Studies on Martensite Transformation in a Metastable Austenitic Cr-Mn Stainless Steel

The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 degrees C at various heating rates between 5 degrees C/s and 150 degrees C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 - 620 degrees C for different durations between 0.1 - 600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 degrees C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, alpha'-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the pre-existing alpha'-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.