Austenite Grain Refinement by Reverse α' → γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investigated using optical microscopy, X-ray diffraction (XRD), electrical resistivity and hardness testing. Austenite grain refinement was successfully achieved by a two-step heat treatment. First, martensite was produced by cooling the solution-treated samples to -196 °C. Then, the deep cryogenic treated samples were heated to 850 °C upon slow or rapid heating. The mean size of original austenite grain was about 400 μm. But the mean size of equiaxed reversion austenite was refined to 50 μm. Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating, and the residual austenite was decomposed, resulting in the formation of pearlite nodules at the austenite grains boundaries. The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite. However, the reverse transformation upon rapid heating was predominated by displacive manner. The residual austenite was not decomposed. The plate a-phase was carbon-supersaturated until the starting of reverse transformation. The reverse transformation was accompanied by surface effect, resulting in the formation of plate austenite with high density dislocations. The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.