Microstructural Characterization and Hardening Mechanism of Steel for Large Size Bearing Ring Under Fast Heating and Short Soaking Time Condition

The microstructure changes of steel for large size bearing ring with different Ni contents at different cooling rates under fast heating and short soaking time condition are analyzed by using thermal simulation test, scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), electron‐backscattering diffraction (EBSD), and transmission electron microscope (TEM). The hardness is evaluated by Rockwell hardness test and the hardening mechanism is discussed. Full martensite cooling rate can be lowered from 10 to 3 °C s−1 by the addition of 0.59% Ni. At full martensite cooling rate range, the martensitic lath structures are refined by adding 0.59% Ni or increasing the cooling rate. The hardness increases progressively with the cooling rate due to the refinement of martensitic lath structures and the precipitation strengthening of M23C6‐type carbides. But the grain boundary strengthening has predominance. The highest hardness values of steel No. 2 with higher Ni is 60 and 2.5 HRC higher than that of steel No. 1 at the cooling rate of 100 °C s−1. This increase of hardness in steel No. 2 can be attributed to the finer martensitic lath structures and the solid solution strengthening. The grain boundary strengthening is the main influencing factor.