Metallurgical Investigation into Ductility Dip Cracking in Ni-Based Alloys: Part II

In this second of two papers, the mcrostructural and microchemical evolution of Alloy 600 (A600), Alloy 690 (A690), Filler Metal 82H (FM82H), and Filler Metal 52 (FM52) during the weld thermal cycle was investigated and compared to the hot ductility data presented in the first paper (Ref. 1). The Gleeble (R) hot ductility test was used to subject these four alloys to a simulated weld thermal cycle. Water quenching was conducted at select temperatures so that the elevated temperature microstructure could be subsequently characterized. Microstructural and microchemical characterization was carried out using scanning electron microscopy, transmission electron microscopy, and analytical electron microscopy techniques. Complete dissolution of intergranular carbides was observed in A690 and FM52 at similar to 2400 degrees F (1316 degrees C), both of which exhibit an on-cooling ductility minimum at 1600 degrees F (871 degrees C). Of all four alloys, the greatest resistance to ductility dip cracking (DDC) was observed in A600 and A690 during on-heating, which had coarse, homogenously distributed intergranular carbides. FM82H, which formed NbC intergranular carbides, had the most stable intergranular microstructure and serrated grain boundaries, which corresponded to the best overall DDC resistance. Modifications to the thermal cycle that resulted in increased intergranular carbide coverage in FM82H and FM52 also reduced DDC susceptibility. AEM analysis did not reveal any sulfur or phosphorous segregation in FM52 at 1600 degrees F (871 degrees C) on-heating, on-cooling, or after a 60s hold. Samples with microstructures that consisted of coarsened carbides and/or serrated grain boundaries, which are expected to decrease grain boundary sliding, were found to be resistant to DDC. Based on the results of this work and the results previously presented in Paper I (Ref. 1), grain boundary sliding contributes to DDC. Conversely, sulfur and phosphorous embrittlement do not play a role in DDC of FM52 at the concentrations investigated. The dynamic precipitation of partially coherent intergranular M23C6 carbides at intermediate temperatures may exacerbate DDC in A690 and FM52, but requires further investigation.