The effects of the heating rate on the reverse transformation mechanism and the phase stability of reverted austenite in medium Mn steels

Both the mechanism of the reverse transformation from martensite (α′) to austenite (γ) and the stability of reverted γ were systematically investigated as a function of the heating rate using cold-rolled Fe-(5-9)Mn-0.05C (wt.%) steels. When the specimens were slowly heated at rates below 15 °C s−1, cementite formed along various boundaries, then the reverse transformation from α′ to γ occurred near the cementite particles. The critical temperatures for both cementite precipitation and the reverse transformation increased as the heating rate increased to 15 °C s−1. These results indicate that slow heating rates resulted in diffusive reverse transformation from α′ to γ. However, when the heating rate was greater than 15 °C s−1, rapid reverse transformation from α′ to γ occurred without cementite precipitation, and the critical temperatures for the reverse transformation were not significantly changed. These results indicate that rapid heating rates caused the reverse transformation to occur without diffusion. The diffusively reverted samples exhibited globular-shaped γ grains (γG) of 200-250 nm in diameter. γG had a low density of dislocations and a higher concentration of Mn than the tempered α′ matrix. Diffusionlessly reverted specimens exhibited lath-shaped γ grains (γL) of 200-300 nm in width and 400-700 nm in length. γL had a high density of dislocations, and the Mn concentration was nearly the same as that of the α′ matrix. γG had a higher phase stability than γL because of its fine grain size and high Mn concentration.