Weld cracking in Ta-modified cast Inconel 718

Scripta

METALLURGICA

Vol. 22, pp. 7 2 9 - 7 3 4 , 1988 P r i n t e d in the U.S.A.

Pergamon Press plc All rights reserved

WELD CRACKING IN TA-MODIFIED CAST INCONEL 718

W.A. BAESLACK III,* S. L. WEST* and T.J. KELLY** *Department of Welding Engineering The Ohio State University Columbus, OH 43210 USA **General Electric Company Cincinnati, OH 45215 USA (Received

February

29,

1988)

Introduction Inconel 718 is a precipitation-strengthened, Ni-base alloy originally developed to provide a combination of high strength at intermediate temperatures and good weldability. Although highly resistant to strain-age cracking during weld stress relief, the alloy can be susceptible to both liquation cracking in the weld heat-affected zone (HAZ) and, under conditions of high restraint, solidification cracking in the weld fusion zone (1,2). HAZ liquation cracking results from the formation of a liquid film at grain boundaries during the weld thermal cycle and the inability of this fill to accommodate thermally-induced stresses during weld cooling. In cast Inconel 718, this liquid film originates primarily from the constitutional liquation of a Nb-rich Laves phase at casting dendrite interstices (1,3). Fusion zone solidification cracking occurs during the final stages of weld solidification due to an inability of the nearly-solidified weld metal to accommodate solidification shrinkage stresses. In Inconel 718, non-equilibrium solidification in the fusion zone promotes partitioning of Nb to dendrite interstices. This segregation extends the alloy solidification range and promotes final solidification of a Nb-rich Laves phase/gamma eutectic. Both of these factors are known to increase solidification cracking (4,5). Previous investigators (6) have shown that the partial substitution of Nb with Ta in Inconel 718 weld filler metal reduces the quantity of Laves phase in the weld fusion zone and thereby improves weld metal bend ductility. In addition, a comparison of the Ni-Nb and Ni-Ta binary phase diagrams (albeit the solidification behavior of Inconel 718 is more complex than that of the binary systems) suggest that Ta substitution could reduce the solidification range and improve cracking resistance in both the fusion and HAZ regions. Based on these observations, the present work was initiated in which Ta was substituted for Nb on a one-for-one atomic percent level. Cracking susceptibilities and morphologies for standard and Ta-modified Inconel 718 were determined and comparatively evaluated. Experimental Procedure Three standard heats and one Ta-modified heat of Inconel 718 were received in the form of cast-to-size weldability test specimens 5 mm in thickness. Chemical compositions are provided in Table I. Prior to weldability testing, all specimens were heat treated in vacuum at i095°C for one hour and cooled to 650°C in twenty minutes. The Spot-Varestraint and Mini-Varestraint weldability tests were utilized to evaluate }{AZ liquation and fusion zone solidification cracking susceptibilities, respectively (Figs. I and 2) (7,8). During Varestraint testing, cracking is induced by surface strain enhancement via deformation around a die block of predetermined radius. In the Spot-Varestraint test, strain is applied to a gas tungsten-arc spot weld immediately after extinguishing the arc, thereby restricting cracking to the weld HAZ. During Mini-Varestraint testing, straining occurs during the generation of a continuous gas tungsten-arc weld, with cracks forming primarily in the

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previously solidified fusion zone. Total crack length (TCL) was utilized as the quantitative measure of cracking susceptibility. After testing, specimens were metallographically prepared for examination using light and scanning-electron microscopy (SEM) and energy-dispersive X-ray analysis (EDS). In addition, crack surfaces were exposed for fractoKraphic examination using

