NUCLEATION OF INTERGRANULAR CRACKS DURING HIGH TEMPERATURE LOW CYCLE FATIGUE OF SUS316 STAINLESS STEEL

Fundamental behaviour of intergranular cracking of SUS316 stainless steel which characterises the high temperature low cycle fatigue process has been studied with a special emphasis on the interaction between oxidation and a grain boundary sliding. Two types of specimens were prepared for fatigue experiment to extract a sole effect of surface oxidation on crack nucleation. One was heat-treated in air and the other was in vacuum so that the specimens had the same history of heat treatment except oxidation to the surface. Results of fatigue tests of these specimens well explain the relationship between oxidation and surface cracking as follows. The morphology of the oxidised surface of the specimen subjected to low-cycle fatigue at 700°C is quite different from that of the oxidised surface caused by simply holding at the same temperature in air for several hours with no applied stress. Localised oxidation along the grain boundary is a characteristic feature for the specimen fatigued at 700°C, while no localised oxidation was observed when the specimens were simply held at the same 700°C, i.e. with no fatigue loading. Accordingly, intergranular cracking in high temperature low cycle fatigue in air occurs when grain boundary sliding due to cyclic loading is accelerated by localised oxidation along the grain boundary.