Microstructure and microchemistry of hard zone in dissimilar weldments of Cr-Mo steels

This paper reports the microstructure and microchemistry of "hard" and "soft" zones in a dissimilar weldment between 9Cr-lMo and 21/4Cr-1Mo ferritic steel subjected to postweld heat treatments (PWHT). Diffusion of carbon driven by the activity gradient across the weld interface is found to result in the formation of a soft zone in the low-Cr side and a carbide-rich hard zone adjoining the soft zone in the high-Cr side of the weldment. Detailed analytical transmission electron microscopy (ATEM) studies were essential to evaluate a number of microstructural parameters based on which the variation of concentration of carbon across the hard zone was derived using mass balance equation. The concentration of carbon in the hard zone thus arrived follows the same trend as that of the variation of area fraction of carbides with distance across the hard zone.