Effects of stress state and temperature on martensitic phase transformation in TRIP 780 steel

Abstract

The effects of the stress state and temperature on the martensitic phase transformation behavior of TRIP 780 steel sheets was investigated using developed experimental techniques. To investigate the effect of the stress state on phase transformation, the four different stress states were performed

i.e., uniaxial tension, hydraulic bulge test, plane-strain tension and simple shear. The temperature effect was also considered with the range of temperature from room temperature to 100°C for each stress state condition. The digital image correlation techniques were used to measure the strain for hydraulic bulge test and simple shear test. For the plane strain tension the FE analysis was used to measure the major strain, because the conventional mechanical extensometer could not be used. In addition, in situ magnetic measurements were performed to monitor the evolution of martensite content throughout each experiment. A stress-state and temperature dependent transformation kinetics law was proposed that incorporates the non-linear function of the Lode angle parameter in addition to the stress triaxiality and also the temperature effect was included. The newly proposed model could reproduce an accurate description of the martensitic phase transformation kinetics of the TRIP 780 steel over a wide range of stress states and temperature.