Creep and Creep-Fatigue Intergranular Damage in Austenitic Stainless Steels: Discussion of the Creep-Dominated Regime

Abstract

Creep and strain controlled low-cycle fatigue tests with an imposed hold time at maximum tensile strain were carried out, mainly at 600°C, on two heats of 316L stainless steel. Very long times to failure (≃ 104 h) corresponding to low applied strain and long dwell period (24 h) were investigated. Both fatigue and creep damage were measured by quantitative metallography. Creep intergranular surface cracking shortens the initiation stage of fatigue cracks, while creep intergranular bulk cracking accelerates the average fatigue crack growth rate. These observations are the basis for a creep-fatigue interaction model previously proposed using intergranular damage as a life correlating parameter. The applicability of this model is tested with results of 104 h creep-fatigue tests showing a saturation effect. The saturation in creep-fatigue life observed for long dwell periods is explained by an improvement in creep ductility. This intergranular damage approach is discussed in the light of the creep dominated regime concept. It is shown that pure creep failure data cannot provide reliable prediction of creep fatigue life. The need to use a model that takes into account the effect of creep intergranular damage on fatigue crack propagation is emphasized.