Temperature-dependence of the TRIP effect in a metastable austenitic stainless steel
The effect of test temperature in the range 20 - 400°C on mechanical properties under quasi-static tension and phase composition in metastable austenite-martensite TRIP steel in the form of a sheet of 0.3 mm thickness is investigated. Here strength and ductility increases are caused by austenite to martensite phase transformation. As supplied, the steel contained 65 % martensite and 35 % austenite. Due to tensile testing at room temperature, the amount of martensite increases to 84 %, which leads to high strength (1600 MPa) and a relative elongation to failure of 20 %. There is no neck formation. With an increase in test temperature from room temperature to 400°C, the strength characteristics, especially yield strength, sharply decreases to 700 MPa and below. At the same time, the elongation to failure decreases by an order of magnitude to 2 %. Above a test temperature of 100°C, deformation localization begins, the length of the yield plateau on the stress-strain curve decreases, and then disappears. X-ray structural analysis of the sample surface after tension showed that the volume fraction of martensite in the microstructure decreases from 84 % at 20°C to 42 % at 400°C. With an increase in temperature in the range of 20 - 400°C the influence of the TRIP effect on the mechanical properties of austenitic steel gradually decreases, and the direct transformation of austenite to martensite eventually changes to the reverse transformation of martensite to austenite. It is assumed that the embrittlement of steel with increasing temperature is associated with inhomogeneous martensite decomposition and precipitation of fine carbides at grain boundaries.