Effect of isothermal temperature on bainite transformation, microstructure and mechanical properties of LSFed 300M steel

Abstract The influence of isothermal treatment on the microstructure and mechanical properties of laser solid formed (LSFed) 300 M steel was investigated within the isothermal temperature range of 280–440 ℃. The result showed that the microstructure of LSFed 300 M steel isothermal with treatment of 440 ℃ consisted of upper bainite and small amount of martensite. As the decreasing isothermal temperature, the amount of upper bainite gradually decreased, the content of the martensite and lower bainite gradually increased. In addition, the superledges have been formed in the lower bainite. Microstructure of LSFed 300 M steel was almost martensite when the isothermal temperature was 280 °C. The hardness in LSFed 300 M steel presented a significant change with the decrement of isothermal temperature. The hardness decreased positively with isothermal temperature when it is more than 400 ℃; while, it increased inversely with the decreasing the isothermal temperature when the temperature is lower than 400 °C. The tensile strength and yield strength changed little when the isothermal temperature decreased from 440 °C to 320 °C, while sharply increased when the isothermal temperature was 280 °C. The elongation of LSFed 300 M steel was the highest (27 %) when the isothermal temperature was 320 °C.