Analysis of Heat Treatment Techniques for Bearings

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Heat treatment plays a crucial role in the performance and longevity of bearings. While many are familiar with the general concept, not everyone understands how to apply these techniques effectively. In this article, I’ll share some insights on the proper use of heat treatment in bearing manufacturing. (1) Expanding the application range of GCR15 steel. Typically, GCR15 steel is suitable for quenching with a wall thickness of up to 12mm. However, using BL quenching with nitrate salt provides a stronger cooling effect. By mixing, stringing, or adding water, the effective wall thickness can be extended to approximately 28mm, allowing for more versatile applications. (2) Achieving stable and uniform hardness. The BL process is a slow transformation, usually taking around 4 hours for GCR15 steel and 5 hours for GCR18MO steel. During this time, the ferrule undergoes isothermal treatment in nitrate salt, which allows the microstructure to change uniformly. This results in a consistent hardness of 59–61 HRC after BL quenching, with a variation of no more than 1 HRC. In contrast, traditional quenching may lead to uneven hardness, soft spots, and poor overall quality, especially when the wall thickness increases. (3) Reducing quenching and grinding cracks in railway and mill bearings. Bearings used in heavy-duty applications often have large sizes and high weights. When oil quenched, M structure becomes brittle, and strong cooling is typically used to achieve high hardness. This can cause microcracks. However, BL quenching offers better structural stability. It creates a compressive stress of up to -400 to -500 MPa on the surface, significantly reducing the risk of cracking. Additionally, during grinding, this compressive stress helps offset grinding-induced stresses, lowering the overall stress level and minimizing crack formation. (4) Improving the service life of bearings. Bearings subjected to heavy impact loads, such as those used in railways or rolling mills, often fail due to inner ring cracking during installation or outer ring fragmentation during operation after M quenching. Austempered bearings, on the other hand, offer superior impact resistance and surface compressive stress. This means that even if the inner sleeve cracks during installation, it is less likely to break apart during use, greatly reducing the risk of failure. As a result, isothermal quenching leads to improved average life and reliability compared to conventional methods.

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