Effect of Quench and Partitioning Treatment on Microstructure, Tensile Properties, and Low Cycle Fatigue Behavior of Low-Carbon High Strength Steel

Abstract

The study investigates the effect of quenching temperature on the microstructure, tensile properties, and low-cycle fatigue behavior of low-carbon, high strength steel. Initially, the steel was prepared in a vacuum induction furnace, processed through forging and hot rolling, and underwent quenching and partitioning treatment to optimize its mechanical and fatigue performance. Hot-rolled plates were heated at 950 °C and subsequently quenched in a salt bath, maintaining temperatures of 250, 300, and 350 °C, followed by partitioning at the same temperatures for 30 min and cooled in air to room temperature. Microstructural examinations reveal martensite, tempered martensite, bainite and retained austenite after post quenching and partitioning treatment, confirmed through XRD, EBSD, and TEM analyses. The hardness and tensile strength improve significantly but reduce the ductility after Q&P treatment as compared to the rolled sample. Specifically, tensile strength decreases while ductility increases with higher quenching temperatures or partitioning temperatures. Steel treated at 250 °C exhibits the most pronounced enhancement in tensile strength. Moreover, the sample treated at 250 °C exhibits superior fatigue life compared to rolled and heat-treated samples at 300 and 350 °C for 30 min. Observations from fatigue crack growth studies using FESEM indicate increased crack initiation sites and secondary cracks with higher quenching temperatures. Fatigue crack propagation is typically initiated at the surface and propagated through the martensite-retained austenite interface, with changes in crack direction observed upon reaching the central part of the sample.