Effect of postweld heat treatment on development of interfacial structures in nickel-based transition joints

Abstract

The interfacial structures between 2·25Cr–1Mo steel and Inconel 182 weld metal have been studied after post-weld heat treatments (PWHTs) at 700°C and subsequent aging at 630°C. This aging temperature accelerates the changes in interfacial structure that occur during power-station operation, and thus provides a method of studying the effects of the initial PWHT. The paper shows that during PWHT for ≤8 h at 700°C, arrays of carbide particles develop in the ferritic steel, parallel and very close to the weld metal interface, and that these arrays continue to grow during aging at 630°C for 6000 h. However, the precipitate sizes after PWHT are small compared with those developed during the subsequent aging. For longer heat treatments, up to 100 h at 700°C, the I interfacial precipitates develop to significant sizes, but growth then ceases because of the limited carbon migration from the 2·25Cr–1Mo steel. Aging at 630°C then causes carbon redistribution in the heat-affected zone, resulting in an incubation period before further interfacial precipitate growth can occur. The net result is that after aging for ≥500 h at 630°C, PWHTs of 2–100 h at 700°C have negligible effects on the interfacial-precipitate sizes and distributions, compared with those found in similarly aged as-welded specimens.MST/119