Smoothing ofa Cavitated Creep-Fracture Surface by Surface Self-Diffusion

In-service smoothing of fractured grain-boundary cavities behind an advancing intergranular crack in the heat-affected zone of a weld in a cast CrMoV steel steam valve is attributed to capillarity-motivated mass transport by surface self-diffusion. Comparison of the observed attenuation kinetics with predictions from a simplified model implies an operative surface self-diffusion coefficient, D _δ, substantially less than that reported for pure α-Fe. This reduction in D _δ might possibly be associated with the segregation to grain boundaries of tramp elements, which are expected to reduce grain-boundary energy and lead to the temperembrittlement observed for such steels.