Anomalous stabilization of austenitic stainless steels at cryogenic temperatures

The deformation-induced formation of alpha' martensite was investigated by tensile testing of a Fe-19Cr-3Mn-4Ni-0.15C-0.17N cast austenitic steel between -196 degrees C and -400 degrees C. The steel did not exhibit spontaneous a' martensite formation at temperatures as low as -196 degrees C. Therefore, the critical driving force for the formation of alpha' ( -2780J/mol) was obtained by determining the complementary mechanical energy necessary to trigger the deformation-induced alpha' martensite at 0 degrees C. Driving forces for the gamma ->alpha' transformation at other tensile test temperatures associated with the deformation-induced alpha' 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 gamma ->alpha' transformation. The driving forces for the gamma ->alpha' 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 Neel temperature. The method can be used to calculate modified driving forces for the occurrence of the gamma ->alpha' phase transformation. (C) 2016 Elsevier B.V. All rights reserved.