AbstractThe objectives of this study are (1) to investigate the microstructural changes in structural steels that are exposed to fire accidents; and (2) to understand their influence on post-fire mechanical properties. Specifically, three structural steels—ASTM A36, ASTM A572, and ASTM A992—are employed, and three post-fire mechanical properties—yield strength, ultimate tensile strength, and ductility—are determined. Structural steel specimens are subjected to high temperatures ranging from 500°C to 1,000°C for 1 h and are subsequently air-cooled. Post-fire mechanical properties of air-cooled specimens are determined using uniaxial tension tests. Metallographic specimens are prepared, and microstructural analysis is carried out. Changes in metallurgical phases are tracked, and the grain sizes of the corresponding metallurgical phases are evaluated as a function of exposed temperature. By observing the post-fire mechanical properties of steels along with the microstructural analysis, an increase in ferrite volume fraction and ferrite grain size clearly leads to a reduction in both the post-fire yield strength and ultimate tensile strength of ASTM A36 steels. The increase in ferrite grain size alone resulted in an increase in the ductility of ASTM A36 steels. Based on the results obtained in this study, multivariate linear regression equations are proposed to evaluate the post-fire yield strength of all three structural steels as a function of ferrite grain size and pearlite colony size. The results of this study will be useful to forensic engineers when evaluating the residual strength of fire-affected steel structures based on microstructures, especially when elevated temperature values are not available.