Abstract 42CrMo4 steel is one of the hardest and most brittle steels in the structural grade steel family. This research investigates the mechanical characterisation and microstructural behaviour of 42CrMo4 steel subjected to isothermal annealing heat treatment. Two key variables are analysed using the Taguchi method and their influence on the tensile strength and hardness of the material. The variables are isothermal temperature and soaking time, their effect on the tensile strength, hardness and microstructural behaviour using experimentation for the L27 orthogonal array. There is a significant reduction in tensile strength and hardness as the temperature and soaking time increase, which is attributed to the formation of pearlitic structure and grain coarsening. A huge reduction of 46.74% in tensile strength and a 75.15% reduction in hardness when comparing with high isothermal temperature of 600 °C, 1 h soaking time and 700 °C, 3 h soaking time. Using the Analysis of Variance (ANOVA), temperature is the dominant factor contributing to both tensile strength and hardness variations and soaking time plays a minor role. Contour plots have shown that the tensile strength and hardness are inversely proportional to temperature and time with a diagonal gradient. Multiple linear regression equations for tensile strength and hardness are generated with very low residuals and with an R-squared value above 96% for tensile strength and above 99% for hardness value, thereby offering a reliable method for validating the mechanical properties and microstructural behaviour of isothermal annealed 42CrMo4 steel. The isothermal temperature of 600 °C with 1 h soaking time is the most stable parameter for tensile strength and the isothermal temperature of 650 °C with 1 h soaking time is the least deviation for hardness.
Kukkila et al. (Fri,) studied this question.
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