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Development direction of bearing materials and heat treatment

Development direction of bearing materials and heat treatment

2020-05-14

1. Improve the smelting technology to improve the cleanliness and uniformity of steel

Compared with industrialized countries, although the oxygen content of bearing steel in China is close to the foreign advanced level, but the size and distribution of inclusions and carbide homogeneity, composition uniformity and a large gap compared with abroad, such as large size of inclusions and uneven carbide is more, the basic composition form of black and white area, etc., caused by the deficiency of bearing parts quality, seriously affect the service life of bearing, reliability and consistency. 

In addition, the existence of large size inclusions on the rolling contact surface also seriously reduces the surface accuracy and increases the bearing noise.Therefore, the bearing industry should negotiate with metallurgical industry, prompted in metallurgical industry, on the basis of further reduce the oxygen content in casting solidification technology and rolling technology, inclusion control and testing technology research, such as improvement of electromagnetic stirring continuous casting, increase the size of the continuous casting billet, strengthen the high temperature diffusion annealing, etc., in order to improve the inclusion and uniformity of size and distribution of the carbide.

Development and promotion of new types of steel to meet the requirements of different bearings

With the miniaturization, lightweight and high-speed of the main engine, the use environment of the bearing is becoming more and more diversified, and the requirements for the bearing are becoming more and more demanding. At present, the existing steel types in China can no longer meet or fully meet the requirements of the main engine for the bearing. Therefore, the development and promotion of new materials should be actively carried out.Such as the development of large size bearings with high hardenability steel, overloading and clean lubrication condition or under the condition of small, lightweight use bearing steel, under the condition of pollution using bearing in carburizing steel and high carbon steel, high temperature (working temperature below 200 ℃) under the condition of use in bearing steel and special conditions of bearing steel (stainless steel, high temperature steel).

Research and promotion of new heat treatment technology

1. Bainite quenching

Due to good impact toughness and compressive stress on the surface of bainitic isothermal quenched bearings, the tendency of the inner sleeve cracking during assembly, the outer sleeve dropping blocks and the inner sleeve breaking in the process of use are greatly reduced, and the edge stress concentration of the rollers can be reduced.Therefore, the average life and reliability of bearing after isothermal quenching are significantly improved compared with that after conventional quenching.The process is widely used in railway bearings, rolling mill bearings and bearings used under special working conditions.Compared with other life-extending measures, the process is simple and the cost is lower.In recent years, China has developed a new type of steel GCr18Mo bainite quenching special steel to promote the application of bainite quenching in large-sized bearing parts.In view of the many advantages of this process, it is recommended to promote it in bearings with poor service conditions (large impact load, poor lubrication, etc.) or requiring high reliability, and further study the wear resistance and fatigue life after bainite treatment.

2. Surface carbonitriding

Luoyang bearing research institute in the 1980 s the bearing steel quenching martensite stress study, through the study of the special of high carbon chromium bearing steel parts after the carbonitriding of hardening, improve surface residual austenite content, improve the stress state of the surface has greatly increased the transmission with the carbonitriding, without reducing the surface hardness on the basis of improving surface residual austenite content, in order to improve the bearing fatigue life and reliability of the lubrication condition in pollution.

Surface modification technology

The surface properties are improved by proper surface treatment to meet the performance requirements of bearings under special conditions.Such as the application of gas deposition technology in the bearing raceway coating diamond steel stone coating can achieve the effect of friction reduction, wear resistance, greatly improve the bearing wear life and precision maintenance performance, can be used in household electrical bearings, computer hard drive bearings;The thermal coating technology can be used to coat aluminum oxide ceramic material on the cylinder outside the outer ring of the bearing, which can improve the electrical insulation performance of the bearing, prevent electric damage, and improve the life and reliability of the motor bearing.The anti-friction lubrication can be achieved by soiling or depositing M0S2 on the surface of bearing parts.

Heat treatment equipment and related technologies

1. Atmosphere and control

From using protective atmosphere heating to accurately controlling carbon potential and nitrogen potential controlled atmosphere heating, the performance of parts is improved after heat treatment, and the heat treatment defects such as decarbonization and cracking are greatly reduced.Combined with the quenching deformation control technology of heat treatment, it can reduce the finishing allowance after heat treatment, improve the material utilization rate and machining efficiency, and improve the surface state of bearing parts after finishing, such as the surface carbon content, structure, hardness and stress state.

2. Automation and intelligence

On the one hand, according to the requirements of the parts, materials, structural dimensions, using the knowledge of physical metallurgy and advanced computer simulation and detection technology, optimize the process parameters, to achieve the required performance or maximize the potential of the materials;On the other hand, it can improve the automation degree and stability of heat treatment, fully guarantee the stability of the optimized process, and realize the goal of small (or zero) dispersion of product quality, so as to meet the performance requirements of the main engine under different conditions of use and improve the reliability and life of the bearing.

Vi. Deformation and dimensional stability

In the process of martensite quenching, due to the uneven cooling of the parts, the thermal stress and the microstructure stress inevitably appear and lead to the deformation of the parts.Quenching tempering after deformation of parts (including the size and shape changes) is influenced by many factors, is a fairly complex problems, such as parts of shape and size, the uniformity of the original organization, before quenching state of rough machining (turning feed quantity, the size of the machining residual stress, etc.), quenching temperature and heating speed, workpiece put way, into the oil way, the characteristics of quenching medium and circulation way, the temperature of the medium, etc all can affect the deformation of the parts.The deformation should be studied in combination with the specific equipment and products, and the measures to control the deformation should be put forward, such as adopting rotary quenching, die quenching, controlling the oil input of parts, etc. to reduce the deformation of heat treatment and improve the processing efficiency and performance of parts.

After martensitic quenching, the dimensional stability of the part is mainly affected by three different transformations: carbon migration from the martensitic lattice to form -carbide, residual austenitic decomposition and fe3c, which are superimposed on each other.Between 50 ° c and 120 ° c, the volume of the part is reduced due to the precipitation of - carbide. Generally, the part has completed this transformation after tempering at 150 ° c, and its influence on the dimensional stability of the part in the later use process can be ignored.Between 100 ℃ and 250℃, the residual austenite is decomposed and transformed into martensite or bainite, which will be accompanied by an increase in volume.At above 200℃, the - carbide converts to the cementite, resulting in a reduction in the volume of the part.Studies have shown that residual austenite decomposes under external loading or at low tempering temperatures (even at room temperature), resulting in dimensional changes in parts.Because of this, in the actual use, the heating temperature of all bearing parts should be higher than the service temperature of 50℃, the dimensional stability of the parts with higher requirements to reduce the content of residual austenite, and improve the dimensional stability, accuracy, life and reliability in storage and use.


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