A Novel Cast Multiphase Stainless Steel With High Strength and High Toughness

Multiphase stainless steels offer very attractive combinations between strength, toughness and corrosion resistance due to the coexistence of different microstructural components and their interactions. In this study, a novel cast multiphase stainless steel containing martensite, ferrite and austenite was fabricated by a combination of alloy optimization and appropriate heat treatment. The effect of aging temperature on microstructure and mechanical properties was systematically studied based on the stability of austenite and the variations of nanoscale precipitations. As the aging temperature increases, the volume fraction of reversed austenite shows a parabolic pattern of first increasing and then decreasing due to the diffusion of Ni, while the lath martensite undergoes gradual decomposition. Appropriate aging treatment can significantly improve the strength and toughness of the steel. This enhancement is due to the TRIP effect of austenite and the precipitation of the nanoscale Cu/NiAl co-precipitates pinning dislocations in ferrite. However, with increasing aging temperature, the precipitates in ferrite and ferritic/martensitic phase boundaries are mainly Laves phase and R phase (molybdenum-rich intermetallic compound), which are detrimental to the properties of the investigated steel. A detailed analysis of the fracture morphologies has been conducted to reveal the fracture mechanism of the cast high strength multiphase stainless steel. Voids caused by the deformation of NbC particles and cracks propagating along the ferritic/martensitic phase boundaries are the main cause of fracture.