The Effects of Phosphorus, Sulfur and Ferrite Content on Weld Cracking of Type 309 Stainless Steel

Transcription

1 The Effects f Phsphrus, Sulfur and Ferrite Cntent n Weld Cracking f Type 0 Stainless Steel The ferrite level that prevents cracking is fund t be dependent n impurity level wherein, fr lw P + S, little r n ferrite is required t prevent cracking while, with ferrite cntents greater than Ferrite Numbers t, large amunts f P and S can be accmmdated withut cracking By J. A. BROOKS AND F. J. LAMBERT, JR. ABSTRACT. The variatin in weld cracking f different heats f Type 0 stainless steel is reviewed. Data are presented which shw the cmbined effects f phsphrus plus sulfur level and ferrite cntent n weld cracking. The data demnstrate that chemistry specificatins mre restrictive than the current AISI specificatin are required t ensure crack-free autgeneus welds. Intrductin As pinted ut by Lundin et al., the cnstructin f nuclear pwer reactrs using large amunts f austenitic stainless steels has renewed interest in the subject f ferrite and weld micrfissuring. The ptent effect f several percent ferrite in reducing cracking has been knwn fr years 2 and resulted in the develpment f a number f diagrams, such as that published by Schaeffler, and later revised by DeLng et al., which predicts weld ferrite cntent frm chemical cmpsitin. The relatinship between ferrite cntent and weld cracking has led t the develpment f cdes which require weld ferrite abve a specified minimum level in an attempt t eliminate cracking. Hwever, in sme cases designers wuld like t minimize, r eliminate, ferrite fr reasns such as preventing transfrmatin f delta ferrite t sigma phase, r excluding magnetic phases, fr example, in cnstructing hardware fr super cnductivity. Therefre, cnflicts ften arise between the designer and welding engineer. Lundin and c-wrkers determined the minimum ferrite cntent required t eliminate weld fissuring f several AISI 00 series ally wire filler metals and thrughly characterized the micrfissures f "fissure bend" test samples. In their study they did nt cnsider the effect f impurity elements which can alter the relatinship between ferrite cntent and cracking. This relatin was, hwever, clearly shwn by ne f the present authrs fr high nitrgen, high manganese stainless steels. 5 In bth weld wire filler metals and wrught material, phsphrus and sulfur levels in particular can vary cnsiderably, and have a significant effect n weld cracking. Impurity levels in wrught material cannt be ignred, since cracking can Paper based n infrmatin included in presentatin made under title f "Effects f Impurities n the Weldability f Electrfrmed Nickel" in a sessin spnsred by the Wrught Nickel Allys Subcmmittee f the Welding Research Cuncil at the AWS 57th Annual Meeting held in St. Luis, Missuri, during May 0-, 77. /. A. BROOKS is with the Materials Develpment Divisin, Sandia Labratries, Livermre, Califrnia, and F. I. LAMBERT, JR., nw retired, was with the Unin Carbide Crp., Y- Plant, Oak Ridge, Tennessee. result in autgeneus welds r the rt pass f multiple pass welds. The purpse f this paper is t demnstrate the imprtance f (P + S) cntent in the relatinship between weld cracking and ferrite cntent f autgenusly welded AISI 0 austenitic stainless steel, ne f the allys studied by Lundin. Material and Testing Studies were cnducted n 2 different heats f Type 0 austenitic stainless steel which cntained varius phsphrus and sulfur levels. The heat chemistries are shwn in Table. Allys t were prduced frm kg (2.2 lb) vacuum inductin melted (VIM) heats, allys 7 t 0 were frm.8 kg ( lb) VIM heats, and allys and 2 were frm cmmercial heats. All material was hmgenized and ht rlled t 0.25 mm (0.0 in.) thick strip. These strips were annealed at 00 C (850 F), air cled, pickled, and finished with a light cld-rll pass. All experimental heats were made frm high purity starting material with the main allying elements balanced t prvide a range in ferrite cntent when welded. Hwever, it was intended that all heats shuld be within the cmmercial AISI specificatin shwn in Table 2. When the allwable chemistry range f the Type 0 stainless steel is pltted n the DeLng Diagram (Fig. ), it is easily seen that autgeneus GTA welds in this ally culd exhibit ferrite levels frm ferrite WELDING RESEARCH SUPPLEMENT I -s

