Abstract
The method of surface observation for creep voids was discussed for SUS304H, 316H and 321H austenitic stainless steels, comparing with cross sectional observation. The creep tests were carried out up to about 60000h at 600-750°C. After machining, the surface of the specimen was polished and etched, then the microstructures of the specimen surface were metallographically observed by optical microscope as a function of depth from the surface and compared with those of vertical section (stress direction of specimen). The precipitation free zone of σ phase was observed beneath the specimen surface and its depth was about 60μm after about 20000h at 700°C. The observation of σ phase precipitates along grain boundaries is important to the evaluation of creep damage for austenitic stainless steels, because σ phase has roles to accelerate the nucleation and growth of creep voids. The polishing of specimen surface up to 70μm in depth lead the same microstructure as that at the vertical section of the center part of specimen. For SUS321H steel, the surface cracks were formed at the grain boundaries of specimen surface and propagated into the inner part of specimen. The length of the surface cracks observed after polishing the surface layer was about twice of that observed at vertical section.
