Microstructures evolution and enhanced performance of hot-rolled 40CrNiMoA steel affected by cyclic quenching

Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), as well as tensile, hardness, and friction-wear tests were utilized to investigate the microstructural evolution, mechanical properties, and wear resistance of hot-rolled (HR) 40CrNiMoA steel after cyclic quenching (CQ) heat treatment following an initial water quenching (WQ) at 930 °C / 30 min. The CQ process involved 860 °C / 2 min + WQ. The results revealed that mutual transformation between austenite and martensite phases occurs during the CQ process. Compared to the HR sample, the austenite grain size in the CQ samples was significantly reduced and became more uniform. With increasing CQ passes, the strength and hardness of the 40CrNiMoA steel first increased and then decreased, reaching peak values after 2 passes of cyclic quenching (CQ2). Compared to the HR sample, these peak values (CQ2 sample) represented increases of 72.6%, 73.8%, and 63.3% for yield strength, ultimate tensile strength, and hardness, respectively. The elongation first decreased, then increased (in CQ2 sample), and subsequently slightly decreased with increasing CQ passes. The elongation of the CQ2 sample was 19% lower than that of the HR sample. Th e wear rate of the CQ2 sample decreased by 19.56% compared to the HR sample. CQ heat treatment effectively enhances the strength and wear resistance of 40CrNiMoA steel while maintaining plasticity without significant deterioration.