What are the Causes and Countermeasures of Quality Problems in the Heat Treatment of Bearing Parts?

● Quenching crack

This crack is caused by the fact that the internal stress formed on the bearing parts is greater than the fracture strength of the material when the quenching temperature is too high or the cooling speed is too fast. The existence of internal stress is the main reason for the quenching crack. If the bearing steel has internal defects, such as slag inclusions, serious non-metallic inclusions or carbide heterogeneity, shrinkage residues, white spots, etc., it is easy to cause stress concentration and form quenching cracks at these defects during quenching. Defects left over from the process before heat treatment and quenching can also lead to stress concentration during quenching and form quenching cracks. In addition, surface decarbonization is also the cause of quenching cracks. The main difference between quenching cracks, material cracks and forging cracks is that there is no decarbonization on both sides of the quenching crack.

● Quenching soft spots

After quenching, the hardness of the local area of the bearing part is lower than the normal hardness of other areas on the same set of rings, which is called the quenching soft point. Due to the low hardness and strength of the soft point area, it is often a fatigue source that the bearing is prone to excessive wear or early contact fatigue peeling when working. Therefore, soft spots are not allowed on finished bearing parts.

The soft point is caused by incomplete quenching, that is, insufficient heating, insufficient insulation time or poor cooling. The way to avoid the occurrence of quenching soft points is to appropriately increase the heating temperature, extend the insulation time, strive to heat evenly, increase the cooling speed of the cooling medium or choose an appropriate quenching method.

● Heat treatment decarbonization

When bearing parts are heat treated at high temperature, it is easy to reduce the carbon content below the normal carbon content at a certain depth of the surface. This phenomenon is called decarbonization. If the depth of the decarbonization layer caused by heat treatment exceeds the grinding retention amount, the hardness of the residual decarbonization layer surface retained on the surface of the finished bearing parts will be reduced, thus reducing wear resistance, and it is easy to cause quenching cracks, soft spots and other quenching defects during the heat treatment quenching process. In addition, due to surface decarbonization, the martensite phase transition point Ms of the surface layer is also raised accordingly, resulting in a large tensile stress value on the surface of bearing parts and reducing contact fatigue strength.

At present, the equipment for heat treatment of bearing parts abroad has generally adopted controllable atmosphere heat treatment automatic production line, vacuum heat treatment or salt bath furnace heat treatment, which has fully realized non-oxidation and non-poverty carbon heat treatment. China is still in the stage of protective atmosphere heat treatment. The proportion of controlled atmosphere heat treatment automatic production lines and vacuum heat treatment furnaces is low, but there is basically little serious decarbonization in the heat treatment process, and there is mainly shallow carbon poverty.

The author of this article is Shaanxi Gainhui Heavy Industries Co., Ltd., which is a professional bearing manufacturer and supplier in China. The company supplies many types of the bearings, such as cylindrical roller thrust bearings, radial insert ball bearings sa series,etc. With high quality and competitive bearing products, we sincerely provide services to customers all over the world.

Contact us for more details:

E-mail: emily@gainhui.com

Tel: +86-029-88692376

Mob/WhatsApp: +86-18710850036