**Martensitic Quenching and Tempering of High Carbon Chromium Bearing Steel**
Home > Bearing Knowledge > Martensitic Quenching and Tempering of High Carbon Chromium Bearing Steel
Source: China Bearing Network | Time: 2013-01-16
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As the speed and weight of modern engines continue to increase, the working conditions for bearings have become more demanding. The functional requirements for bearings are constantly rising, including smaller size, lighter weight, greater load capacity, longer life, and higher reliability. In recent years, the lifespan and reliability of domestic bearings have become critical issues, driving the development of new heat treatment technologies and improvements in heat treatment quality. This has become a central focus for both domestic and international bearing manufacturers and research institutions.
### 1. Annealing Process of High Carbon Chromium Bearing Steel
The annealing structure of high carbon chromium bearing steel is characterized by fine, uniform, and round carbide particles distributed on a ferrite matrix. This microstructure prepares the material for subsequent cold working and final quenching and tempering. Most companies now use a single-channel pusher isothermal annealing furnace with an inert atmosphere, which offers better control over the annealing structure and hardness compared to older methods. According to JB1255 standards, the ideal structure should be 2–3 points or finer. However, this process is energy-intensive, and oxidation and decarburization after annealing remain significant issues. Recently, energy-saving technologies such as oil-electric composite heating and double-chamber isothermal furnaces have been developed and show promising results. These innovations help reduce oxidation, lower raw material costs, and improve machining efficiency.
### 2. Development of Martensitic Quenching and Tempering Process
The martensitic quenching and tempering process for high carbon chromium bearing steel mainly involves three aspects:
- **Microstructure and Property Analysis**: Studies focus on phase transformation during quenching and tempering, residual austenite content, and the impact on fatigue resistance.
- **Process Function Research**: This includes the effects of quenching conditions on scale, deformation, and stability.
- **Controlled Atmosphere Heating**: Efforts are being made to replace oxidative heating with controlled atmosphere techniques.
#### 2.1 Microstructure and Mechanical Properties After Quenching and Tempering
After martensitic quenching, the microstructure consists of martensite, residual austenite, and undissolved carbides. The carbon content in the martensite matrix is typically around 0.55%. The shape is often a mixture of lath and flake martensite, or a jujube nucleus structure. As quenching temperature or holding time increases, the structure evolves from cryptocrystalline to crystalline and eventually to fine needle-like martensite. A well-balanced microstructure includes a mix of cryptocrystalline, crystalline, and fine acicular martensite. Excessive acicular martensite can lead to substandard structures and should be avoided.
Studies, such as those conducted by Luoyang Bearing Research Institute in the 1980s, showed that quenching at 835–865°C and tempering at 150–180°C provides optimal mechanical properties and fatigue life. Higher quenching temperatures (e.g., 845°C) result in higher compressive strength and longer fatigue life. Tempering at higher temperatures reduces hardness but improves strength and durability. Special treatments like cold treatment between -40°C and -78°C can enhance dimensional stability, while isothermal quenching at 250°C followed by tempering at 180°C can improve impact resistance.
Currently, foreign bearings often undergo personalized heat treatment based on specific working conditions. Domestic processes, however, still rely heavily on standardized specifications like JB1255, which lack personalization. Improving this approach by aligning with actual bearing conditions and focusing on inner and outer ring hardness is essential.
#### 2.2 Quenching Deformation Control
Conventional quenching uses equipment like chain furnaces or mesh belt furnaces. While these systems offer good control, reducing quenching distortion remains a major challenge. Distortion affects the performance of dust-proof bearings, especially when it impacts the sealing function. Effective methods include optimizing workpiece placement, controlling quenching oil temperature, and mixing oils to achieve minimal distortion.
#### 2.3 Residual Stress and Austenite Management
Residual stress and retained austenite are not currently regulated in Chinese thermal inspection standards. However, studies suggest that proper residual compressive stress can enhance fatigue life and prevent grinding cracks. Foreign companies often monitor these parameters closely. Future research should focus on understanding their impact and developing targeted control strategies based on bearing operating conditions.
### 3. Bainite Austempering
Bainite austempering has gained attention in the bearing industry since the 1980s. It was first applied to railway bearings and later extended to rolling mill and locomotive bearings. The process significantly improves impact resistance, wear resistance, and dimensional stability. Bearings treated with bainite austempering are suitable for harsh environments such as railways, mines, and cranes.
In production, bainite quenching helps avoid quenching cracks by forming compressive surface stresses. SKF has successfully applied this technique to railway and rolling mill bearings, using specialized steels like SKF24, SKF25, and 100Mo7. Their new grade 775V achieves even better performance after special austempering.
Despite its benefits, challenges remain, such as determining optimal bainite content and managing increased production costs. Research into alternative nitrate-based equipment and environmental protection is also ongoing.
### 4. Special Heat Treatment
Special heat treatments like carburizing, nitriding, and carbonitriding are used to enhance surface properties. These methods increase surface hardness, reduce the Ms point, and create compressive stresses that improve wear resistance and fatigue life. NSK and KOYO have developed advanced techniques that significantly extend bearing life under contaminated conditions.
### 5. Surface Modification Techniques
Ion implantation and coating technologies are increasingly used to improve bearing performance. Ion implantation with chromium, boron, or nitrogen enhances corrosion and wear resistance. Coating methods like PVD and CVD apply hard or soft films to improve friction characteristics and reduce wear. SKF’s Diamond-Like Carbon (DLC) coating and aluminum spray coatings demonstrate exceptional performance in high-stress applications.
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