Quantification of Residual Plastic Strains in Ni-Cr-Mn-Nb GTAW Welds via Electron Backscatter Diffraction

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1 Trends in Welding Research: Proceedings of the 6th International Conference Stan A. David, Tarasankar DebRoy, John C. Lippold, Herschel B. Smartt, John M. Vitek, editors, p DOI: /cp02twr0912 Copyright 03 ASM International All rights reserved. rl h International Trends in Welding Research Conference Proceedings, April 02. Pine Mountain, GA, ASM International, 03 Quantification of Residual Plastic Strains in Ni-Cr-Mn-Nb GTAW Welds via Electron Backscatter Diffraction G.A. Young, N. Lewis, C.K. Battige, R.A. Somers, and M.A. Penik Lockheed Martin Corporation, Schenectady, NY L. Brewer and M. Othon General Electric Corporate Research and Development Laboratory, Niskayuna, NY Abstract Electron backscatter diffraction (EBSD) was used to investigate the plastic strain distribution in a highly constrained, multipass automatic gas-tungsten-arc-weld (GTAW). The parameter used to quantify the plastic strains was the average intra-grain misorientation, which was averaged over all analyzed grains for a given region (i.e. the "amis" value). The "am is" parameter quantifies the extent to which dislocation subcells rotate the internal lattice orientation (and hence the electron diffraction pattern) from that ofneighboring subcells. Quantification ofthe plastic strains was estimated by constructing a calibration curve from uniaxially strained tensile bars between 0 and % plastic strain. Similar to previous studies, results show that the amis parameter exhibits a linear correlation with plastic strain. In a highly constrained GTAW weld, residual strains ranged from near zero to ~14.3%. The highest strains were measured toward the root of heavy section (2" thick) narrow groove welds. Regions ofhigh strain measured by EBSD correlate well with observations of microstructural damage and with welding induced defects. Results indicate that EBSD is a reliable method to quantify the residual plastic strains in weldments. Background State of the art scanning electron microscopy offers powerful tools to characterize the microstructure, crystallography, and microchemistry of materials. In the past decade, advances in data collection and analysis have made electron backscatter diffraction (EBSD) the preferred method to characterize the microtexture of materials [I]. In addition to crystallographic information, EBSD has been used to quantify the residual plastic strain in wrought alloys via measuring either the quality of the diffraction pattern or the rotation ofthe diffraction pattern [2-4]. Recent work in wrought alloys has focused on the rotation ofthe diffraction pattern as the more reliable and quantitative predictor of residual plastic strain [2, 3, 5]. Plastic strains are typically quantified via a calibration curve that relates a parameter that characterizes the lattice rotation to the residual plastic strain. The present work seeks to characterize and quantify the residual plastic strain in Ni-Cr-Mn-Nb weld metal (EN82H) via the intragrain misorientation or "amis" parameter. The amis parameter is a measure of the extent to which dislocation subcells rotate the internal fcc lattice and the EBSD pattern ofen82h. While EBSD techniques have been used to measure the plastic strains in wrought fcc and bee alloys [2, 3, 6], little work has been done to quantify the residual plastic strain in a weld or casting with a dendritic structure. As shown in another paper in these proceedings [7], residual stresses and plastic strains in welds can have a large effect on the hydrogen embrittlement resistance of welds. The purpose of the present study is to explore the relationship between the amis parameter and applied plastic strain as a function of orientation and temperature for EN82H weld metal. Additionally, EBSD is used to explore the strain distribution in a highly constrained EN82H weld. Experimental Procedure Materials Used The filler wire used was 0.045" diameter Alloy EN82H where "H" denotes a high carbon variant (0.03 wt.% min.) of Alloy EN82. Note that EN82H is the military designation for the commercial filler metal ERNiCr-3. The nominal composition of EN82H is given in Table 1. Table 1. Nominal Composition offiller Metal EN82H Ni Cr Mn Nb Fe Ti ~~ ~~~ ~~ Si Cu CPS Other max max 0.10 max max max Welding Parameters Multi-pass, hot wire, automatic gas tungsten arc welding (GTAW) was used to fabricate a weld buildup approximately 3" high x 4" wide x 24" long. The weld buildup was made on a low carbon steel backing plate. The shielding gas used was 95%Ar/5%H 2 A summary of the welding parameters is provided in Table

