Effect of modified heat treatments on the microstructure and mechanical properties of a low alloy high strength steel

Isothermal, successive and up-quenching heat treatments were used to improve the mechanical properties of an AISI 4130 steel. Scanning electron and optical microscopy were used to study the morphology of lower and upper bainite formed by the different heat treatments. Mechanical properties were determined by testing sub-sized tensile and Charpy impact specimens. The results showed that successive austempering improves the mechanical properties compared with continuous cooling and conventional austempering. However, it was shown that the best combination of mechanical properties is achieved when an up-quenching heat treatment is used. The microstructural studies showed that partition of grains by lower bainite is probably the main reason for this improvement.