Austenite reversion in lean duplex steel: Microstructural, dilatometric and magnetic characterization

Austenite reversion, i.e., a’-martensite → γ phase transformation in UNS S32304 lean duplex steel was investigated. The material was cold rolled to a true strain (ε) of 1.61 and subjected to continuous annealing up to 1000 °C with a heating rate of 3 °C/min. From the dilatometric curve, an unexpected thermal expansion at around 545 °C was found within the temperature range where the austenite reversion occurs. Based on this unexpected behavior, additional samples were annealed at the same heating rate until key temperatures. Besides dilatometry, microstructural changes were followed by means of magnetic measurements at room temperature, Vickers microhardness testing, high-resolution electron backscatter diffraction (EBSD), and electron channeling contrast imaging (ECCI). From EBSD data, a protocol was developed to distinguish the different constituents in the material during the austenite reversion, i.e., α'-martensite, ferrite, and both reversed and untransformed austenite. The driving force for the austenite reversion was calculated using thermodynamic calculations. The a’-martensite → γ transformation begins at about 545 °C and ends at almost 800 °C. In the early beginning, the austenite reversion is governed by a shear mechanism. At higher temperatures, at about 725 °C, KAM (kernel average misorientation) distributions and texture of both reversed and untransformed austenite show evidence of a diffusion-controlled austenite reversion mechanism. These results are supported by thermodynamic calculations and microstructural evidence revealed by ECCI.