Effects of solution annealing on the precipitation of dendrite-like carbides during continuous cooling in Alloy 690

The precipitation characteristics of intergranular carbides generally have a significant impact on the microstructural homogeneity and properties of Alloy 690. M23C6 carbides with distinct dendritic morphology are found precipitating in the continuously cooled Alloy 690, and this study focuses on the effects of solution heat treatment on the morphology evolution of M23C6 carbides during cooling. Results show that if the solution heat treatment is sufficient enough, carbides tend to precipitate in a dendritic morphology during cooling, otherwise rod-like carbides would be dominant on the grain boundary. The precipitation morphology of carbides basically depends on the critical nucleation radius R∗ and the relative supersaturation S which is closely related to the solute concentration on the grain boundary. Insufficient solution annealing will leave undissolved original carbides on the grain boundary, which can grow prior to the precipitation of new carbides and consume lots of solute elements in advance. This earlier solute consumption and the consequent element diffusion could prevent the formation of dendritic carbides, and then rod-like carbides form instead. This study will help to regulate the precipitation morphology of grain boundary carbides in Alloy 690 during continuous cooling, so as to obtain a microstructure conducive to properties.