Common Quality Defects in Bearing Parts After Heat Treatment

• Overheating

Overheating of the quenched microstructure can often be observed from the rough surfaces of bearing parts, but the exact extent of overheating must be determined through microstructure observation. For example, in GCr15 steel, the presence of coarse needle-like martensite in the quenched structure indicates overheating. This can be caused by excessively high quenching temperatures or prolonged heating times, leading to overall overheating. It can also result from severe banded carbides in the original structure, where coarse needle-like martensite forms in low-carbon zones between bands, leading to localized overheating. Overheated microstructures increase retained austenite, reducing dimensional stability. The coarse crystalline structure from overheating reduces toughness, lowers impact resistance, and shortens bearing life. Severe overheating can even cause quenching cracks.

• Underheating

Insufficient quenching temperature or poor cooling can result in a microstructure containing troostite exceeding standard specifications, known as underheated microstructure. This leads to reduced hardness, significantly lower wear resistance, and negatively impacts bearing life.

• Quenching Cracks

Quenching cracks are cracks formed during the quenching cooling process due to internal stress. Causes include excessively high quenching temperature or overly rapid cooling, where thermal stress and structural stress exceed the fracture strength of the steel. Other contributing factors include pre-existing surface defects (e.g., microcracks or scratches), internal material defects (e.g., inclusions, severe non-metallic inclusions, white spots, or shrinkage residues), severe surface decarburization, carbide segregation, insufficient or delayed tempering after quenching, or excessive residual stress from prior processes (e.g., cold stamping, forging folds, deep machining marks, or sharp oil grooves). In summary, quenching cracks can result from one or more of these factors, but internal stress is the primary cause. Quenching cracks are deep, thin, straight, with a smooth fracture surface and no oxidation coloring. On bearing rings, they usually appear as longitudinal or circumferential cracks, while on steel balls, they form S-, T-, or ring-shaped patterns. Microstructural characteristics of quenching cracks show no decarburization around the crack, distinguishing them from forging or material cracks.

• Heat Treatment Deformation

During heat treatment, the internal stresses (thermal and structural) in bearing parts interact, either adding to or partially canceling each other. These stresses vary with heating temperature, heating speed, cooling method, cooling rate, and the shape and size of the part, making heat treatment deformation unavoidable. Understanding and controlling these variables can minimize deformation (e.g., ring ovality or size increase) within acceptable limits, benefiting production. Mechanical collisions during heat treatment can also cause deformation, but these can be minimized through improved handling practices.

• Surface Decarburization

When bearing parts are heated in an oxidizing medium during heat treatment, oxidation can reduce the carbon content on the surface, causing surface decarburization. If the depth of the decarburized layer exceeds the finishing allowance, the part becomes scrap. The depth of the surface decarburization layer can be measured using metallographic methods or microhardness testing. The microhardness distribution curve of the surface layer is the standard method for arbitration.

• Soft Spots

Soft spots refer to areas of insufficient surface hardness on bearing parts caused by insufficient heating, poor cooling, or improper quenching operations. Similar to surface decarburization, soft spots significantly reduce surface wear resistance and fatigue strength, leading to poor part performance.

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