Anomalous stabilization of austenitic stainless steels at cryogenic temperatures

The deformation-induced formation of α′ martensite was investigated by tensile testing of a Fe-19Cr-3Mn-4Ni-0.15C-0.17N cast austenitic steel between −196 °C and 400 °C. The steel did not exhibit spontaneous α′ martensite formation at temperatures as low as −196 °C. Therefore, the critical driving force for the formation of α′ (−2780 J/mol) was obtained by determining the complementary mechanical energy necessary to trigger the deformation-induced α′ martensite at 0 °C. Driving forces for the γ→α′ transformation at other tensile test temperatures associated with the deformation-induced α′ formation were then obtained by subtracting the mechanical energies applied to trigger the martensitic transformation from the critical driving force. The triggering mechanical energies were obtained from in-situ magnetic measurements which enabled to mark the onset of the γ→α′ transformation. The driving forces for the γ→α′ transformation calculated using the preceding method indicated an increase in the stability of austenite which was attributed to changes in the mechanical and physical properties of austenite in the vicinity of the Néel temperature. The method can be used to calculate modified driving forces for the occurrence of the γ→α′ phase transformation.