What are the properties and requirements of steel for rolling bearings?
Rolling bearing materials include rolling bearing parts,cages, rivets and other auxiliary materials. The vast majority of rolling bearings and their parts are made of steel which is usually high-carbon chromium steel and carburizing steel. With the development of modern science and technology and the increasing use of rolling bearings, the requirements for bearings are becoming higher and higher, such as high precision, long life,high reliability and so on. For bearings with some special purposes, bearing materials are also required with high temperature resistance, corrosion resistance, non-magnetic, ultra-low temperature, radiation resistance and other properties. In addition, bearing materials also include alloy materials, non-ferrous metals and non-metallic material and so on. Bearings made of ceramic materials are now used in locomotives, automobiles, subways, aviation, aerospace, chemical and other fields.
The basic requirements for rolling bearings materials depend to a large extent on the working performance of bearings. Whether the material selected for manufacturing rolling bearings is suitable,it will have a great impact on the
service performance and life. Under normal circumstances, the main form of failure of rolling bearings is fatigue flake under the action of alternating stress and the loss of bearing accuracy due to friction and wear. In addition, there are cracks, indentation, corrosion and other reasons which cause abnormal damage to the bearings. Therefore, rolling bearings should have high resistance to plastic deformation, less friction and wear, good rotation accuracy, good dimensional accuracy and stability, and long contact fatigue life. Many of these properties are determined by the materials and the heat treatment process together. Since the basic requirements for rolling bearings materials are determined by the damage form of bearings, the materials required to manufacture rolling bearings should have the following properties after a certain heat treatment of the post-process:
a High contact fatigue strength
Contact fatigue failure is the main form of normal failure of bearings. When the rolling bearing is running, the rolling body rolls between the raceways of the inner and outer rings of the bearing. The contact surface bears periodic alternating load which can reach hundreds of thousands of times per minute. Under the repeated action of periodic alternating stress, the contact surface appears fatigue flake. After fatigue flake, it will cause bearing vibration, noise increase and the working temperature to rise sharply, resulting in the final damage of the bearing which is called contact fatigue damage. Therefore, it is required that the steel used for rolling bearings should have high contact fatigue strength.
b High wear resistance
When the rolling bearing is working normally, in addition to rolling friction, it is also accompanied by sliding friction. The main parts of sliding friction are: the contact surface between the rolling body and the raceway, the contact surface between the rolling body and the cafe pocket , the contact surface between the cage and the ring guide guard, and the end face of the roller and the ring guide guard. The existence of sliding friction in rolling bearings inevitably causes bearing parts to wear. If the wear resistance of the bearing steel is poor, the rolling bearing will lose accuracy prematurely due to wear or the bearing vibration will increase and the life will be reduced due to the reduction of rotation accuracy. Therefore, the bearing steel is required with high wear resistance.
c High elastic limit
When the rolling bearing is working under load, especially under large load, the contact pressure of the contact surface is very large because the contact area between the rolling element and the raceway of the ring is very small. In order to prevent excessive plastic deformation under high contact stress resulting in loss of bearing accuracy or surface cracks, bearing steel is required with a high elastic limit.
d Suitable hardness
Hardness is one of the important indexes of rolling bearing. It is closely related to the contact fatigue strength, wear resistance and elastic limit of the material which directly affects the life of the rolling bearing. The hardness of the bearing is usually determined according to the way and size of the bearing load, bearing size and the overall situation of the wall thickness. The hardness of the steel used for rolling bearings should be appropriate. Large or small hardness will affect the service life of the bearings. As we all know, the main failure forms of rolling bearings are contact fatigue failure and the loss of bearing accuracy due to poor wear resistance or dimensional instability; If the bearing parts lack a certain toughness, they will be damaged due to brittleness when bearing large impact loads. Therefore, it is necessary to determine the hardness of the bearing according to the specific situation of the bearing and the way of damage. For the loss of bearing accuracy due to fatigue flake or poor wear resistance, bearing parts should be selected with higher hardness; For bearings bearing large impact loads (such as rolling mill bearings, railway bearings and some automobile bearings, etc.), it is necessary to appropriately reduce the hardness to improve the toughness of the bearing.
e Certain impact toughness
Many rolling bearings will be subjected to a certain impact load during use, so the bearing steel is required with a certain toughness to ensure that the bearing is not damaged by impact. For bearings bearing larger impact loads such as rolling mill bearings, railway bearings and so on which are required with relatively high impact toughness and fracture toughness, they use bainite quenching heat treatment process and some use carburizing steel materials in order to have better impact toughness.
f Good dimensional stability
Rolling bearings are precision mechanical parts and their accuracy is calculated in microns. During the long-term storage and use, changes in the internal structure or stress changes will cause changes in bearing size, resulting in loss of bearing accuracy. Therefore, in order to ensure the dimensional accuracy of the bearing, the bearing steel should have good dimensional stability.
g Good rust resistance
There are many production processes for rolling bearings with long production cycle.Some semi-finished or finished parts need to be stored for a long time before assembly, so bearing parts are prone to certain rust in the production process or in the storage of finished products, especially in the humid air. Therefore, the bearing steel is required to have good anti-rust properties.
h Good process performance
In the production process, the parts of rolling bears have to go through a number of cold and hot processes. This requires that bearing steel should have good process properties, such as cold and hot forming properties, cutting, grinding processing properties and heat treatment properties, etc., in order to meet the needs of high-volume, high-efficiency, low-cost and high-quality production of rolling bearings.
For bearings used under special working conditions, in addition to the above basic requirements, the steel must have the corresponding special performance, such as high temperature , high-speed, corrosion resistance, magnetic resistance and so on.
