Surface hardening technologies: methods for hardening the surface of bearing steel

Surface hardening technologies: methods for hardening the surface of bearing steel

There are significant differences between the working surfaces and the core of bearing components in terms of condition, structure and performance requirements; however, conventional heat treatment often fails to address both aspects simultaneously, thereby preventing the material’s full potential from being realised. The application of surface hardening techniques not only effectively resolves the discrepancies between the surface and the core in terms of structure and requirements, but also enables the surface to acquire certain specialised working properties, thereby meeting the performance requirements of bearing working surfaces operating under specific conditions. This is of great significance in the development of modern science and technology.

Traditional surface hardening methods fall within the scope of heat treatment in terms of process. However, modern surface hardening methods such as laser, electron beam and ion beam hardening not only apply advanced technologies to surface hardening but have also transcended the scope of traditional heat treatment, forming a new technical field. Consequently, current surface strengthening technologies can be classified in various ways from different perspectives. Based on the physicochemical processes involved, they can be broadly categorised into five main types: surface deformation strengthening, surface heat treatment strengthening, chemical heat treatment strengthening, surface metallurgical strengthening, and surface film strengthening.

1. Surface Deformation Hardening

This method involves inducing plastic deformation in the surface layer of a metal through mechanical means, thereby forming a hardened layer with high hardness and high strength. This surface hardening technique is known as surface deformation hardening, also referred to as work hardening. It includes shot peening, sandblasting, cold extrusion, rolling, cold rolling, impact hardening and explosive impact hardening. These methods are characterised by an increased dislocation density in the hardened layer and a refined sub-grain structure, thereby enhancing hardness and strength whilst reducing surface roughness. They can significantly improve the surface fatigue strength of components and reduce the sensitivity of fatigue notches. This strengthening method is simple to implement and highly effective; there is no distinct boundary between the hardened layer and the matrix, the structure is continuous, and it is not prone to peeling off during use. Most of these methods have already been applied in the bearing industry: impact strengthening of rolling element surfaces is an application of this type of method, whilst precision rolling has become a new method for processing and strengthening bearing rings.

2. Surface Heat Treatment Hardening

Surface heat treatment, commonly known as surface hardening, involves the quenching of a component’s surface layer through rapid heating utilising solid-state phase transformations. This includes flame heating and quenching, high (or medium) frequency induction heating and quenching, and laser or electron beam heating, which yield significant improvements in the wear resistance and fatigue strength of large and extra-large bearing components.

Quenching, etc. These methods are characterised by: localised surface heating and quenching, resulting in minimal workpiece deformation; rapid heating rates, leading to high production efficiency; and short heating times, resulting in only very slight surface oxidation and decarburisation. This method is particularly effective for enhancing the ability of components to withstand certain impact loads.

3. Chemical Heat Treatment Hardening

A method known as chemical heat treatment hardening—also referred to as diffusion heat treatment—involves altering the chemical composition of the metal’s surface layer through the solid-state diffusion of specific elements to achieve surface hardening. These include boron diffusion, metal diffusion, carburisation and carbonitriding, nitriding and nitrocarburising, sulphur diffusion and sulphur-nitrogen-carbon diffusion, chromium diffusion, aluminium diffusion and chromium-aluminium-silicon diffusion, graphitisation diffusion layers, and so on; there are numerous types, each with distinct characteristics. The diffusing elements either dissolve into the matrix metal to form a solid solution, or combine with other metallic elements to form compounds. In summary, the diffused elements not only alter the chemical composition of the surface layer but also result in different phase structures. The treatment process for carburised bearing steel components and the surface nitriding hardening treatment of needle roller bearing sleeves both fall under this category of hardening methods.

4. Surface Metallurgical Hardening

Surface metallurgical hardening refers to surface hardening techniques that utilise the remelting and solidification of the metal in the surface layer of a workpiece to achieve the desired composition or microstructure. It includes methods such as surface self-fusing alloy or composite powder coatings, surface melting with crystallisation or amorphous treatment, and surface alloying. A characteristic feature of this process is the use of rapid heating with high energy density to melt the surface layer of the metal or the alloying material coated onto it, followed by self-cooling and solidification to produce a strengthened layer with a special microstructure or specific properties. This special microstructure may consist of a refined crystalline structure, or it may comprise supersaturated phases, metastable phases, or even an amorphous structure, depending on the process parameters and methods employed in surface metallurgy.

The rolling bearing industry has conducted research into laser heating strengthening on the working surfaces of miniature bearings, with favourable results.

5. Surface Thin-Film Strengthening

The application of physical or chemical methods to deposit strengthening film layers with properties different from those of the substrate material onto a metal surface is termed surface film strengthening. This includes electroplating, electroless plating (chromium, nickel, copper and silver plating, etc.), as well as composite plating, brush plating and conversion treatments. It also encompasses high-tech methods that have developed rapidly in recent years, such as CVD, PVD, P-CVD and other vapour deposition film strengthening methods, as well as ion implantation surface strengthening technology (also known as atomic metallurgy). A common feature of these techniques is that they significantly improve wear resistance, fatigue resistance and contact fatigue resistance, thereby multiplying the service life of the bearings.

All these methods can form thin films with specific properties on the working surface to enhance the surface’s wear resistance, fatigue resistance, corrosion resistance and self-lubricating properties. For example, the use of ion implantation technology to strengthen the working surface of bearings can improve the bearing’s wear resistance and corrosion resistance.