Characterization of Microstructures and Creep Fracture Characteristics of Dissimilar Metal Welds between Inconel 740H Ni-Based Superalloy and Gr.92 Ferritic Steel

Microstructure evolution and the creep fracture characteristics of dissimilar welds between Ni-based superalloy Inconel 740H and High Cr ferritic steel Gr.92 were investigated.The dissimilar welds were fabricated by shielded metal arc (SMA) welding process with Ni-based austenitic Inconel 182 superalloy.Postweld heat treatment at 760 o C for 4 hours was conducted to temper the fresh martensite formed in the heat-affected zone (HAZ) of Gr.92, and to form γ΄ strengthener in Inconel 740H.The weld interface between the Gr.92 and weld metal was characterized by a discrete line with minimal inter-diffusion, whilst the weld interface on the Inconel 740H side showed an unmixed zone having epitaxial columnar growth from the base metal.It was observed that the fresh untempered martensite formed at the Gr.92-sided coarse-grained HAZ next to the weld interface during PWHT.This may be due to higher Ni contents diffused from the Ni-based weld metal.A sharp increase in hardness was detected at the coarse-grained HAZ of Gr.92 while the hardness minimum occurred at the fine-grained and intercritical HAZ (FG/IC HAZ).Fracture after creep at 650 o C/80 MPa and 700 o C/35 MPa occurred at the FG/IC HAZ of Gr.92, commonly known as a type IV fracture.This may be attributed to the strain localization at the FG/IC HAZ due to the smaller grain size as well as the lowest hardness.The elemental migration across the weld interface was not significant.It was also found that a needle-like Nb-rich δ-phase formed at the interdendritic region of the weld metal after creep at 700 o C/35 MPa.