SmlF~S. Results and Discussion The cast + heat treated Alloy 718-Ta macrostructure appeared similar to that of the standard alloy, exhibiting relatively coarse gamma dendrites and dark-etching interdendritic regions (Fig. 3A). SEM analysis of the interdendritic regions revealed several different phases which formed during solidification and on cooling in the solid state (Fig. 3B). Ti-rich carbonitrides and Ta-rich carbides which solidified via eutectic-type reactions were observed near the dendrite interstices, with the carbonitrides typically serving as growth centers for the carbides. The final regions to solidify, which were occasionally associated with solidification shrinkage cavities, exhibited a globular Ta-rich phase morphologically similar to a (Ni,Fe,Cr)2(Nb,Ta,Mo) Laves phase observed in the standard alloy. EDS analysis revealed a near (Ni,Fe,Cr)2(Ta,Ti,Mo) stoichiometry and appreciable Mo enrichment, which is characteristic of Laves phase in standard 718. Quantities of Laves phase in the standard and Ta-modified alloys appeared approximately the same. Solid-state phase transformations in the Ta-rich interdendritic gamma occurred to an acicular "delta" phase (Ni3Ta, orthorhombic) and extremely fine precipitates which may have been Ta-rich gamma-double prime (Ni3Ta, body-centered tetragonal). Compositionally, the delta phase was distinguished from the Laves by an appreciably lower Mo content (1.5 versus 9.0 wt%). Figures 4A and 4B are plots of TCL versus % augmented strain for the Spot-Varestraint and Mini-Varestraint tested Inconel 718 alloys, respectively. As shown, the Ta-modified alloy TABLE I Chemical Compositions of Standard and Ta-Modified Inconel 718 Alloys (wt%) HEAT Fe Ni Cr Nb* Ta 718-i bal 53.13 19.03 5.01 -718-2 hal 53.88 18.34 5.20 -718-3 bal 53.01 18.83 5.07 -718-Ta bal 48.60 19.20 0.02 9.10 (Nb + Ta) for standard 718 heats

Ti 0.75 0.94 0.79 1.04

AI 0.41 0.41 0.39 0.47

Si 0.01 0.14 0.03 0.02

Mo 3.04 3.02 3.08 3.00

Mn 0.01 0.01 0.01 0.01

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FIG. 1 Spot-Varestraint weldability test apparatus Test parameters were: 65 A, 15 V, 6.3 mm arc length, argon shielding, 15 s arc time, 50 ms delay time.

FIG. 2 Mini-Varestraint weldability test apparatus. Test parameters were: 70-90 A, 10.5 V, 6.3 mm arc length, 1.7 mm/s travel rate, argon shielding.

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FIG. 3 Light (A) and SEM (B) micrographs of cast + heat-treated Inconel 718-Ta. in (B) indicate Laves Phase and Ta-rich carbides, respectively.

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exhibited the lowest susceptibility to both forms of cracking over the entire range of strain levels. Specimen-to-specimen variations in TCL were less than 5% from the average. The fusion boundary macrostructure of the Inconel 718-Ta Spot-qarestraint test specimen is illustrated in Fig. 5A. A shallow temperature gradient at the solid-liquid interface and a relatively large difference between the melting temperature of the gamma dendrite cores and the liquation temperature of the interdendritic Laves phase promoted a highly irregular fusion boundary and relatively wide partially-melted region (PMR). However, the PMR width in the Tamodified alloy was noticeably narrower than that in the standard cast alloys. SEM analysis of the PRM outer periphery showed liquation to initiate at the interface between the Laves phase and the gamma matrix, which was indicative of a constitutional-liquation phenomena. At locations nearer the fusion boundary, higher peak temperatures promoted complete liquation of the Laves phase and additional melting into the gamma matrix. On cooling, this Ta-rich liquid solidified to a lamellar Laves phase/gamma eutectic (Fig. 5B). Also apparent in Fig. 5B are Tarich carbides which did not react with the gamma matrix due to their higher constitutionalliquation temperature. During cooling of the original cast structure, gamma grain boundaries

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Total crack length versus % augmented strain for (A) Spot-Varestraint and (B) MiniVarestraint tested standard and Ta-modified Inconel 718 alloys.

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migrated and intersected interdendritic Laves phase. The coincidence of the grain boundaries and Laves phase promoted rapid wetting by the eutectic liquid, as shown in Fig. 5C. Figure 5A also shows HAZ liquation cracks which propagated intergranularly through the PMIR. A shorter maximum crack length in the Inconel 718-Ta versus the standard alloys was consistent with the narrower PMR. The SEM micrograph in Figure 5D shows solidification products in a backfilled crack directly adjacent to the fusion boundary. The solidification of this Ta-rich liquid occurred to gamma, carbides and the Laves phase/gamma eutectic. Figure 6A illustrates the top surface of a Mini-Varestraint test specimen. At high strain levels, fusion zone solidification cracks propagated across the fusion boundary into the HAZ. SEM analysis of the fusion zone revealed a fine dendritic gamma structure and the presence of Ta-rich Laves phase and fine carbides at interdendritic regions (Fig. 6B). The quantity and morphology of the Laves phase appeared very similar to that observed in the standard alloys. Crack propagation appeared to be associated with the low-melting Laves/gamma eutectic at grain boundaries (Fig. 6C).

