Creep-fracture initiation in 2¼%Cr-1%Mo steel

Abstract

A detailed study has been made of grain-boundary damage in a 2¼%Cr-1%Mo steel after creep at 565°C. By subjecting the material to specific heat-treatments, a quantitative description of the resultant structures together with an assessment of creep damage has enabled creep cavitation to be correlated with certain microstructural features. Large numbers of cavities were distributed heterogeneously on prior austenite grain boundaries and an association with coarse carbide particles was evident. The cavity population was relatively insensitive to changes in the prior austenite grain size but was found to be inversely related to the width of precipitate-free zones (PFZ's) at the prior austenite grain boundaries. This result was linked with surface observations of zone shear and suggested that stress concentrations necessary for cavity nucleation were induced by local accommodation strains adjacent to the grain boundaries. On these lines, a theoretical nucleation model has been proposed and discussed in the light of results obtained.