Dissimilar Metal Welds in Grade 91 Steel

Abstract

Abstract Unpredictable failures near the phase boundary in Grade 91 dissimilar metal welds (DMW) with nickel based filler metals represent a significant problem for the power generation industry. In order to determine the root cause for these failures, it is necessary to understand the formation of the microstructure in the weld regions around the site of failure. Thermal histories were therefore measured inside the Grade 91 steel heat affected zone (HAZ) of an autogenous weld and of a DMW in the form of bead on plate with Alloy 625 to study the effect of the weld thermal cycle on microstructural formation. It was found that the HAZ in the DMW experienced longer dwell time at high temperatures because of the latent heat of fusion released during Alloy 625 solidification (1350 - 1125 °C). This allowed longer time for carbide dissolution and phase transformations in the DMW than in the autogenous weld. Additionally, the 625 filler metal created a large chemical potential gradient for carbon, which when combined with longer dwell times, yielded carbon depletion in the heat affected zone. Retention of δ ferrite in the coarse grained HAZ (CGHAZ) of DMWs was found to be an indicator for these mechanisms.