This paper aims to experimentally investigate the post-fire mechanical properties of Grade 12.9 high-strength bolts. To evaluate the effects of elevated temperatures and the bolt diameter on the post-fire mechanical properties, seven different diameters of the Grade 12.9 bolts were heated based on a standard temperature-time diagram with the ranges of 20 to 900 °C. Upon air-cooling and reaching room temperature (a heating and cooling cycle), 105 bolt specimens were subjected to the standard tensile test. The experimental results, including the post-fire stress-strain curve, yield strength, tensile strength, modulus of elasticity, and ductility, were presented. The results indicated that changes in mechanical properties were noticeable for temperatures higher than 400 °C. Although the post-fire modulus of elasticity did not significantly change, the post-fire yield and tensile strength decreased substantially from the temperature above 400 °C. After heating up to 900 °C and then cooling down and reaching ambient temperature, Grade 12.9 steel bolts showed a 60% reduction in strength. The comparison of the results showed that the effect of diameter on the post-fire yield and tensile strengths of Grade 12.9 bolts was insignificant and could be neglected, but on elastic modulus and ductility, the diameter effect needs to be considered. In addition, the scanning electron microscope (SEM) images indicated that the elevated temperatures remarkably changed the microstructural characteristics of the Grade 12.9 bolts. Finally, based on the test results, the prediction equations of the post-fire mechanical properties of the Grade 12.9 high-strength bolts were proposed.
