Microstructural evolution of a low-alloy steel / nickel superalloy dissimilar metal weld during post-weld heat treatment

TL;DR

This study investigates how the microstructure of a dissimilar metal weld between low-alloy steel and nickel alloy evolves during post-weld heat treatment, revealing key microstructural changes and compositional gradients.

Contribution

It provides detailed insights into microstructural evolution and chemical potential differences in dissimilar metal welds during PWHT, using advanced characterization techniques.

Findings

Microstructural changes occur at the fusion boundary during PWHT.

Chemical potential differences influence carbon migration across the weld.

Gradient of composition and hardness varies post-PWHT.

Abstract

The microstructural evolution of a dissimilar metal weld (DMW) obtained by narrow-gap gas tungsten arc welding (NG-GTAW) was investigated after it was subjected to a post-weld heat treatment (PWHT). The case studied here is a joint between low-alloy steel pipes and a stainless steel steam generator using a nickel based alloy as filler material. The fusion boundary that was the focus of this work was that between the low-alloy steel (2.25Cr-1Mo) and the nickel alloy (alloy 82). The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical potential for carbon, which is mobile at the PWHT temperature. A number of advanced characterization techniques were used to assess the gradient of composition, hardness and microstructures across the fusion line, both as welded and after PWHT. This complete analysis permits to highlight and understand the main microstructural changes occurring during the PWHT.