2 . Table 2 Summary ofwelding Parameters Voltal!e 12.5 V Current 300 A Travel Sneed 6.5 in./min. Oscillation Frequency 85 cvc.zmin, Oscillation Width 0.5 in. Hot Wire Voltal!e V Hot Wire Current A Wire Feed Rate 180 in./min. Meehanical Testing Tensile samples for EBSD strain calibrations were machined in three orthogonal directions as shown in Figure 1. The welding direction was denoted as 'L', transverse 'T' and the short direction as's'. For elevated temperature tests, tensile samples were heated to the desired test temperature, allowed to soak for - minutes to ensure a homogenous temperature distribution across the sample and strained at a stroke rate of 0.02 in./min. until the target strain was reached. During the loading, strain was measured with an extensometer. After the target strain was achieved, the sample was immediately unloaded, removed from the fixtures and rapidly cooled to room temperature via water quenching. Time between test completion and cooling to room temperature was approximately 5 minutes. The conditions tested aresummarized in Table 3. Table 3. Conditions Tested for EBSD Calibration Curve Heat Plastic Treatment Test Orientations Strain Prior to Temp. Tested (%) Strainine F S,L,T As F L Welded 15 I 800 F L 0 00 F 5 70 F S,L,T 4 hours F L prior to 15 I 800 F L straining Results Mechanical Testing Engineering yield strength data are summarized in Figure 2. As expected, as-welded tensile bars exhibited higher yield strengths than annealed (00 F/4hrs) samples. The yield strength of EN82H also decreased with increasing test temperature. Note that % strain was below the ultimate tensile strength for all samples investigated Error bare art for one standalddevl8llon 1- "l"_oiienlalion Welding Direction 10 Figure 1. Schematic illustration of the three orientations tested in this study. EBSD Measurements EBSD Experiments were performed using a CamScan CS44 SEM with a tungsten filament at key beam voltage and an automated stage. The EBSD data were collected using the HKL Inc. EBSD acquisition system and analysis software along with custom software written for calculating amis. The samples were prepared by mechanical grinding and polishing. Final polishing was done on an automated, vibratory with a finishing step of 0.05 J1Ill colloidal silica. The calibration data scans consisted of 25 lines of 00 um x I urn steps across the center of the sample. The distance between the lines was 15 urn. o Teat Temperature ('F) 1600 Figure 2. Comparisons of 0.2% engineering yield strength data as a function ofheat treatment and test temperature. Strain Calibration Summary calibration curves for the amis parameter and the % plastic strains are shown in Figure 3. Calibration curves are given for two different starting conditions (1) as-welded prior to straining (left column) and (2) annealed prior to straining (right hand column) and three different straining conditions (70 F, 1000 F, and 1800 F). At 70 F, the three different orientations (L, S, and T) were investigated. At the higher temperatures, only the "L" orientation was investigated. 913

3 As Welded Prior to Straining 00 F Annealed Prior to Straining Strained at 70 F Strained at 70 F intercept=1.040 slope= r 2= A intercept= slope=0.52 r 2= !!2 3~ :!: «c- 5[- i- f- f- 4t I 2[ ~ r r- 1 f-:; intercept= slope= r 2= Strained at 1800 F (Partially Recrystallized) intercept= slope=o.1196 r 2= o Plastic Strain (%) Plastic Strain (%) Figure 3. Summary ofstrain calibration curves for as-welded material (left) and material that was annealed at 0(/Ffor 4 hours prior to straining. The solidlines are the linear regressions through the data. 914

