Retention of Delta Ferrite in the Heat-Affected Zone of Grade 91 Steel Dissimilar Metal Welds

This study aimed to determine the mechanism of δ-ferrite retention in the coarse-grained HAZ (CGHAZ) of Grade 91 steel dissimilar metal welds (DMWs) with Ni-based filler metals. This phenomenon was investigated in four DMWs made with cold-wire gas tungsten arc process using Alloys 625, 617, 82, and P87 filler metals. A narrow band of δ-ferrite grains was identified in the CGHAZ in all welds. It was hypothesized that δ-ferrite retention was caused by local carbon depletion in the CGHAZ, which was validated through extensive thermodynamic and kinetic simulations and metallurgical characterization. Carbon diffusion across the fusion boundary was driven by the carbon chemical potential gradient between Grade 91 steel and the Ni-based filler metals, which was facilitated by long high-temperature dwell times resulting from a difference in heat capacity and thermal conductivity between the base and filler metals. A linear relationship was established between the amounts of retained δ ferrite and the predicted carbon depletion in the CGHAZ of each DMW. Alloy 625 filler metal generated the largest extent of carbon depletion and the most retained δ ferrite, followed by Alloys 617, 82, and P87. The carbon depletion resulted in local softening of the CGHAZ martensite.