Initiation of intergranular cracks in SUS316 stainless steel subjected to low-cycle fatigue below 750°C

摘要

The initiation of intergranular cracks under low-cycle fatigue conditions was studied by comparing the morphologies of crack initiation both in air and in vacuum environments within a temperature range from 200 to 750°C. The most frequent mode of crack initiation below 300°C in air is transgranular, while it is intergranular above 500°C. Reductions in fatigue life at higher temperatures (T>450°C) could be closely related to the behaviour of intergranular crack development. Possible mechanisms involved in the initiation of intergranular cracks are discussed with a particular emphasis on the contribution of high temperature oxidation at the surface. The initiation of intergranular cracks is associated with the combined effects of grain boundary sliding (a mechanical effect) and localised oxidation along the grain boundary (a chemical effect) above temperatures of 600°C. An essential characteristic of high temperature fatigue crack initiation and its early stage of propagation is characterised by preferential oxidation along the newly exposed fresh surface brought about by mechanical sliding. On the other hand, the initiation of intergranular cracks at medium temperatures of approximately 400°C involves the blocking effect of grain boundaries on the development of the transgranular slip bands, with no appreciable involvement of oxidation and grain boundary sliding.