4 Microstrucutural Characterization The dendritic microstructure typical of tensile bars strained at either 70 F or 1000 F is shown in Figure 4a. For tensile bars strained at 1800 F, a significant amount of recrystallization occurred as shown in Figures 4b and 4c. Recall that the relatively slow cooling rates from l800 F to room temperature (~5 minutes) make recrystallization after straining likely. Recrystallization was not observed in tensile bars strained at 70 F or 1000 F. However, small areas (~l00 x 0 IJID) ofinterbead recrystallization were observed in similar welds in the aswelded condition [7]. Discussion The Effect oforientation Statistical analysis of the 70 F tensile data in the S, L, and T orientation showed no significant difference between orientation for the condition tested (0-% uniaxial tensile strain). The regression lines for the as-welded and annealed calibrations at 70 F reflect fits through all ofthe data. At 1000 F and l800 F only samples in the "L" orientation were tested. Effect ofthe Temperature ofstraining As shown in Figure 3 and summarized in Table 4, the regression lines at 70 F and 1OOO F show similar slopes and intercepts. Note that in Figure 3, the data were plotted as % strain (abscissa) vs. amis (ordinate) while in Table 4, the correlation has been solved for % strain as a function of amis. However, for tensile bars strained at l800 F, markedly different calibration lines are obtained for both the as-welded and annealed conditions. Metallographic examination ofthe 1800 F tensile samples shows that a significant amount ofrecrystallization occurred at this test condition (Figures 4b and 4c). Recrystallization, of course, recovers plastic strain leading to the lower measured amis values relative to tests where recrystallization or recovery did not occur. Evidence of annealing twins (arrows in 4b and 4c) proves that these are recrystallized grains and not equiaxed dendritic grain boundaries [7]. Clearly, at temperatures where recovery or recrystallization can occur, the thermal history ofthe sample can influence the correlation between the amis parameter and the % plastic strain. Table 4. Summary ofstrain Calibration Parameters. The Regression Lines in Figure 3 Were Solvedfor % Strain as a Function ofthe amis Parameter. %strain=m -amis-v b Temperature of Condition Straining m b.-z (OF) As Welded F/4 hours prior to straining Samples were partially recrystallized Figure 4 a.) Typical dendritic microstructure oftensile bars strained at room temperature or 100(j'F, b.) the dendritic + recrystallized microstructure ofa tensile bar strained 10% at 180(j'F, and c.) a higher magnification view of the recrystallized grains in b.). The arrows point to annealing twins. All samples were electrolytically etched with 8:1 phosphoric acidin water, Nomarski DIe. 915

5 Comparison ofen82h with Wrought Alloys For EN82H weld metal, the relationship between the amis parameter and the residual plastic strain is well described by a straight line for plastic strains between 0 and %. This finding is in good agreement with previous studies of amis vs. plastic strain for wrought alloys {304L and 316L stainless steel and oxygen free, high conductivity copper (OFHC)} as shown in Figure 5 [2, 6]. Note that for strains above the ultimate tensile strength, increasing triaxiality may cause deviations from this linear behavior as shown by the biaxial tension study of Lehockey et al. on a wrought Ni-Cr-Fe alloy (A600) [3]. For a given magnitude of plastic strain, more complicated strain states (i.e. biaxial vs. uniaxial) require increased dislocation density and may result in higher amis values [3]. In Figure 5, comparison of the wrought alloys with annealed EN82H shows that for a given strain, the amis value increases from austenitic stainless steel, to EN82H to the OFHC copper. It is also notable that stacking fault energy also increases from 304 stainless steel (~ mj/m 2 ) [8] to EN82H (~60 ml/m') [9] to copper (~80 m.l/rrr') [8]. As stacking fault energy (SFE) increases, the tendency to form dislocation subcells increases although the relationship between subcell formation and amis value is unclear. While this type of correlation may be useful in developing a universal calibration curve between amis and % plastic strain applicable for many different metals and alloys (Figure 6); additional research into the effect of dislocation substructure and the amis parameter on additional alloys is required l!