Development of Optimized Mechanical Properties of AISI 4340 Steel: Role of Quenching and Partitioning Process

The current research is based on an attempt to achieve ultra-high strengths in AISI 4340 steel, by the quench and partitioning (QP) heat treatment processes. In this study, the microstructure and mechanical properties of the advanced QP process are compared with the quenching and tempering (QT) and the austempering processes. The microstructural investigations are performed with optical microscopy, scanning electron microscopy, and electron backscattered diffraction. Tensile testing is carried out to measure ultimate tensile strength, yield strength, and elongation. The hardness measurement was done on the Rockwell C scale. The hardness and tensile results were used to compare mechanical properties. Conventional QT processes lead to compromise in ductility with increased strength. The austempering process shows the bainitic structure with mechanical properties comparable to specimens quenched and tempered at 560 °C. The QP process comes up with optimum mechanical properties with exceptional UTS (1802 MPa) and elongation of 12.67%. The hardness of the QP process is also higher than other processes. When compared with conventional processes like QT, the strength in the QP process is much higher with good ductility. The microstructure in the QP process is reformed to fine-grained austenite within the inter-lath positions of martensite. This morphology of retained austenite plays an important role in achieving these properties in QP.