Effectiveness of Ni-based diffusion barriers in preventing hard zone formation in ferritic steel joints

A numerical procedure based on finite difference method was used to simulate the formation of 'hard' and 'soft' zones, in dissimilar weldments of 9Cr-1Mo and 2 1/4Cr-1Mo steels during high temperature exposure. Kinetic analysis of the calculated diffusion profiles showed that the activation energy for carbon diffusion in 9Cr-1Mo steels is marginally higher than that in Fe - C system. Calculations were extended to incorporate the effect of Ni-based interlayers between 2 1/4 Cr-1Mo and 9Cr-1Mo ferritic steels. The presence of a diffusion barrier was found to reduce the propensity for formation of hard and soft zones, which is related to the interaction parameter epsilon(M)(C) : Thickness of the interlayer required to suppress the formation of hard zone was optimized by the calculations. Transition joints of ferritic steels with Inconel 182 as the interlayer of thickness close to that predicted by the computations were fabricated and exposed to elevated temperature. Microstructural studies and hardness measurements further confirmed the effectiveness of Ni-based interlayers in preventing hard zone formation.