Effect of various heat treatment methods and optimization of their parameters on mechanical properties of AISI 4140 steel

AISI 4140 steel is one of the important category in the steels with wide range of applications including but not limited to automotive, general machinery, oil and gas industry. In the current study, an effort is made to understand the effects of heat treatment parameters, such as heat treatment temperature and holding time, on the mechanical properties of AISI 4140 steel, and to optimize these parameters to obtain the superior combination of mechanical properties. The three important heat treatments which are used in this study are annealing, normalizing and oil quenching. The heat treatment parameters such as temperature and time are varied at three different levels of 900, 925, and 950 °C, and 1, 1.5, and 2 h respectively. Using the full factorial method, total 9 experiments were carried out with all the possible combination of temperature and time as the variants. In each of the tests, hardness and impact energy values were evaluated using appropriate tests, while microstructural changes were analyzed through a scanning electron microscope (SEM). The results obtained through statistical analysis have shown that combination of 900 °C with 2 h for annealing, 919 °C with 2 h for normalizing and 944 °C with 1 h for oil quenching as the optimum combination of heat treatment parameters for superior combination of hardness and impact energy. Results showed that increasing temperature led to grain coarsening, reducing hardness but improving impact energy. Regression equations generated in this study which have R square value more than 90% may be used to predict the hardness and impact energy for any value of temperature and time which is within the range of values considered for this study.