Effect of quenching temperature on microstructure and yield strength of Q-T-treated X100Q bainitic steel

The experimental X100Q bainitic steel was thermally processed through quenching (Q) at 875 degrees C-975 degrees C and tempering (T) at 650 degrees C. The microstructure of each sample was characterized via optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), transmission electron microscope (TEM) and x-ray diffractometry (XRD), and the tensile properties were measured. The results indicate that a microstructure consisting of granular bainite (GB) and lath bainite (LB) formed in each sample. With the increased quenching temperature (T-q), the starting temperature (Ar-3) for gamma -> GB + LB transition decreased. Consequently, the amount of LB increased at the expense of GB, the effective grain size of GB + LB decreased, the dislocation density and precipitate quantity increased, leading to increases in yield strength (YS), tensile strength (TS) and yield-to-tensile strength ratio (YR). The mechanisms dominating the T q effect on the microstructure and tensile properties were discussed.