrstructural and hardness variatins indicate that, even with slw cling rates and similar steel cmpsitins, Weld B cnsists f harder bainitic and martensitic micrcnstituents. The abve result apparently cntradicts the well-established fact that a slwer weld cling rate will lead t a sfter ferritic micrstructure fr a given steel cmpsitin. Since there was uncertainty in the weld thermal cycle experienced by Welds A and B during resistance seam welding, the austenite-t-ferrite transfrmatin temperature was studied during simulated cling cnditins. Mrever, predictive mdels were used t esti mate the austenite-t-ferrite decmpsitin behavir as a functin f steel cmpsitin. Gleeble Thermal Simulatin Bth Steels A and B were prgrammed t identical thermal cycling. The actual n-cling temperatures f

3 WELDING RESEARCH SUPPLEMENT I 251-s the samples were measured during the cling cycle -- Fig. 4. With the initiatin f transfrmatin, the heat f transfrmatin is released. This heat reduces the cling rate f the sample because there are n artificial cling methds in the present experiment. The transfrmatin start temperatures (at which the change in cling rate ccurs) were derived frm these measurements. The transfrmatin start temperature f Steel A is fund t be ~820 C and that f Steel B -700 C. The results demnstrate that the hardenability f is higher than that f. Therefre, it is necessary t delineate the pssible cause fr this difference. Calculatin f Austenite-t-Ferrite Decmpsitin The austenite decmpsitin kinetics are influenced by the cncentratin f varius elements including C, Mn, Ni, Si, Cr, M and B. Careful examinatin f the cmpsitin f Steels A and B shwed differences in cncentratins f C, Mn, P, S, Ni and B. The small differences in cncentratins f Mn, P, S and Ni are nt expected t change the decmpsitin kinetics t a great extent (Ref. 3); hwever, it is well knwn that small changes in C and B may have a large influence n the austenite-t-ferrite decmpsitin kinetics (Ref. 6). The effect f varius elements n the austenite-t-ferrite decmpsitin can als be inferred frm carbn equivalent frmulas (Ref. 7). The riginal mdel develped by Bhadeshia (Ref. 3) allws ne t estimate the TTT diagrams fr a steel cmpsitin as a functin f C, Si, Mn, Ni, Cr and M cncentratin. In this mdel, the effect f brn was nt cnsidered. Therefre, the calculatins fr Steels A and B (Fig. 5) will reflect nly the effect f small differences in carbn cncentratin. The plt shws that the difference in carbn cncentratin des nt change the TTT behavir significantly. Previus wrk by Bhadeshia, etal. (Ref. 8), has shwn that the grwth rate f ferrite can be suppressed a t large extent by increasing carbn cncentratin, thereby reducing alltrimrphic ferrite fractins. AIItrimrphic ferrite thickness was estimated fr bth Steels A and B, fr a dwell time f 1 s at 700 C, with the methd given in Ref. 8. As expected, the calculated ferrite thickness fr was 6.2 lam and that f was 3.6 IJm. Hwever, in the present wrk, the welds shwed a negligible amunt f alltrimrphic ferrite. Therefre, the change in carbn may nt be sufficient enugh t induce the bserved difference in weld micrstructure. Recently, the riginal TTT mdel has been mdified by Bhadeshia and Svenssn (Ref. 9) t cnsider the brn effect. In this mdel, the initiatin f transfrmatin t alltrimrphic ferrite frm austenite is retarded as a functin f brn cncentratin in slid slutin in the austenite. In their wrk, the brn in slid slutin was estimated by cnsidering thermdynamic equilibrium between austenite, TiN and BN at 900 C. Mrever, abve wt ppm brn in slutin, the brn effect was fund t be cnstant. In the present experiment, the amunt f brn in slutin will depend n the austenitizatin temperature. T estimate the brn in slutin within the austenite, equilibrium calculatins were perfrmed with ThermCalc. Recently, Bang and Ahn (Ref. ) als estimated the sluble brn by slving the slubility equatins f TiN, AIN and BN. Hwever, in this wrk, the phase equilibria between austenite, carbnitrides (Ti,V[C,N]), AIN and BN were cnsidered and it allwed fr estimatin f the vlume fractins f these phases. The calculated brn cncentratins in slutin as a functin f temperature fr Steels A and B, with different nminal brn levels, are shwn in Fig. 6. The brn in slutin fr the austenitizing temperature f 00 C fr (with 20 wt ppm B) was fund t be 0.34 wt ppm. In cntrast, if the nminal cncentratin f brn is 40 wt ppm B, the brn in slutin increases t wt ppm. Bth 20 and 40 wt ppm brn were measured in different lcatins in. The nminal cncentratin f brn in Steel A was assumed t be 5 wt ppm because the cncentratin belw 5 wt ppm f brn culd nt be detected experimentally. With this assumptin, the brn ).~lan f'~ ~ensitc~ ~ ~ 40 pm >/- t "// Bainite..... Fig Cmparisn f weld micrstructure f Welds A and B. A and B -- Macrstructure; C and D -- micrstructure. (The white rectangular bundary shws the regin f hardness measurements.)

