Microstructural Evolution of a Low-Carbon Steel during Application of Quenching and Partitioning Heat Treatments after Partial Austenitization

The quenching and partitioning'' (Q&P) process has been studied in a low-carbon steel containing 1.1 wt pct aluminum by heat treatments consisting of partial austenitization at 900 degrees C and subsequent rapid cooling to a quenching temperature in the range between 125 degrees C and 175 degrees C, followed by an isothermal treatment (partitioning step) at 250 degrees C and 350 degrees C for different times. Characterization by means of optical and scanning electron microscopy, electron backscattered diffraction (EBSD), magnetization measurements, and X-ray diffraction (XRD) has shown a multiphase microstructure formed by intercritical ferrite, epitaxial ferrite, retained austenite, bainite, and martensite after different stages of tempering. A considerable amount of retained austenite has been obtained in the specimens partitioned at 350 degrees C for 100 seconds. Experimental results have been interpreted based on concepts of the martensite tempering, bainite transformation, and kinetics calculations of the carbon partitioning from martensite to austenite.