FIG. 5 Light (A) and SEM (B-D) micrographs of Inconel 718-Ta Spot-Varestraint specimen tested at 1.1% augmented strain: (A) top surface at low magnification, large arrow indicates fusion boundary, small arrows indicate HAZ liquation cracks; (B) liquated Laves phase at cast dendrite intersticy, note unaffected carbides; (C) liquated Laves phase at cast dendrite intersticy and wetting of gamma grain boundary with eutectic liquid, (D) backfilled crack tip nearest to fusion boundary. In B-D, large arrows indicate Laves phase/gamma eutectic constituent, small arrows indicate Ta-rich carbides.

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FIG. 6 Light (A) and SEM (B,C) micrographs of Inconel 718-Ta Mini-Varestraint specimen tested at 1.2Z augmented strain: (A) top surface at low magnification, large arrow indicates fusion boundary, small arrow indicates solid-liquid interface at instant of straining; (B) fusion zone surface, large arrow indicates Laves phase, small arrows indicate Ta-rich carbides, (G) solidification crack. SEM analysis of an exposed Mini-Varestraint specimen crack surface clearly revealed a dendritic-appearing fracture in the fusion zone and liquation-related intergranular fracture in the HAZ (Fig. 7). These observations confirmed the liquation versus solid-state nature of these cracking phenomena. Quantitative cracking results obtained in this investigation demonstrated a significant improvement in HAZ liquation and fusion zone solidification cracking resistance with Tasubstitution for Nb in cast Inconel 718. Metallurgical characterization, however, revealed similar as-cast microstructures and comparable phase transformations in the weld heat-affected and fusion zone regions. Both alloy chemistries exhibited a Laves phase/gamma eutectic which served as the constitutional-liquation reaction in the HAZ and the terminal solidification reaction in the fusion zone. As suggested previously, the greater solidification cracking resistance of the Ta-modified alloy may be attributed in part to a narrower solidification range versus the standard alloy. Gorrespondingly, an improvement in HAZ liquation cracking resistance may result from a smaller difference between the solidus of the cast dendrite cores and the Laves/gamma eutectic, which was evidenced by a narrower PMR and shorter maximum crack length in the Ta-modified alloy. Simulated weld HAZ studies performed using a Gleeble 1500 system also

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indicated a higher Laves phase liquation temperature for the Ta-Modified versus the standard Inconel 718 alloys (1225 versus i175°C). An alternate and/or complementary explanation for cracking differences may be variations in the interaction of the Ta and Nb-rich Laves phase/gamma eutectics with grain boundary active elements such as boron, which could influence grain boundary wetting. References i. 2. 3. 4. 5. 6. 7. 8.

D. Phillips, W.A. Baeslack III and T.J. Kelly, to he published in Weld. J. G.A. Knorovsky, M.J. Cieslak, T.J. Headly, A.D. Romig, Jr., and W.F. Hammetter, to be published in Met. Trans. W.A. Baeslack III and D. Nelson, Metallography, 19, 371 (1986). J.C. Borland, Brit. Weld. J., 7, 508 (1960). M.J. Cieslak, G.A. Knorovsky, T.J. Headly, and A.D. Romig, Jr., Met. Trans., 17A, 2107 (1986). A.C. Lingenfelter, Private Communication (1987). G.M. Goodwin, W.F. Savage and E.F. Nippes, Weld. J., 56, 283-s (1977). W.F. Savage and C.D. Lundin, Weld. J., 44, 433-s (1965). Acknowledgements

The authors are indebted to General Electric Co., Cincinnati, Ohio for their financial support of this work. Appreciation is also expressed to Mr. Rick Bacon of Systems Research Laboratories at WPAFB, Ohio for his assistance in quantitative electron microscopy.

FIG. 7 SEM fractographs of crack surface in Inconel 718-Ta Mini-Varestraint specimen tested at 1.8% augmented strain: (A,B) fusion zone solidification crack surface; (C,D) HAZ liquation crack surface, arrow in (D) indicates Ta-rich second phase.

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