Physical properties of high-strength bolt materials at elevated temperatures

Carbon steel bolts have been broadly utilized in modern building construction. Fire performance of these components has been investigated in the published journals aiming at deriving their reduction factors at elevated temperatures. However, it should be noted that materials employed for manufacturing high-strength structural bolts may be different from country to country. Meanwhile, the present research efforts mainly focused on fire performance of carbon steel bolts instead of their material properties. Therefore, this paper developed an experimental program for investigation of the elevated-temperature material properties for three types of highstrength bolts with property classes 8.8, 10.9 and 12.9. Standard coupon specimens were milled from carbon steel bolts. The temperatures of tensile testing were in the range of 20-900 degrees C. The key material properties were extracted from the recorded stress-strain curves. These tested results were compared with those of high-strength bolts and steels from the literature. It was found that different materials for bolt making has a negligible influence on the deterioration of material properties at elevated temperatures, not including strains (epsilon(u) and epsilon(f)). In contrast, the strength reduction was generally faster than the majority of high-strength steels at elevated temperatures (over 300 degrees C). In addition, the analytical models were proposed for describing the deterioration of yield strength (f(yT)), Young's modulus (E-sT), ultimate strain (epsilon(uT)) and fracture strain (epsilon(fT)) at elevated temperatures.