Some Aspects of Cracks in Austenitic Stainless Steels Subjected to High Temperature, Low-Cycle Fatigue Test

Abstract

Observation was made on cracks on the surface and on the cross-sections of austenitic stainless steel specimens (Type 304, 316 and 321) subjected to a low-cycle fatigue test at high temperatures. The specimens were cycled in axial loading under a constant total strain amplitude. The test variables were the total strain range of 0.5 to 2.5%, the strain rate of 0.4 to 40%/min., and the temperature from room temperature to 800°C. The results obtained are summarized as follows:
(1) Initiation of cracks on the surface occurred at the twin boundaries at R.T. and 450°C, and at the grain boundaries at 700°C.
(2) Surface cracks initiated at the twin boundaries where slip bands were formed parallel to them or impinged on them with angles.
(3) Surface cracks observed on the fractured specimen propagated along the grain boundaries when the temperature was higher, the strain rate was lower and the strain amplitude was larger. The relative magnitude of creep damage per cycle showed a correlation with the mode of crack propagation. That is, the larger the creep damage is, the more the mode of propagation tended to be intergranular than transgranular.
(4) Cracks observed on the cross-section of failure specimens were dependent on crystal orientations at 450°C and 600°C (ε=40%/min).