2 Table - Chemistries f Type 0 Stainless Steel Heats Studied, Wt-% Heat Cr Ni Mn Si C P S N heat N weld F.N.'*» Crack rating ~ ,,, 2 2, 2,,,,, 2,, 2, 2, "F.N.-Ferrite Number calculated frm the DeLng Diagram using nitrgen f weld analysis, r 0 if nt analyzed. number* (F.N.) = 0 t F.N. > 8 depending upn heat chemistry. A weld test was established 7 t determine the cracking sensitivity f the 0.25 mm (0.0 in.) thick sheet. Strips, each 00 mm wide by 200 mm lng, ( 5 /x7'/ s in.) were welded endt-end t a standard Type 0L stainless steel strip f equal size. The test was perfrmed by making a series f fusin passes side by side alng the length f the cmbined standard and test specimen strips. The standard Type 0L stainless steel strip was used t help detect any sutble changes in weld parameters r weld technique which may effect weld bead gemetry *Ferrite cntent is reprted in ferrite number as preferred by the Welding Research Cuncil see "Technical Nte: Calibratin Prcedure fr Instruments t Measure the Delta Ferrite Cntent f Austenitic Stainless Steel Weld Metal" by W. T. DeLng, Welding Jurnal, 52 (2), Feb. 7, Research Suppl., pp. -s t 72-s. t test results. The samples, secured in a seam welding unit, were welded with the fllwing parameters: 0 A, 8 V, 0 mm/min travel speed with argn shielding. Twelve t sixteen weld passes were usually made unless the test piece prved t be very crack sensitive, in which case the test was terminated early. This test was perfrmed twice n each test panel. If the test specimen shwed n visible Table 2 AISI Specificatin fr Type 0S'"' Stainless Steel, Wt-% c Ni Cr Mn Si P S "S = lw carbn grade (max) (max) 0.0 (max) 0.0 (max) cracks, the back side f the weld was painted with dye penetrant and the tp side with a develper which culd easily detect the thrugh thickness cracks. A qualitative rating f - was established: () n cracks, (2) n mre than tw cracks, () mre than tw cracks, and () the dye check was nt needed since it was pssible t see thrugh cracks. The ratings fr each heat are shwn in Table. A cmpleted test specimen which prved t be very crack-sensitive is shwn in Fig. 2. All cracks were centerline in nature and in sme severe cases extended the ttal length f the test specimen. A sectin f test plate shwing the weld gemetry and spacing is shwn in Fig.. The weld ferrite cntent was calculated frm the DeLng Diagram since the specimens are much t thin fr magne gage readings. When available, the nitrgen analysis frm the actual welds were used in these calculatins since in sme cases nitrgen was 0-s I MAY 78

3 CONSTITUTION DIAGRAM FOR STAINLESS STEEL WELD METAL br William T. D.tng revised January, CHROMIUM EQUIVALENT = '/.Cr + %M +.5 X "/.Si X */.Cb Fig. 7 DeLng Diagram shwing calculated ferrite levels f welds within the Type 0 stainless steel cmpsitin range f Table 2 and 0.05% maximum N absrbed during welding. This can be seen in Table by cmparing the heat analysis with the nitrgen analysis f the weld metal. The effect f nitrgen pick-up during welding has been discussed by Lng and DeLng. 8 A nitrgen level f 0.02% was used fr ferrite predictin when the weld metal analysis was nt available. As a check between predicted and actual ferrite cntent, the ferrite cntent was estimated fr welds f five allys using the line intercept technique. Scanning electrn micrgraphs at X000 were used t clearly distinguish the fine ferrite mrphlgy. The measured and calculated values are shwn in Table. As shwn, fair agreement existed fr all welds except heat, which had a measured ferrite cntent f % (F.N. ), but a calculated value f 0.7% (F.N.). As discussed later, weld test behavir f this heat was mre typical f F.N. than F.N. Results and Discussin In this study nly phsphrus and sulfur cntent were cnsidered in studying the cracking behavir even thugh ther elements such as silicn and carbn can have an effect. ' 0 Ht Fig. 2 Weld test sheet after dye penetrant inspectin WELDING RESEARCH SUPPLEMENT I -s

