Bearing Heat Treatment Methods in the Foreign Machinery Industry (Part 2)

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3. Bainite Austempering

3.1 Bainite Quenching Arrangement and Mechanical Function
When high-carbon chromium bearing steel is quenched using lower bainite, its microstructure consists of lower bainite, martensite, and residual carbides. The bainite forms irregularly intersected strips that are carbon supersaturated alpha structures. These strips are interspersed with granular or short rod-shaped carbides at an angle of approximately 55 to 60 degrees relative to the sheet’s long axis. The spatial shape resembles a convex lens, and the sub-structure contains dislocation entanglements without any tantalum sub-structures. The morphology and quantity 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 angle between the bainite strips also changes accordingly. The lower bainite structure in high-carbon chromium bearing steel enhances the steel's yield strength, flexural strength, and section reduction rate. Compared to quenched and tempered martensite, it offers higher impact resistance, fracture resistance, and dimensional stability, with a compressive stress state on the surface. The high ΔKth threshold value and low crack growth rate (da/dN) indicate that the bainite structure is less prone to crack initiation and propagation. This makes it ideal for applications where fatigue resistance is critical. It is generally believed that the wear resistance and contact fatigue performance of full bainite or shell-like composite structures are lower than those of low-temperature tempered martensite. However, under smooth conditions such as coal slurry or water, the full bainite arrangement shows significant superiority, offering a longer service life compared to low-temperature tempered martensite. For example, in smooth water conditions, the L10 life of the full bainite arrangement reaches 168 hours, while that of the tempered martensite arrangement is only 52 hours. 3.2 Production and Use
3.2.5 Effects
The key features of the bainite structure include high impact resistance, fracture resistance, wear resistance, and dimensional stability. The surface residual stress is compressive, making it suitable for bearings operating under heavy interference and harsh conditions, such as railway, rolling mill, and crane bearings, as well as mine transportation systems and coal mine bearings. The bainite austempering process has been successfully applied in railway and rolling mill bearings. (1) It expands the application range of GCr15 steel. Typically, GCr15 steel is limited to wall thicknesses of 12 mm or less when quenched in martensite. However, bainite quenching allows for thicker sections due to salt cooling, extending the useful wall thickness to about 28 mm. (2) The hardness is stable and uniform. The bainite transformation is a slow process, taking about 4 hours for GCr15 and 5 hours for GCr18Mo. During isothermal treatment in nitrate salt, the ferrule undergoes a simple transformation, resulting in good stability and uniformity. After bainite quenching, the hardness of GCr15 is typically 59–61 HRC with a uniformity of ≤1 HRC. In contrast, martensitic quenching can lead to soft spots and poor uniformity in thicker parts. (3) It reduces quenching and grinding cracks. In the production of railway and rolling mill bearings, large and heavy ferrules are prone to brittle cracking during oil quenching. To achieve high hardness, strong cooling is often used, which increases the risk of quenching microcracks. Additionally, the tensile stress on the surface combined with grinding stress can cause grinding cracks. With bainite quenching, the external surface experiences compressive stress of up to -400 to -500 MPa, significantly reducing the tendency for quenching cracks and lowering the overall stress level during grinding. (4) It improves bearing life. Bearings subjected to large impact loads, such as those in railways and rolling mills, often fail due to inner sleeve cracking after martensitic quenching. However, bainite-quenched bearings exhibit excellent impact resistance and compressive stress, preventing cracking during installation and operation. This leads to improved average life and reliability. SKF applies the bainite austempering process to railway and rolling mill bearings, as well as bearings used in special conditions. They have developed specialized steel grades like SKF24, SKF25, and 100Mo7. A new steel grade, 775V, was developed by SKF, offering more uniform lower bainite after special austempering. The hardness after quenching is 60% higher than conventional austempering, with three times better wear resistance and a treated ferrule wall thickness exceeding 100 mm. While the performance of M/BL composite arrangements remains debated, especially regarding optimal BL content, the benefits of bainite quenching are widely recognized.

