Evolution of Carbides in GCr15 Bearing Steel during the Quenching Process

The evolution of carbides and the hardness of GCr15 bearing steel specimens during the quenching process were investigated using scanning electron microscopy (SEM) and hardness testing. The volume fraction and size of undissolved carbides under different quenching parameters were analyzed. The results indicated that as the austenitizing time increased from 0 to 25 minutes, the volume fraction of undissolved carbides decreased from 10.77 to 5.72 %, while the mean diameter of undissolved carbides first decreased from 0.264 to 0.214 μm, and then increased to 0.227 μm; as the austenitizing temperature increased from 810 to 930 ℃, the volume fraction decreased from 9.25 % to 0.53 %, while the mean diameter first decreased from 0.238 to 0.222 μm, and then increased to 0.248 μm; as the heating rate increased from 2 to 8 ℃/min, the volume fraction increased from 5.10 to 6.76 %, while the mean diameter decreased from 0.234 to 0.217 μm. It is found that the energy absorbed during austenitization plays a crucial role in the volume fraction and mean diameter of undissolved carbides. That is, before the heat treatment reaches equilibrium or the carbides are fully dissolved, as the energy absorbed during the heat treatment process increases, the volume fraction of undissolved carbides decreases, while the mean diameter of undissolved carbides tends to increase. It is demonstrated that the hardness of the GCr15 bearing steel specimens is closely related to the volume fraction and mean diameter of undissolved carbides. That is, before the coarsening of carbides, the hardness of GCr15 steel specimens gradually increases as the volume fraction of carbides decreases.