Bearing Heat Treatment Methods in the Foreign Machinery Industry (Part 2)
### 3. Bainite Austempering #### 3.1 Bainite Quenching Arrangement and Mechanical Properties When high-carbon chromium bearing steel is quenched to form lower bainite, the microstructure consists of lower bainite, martensite, and residual carbides. The bainite appears as irregularly intersected strips with a carbon supersaturated α-iron structure. These strips are interspersed with granular or short rod-shaped carbides, oriented at an angle of 55–60° to the sheet's long axis. The spatial shape of bainite is convex lenticular, and it contains dislocation entanglements without any sub-lattice structure. The morphology and distribution of bainite vary depending on the process conditions. As the quenching temperature increases, the bainite strips become longer. When the isothermal temperature rises, the width of the bainite strips changes, and the carbide particles grow larger. The intersection of bainite strips becomes more pronounced under these conditions. Lower bainite improves the yield strength, flexural strength, and section reduction rate of the steel compared to quenched and tempered martensite. It also offers higher impact resistance, fracture toughness, and dimensional stability. Additionally, the surface stress state is compressive, which enhances its performance under external stress. The high threshold value ΔKth and low crack growth rate da/dN indicate that the bainite structure is less prone to crack initiation and propagation. This makes it suitable for applications where fatigue resistance is critical. Although full bainite or horse-shoe composite structures may have slightly lower wear resistance and contact fatigue properties compared to low-temperature tempered martensite, they perform better in smooth environments like coal slurry or water. For example, under such conditions, the full bainite arrangement can last up to 168 hours, while the low-temperature tempered martensite lasts only 52 hours. #### 3.2 Production and Application ##### 3.2.5 Effects and Benefits The key advantages of the bainite structure include excellent impact resistance, fracture resistance, wear resistance, and dimensional stability. The compressive surface stress makes it ideal for bearings operating under heavy interference or poor service conditions, such as those found in railways, rolling mills, cranes, and mining equipment. High-carbon chromium bearing steel undergoing bainite austempering has been successfully applied in railway and rolling mill bearings. Some specific benefits include: 1. **Expanded Application of GCr15 Steel**: GCr15 steel typically has a useful wall thickness of 12mm or less when quenched in martensite. However, bainite quenching using salt cooling allows the wall thickness to extend up to about 28mm. 2. **Stable and Uniform Hardness**: The bainite transformation is a slow process, usually taking 4 hours for GCr15 steel and 5 hours for GCr18Mo steel. During this time, the ferrule is isothermally treated in nitrate salt, leading to uniform hardness of 59–61 HRC with a variation of ≤1 HRC. 3. **Reduced Quenching and Grinding Cracks**: In large bearings like those used in railways and rolling mills, M-quenched parts tend to develop microcracks due to high cooling rates. Bainite quenching reduces this risk by creating a compressive surface stress of up to -400 to -500 MPa, significantly reducing the tendency for cracks. 4. **Improved Bearing Life**: Bearings subjected to large impact loads benefit from bainite's superior impact resistance and compressive surface stress. This results in a longer service life and increased reliability compared to traditional quenching methods. SKF has applied bainite austempering to railway, rolling mill, and special condition bearings. They have developed specialized grades like SKF24, SKF25, and 100Mo7 for bainite quenching. A new grade, 775V, has been introduced with improved lower bainite uniformity, higher hardness, and enhanced wear resistance. Despite the potential for M/BL composite structures, the optimal balance between bainite and martensite content remains a topic of debate. --- ### 4. Carburizing, Nitriding, and Carbonitriding Low-carbon steel carburizing, nitriding, and carbonitriding are traditional surface heat treatment processes. After carburizing and quenching, the surface becomes hard and wear-resistant, while the core remains tough. Recent developments in vacuum technology have led to vacuum low-pressure carburizing and plasma carburizing. For example, Ipsen’s low-pressure acetylene carburizing process operates at 10 mbar, offering fast diffusion, uniform layers, and reduced carbon black. Plasma carburizing improves penetration speed and reduces coarse carbide formation on the surface. Nitriding and carbonitriding enhance wear and corrosion resistance, especially for needle roller bearings made from low-carbon steel. Carburizing or carbonitriding high-carbon chromium bearing steel increases the surface carbon/nitrogen content, lowering the Ms point and forming compressive surface stress. This improves wear resistance and rolling contact fatigue performance. Studies show that carburized or carbonitrided high-carbon chromium bearing steel can significantly improve bearing life. Process control is crucial in carburizing, nitriding, or carbonitriding. Modern systems use oxygen probes and CO₂ infrared analyzers for real-time monitoring of carbon or nitrogen potential. Computer simulation software like Carb-o-Prof helps optimize parameters and adjust them dynamically during the process. --- ### 5. Surface Modification Techniques Ion implantation offers several advantages over other surface reinforcement techniques. It preserves original dimensions and surface roughness, making it ideal for precision components like aerospace bearings. Ion implantation is not limited by metallurgical phase diagrams and allows flexible element selection based on application needs. Research shows that chromium ion implantation improves the corrosion and contact fatigue resistance of M50 steel. Boron ion implantation enhances wear resistance, while nitrogen plasma ion implantation (PSII) forms a thin nitride layer on 52100 bearing steel, improving corrosion resistance and reducing micro-vibration wear. Coating technologies like PVD, CVD, RF sputtering, and electroless plating are widely used. SKF has developed diamond-like carbon (DLC) coatings for bearing surfaces, offering high hardness and self-lubrication. They also use PSC to spray alumina on outer rings, improving insulation and corrosion resistance. Low-temperature ion sulphurization and phosphating are effective for reducing friction and improving wear resistance. Dispersed chromizing at high temperatures enhances heat, corrosion, and wear resistance. --- ### 6. Induction and Laser Hardening Induction heating surface quenching is widely used in the former Soviet Union for railway bearings. It produces a hard, wear-resistant martensitic surface with compressive stress, significantly improving service life and reliability. Laser hardening is a modern technique that provides precise control over hardened layer depth and geometry. It is particularly useful for fine bearing parts, offering high hardness, uniform carbide dispersion, and improved wear resistance. These advanced heat treatment methods continue to evolve, offering better performance, cost-effectiveness, and versatility in bearing manufacturing. --- **Related Articles** - Replace the Lower Bearing of the Door Machine Using the Jacking Method (4) - SKF Bearing Excitation Attack Phase and Its Repair - Rolling Bearing Cooperative Die Casting Process Characteristics Analysis - Overview of Public Service and Cooperation Options for Imported Bearings This article is linked to http:// Please cite China Bearing Network: http:// Previous: Mechanical Vocabulary (16) Next: Aviation Initiator Bearing New Information
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