Austenite reversion in AISI 201 austenitic stainless steel evaluated via in situ synchrotron X-ray diffraction during slow continuous annealing

In situ synchrotron X-ray diffraction was used to track real-time austenite reversion in AISI 201 austenitic stainless steel deformed to a true strain of 0.34 under tensile testing. The deformed material was continuously annealed from 100 degrees C up to 800 degrees C at a heating rate of 0.05 degrees C s(-1). Phase changes and microstrain partitioning were evaluated by means of the direct comparison method and modified Williamson-Hall plots, respectively. The microstructure of the deformed steel consists of gamma, epsilon- and alpha'-martensite with volume fractions of 0.24, 0.07, and 0.69, respectively. epsilon -> gamma reversion occurs within the temperature range of 150-400 degrees C through a shear mechanism. The starting (A(s)) and finishing (A(f)) temperatures for alpha' -> gamma reversion are 486 degrees C and 770 degrees C, respectively. Three stages were distinguished for this reaction. By evaluating the crystallite size of both gamma- and alpha'-phases, it can be inferred that alpha' -> gamma reversion is diffusion-controlled. Such results were corroborated by thermodynamic simulations to assess the driving force for gamma-formation. Microstructural aspects such as gamma-nucleation sites, in-grain misorientation, grain refinement, and crystallographic texture were investigated by means of electron backscatter diffraction. Fresh formed ultrafine austenite grains hold different crystallographic orientations than those of untransformed austenite.