Effect of liquid nitrogen and warm deformation on the microstructure and mechanical properties of 321-type metastable austenitic steel

Abstract

New thermomechanical treatments combining plastic deformation with cooling in liquid nitrogen followed by warm deformation and annealing are applied to form fine-grained structure in the 321-type metastable austenitic steel. The structural-phase transformations during these treatments are studied by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy with Electron Backscatter Diffraction (SEM EBSD), X-ray Diffraction (XRD) and magnetic investigations. During liquid nitrogen deformation (LND) with a true strain of e = 0.2 a band-like deformed structure with a volume fraction of α′-martensite ≈ 70% is formed. The subsequent warm deformation (WD), with e = 0.4, i.e. LND + WD treatment at 600 °C, leads to a partial reverse α′ → γ transformation under plastic deformation. A fine-grained structure with high fraction of low-angle boundaries was formed by the shear reversion mechanism. The α′-martensite content decreases to ≈50%. The mechanical properties of the material in different stages of the treatments are studied under static tensile testing. It is demonstrated that these treatments give rise to an increase in the yield strength of more than 1250 MPa. Additional annealing provides a more complete (up to 85–93%) martensite-to-austenite transformation and allows maintaining a fine-grained structure. After annealing, the fraction of high-angle and twin boundaries in the austenite increases. The diffusion-type mechanism of austenite reversion is involved in the formation of microstructure features under these conditions. Annealing allows changing tensile and ductile properties (yield strength up to ≈ 830–945 MPa at elongation of ≈10–19%) through modifying the microstructure and phase composition of fine-grained steel.