Some Direct Observations on the Metallography of Creep-Cavitated Grain Boundaries

Abstract

Examination by transmission electron microscopy of creep cavities retained in thick foils and by scanning electron microscopy of creep cavities exposed by fracture at − 196° C (77 K) has provided direct evidence for the association of certain microstructural features with creep cavitation in a Cr–Mo–V pipe steel heat-treated to give a bainitic structure similar to that obtained on welding. The cavities occurred on prior austenite grain boundaries in proximity to one or more large carbide particles and in areas containing fewer dislocations and much less of the fine precipitate than in the bainitic matrix. Nucleation and growth of cavities was widespread and continuous throughout the creep tests on grain surfaces subjected to both shear and normal stress components. Some coalescence possibly took place but many cavities still retained their identity and appeared in a “honeycomb” network on the creep-fracture surface.