4 Table Measured and Calculated Values f Weld Ferrite Cntent Heat number 5 Cal F.N Ferrite culated % Measured, % Fig. Tp surface f weld test sheet shwing sectins f tw test runs. Each test includes - GTA weld passes Hwever, phsphrus and sulfur are knwn t be the mst detrimental impurities cmmn t this type ally, and silicn was intentinally held cnstant at a nminal range f -0.7%. Using "Relative Ptency Factr" calculatins prpsed by Brland and discussed by Cannnic, et al.,'" sulfur in irn is predicted t be much mre detrimental than phsphrus in frming lw melting eutectics respnsible fr cracking. Hwever, frm several studies discussed by Brland and Yunger, bth elements seem t exhibit apprximately the same ptency in prducing cracking. Thus, phsphrus and sulfur were cnsidered t have equal effects n cracking and the sum f P + S was used fr cmparing the heats in this study. Hwever, at lw levels, the detrimental effects f sulfur are likely t be reduced by the frmatin f manganese sulfide. 5 The relatinship fund between (P + S) cntent and ferrite number n weld cracking is shwn graphically in Fig.. The data can be separated int three regins:. A regin where cracking is severe. 2. A regin marginal in cracking resistance where a small amunt f cracking resulted.. A regin where n cracking ccurred. Figure demnstrates that fr very lw levels f P + S little r n ferrite was required t prevent cracking. Hwever, as P + S level increased the ferrite cntent als had t be increased t resist weld cracking. At lw ferrite levels, fr each 0.0% increase in P + S cntent, an increase f abut F.N. was required t maintain the same cracking resistance. At Weld Cracking ( rating 2,--.) Q Marginally Crack Sensitive) rating,2-2,2) O N Cracking! rating ) * n : Cracking i- ** 8 ' J I / N. / O / t / * 7 / #f 7 / / ' 0 NO CRACKING Q Ferrite Number (F.N.) Fig. Weld test results shwing the relatinship between (P + Sj cntent, F.N. and weld cracking. Ratings crrespnd t thse shwn in Table. Heat measured F.N.-see text -s I MAY 78

