Effect of Strain Induced Martensite Reversal on the Degree of Sensitization of Metastable Austenitic Stainless Steel

Abstract

Effect of reversion of strain induced martensite on degree of sensitization (DOS) of Cr-Mn Austenitic stainless steel (ASS) have been examined. The martensite reversion is known to be effective in refining the grain size of metastable ASS. However, martensitic reversal takes place only after severe cold rolling reductions. In this study, the effect of degree of cold rolling and subsequent thermal ageing on the microstructure, mechanical, and sensitization properties of a metastable Cr-Mn ASS was investigated. Samples were cold worked (25% and 45%) and then thermal aged at different temperature of 700°C and 900°C for 2 hrs. Microstructures were subjected to optical microscopy. Microstructural study shows that with increasing thermal ageing temperature, strain induced martensite undergoes reversal to austenite and is associated with grain refinement. XRD analysis has revealed that at 25% cold work, volume fraction of martensite was 24%, and as cold work was increased to 45%, the volume fraction of martensite was 42%. Whereas after thermal ageing at 700°C, part of strain induced martensite undergo reversal to austenite and only 18% martensite is left in the matrix. On thermal ageing at 900°C, only 2% strain induced martensite was left in the matrix. Hardness also follows the same trend as observed for volume fraction of martensite during cold work and subsequent thermal ageing. As the cold work percentage increases, hardness and volume fraction increases but as thermal ageing temperature increases, both hardness and volume fraction of martensite decrease rapidly. Degree of sensitization (DOS) decreases with increasing thermal ageing temperature.