Heat treatment processes are widely used to achieve high mechanical properties in materials. Additionally, the combination of cold deformation and heat treatment is one of the oldest methods employed to alter the mechanical properties of materials. In this research, a combination of cold deformation and heat treatment was applied to structural steel samples (ASTM A615) to investigate the effect of prior deformation on subsequent heat treatment, specifically focusing on the microstructure and hardness of this steel type. Samples were deformed to different extents (20%, 30%, 40%, and 50%) before being austenitized at 850°C. Following austenitization, some samples were quenched in water, while others were allowed to cool in air. Hardness measurements and microstructural investigations were conducted. The results indicate that the hardness of water-quenched samples exhibited a significant increase compared to that of the as-received samples. This increase was attributed to the transformation of the microstructure from ferrite-pearlite to a bainitic structure. Cold deformation was found to accelerate the transformation processes due to the introduction of structural defects, which acted as preferred sites for precipitation. Both the amount and morphology of the transformation products were influenced by the extent of deformation............. Keywords:.............. Steel bars, Hardness, Heat treatment, Microstructure, ASTM.