5 ferrite number greater than abut -, high levels f P + S were accmmdated withut weld cracking. Heat exhibited mre weld cracking than expected using Fig.. The line intercept technique indicated an actual ferrite cntent f F.N. which wuld be in much better agreement with weld results than the calculated value f. A likely explanatin is that in this heat mre nitrgen was pickedup during welding than indicated by the chemical analysis which sampled -2 cm-. Nitrgen pick-up during welding f 0.0% rather than the assumed value f 0.02% wuld result in the bserved discrepancy in measured and calculated ferrite levels. As shwn in Table, large nitrgen increases actually did ccur in several f the welds. The results frm this study demnstrate that tw chices are available t ensure crack-free welds:. Ensure that sufficient ferrite is prduced in the weld depsit t prevent cracking at the maximum P + S level which can exist. 2. Or, reduce the allwable P + S level such that less ferrite is required t maintain the same cracking resistance. The latter case wuld be mre attractive where large amunts f ferrite may be deterimental t ther prperties fr example, where ferrite may transfrm t sigma. In either case, a specificatin with different chemistry limits than thse allwed in many wrught ally specificatins is required t guarantee crack free welds. This is especially true fr the mre highly allyed type f chemistry f the AISI 00 series stainless steels. It wuld be expected that the relatinship between weld cracking, P + S level, and ferrite cntent may be dependent upn welding cnditins and cnstraint. Hwever, the results frm this study are in gd agreement with thse f Lundin et a/. in that bth studies shw that ferrite number f abut is required t prevent cracking at a P + S = 0.02 level in the Type 0 stainless steel chemistry range. Hwever, this study emphasizes the fact that the ferrite level that will prevent cracking is very dependent upn impurity level. Summary The cmbined effects f phsphrus plus sulfur level and ferrite cntent n weld cracking f Type 0S stainless steel were studied. The ferrite level that prevented cracking was fund t be very dependent upn impurity level. Fr very lw levels f P + S, little r n ferrite was required t prevent cracking. At lw levels f bth ferrite and P + S an increase f 0.0% P + S required an increase f ~ F.N. t maintain the same cracking resistance. Hwever, fr ferrite cntents greater than F.N. - large amunts f phsphrus and sulfur culd be accmmdated withut cracking. Acknwledgments We wish t acknwledge the experimental assistance f bth D. Clark and A. Salmi f SLL, and Billy Crss frm Oak Ridge, Y- plant. The review f the manuscript by D. Dawsn and M. Mte is greatly appreciated. References. Lundin, C. D., DeLng, W. T., and Spnd, D. F., "Ferrite-Fissuring Relatinship in Austenitic Stainless Steel Weld Metals," Welding Jurnal, 5 (8), Aug, 75, Research Suppl., pp. 2-s t 2-s. 2. Brland, ). C, and Yunger, R. N., "Sme Aspects f Cracking in Welded Cr- Ni Austenitic Steels," British Welding lurnal, Vl. 7, 0, pp Schaeffler, A. L., "Cnstitutin Diagrams fr Stainless Steel Weld Metals," Metal Prgress, 5, Nv., pp. 80 and 80-b.. DeLng, W. T., Ostrm, G. A., and Szumachwski, E. R., "Measurement and Calculatin f Ferrite in Stainless-Steel Weld Metal," Welding Jurnal, 5 (), Nv. 5, Research Suppl., pp. 52-s t 5-s. 5. Brks,. A., "Weldability f High N, High Mn Austenitic Stainless Steel," We/ding lurnal, 5 (), lune 75, Research Suppl., pp. 8-s t 5-s.. DeLng, W. T., "Ferrite in Austenitic Stainless Steel Weld Metal," We/d/ng yurnal, 5 (7), July 7, Research Suppl., pp. 27-s t 28-s. 7. Lambert, Jr., F. J., Unpublished Unin Carbide, Oak Ridge Y- Plant, Oak Ridge, Tenn. 8. Lng, G )., and DeLng, W. T., "The Ferrite Cntent f Austenitic Stainless Steel Weld Metal," Welding Jurnal, 52 (7), July 7, Research Suppl., pp. 28-s t 27-s.. Brland, J. C, "Suggested Explanatin f Ht Cracking in Mild and Lw Ally Steel Welds," Welding Research Abrad, Vl. VIM, N. 2 (Feb. 2), pp Cannic, D. A., Savage, W. F., Werner, W. )., and Gdwin, G. M., "Effects f Minr Elements n Weldability f Incly 800," Effects f Minr Elements n the Weldability f High-Nickel Allys, Welding Research Cuncil Sympsium,. pp Brland,. C, and Yunger, R. N., "Sme Aspects f Cracking in Welded Cr- Ni Austenitic Steels," British Welding Jurnal, Vl. 7, 0, pp PVRC/MPC Task Grup n Fracture Tughness Prperties fr Nuclear Cmpnents Final Reprt This publicatin, dated Octber 77, was spnsred jintly by the Metal Prperties Cuncil, Inc. and the Welding Research Cuncil. The fur reprts in this 22-page mngraph summarize several years wrk n the analysis and applicatin f fracture mechanics perfrmed under the jurisdictin f the Task Grup. The titles f the reprts are:. PVRC-MPC Prpsed Initial Prgram n Fracture Tughness Prperties f Nuclear Cmpnents Material-Reprt t USAEC. 2. Crack Arrest Analysis and Testing fr Nuclear Cmpnent Materials.. Elastic Plastic Analysis fr Nuclear Cmpnent Materials.. Instrumented Precracked Charpy Testing fr Medium Strength Nuclear Pressure Vessel Steels. Part I Test Prcedure, Part II Rund Rbin Test Prgram. The price f the mngraph is $25 per cpy. Discunt prices: 5 t 20 cpies, 20% discunt; 20 t 50 cpies, V% discunt; ver 50 cpies, 50% discunt. Orders shuld be sent with payment t the Welding Research Cuncil, United Engineering Center, 5 East 7th St., New Yrk, NY 007. WELDING RESEARCH SUPPLEMENT -s

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