Heat treatment of AISI 1040 and AISI 4140 steels: microstructure-mechanical property relationships for normalization, spheroidization and quenching-tempering

The mechanical properties of steels are enhanced through heat treatment according to their intended applications. In this study, normalization, pearlite spheroidization, quenching, and tempering at various temperatures were performed on AISI 1040 and AISI 4140 steels, which are widely used industrially. The effects of these heat treatments on microstructure and mechanical properties were examined. The impact of varying tempering temperatures on mechanical properties such as hardness, tensile strength, yield strength, impact toughness (at -30 °C, 0 °C, and room temperature), and elongation was analyzed and correlated with the microstructures. Additionally, examinations were conducted on AISI 4140 steel to assess the effect of alloying elements on the efficacy of heat treatment. The results revealed that increasing the tempering temperature caused morphological changes in the martensitic structure, leading to a significant decrease in hardness and strength up to 500 °C, with a notable increase in ductility beyond this point. AISI 4140 steel exhibited more pronounced changes in mechanical properties after tempering compared to AISI 1040 steel, highlighting the role of alloying elements in enhancing heat treatment efficiency. Impact tests indicated that the minimum tempering temperatures required to achieve an impact energy above 27 J were 500 °C for AISI 1040 steel and 450 °C for AISI 4140 steel. The study identified the appropriate heat treatment conditions to achieve the expected properties for AISI 1040 and AISI 4140 steels based on their application areas.