Effect of microstructural transformation during tempering on mechanical properties of quenched and tempered 38CrSi steel

The investigation was conducted to explore the hidden potential of 38CrSi steel under various tempering temperatures to widen its field of applications. The current work is mainly concerned to appraise the impact of microstructural transformation occurred during tempering on the mechanical properties of 38CrSi steel. Light optical and scanning electron microscopy validated the presence of packets and blocks of lath martensite with retained austenite. Similarly, energy dispersive spectroscopy verified the carbon enrichment of martensite at tempering temperature of 280 °C. This microstructure provided higher hardness, tensile strength while lower elongation and impact absorption energy as compared to non-heat treated steel. The precipitation of secondary phase meta-stable -carbides (Fe2.4C) by the diffusion of carbon at tempering temperature of 380 °C caused lattice relaxation, reduction in tensile strength, hardness, and amelioration in elongation, impact absorption energy. The transformation of meta-stable -carbides (Fe2.4C) to cementite (Fe3C) was observed at 480 °C. Tempering at 580 °C and 680 °C caused spheroidizing of cementite (Fe3C) and recovery, recrystallization of ferrite. This transformation resulted in a maximum reduction in carbon enrichment, hardness, tensile strength and a maximum amelioration in elongation, impact absorption energy. 38CrSi observed to be moderately brittle material and its mechanical properties were found to be highly sensitive to microstructural transformation during tempering.