The Stability of Fine, Sub-Grain Microstructures Within Carbon Depleted Regions of Dissimilar Metal, Ferritic, Creep Resistant Welds

The duration of post weld heat treatments (PWHT) applied to thick section multi-pass dissimilar metal welds (DMW), involving ferritic creep resistant steels of differing chromium content, are shown to have a considerable impact on the performance of the welded joint. Welding consumables of alloy types P22 and P24 have been used to form joints with P91 base alloy which were subsequently post weld heat treated for varying durations. High resolution transmission electron microscopy (TEM) has been exploited in the characterisation of precipitation in the weld material and the heat affected zone. It has been shown that uphill diffusion of carbon from the low to the higher alloy material during PWHT and creep test conditions occurs in all specimens. Selected area diffraction (SAD) and convergent beam electron diffraction (CBED) studies of carbon extraction replicas reveal extensive dissolution of M₂₃C₆ and M₇C₃ carbides in the decarburised zone of the weld alloy subsequent to post weld heat treatments. However, welds completed using Nb and V containing consumables retain a fine distribution of MX precipitation in the carbon depleted regions after PWHT. The retention of these microstructure stabilising carbonitrides facilitates the preservation of an ultra fine sub-grain microstructure, thus avoiding recrytallisation which is invariably observed in post weld heat treated P22:P91 DMWs. Cell size comparisons of the sub-grain microstructures have been investigated utilising channelling contrast back scattered scanning electron images of as welded and post weld heat treated material.