AbstractIn this study, we investigated the composition and mechanical properties of metallurgical phases present in the ASTM A36 steels subjected to postfire temperatures using nanoindentation testing in conjunction with the K++ clustering method. The specimens are exposed to target temperatures from 500°C to 1,000°C, with increments of 100°C. We extracted two nanomechanical properties, namely, hardness and Young’s modulus, from the nanoindentation tests and used them as descriptive features for the clustering analysis. Results obtained from this analysis show that average volume fractions of ferrite and pearlite were 84% and 16%, respectively. The results also revealed that the mean hardness values were in the range of 2.46 to 3.01 GPa for ferrite and 3.11 to 4.27 GPa for pearlite for the different temperature exposures. The Young’s moduli of ferrite ranged from 171.7 to 203.3 GPa, whereas the pearlite phase ranged from 181.1 to 206.8 GPa for the different temperature exposures. The obtained results also indicated the existence of a quadratic relation between the pearlite’s mean nanoindentation hardness and the yield and tensile strengths of different postfire ASTM A36 steels.