4. Carburizing, Nitriding, and Carbonitriding

Low carbon steel carburizing, nitriding, and carbonitriding are traditional surface chemical heat treatment processes. Carburized steels (low carbon low alloy steel, low carbon high alloy high-temperature carburizing steel) are hardened through carburizing and quenching, resulting in a hard, wear-resistant surface and a tough core. Improvements in carburizing media, such as additives to increase diffusion speed and optimized cycles, enhance penetration efficiency and layer structure. With the development of vacuum technology, vacuum low-pressure carburizing and plasma carburizing have emerged. Acetylene-based low-pressure carburizing, as developed by companies like Ipsen, offers fast diffusion, uniform layers, and minimal carbon black. This method is particularly useful for thin-walled needle roller bearings requiring precise control over penetration depth and composition. Plasma carburizing of high-alloy carburized steel can improve diffusion rates and reduce coarse carbides on the surface. Nitriding or carbonitriding is often used for the inner and outer rings of needle bearings and low-carbon steel needle bearings, enhancing wear and corrosion resistance while reducing friction coefficients. For high-carbon chromium bearing steel, carburizing, nitriding, or carbonitriding can increase surface carbon and nitrogen content, decrease the Ms point, and create compressive stress after quenching. This improves wear resistance and rolling contact fatigue performance. Studies show that carburizing or carbonitriding can significantly extend bearing fatigue life. However, if the carbon potential exceeds 2%, additional machining allowance may be required to avoid coarse carbides. 3.4 Process Control
Controlling the atmosphere during carburizing (nitriding or carbonitriding) is crucial. Early methods included dew point meters and CO2 infrared analyzers, but oxygen probes are now preferred for faster response and real-time monitoring. Combined with other measurement techniques, such as the HydroNit probe, accurate carbon (or nitrogen) potential control is achievable. Computerized control of the carburizing process has advanced significantly. Software like Carb-o-Prof simulates carbon transfer and distribution, allowing real-time adjustments to process parameters. This software integrates metallurgical knowledge, equipment functions, and workpiece requirements, enabling efficient and accurate process planning.

5. Surface Modification Techniques

Ion implantation offers several advantages over other surface reinforcement methods. It preserves the original dimensions and surface roughness, making it ideal for precision components like aerospace bearings. It is not limited by metallurgy or phase diagrams, allowing flexible element selection based on application needs. The implanted layer is firmly bonded to the substrate, ensuring no delamination during use. Ion implantation is a low-temperature process, avoiding tempering, deformation, and oxidation. It also provides good controllability and reproducibility. Research by the US Naval Laboratory since 1979 has shown that chromium ion implantation improves the corrosion resistance and contact fatigue performance of M50 steel. Boron ion implantation enhances wear resistance on the outer bearing surfaces. Nitrogen plasma ion implantation (PSII) on 52100 bearing steel forms a thin nitride layer, improving corrosion resistance and reducing micro-vibration wear. (Ti + N) or (Ta + N) plasma immersion ion implantation (PSIII) significantly boosts microhardness, wear resistance, and bearing life. Surface coating techniques include physical vapor deposition (PVD), chemical vapor deposition (CVD), radio frequency sputtering (RF), ion spraying (PSC), and electroless plating. PVD is widely used due to its low processing temperature and lack of post-plating heat treatment. Coatings like TiC, TiN, and TiAlN on bearing parts improve wear resistance and reduce friction coefficients. SKF has developed two coating technologies: Diamond-Like Carbon (DLC) for bearing rings and tumbling surfaces, offering high hardness and self-lubrication; and PSC-sprayed alumina for insulation, improving dielectric properties and corrosion resistance. Low-temperature ion sulfurization, introduced in the late 1980s, creates a FeS sulfide layer that reduces friction and improves wear resistance under heavy load. Similarly, low-temperature phosphating forms a Fe2O3/Fe4P2O7 layer that reduces friction and enhances wear resistance. Dispersed chromizing via gas or powder methods at 850–1100°C improves heat resistance, corrosion resistance, and wear resistance after chrome and heat treatment.

6. Surface Hardening by Induction Heating

Induction heating surface quenching is one of the most widely used methods. The former Soviet Union conducted extensive research on this process. Its primary applications include railway bearing surface induction heating quenching, where the ferrule made of XG4 steel becomes a hard, wear-resistant martensite structure with a compressive stress of up to 500 MPa. This results in a service life twice that of ШХ15СГ bearings, eliminating brittle fracture issues. Induction heating is also used for large bearings, reducing quenching deformation and hardness non-uniformity. Japan has successfully applied this technique to car constant velocity joints, achieving cost reductions and improved reliability. High-energy beam treatments, such as laser, offer precise control over hardened layer depth and azimuth, with no deformation. Laser-hardened high-carbon chromium bearing steel exhibits fine martensite, uniform carbide dispersion, and reduced retained austenite, offering higher hardness and wear resistance than conventional quenching and tempering. These advanced heat treatment methods continue to evolve, providing enhanced performance and reliability for bearings in demanding industrial environments.

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