Metal interlayers to prevent ‘hard zone’ formation in dissimilar weldments of Cr-Mo steels — A comparison between Cu, Co and Ni

Abstract

High temperature exposure of dissimilar ferritic steel weldments either during service or during post weld heat treatment has been found to result in the formation of deleterious ‘hard’ and ‘soft’ zones near the weld interface. Activity gradient driven carbon diffusion was found to be responsible for the formation of the zones. One of the methods suggested for suppression of the zone formation is by use of carbon diffusion barriers. The effectiveness of this method was investigated in this study using numerical simulations based on finite difference method. Diffusion barriers like copper, cobalt and nickel were evaluated for this purpose due to their positive interaction parameter, ɛ C M with carbon. It was found that these interlayers if introduced between 9Cr-1Mo and 2¼Cr-1Mo ferritic steels reduce the propensity for formation of hard and soft zones. It was found by computation that their efficiency correlates well with the differences in interaction parameter with carbon.