HIGH-TEMPERATURE BRITTLE INTERGRANULAR FAILURE IN AUSTENITIC STAINLESS-STEELS

Crack growth tests have been carried out between 550 and 750-degrees-C on AISI 316 and 347 stainless steels containing controlled additions of P, Sn, and S, after a heat affected zone simulating heat treatment. The crack growth rate was found to be controlled by a damage input rate parameter C*. At low values of C*, crack growth was found to be featureless and intergranular in all alloys. Increasing C* led to growth by intergranular microvoid coalescence, and then transgranular microvoid coalescence. Transmission electron microscopy (TEM), scanning TEM, and Auger electron spectroscopy studies showed that, as in the high temperature brittle intergranular fracture mechanism found in ferritic alloys, the slow, brittle fracture mode in austenitic steels is associated with the stress assisted flow of S to grain boundaries ahead of the crack tip. A component of finite strain rate is also required, leading to a greater brittle crack growth resistance in strain aging alloys.