There are great differences between the working surface and the core of bearing parts in terms of state, structure and performance requirements, and the overall heat treatment often makes the two can not take into account, and the potential of the material can not be fully exploited.Using surface hardening technology can not only solve the differences between surface and core in structure and requirements, but also make the surface obtain some special working properties to meet the requirements of working surface of bearings working under specific conditions.This is of great significance in the development of modern science and technology.
The traditional surface strengthening method belongs to the category of heat treatment technologically.However, the surface strengthening methods such as laser, electron beam and ion beam developed in modern times not only apply some high and new technologies to the surface strengthening of materials, but also have gone beyond the scope of traditional heat treatment and formed a new technical field.Therefore, the surface strengthening technology can form a variety of classification methods from different perspectives. According to the physical and chemical process of surface strengthening technology, it can be roughly divided into five categories: surface deformation strengthening, surface heat treatment strengthening, chemical heat treatment strengthening, surface metallurgy strengthening, surface thin film strengthening.
1. Surface deformation strengthening
Through the mechanical method to make the metal surface layer plastic deformation, thus forming a high hardness and high strength hardening layer, this kind of surface strengthening method is called surface deformation strengthening, also known as work hardening.Including shot peening, sand blasting, cold extrusion, rolling, cold rolling and impact, explosion impact strengthening, etc.The characteristics of these methods are as follows: the strengthening layer dislocation density is increased, the subcrystalline structure is refined, so that the hardness and strength are increased, the surface roughness value is reduced, the surface fatigue strength of parts can be significantly improved and the sensitivity of fatigue notch can be reduced.This strengthening method has the advantages of simple technology and remarkable effect. There is no obvious boundary between the hardened layer and the matrix. The structure is coherent and not easy to fall off in use.Most of these methods have been used in bearing industry: the surface impact strengthening of rolling body is one of these methods, and precision rolling has become a new method of processing and strengthening the ring.
2 Surface heat treatment enhancement
The use of solid phase change, through the rapid heating of parts of the surface layer quenching treatment is called surface heat treatment, commonly known as surface quenching.Including flame heating quenching, high (medium) frequency induction heating quenching, laser heating or electron beam heating quenching.The characteristics of these methods are: surface local heating and quenching, workpiece deformation is small;The heating speed is fast and the production efficiency is high.The heating time is short, and the surface oxidation and decarbonization are very slight.This method is particularly effective in improving the wear resistance and fatigue strength of large and extra-large bearing parts under certain impact load.
3. Chemical heat treatment enhancement
The use of solid diffusion infiltration of some elements to change the chemical composition of the metal surface layer, in order to achieve the surface strengthening method is called chemical heat treatment strengthening, also known as diffusion heat treatment.Including boronizing, metallization, carburizing and carbonitriding, nitriding and nitrogen carburizing, sulfurizing and sulfur nitrogen carburizing, chromizing, aluminizing and chromium-aluminizing, graphitized carburizing layer and so on, there are many kinds and different characteristics.Infiltrate elements or dissolve into the host metal to form a solid solution, or combine with other metal elements to form compounds.In short, infiltration elements can not only change the chemical composition of the surface layer, but also obtain different phase structures.The treatment technology of carburized bearing steel parts and the surface nitriding strengthening treatment of needle roller bearing sleeve belong to this kind of strengthening methods.
4 Surface metallurgical strengthening
Surface hardening is a technique that uses the re-melting and solidification of the metal on the surface layer of the workpiece to obtain the desired composition or structure.Including surface autolytic alloy or composite powder coating, surface melting crystallization or amorphous treatment, surface alloying and other methods.It is characterized by the rapid heating with high energy density to melt the metal surface layer or the alloyed material coated on the metal surface, and then solidify by self-cooling to obtain a special structure or a specific performance of the strengthened layer.Depending on the process parameters and method of surface metallurgy, this particular structure may be refined crystalline, supersaturated, metastable, or even amorphous.
In the rolling bearing industry, laser heating has been used to strengthen the working surface of miniature bearings, and the results are satisfactory.
5 surface film enhancement
The application of physical or chemical methods, the metal surface coated with different properties of the matrix material of the strengthening film, called surface film strengthening.It includes electroplating, electroless plating (chromium, nickel, copper, silver, etc.) and composite plating, brush plating or conversion treatment, but also includes the recent rapid development of high and new technologies, such as CVD, PVD, P-CVD and other vapor deposition film strengthening methods and ion implantation surface strengthening technology (also known as atomic metallurgy technology) and so on.Their common feature is that they can form a specific performance of the film on the working surface, to strengthen the surface of wear resistance, fatigue resistance, corrosion resistance and self-lubrication and other properties.For example, ion implantation technology can strengthen the bearing working surface, which can significantly improve the wear resistance, corrosion resistance and contact fatigue resistance of the bearing working surface, so that the service life of the bearing can be multiplied.