NSK Bearings in the heat treatment process will produce a great internal stress (organizational stress and thermal stress), when the stress exceeds the yield strength of steel, NSK Bearings will cause deformation; when the stress greater than the tensile strength of steel will result in cracking of NSK bearings. NSK bearing on the role of stress in two ways: compressive stress and tensile stress. Quenching the formation of quenching cracks caused by tensile stress is the main reason.But when the plasticity of steel is high, even with greater tensile stress does not cause cracking of NSK bearings, such as organizational change did not occur to stress annealing, to obtain more residual austenite isothermal quenching. Only in the stress is large, but also has a high hardness, brittleness of the organization, it is likely to cause cracking of NSK bearings. Therefore, the formation of quenching cracks two conditions must exist: First, the organization has a brittle; the second is the tensile stress exceeds the tensile strength of steel at this time (of course, other conditions can also promote the crack occurred NSK bearings, such as defects in materials, design and defects caused by improper machining, etc.).