Bearing Heat Treatment:
Superior material quality is a prerequisite for all subsequent production steps. Following the soft machining of components, heat treatment is the next step. This process is crucial in matching the performance of the component to the specific application.
The classic heat treatment process for bearing rings made of integral hardened steel is martensitic hardening followed by tempering.
The part is heated to a defined temperature, held for a period of time, and then quenched. After quenching, the part then undergoes what is called hardening – quenching is carried out in an oil bath, or nowadays in a special polymer compound. After cooling, the part is tempered again. In a suitable furnace, they are reheated. The tempering temperature depends on the desired properties, such as heat resistance. The selected temperature during tempering determines the highest operating temperature the part can withstand, within which the material has a stable structure. Tempering can affect various variations and performance characteristics of chromium steel; for example, within a certain temperature range, toughness can be significantly improved while hardness loss is minimal.
Another proven method is bainitrification (isothermal hardening), which creates a highly favorable residual stress distribution within the material.
This is achieved by holding the quenched component at a quenching temperature relevant to the material's properties. After several hours, the microstructure gradually transforms, and residual compressive stresses form on the surface. These compressive stresses reduce the material's fatigue performance under overload conditions and inhibit cracking that may develop after high-load cycles.
A third hardening process is surface hardening.
In many applications, rolling element bearings are subjected to high impact or flexibility. Due to fatigue or poor installation conditions, bearing rings may become loose. This can lead to relative movement between bearing components and on the shaft, resulting in localized overheating and cracking. For such applications, surface-hardened steel and appropriate hardening processes are recommended. Following guided gas carburizing, each component undergoes martensitic heat treatment. This not only creates a hardened edge region around the viscous core but also generates residual compressive stress within the surface layer. The hardness and strength of the hardened layer gradually decrease at the center of the workpiece cross-section due to surface hardening, necessitating the determination of the surface hardening depth.
Based on the load and contact geometry, this depth is designed to counteract plastic deformation at the contact points or fatigue damage occurring during static or dynamic operation. The hardening depth is calculated using mathematical formulas. A maximum depth of approximately 2 mm is the standard value; deeper depths are extremely costly, depending on the component size, with furnace heating times reaching up to 90 hours.
Furthermore, flame or induction hardening methods are mentioned. These methods can only treat portions of the component. In recent years, several mechanical treatment methods aimed at improving component performance have been established, namely shot peening or solid or smooth rolling. These methods can generate a beneficial residual stress state close to the surface. Last but not least, there are surface treatment technologies—coating processes. These methods can also influence the characteristics of the component—related to practical applications. Browning and phosphating processes, in particular, have been widely applied. These methods improve break-in performance and emergency operating performance.
Meanwhile, in recent years, the aerospace bearing industry has also incorporated cryogenic treatment technology to enhance bearing stability. This will be explained in a separate chapter.