4 252-s I IUNE O... i- -i- 0-9OO G" "~,.:..,~"~'-~'.,_:.:.",,:... :.%.. " ~:~.'; " ~ I ~ = l WeldA I I 0 -- Weld B l I I I I I I I =E I O0 400 I I I Death (~.m) Fig Cmparisn f Vickers pyramid hardness variatins in Welds A and B. The plts indicate a harder weld zne in Weld B than in Weld A. Fig The variatin f actual temperature with time fr Steels A and B during cling frm 00 C. The change f slpe (marked by arrws) indicates the transfrmatin start temperature..4 // I " = 8 --i... ~ ~ ~ ~... / (20 wt.ppm B) / (40 wtppm B) ~ 600 -t [ B=(Steal B)-.= 642 C I a,() = 543 (3 M=() = 531 C ' ' '''"1 ' ' ' '''"1 ' ' ' '''"1 ' ' ''''" _E 6-~ i (5 wtppm B) I I I I Temperature ( C) Fig Calculated diagram fr initiatin f transfrmatin fr Steels A and B, withut cnsidering the brn effect. The bainite start (B~) and martensite start (M~) were btained frm the TTT mdel (Ref. 3). The calculatins cnsider the effect f C, Si, Mn, Ni, M and Cr cncentratins. Fig Calculated brn cncentratin (wt ppm) in slid slutin within austenite fr (with 5 wt ppm brn) and B (with 20 and 40 wt ppm brn) as a functin f temperature. in slutin in fr the austenitizing temperature f 00 C was fund t be 0.24 wt ppm. The abve estimates were used t calculate the mdified TTT diagrams -- Fig. 7. The calculatins demnstrate that the small brn level in retards the austenite-t-ferrite decmpsitin kinetics. The initiatin time fr ferrite frmatin is retarded t such an extent that austenite decmpsitin will ccur at significantly lwer temperatures. This will lead t large fractins f bainite and martensite in fr similar thermal cycles. The diagrams fr Steels A and B were cnverted int CCT diagrams (Fig. 8) assuming linear cling rates frm 0.1 t,000 Cs -I. The CCT calculatins cmpare very well with the experimen- tally measured transfrmatin start temperatures. The experimental transfrmatin start fr was fund t be in between the ne calculated fr 0 and 5 wt ppm f brn. The experimental transfrmatin start fr was fund t be in between the ne calculated fr 20 and 40 wt ppm f brn. In the abve calculatins, the brn in slutin was calculated fr the austenitizing cnditins. In the case f cntinuusly cled weld metal, the brn in slutin wuld vary as the weld cls. The precipitatin f AIN, BN and Ti,V(C,N) may ccur at higher temperature. Therefre, the amunt f brn in slutin will depend n the thermdynamic equilibrium at r near the transfrmatin start temperatures. Bhadeshia, et al. (Ref. 9), assumed this temperature t be 900 C irrespective f the steel cmpsitin. Significance f the Results The abve results demnstrate the imprtance f brn n micrstructural evlutin. Small amunts f brn cupled with rapid cling cnditins can induce hard bainitic and martensitic micrstructure even in very lw-carbn steel welds. These micrstructural changes will result in large differences in prperties. It is imprtant t nte that the specificatins fr typical steels used fr resistance welds d nt specify the brn levels. Therefre, fr cnsistent weld quality cnsideratins, it is recm-

5 WELDING RESEARCH SUPPLEMENT I 253.~ I (5 wt.ppm B) (20 wt.ppm S) 00 9OO. (20 wt.i ~m B) / / Ste,, Zstee, B / "( -1 i I (,0,,.ppm S) ~, 8 E ~-~'" /... --~/- I B.(Stee, A)= 649 C I -I... I B,() = 642 C I s / I M.() = 543 C I / I M,() = 531 C I 400 "-il... I... I... I... I -2 " % t30 (5 wt.ppm B) i ;-; I ~. "" ' /... 