Structure and properties of high interstitial as-cast austenitic stainless steel CrMnCN for use at subzero temperatures

Austenitic stainless nickel-free as-cast steel with high (C + N) =1.27 mas.% content was studied. Homogenization and the following solution treatment contributed to dissolving Cr2N nitrides in austenite, but Cr3C2 carbides and oxy-carbides were partially preserved. When the heat-treated steel was cooled to subzero temperatures, its lattice parameter initially decreased by 0.3 % at −90°C and remained at this level down to −196°C. This phenomenon was accompanied by an increase in the concentration of stacking faults. It was assumed that thermal compression of the fcc lattice extremely enriched in carbon and nitrogen contributed to greater internal microstresses with lowering temperature, being a trigger for structural changes. The formation of the stacking faults is a result of relaxation of internal microstresses, reducing the RMS displacements of atoms from their equilibrium positions. In the temperature range from 20 down to −80°C, the strength properties of the studied steel were at the high levels, rising with lowering the test temperature (σ0.2 = 570 MPa and σuts = 950 MPa at 20°C in contrast to σ0.2 = 800 MPa and σuts =1200 MPa at −80°C). Elongation was not lower than 10 % in all investigated cases. At the cryogenic temperature, the studied steel fractured at its yield point. Steel cannot be used at cryogenic temperatures due to unsatisfactory low ductility.