Effect of Welding Thermal Cycling on Microstructures and Cryogenic Impact Toughness of Medium-Mn Low-Temperature Steel

This work studied the effect of welding thermal cycling on the microstructures and cryogenic impact toughness of medium-Mn low-temperature steel by single-pass welding thermal simulation test. The microstructures of the heat-affected zone (HAZ) were characterized by optical microscopy, electron backscatter diffraction, X-ray diffraction, and transmission electron microscopy. The results indicated that the microstructures of fine-grain HAZ are fine-lath martensite with film-like retained austenite in the heat input range of 10 similar to 30 kJ/cm. Meanwhile, fine-grain HAZ has excellent low-temperature impact toughness, and the impact energy tested at - 40 degrees C can reach about 245 J. The coarse martensitic packet and block in the coarse grain HAZ seriously deteriorated the cryogenic impact toughness, and the greater the heat input, the worse the cryogenic impact toughness. The impact energy tested at - 40 degrees C was 71 J when the heat input was 10 kJ/cm. The cryogenic impact toughness of HAZ gradually deteriorated with the peak temperature increase because of the decrease of retained austenite content and the increase of martensitic lath width.