Effect of Cooling Rate on Microstructure and Effective Grain Size for a Ni–Cr–Mo–B High-Strength Steel

Abstract

The effect of cooling rate on microstructure and effective grain size (EGS) of a Ni–Cr–Mo–B high-strength steel has been studied by dilatometer, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results show that the microstructure of the Ni–Cr–Mo–B steel is dependent on cooling rate in the following sequence: lath martensite (LM), mixed LM and lath bainite (LB), mixed LB and granular bainite (GB) and GB. The critical cooling rates for appearance of LB and GB are about 10 °C/s and 0.5 °C/s, respectively. The LM (> 10 °C/s) consists of few blocky regions with a width of several micros. Compared with the lath regions, the blocky regions in LM form at higher actual transformation temperatures during cooling. The blocky region area percentage in LM keeps almost constant about 8% at different cooling rates (> 10 °C/s) due to similar martensite transformation starting temperature (M_s). The LB percentage in mixed LM/LB increases gradually with decreasing cooling rate (10–0.5 °C/s). The EBSD results show that different microstructures have different EGS. The mixed LM/LB exhibits the smallest EGS due to the separation of the prior austenite grains by the pre-formed LB and the refinement of the LM. Meanwhile, the mixed LM/LB at different cooling rates (10–0.5 °C/s) exhibits almost the same EGS because the LB and LM in the mixed LM/LB have a similar high-angle grain boundary density and similar EGS. Because the blocky regions contain few high-angle grain boundaries and have similar area percentages in the LM, the LM at different cooling rates (> 10 °C/s) exhibits almost the same EGS. The ferrite in GB exhibits as a whole with few high-angle grain boundaries; thus, the mixed LB/GB exhibits the largest EGS.