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

摘要

A numerical procedure based on finite differ_ence 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 Cr_Mo 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 ε_C~M. Thickness of the interlayer required to suppress the for_mation 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 com_putations 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.