Phase transformation in AISI 410 stainless steel

AISI 410 stainless steel can virtually fully transform to dislocated lath martensite at a very low cooling rate by air cooling. It is found that at the same cooling rate the specimen austenitized at the higher temperature has a lower martensite start temperature (Ms); besides, the difference of Ms becomes much larger at the higher cooling rate. In the continuously cooled specimens investigated, significant amounts of inter-martensite retained austenite film may be imaged, where the martensite laths tend to be in the same crystallographic orientation. On the other hand, micro-twinning is generally detected in martensite laths when the adjacent variants of martensite form in a twin-related manner. Another important result indicates that no bainitic structure can be obtained in this steel for the isothermal transformation just above the martensitic start temperature (Ms), and suggests that the single C curve in the time–temperature–transformation (TTT) diagram of AISI 410 is for a diffusional transformation. The prior heavy deformation of austenite also gives strong evidence to suggest that severe deformation causes mechanical stabilization of austenite against martensitic transformation.