  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.
