Austenite reversion suppression with deep cryogenic treatment: A novel pathway towards 3rd generation advanced high-strength steels

This study investigates the impact of deep cryogenic treatment (DCT) on microstructure and properties of a martensitic stainless steel AISI 431. The reasonably simplistic microstructure tailoring with DCT delivers a novel pathway towards advanced 3rd generation high-strength steels (AHSS), whilst utilizing existing alloys and chemistry. The resulting microstructural changes deliver improved strength and hardness of the material, whilst still sustaining good elongation properties. DCT reduces the material's toughness but improves its deformation resistance and wear resistance. The study reveals that DCT modifies the microstructural evolution during tempering. During the conventional treatment, AISI 431 develops austenite reversion transformation (ART), which allows the high strength and elongation capabilities formed with transformation-induced plasticity (TRIP). In contrast, DCT progresses the microstructure even further by retransforming the reverted austenite formed through ART into tertiary α-martensite, ε-martensite and carbides that increase the strength and deformation behavior of AISI 431. The novel microstructural modification that we have entitled cryogenic austenite retransformation (CAR), is both experimentally and theoretically assessed. The underlying transformation mechanisms of CAR and individual stages of DCT were evaluated with ex-situ electron backscatter diffraction and transmission electron microscopy. The impact of DCT on the modified transformation capabilities of AISI 431 is discussed in relation to possible implementation in automotive industry as a universally applicable treatment procedure.