-en CJ :;:; 10 II).!! Q. ';/! AMIS Figure 5. Comparison ofamis / % plastic strain calibration curves for EN82H weld metal with wrought alloys 304L, 316L stainless steel and OFHC Cu. Straining was performedat room temperature. Application ofebsd to Measuring Strains in Welds A schematic of a highly constrained multipass weld is shown in Figure 7. Welding was performed via automatic GTAW with a 95%Ar/5%H 2 shielding gas. In this weld, two different orientations of buttering were deposited on carbon steel and subsequently jointed to an Alloy 690 component via a multipass, narrow-groove EN82H weld. The weld joint was highly constrained and post-weld inspection via serial metallography revealed extensive cracking in two areas: (1) near the center of the narrow groove weld and (2) near the interface of the type-i and type-ii buttering (Figure 8). &.5 :e_ 5.5 Q) Q. o 5 en rn 4.5 s <C Steel e-, <, ~ <, s-; OFHC SFE(mJ/m 2 ) EN82H 'annealed' A Cu ~ Figure 6. Comparison ofthe slope determined in the amis calibration andstackingfault energy. Figure 7. Schematic ofthe weld joint investigated with EBSD. The joint consisted oftwo types ofen82h buttering (1 and II) and a EN82Hnarrow groove (NG) weld. _ 16,...-.._~ R="""",,=""""lC====""""lC=====""'F====~ 14 H---liOrHl&DL IiOI!IIR6I:L-+,";ng~H BlU!!L.. -I ~ 13!I-,+ -...L_-+-.J.I_--.J~:!hJ;.J-...l----Il !I-'+--~~--+-- ~~--h"l--+--ft l g 11 it-i ,F-- ---\--.f J ~ 10 It: ~'\ i 9 ii-.-..~--- S _ Inches! 6 1l4-\--- ~ 6 II---+I~-----:= i 4 1J-4~'""-- W 3 1J-4-A:--+-' O~..._.r r_..,.~~~_.._.l::;::+::i.r_+:;:::;=:;::::~ UU2UUUU1UUUUO~~~~~ Distance from A690 Fusion Line Figure 8. Correlation ofregions ofweldcracking with residual plastic strain measured via electron backscatter diffraction. Strain was calculated using the as welded, room temperature calibration. Cracking was observed in regions ofhigh plastic strain

6 Subsequent analysis ofthe cracking showed no evidence ofhot cracking. Instead, it is believe that welding with a hydrogen bearing shielding gas lowered the ductility ofthe weld metal to cause fracture in regions of high strain [7]. A typical fracture surface is shown in Figure 9 illustrating the brittle, interdendritic crack path with extensive secondary cracking. This conclusion is supported by the amis measurements summarized in Figure 8. Regions that cracked (gray boxes) correlate well with measurements ofhigh plasticstrain as measured via EBSD. Examination of the plastic strain distribution in a highly constrained weld joint shows that regions of high strain correlate with regions ofcracking. Acknowledgements The Authors wish to thank Dr. John Sutliff of HKL Technologies for his assistance in developing the electron diffraction techniques used in this study and Mr. Lou Maturo of Lockheed Martin for performing the mechanical testing, References Figure 9 a.) low magnification and b.) higher magnification scanning electron images ofthe brittle, interdendritic fracture observed in the EN82Hwelds. Conclusions I. Randle, V., Theoretical Framework for Electron Backscatter Diffraction, in Electron Backscatter Diffraction in Materials Science, A. Schwartz, M. Kumar, and B.L. Adams, Editors. 00, Kluwer Academic/Plenum Publishers: New York. p Sutliff, J.A., Microscopy and Microanalysis Proceedings, 1999: p Lehockey, E.M., Y.-P. Lin, and O.E. Lepik, Mapping Residual Plastic Strain in Materials Using Electron Backscatter Diffraction, in Electron Backscatter Diffraction inn Materials Science, A.1. Schwartz, M. Kumar, and B.L. Adams, Editors. 00, Kluwer Academic / Plenum Publishers: New York. p Orsund, R., 1. Hjelen, and E. Nes, Local Lattice Curvature and Deformation Heterogeneities in Heavily Deformed Aluminum. Scripta Met., : p Wilkinson, A.J., Measuring Strains Using Electron Backscatter Diffraction, in Electron Backscatter Diffraction in Materials Science, A. Schwartz, M. Kumar, and B.L. Adams, Editors. 00, Kluwer Academic / Plenum Publishers: New York. p Angeliu, T. Microstructural Characterization of L-Grade Stainless Steels Relative to the IGSCC Behavior in BWR Environments. in Tenth International Conference on Environmental Degradation of Materials in Nuclear Power Systems. 00. Lake Tahoe, NY: NACE. 7. Young, G.A. Factors Affecting the Hydrogen Embrittlement Resistance ofni-cr-mn-nb Welds, these proceedings, 02. Pine Mt., GA: ASM. 8. Dieter, G.E., Mechanical Metallurgy. 3rd ed. 1986, New York: McGraw-Hill Symons, D.M., Hydrogen Embrittlement ofni-cr-fe Alloys. Met. Trans. A, A: p Electron backscatter diffraction can be used to quantify residual plastic strains in welds. The correlation between the average grain misorientation (amis) and % plastic strain is insensitive to weld orientation between 0 and % uniaxial tensile strain. The correlation between amis and % plastic strain is relatively insensitive to temperature between 70 F and 1000 F but is significantly changed at 1800 F when recrystallization occurs. 917