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

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℃/s and 0.5℃/s,respectively.The LM(>10℃/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℃/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℃/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℃/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℃/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.