Overheating
Evidence of an overheated microstructure following quenching can be observed on the fracture surfaces of bearing components. However, to precisely determine the degree of overheating, a microscopic examination of the microstructure is required. If coarse, acicular (needle-like) martensite is present within the quenched structure of GCr15 steel, it indicates an overheated microstructure resulting quenching. The underlying cause may be *general overheating*—resulting an excessively high quenching temperature a prolonged holding time during heating— it may be *localized overheating*—occurring when severe banding of carbides in the original structure leads to the formation of coarse martensite needles within the low-carbon regions situated between these bands. An overheated microstructure is characterized by an increased volume of retained austenite a consequent reduction in dimensional stability. Furthermore, because the quenched structure is overheated—resulting in a coarse grain size—the component's toughness impact resistance are compromised, thereby reducing the overall service life of the bearing. In severe cases, overheating can even lead to quenching cracks.

Underheating
If the quenching temperature is too low cooling is inadequate, the microstructure will exhibit a proportion of troostite exceeding standard specifications. This is referred to as an "underheated structure"; it results in reduced hardness a drastic decline in wear resistance, thereby compromising the service life of the idler roller bearings.

Quenching Cracks
Cracks that form in bearing components during the quenching cooling process due to internal stresses are known as quenching cracks. The causes of such cracks include: thermal stresses—arising excessively high quenching temperatures overly rapid cooling— structural stresses—resulting volume changes during phase transformations—that exceed the steel's fracture strength; pre-existing defects on the working surface (such as microscopic surface cracks scratches) internal defects within the steel (such as slag inclusions, severe non-metallic inclusions, white spots, residual shrinkage cavities) that act as stress concentration points during quenching; severe surface decarburization carbide segregation; insufficient tempering a failure to temper the component promptly after quenching; excessive cold-working stresses introduced during prior manufacturing stages, as well as defects such as forging laps, deep machining marks, sharp edges within oil grooves. In summary, quenching cracks may result one a combination of the aforementioned factors; fundamentally, however, the presence of internal stress is the primary cause of their formation. Quenching cracks are typically deep slender, featuring a flat, straight fracture surface that exhibits no signs of oxidation discoloration. On bearing rings, these cracks often manifest as straight longitudinal fissures circumferential fractures; on bearing steel balls, they may take on S-shapes, T-shapes, annular forms. A distinct microstructural characteristic of quenching cracks is the absence of decarburization on either side of the crack, which clearly distinguishes them forging cracks material defects.

Heat Treatment Deformation
During the heat treatment of NACHI bearing components, both thermal stresses structural stresses are present. These internal stresses can either superimpose upon partially counteract one another; their nature is complex highly variable, as they fluctuate in response to changes in heating temperature, heating rate, cooling method, cooling rate, the shape size of the component. Consequently, deformation during heat treatment is inevitable. By understanding mastering the laws governing these variations, the deformation of bearing components—such as the ovality of rings dimensional expansion—can be kept within a controllable range, thereby facilitating the manufacturing process. Of course, mechanical impacts occurring during the heat treatment process can also induce deformation in components; however, such deformation can be minimized entirely avoided through improvements in operational procedures.

Surface Decarburization
During the heat treatment of bearing components, if heating takes place within an oxidizing medium, surface oxidation will occur; this reduces the mass fraction of carbon at the surface, resulting in surface decarburization. If the depth of the surface decarburized layer exceeds the machining allowance, the component must be scrapped. In metallographic inspection, the depth of the surface decarburized layer can be determined using either the metallographic method the microhardness method. The measurement method based on the microhardness distribution curve of the surface layer is considered the standard may serve as the basis for arbitration.

Soft Spots
The phenomenon of insufficient localized surface hardness in idler roller bearing components—resulting factors such as insufficient heating, inadequate cooling, improper quenching procedures—is referred to as a "quenching soft spot." Much like surface decarburization, this defect can lead to a severe reduction in both surface wear resistance fatigue strength.