1~" ' ' ''''"1 ' ' '~''"1 ' ' ''''"1... I " Fig Calculated diagram fr initiatin f transfrmatin f Steels A (with n brn and assumed 5 wt ppm brn) and B, with 20 and 40 wt ppm brn. It is imprtant t nte that the change in TTT diagrams are based n sluble brn in the austenite calculated by ThermCalc. Fig Calculated CCT diagram fr initiatin f transfrmatins in Steels A and B. The CCT lines fr steel A with 0 and 5 wt ppm B and with tw brn levels are shwn. The temperature-time data measured fr Steels A and B in Gleeble simulatin are als shwn. The arrws indicate the apprximate transfrmatin temperatures. mended that brn levels in steels be cnsidered. Mrever, with increased use f recycled steels, it is imprtant t quantify and evaluate the effects f ther elements in steels. In this cntext, ne needs t cnsider austenite grain size variatins n the micrstructure develpment. Additinally, t avid the frmatin f hard bainitic and martensitic micrstructures in welds, the welding parameters can be mdified. The mdificatins can induce a slw cling in the temperature range f alltrimrphic ferrite frmatin. This will reduce the bainite and martensite fractin in the micrstructure. Currently, at Oak Ridge Natinal Labratry (ORNL), a cmprehensive thermalmicrstructure-prperty mdel is being develped t ptimize the welding parameters fr the required weld prperties. Cnclusins Tw steels with similar strength levels welded by the resistance seam welding prcess exhibited large differences in micrstructure and hardness. These differences were attributed t the sluggish austenite decmpsitin kinetics in the steel with higher brn. This resulted in the frmatin f bainitic and martensitic cnstituents and a hard weld zne. In cntrast, the steel with lw brn level transfrmed predminantly t alltri- mrphic ferrite and resulted in a cmparatively sfter weld zne. The results are supprted by Gleeble thermal simulatin results. Theretical cntinuus cling transfrmatin predictins, cnsidering the effect f sluble brn, cmpare very well with the experimentally measured transfrmatin behavir. Acknwledgments Research spnsred by the U.S. Department f Energy, Divisin f Materials Sciences, and Assistant Secretary fr Energy Efficiency and Renewable Energy, Office f Industrial Technlgies, Metals Prcessing Labratry User Center (MPLUS), Advanced Industrial Materials Prgram, under cntract DE-AC05-96OR22464 with Lckheed Martin Energy Research Crp. The authrs thank M. W. Richey f LMES, Oak Ridge, and J. F. King f ORNL fr helpful discussins. The authrs als thank D. A. Frederick fr help with Gleeble thermal simulatins and A. Natin f ABB Cmbustin Inc., Chattanga, Tenn., fr perfrming residual brn analyses. References 1. Welding Prcesses. Welding Handbk, Vl. 2, 8th ed Ed. R. L. O'Brien. American Welding Sciety, Miami, Fla. p In, J. C., Easterling, K. E., and Ashby, M. F A secnd reprt n diagrams f mi- crstructure and hardness fr heat-affected znes in welds. Acta. Metall., 32: Bhadeshia, H. K. D. H A thermdynamic analysis f isthermal transfrmatin diagrams, Metal Science, 16: Sundman, B., Janssn, B., and Anderssn, J.-O The ThermCalc databank system. Calphad, 9: Babu, S. S Acicular ferrite and bainite in Fe-Cr-C weld depsits. Ph.D. dissertatin. Cambridge, U.K. University f Cambridge. 6. Pickering, F. B Physical metallurgy and the design f steels. Applied Science, U.K. 7. Yurika, N TMCP steels and their welding. Welding in the Wrld, 35: Bhadeshia, H. K. D. H., Svenssn, L.-E A mdel fr develpment f micrstructure in lw-ally steel (Fe-Si-Mn-C) weld depsits. Acta Metall., 33: Bhadeshia, H. K. D. H., and Svenssn, L.-E Mdel fr brn effects in steel welds. Prc. Internatinal Cnference n Mdeling and Cntrl f Jining Prcesses, ed. 1. Zacharia, pp American Welding Sciety, Miami, Fla.. Bang, K-s., and Ahn, Y-h Weldability f direct-quenched, lw-carbn T-Bcntaining steels. Welding Jurnal, 76(4): 151-s t 157-s.