Creep failure in Type 316 austenitic steel

Abstract

The creep rupture behaviour of a Type 316 austenitic steel has been investigated at 625°C for a wide range of grain sizes and applied stresses. The rupture microstructure has been examined in detail and the modes of failure deduced. Increasing the grain size, at a given stress, leads to a maximum in rupture ductility at a grain size of 25 flm. Failure occurs after the formation of triple-point wedge cracks at all grain sizes and occurs by intragranular tearing of the regions between such cracks. At small grain sizes there is a low density of small cracks, which are elongated in the applied stress direction, and failure appears to be consistent with the operation of the socalled intergranular void sheet mechanism. At large grain sizes there is a greater density of larger cracks and failure occurs after a certain amount of crack growth. Failure appears to depend on the stress concentration developed at the tips of the largest cracks, that is, it occurs by a Griffith-type mechanism. The extent of stress concentration achieved at failure, however, is shown to depend on the material state, and may be related both to the ease with which the stress concentration may be relaxed and tothe effect of grain boundary intermetallic particles on grain boundary surface structure.