Microstructural evolution and flow stress of semi-solid type 304 stainless steel

Thixoforming or semi-solid metal forming offers many advantages in comparison with casting and conventional forging. However, because of the high-melting temperature and other related difficulties, there is relatively less amount of experimental data and investigations available on steels. The present study is aiming to provide the basic deformation characteristics of stainless steel under semi-solid state, which is affected by the change in morphologies of microstructure at elevated temperature. Microstructural evolution and flow stress of semi-solid type 304 stainless steel using strain-induced melt activation method is investigated, and different morphologies of microstructure of type 304 steel are observed at different forming temperatures in the semi-solid state. Hot compression tests for varied combination of deformation rate and deformation temperature around solidus temperature are conducted to obtain flow stress curves of type 304 stainless steel for two different semi-solid states and solid state. Flow stress curves around solidus temperature exhibit abrupt change according to the change in morphologies of microstructure, which is related to the phase change from delta-ferrite/austenite to liquid/delta-ferrite via liquid/delta-ferrite/austenite. The characteristics of deformation in two semi-solid states of type 304 stainless steel are discussed referring the change in flow stress and micrographs of quenched microstructure obtained by the experiments.