Common quality defects in bearing parts after heat treatment

Common quality defects of bearing parts after heat treatment include: quenching microstructure overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc.
Overheat
Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing part. However, the exact microstructure must be observed to determine the degree of overheating. If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is quenched and superheated. The formation may be caused by excessive overheating of the quenching heating temperature or excessive heating and holding time; or it may be due to the serious banding of the original structure, forming a local martensite needle-like coarseness in the low carbon zone between the two zones. Local overheating caused. The retained austenite increases in the superheated structure, and the dimensional stability decreases. Due to the overheating of the quenched structure, the coarse crystal of the steel causes the toughness of the part to decrease, the impact resistance is lowered, and the life of the bearing is also lowered. Excessive heat can even cause quenching cracks.
2. Underheat
If the quenching temperature is low or the cooling is poor, the tortite structure exceeding the standard is formed in the microstructure, which is called the underheated structure, which lowers the hardness and sharply reduces the wear resistance, which affects the bearing life.
3. Quenching crack
The crack formed by the internal stress of the bearing part during the quenching and cooling process is called quenching crack. The causes of such cracks are: due to excessive heating temperature of quenching or too rapid cooling, the microstructure stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; the original defects of the working surface (such as surface micro cracks or scratches) Trace) or steel internal defects (such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage residuals, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; insufficient tempering after quenching of parts Or not tempered in time; the cold punching stress caused by the previous process is too large, forging and folding, deep turning tool marks, sharp edges and corners of the oil groove. In short, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks. The quenching crack is deep and slender, the fracture is straight, and the fractured surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; the shape on the bearing steel ball is S-shaped, T-shaped or ring-shaped. The microstructure of the quenching crack is that there is no decarburization on both sides of the crack, and the difference is obvious with forging cracks and material cracks.

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How to prevent precision imported bearings from rusting

Corrosion occurs, which not only affects the accuracy and surface roughness, but also reduces the service life, even scrapped and cannot be used.

Bearing anti-rust work is an important task, especially the rust prevention between processes, like the bearing in the heat treatment workshop after pickling, cleaning, grinding, there are many processes. When the product is not produced by running water, it is stored in the intermediate tank once it is processed sequentially, so the bearing rings stored between the processes must be rust-proof.

The conventional anti-rust oil anti-rust method similar to rust-proof paint and the methods for preventing rust between the bearing processes are generally the following:

1. Dip in the rustproof tank
The bearing ring is immersed in an aqueous solution of 5% sodium nitrite and 0.6% sodium carbonate. The anti-rust effect is good, but many rust-proof tanks and other equipment occupy a large amount of space, and management is not convenient.

2. Dip coating of concentrated sodium nitrite solution
After washing the product, it is immersed in a solution containing 15%-20% sodium nitrite and 0.6% sodium carbonate, and then stacked. In this way, the rust-preventing period is short, generally only 7-14 days, and in the rainy season, due to the wet weather, it can only be re-treated after 2-3 days, which requires labor and money.

3. Oiling
Apply anti-rust oil like anti-rust paint like finished product. This method must be cleaned before the next step is performed, which is troublesome.

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