Influence of Negative Temperatures on Crystal Structure, Properties, and Fracture of Cr–Mn–C–N Steel

The structure of casting austenitic Cr–Mn–C–N steel at low enviromental and cryogenic temperatures has been investigated by means of X-ray diffraction analysis and transmission electron microscopy. The results demonstrate that the parameters of the crystal structure undergo a change during cooling of the quenched steel. During the cooling process from 20 to –90°C, the austenite lattice parameter undergoes a decrease, whereas the concentration of stacking faults remains unaltered. At lower temperatures, the FCC lattice parameter exhibits a stabilizing effect, while the concentration of stacking faults increases sharply. It was observed that there are temperature-dependent displacements of atoms belonging to the FCC lattice from their equilibrium positions. In close-packed planes with the {111} orientation, the observed displacements are larger than in planes with the {200} orientation, along the entire temperature range. The alterations in the crystal structure parameters are indicative of the relaxation process of internal stresses. The steel exhibited a set of high strength properties within the temperature range from –105 to 20°C, with σ_0.2 of 800 MPa and σ_u of 1100 MPa. Additionally, the steel demonstrated satisfactory ductility of 10% and a ductile nature of fracture. At a temperature of –196°C, steel fracture is alwaya of brittle character.