Analysis of the martensitic transformation at various scales in TRIP steel

The mechanical behavior of transformation induced plasticity (TRIP) steels depends on the martensitic transformation of the retained austenite. The studied material is a three-phase TRIP steel with ferrite base containing less than 1% of bainite and 8% of retained austenite. Tensile tests were performed at various temperatures ranging from −60 to +120 °C. X-ray diffraction was used to determine the kinetics of transformation at various temperatures. First results show that at low temperatures, the austenite transforms into martensite very rapidly, whereas the kinetics of transformation are much slower at high temperatures. The stress state in the austenite at room temperature was also obtained by X-ray diffraction. After pre-strain, it is noticed that retained austenite is in tension probably due to the carbon content which is more important in austenite than in ferrite.