Transition of creep damage region in dissimilar welds between Inconel 740H Ni-based superalloy and P92 ferritic/martensitic steel

Characterization of microstructures and creep fracture characteristics of dissimilar welds between Ni-based Inconel 740H superalloy and ferritic/martensitic P92 steel has been investigated. The welds were produced by shielded metal arc (SMA) welding process with the AWS A5.11 Class ENiCrFe-3 filler metal, commonly known as Inconel 182 superalloy. Postweld heat treatment (PWHT) at 760 °C for 4 h was conducted to temper the martensite in heat-affected zone (HAZ) of P92 and to form γ′ strengthener in Inconel 740H. The deposited Inconel 182 austenitic weld metal had a columnar microstructure, and grew epitaxially from the Inconel 740H base metal. The weld interface between P92 and weld metal was characterized by the discrete line with a minimal inter-diffusion, where the fresh untempered martensite was found to be formed during PWHT due to higher Ni contents diffused from Inconel 182 into P92. The P92 base metal and Inconel 182 weld metal displayed low hardness values (~240 Hv), and the sharp hardness increase was detected at coarse-grained HAZ of P92 while the hardness minimum occurred at the fine-grained HAZ (FGHAZ). Fracture location after creep was found to shift from the P92-sided fusion line to the FGHAZ of P92 with increasing creep temperature. The carbon migration from the P92 into the weld metal was not significant at lower creep temperature (600 °C), where the crack initiation at the P92-sided fusion line appeared to be related with a significant strain incompatibility across the fusion line. At both 650 °C and 700 °C where diffusion-controlled creep was more pronounced, the type IV cracking was observed. This may be attributed to the strain localization at the weak grain boundaries of FGHAZ due to the smaller grain size as well as the lack of grain boundary strengthening by carbides.