Mechanical properties and low-temperature impact toughness of high-strength bolts after elevated temperatures

This paper presents the static mechanical properties and low-temperature impact toughness of high strength bolts after various elevated temperatures. The experimental results show that the increase in the heating temperature would improve the ductility of the high-strength bolts, but cause a rapid reduction in residual strength. However, the yield and ultimate strength of the bolt show a slight increase when the temperature is elevated to 800 °C, which is caused by rapid cooling. Based on the tested data, the prediction formulas for the mechanical properties and the stress-strain relationship model of high-strength bolts are proposed respectively. Under low-temperature, the impact failure mode of high-strength bolts shows an obvious transition trend from ductile fracture to brittle fracture. With the increase in pre-heating temperature, the low-temperature impact toughness and cold brittleness of the bolts are improved to a certain extent, which is caused by the elevated temperature. But when the pre-heating temperature reaches 800 °C, the impact toughness of the bolts degrades with the decrease of the freezing temperature, which may result from the structure of the increased martensite having high dislocation density to limit the plastic deformation of the bolts under low-temperature.