A study of the reverse martensitic transformations in austenitic stainless steels by calorimetry and isothermal heat treatments

The objective of this work was to study the reversion of the strain-induced α-martensite to γ-austenite in three sets of AISI 304, 316-L and 347 stainless steels specimens. To obtaining a high amount of αʹ-martensite (γ → αʹ ), the samples were submitted to 63% reduction in thickness by cold rolling at a temperature of –70 ºC. The reversion of martensite to austenite was made by means of 1-hour isothermal heat treatments between 200 and 1000 ºC (αʹ → γ). Martensite amounts were determined by measurements made with a commercial feritscope. The main analysis of reversion was performed by differential scanning calorimetry (DSC) studies with heating rates of 10, 20 and 40 °C/min. It was determined that under continuous heating the deformation-induced reversion is carried out by a superposition of the diffusional and shear mechanisms, for the heating rates tested. The reversion begins via diffusion-controlled mechanism at temperatures between 414 and 453 °C and ends via non-diffusional mechanism between 557 and 633 °C. On the other hand, the energy per unit mass of the transformation turned out to be between 37 and 63 J/g; and it was found that this depends on the heating rate.

Keywords:
αʹ-martensite; reversion; austenitic stainless steel; calorimetry; isothermal-treatments