Optimization of various heat treatment parameters for superior combination of hardness and impact energy of AISI 1040 steel

AISI 1040 steel which is also known as plain medium carbon steel has wide range of applications especially in structural applications. This study investigated the effects of different heat treatment methods, such as annealing, normalizing, and hardening on the mechanical and microstructural properties of AISI 1040 steel. The results are analyzed statistically using the statistical tool to determine the optimum set of heat treatment parameters to obtain superior combination of hardness and impact energy. Heat treatment temperatures of 900, 925, and 950 °C and holding times of 1, 1.5, and 2 h were used in the study. The hardness and impact energy were evaluated using Vickers microhardness and Charpy impact tests, while microstructural changes were analyzed through SEM. Full factorial method was used to conduct the experiments. Regression equations are generated to predict the hardness and impact energy of the heat treated samples. Results showed that increase in the austenitization temperature has resulted in grain coarsening, reducing the hardness but improving impact energy. Oil quenched hardening resulted in the highest hardness (245 HV), whereas annealing provided superior toughness (38 J), and normalizing offered balanced properties. Optimal heat treatment conditions for superior combination of hardness and impact energy were identified for each processes. For annealing the optimum heat treatment temperature and time were found to be 900°C and 2 h, whereas for normalizing the optimum parameters were determined to be 902°C and 2 h. In the case of oil quenched hardening heat treatment the optimum parameters were found to be 912°C and 2 h to get good combination of mechanical properties.