The production of bearing steel mainly adheres to the GB/T18254-2002 standard and the Lai Steel GCr15JD quality agreement tailored to the needs of precision forging bearing users. The GCr15JD agreement has stricter quality requirements than the GB/T18254-2002 standard, including oxygen content ≤10ppm, center segregation level ≤1.0 grade, component control, fixed length, and dimensional deviation, all of which are more stringent than the GB/T18254-2002 standard.
Bearings withstand significant pressure and friction during operation, necessitating high and uniform hardness, wear resistance, and a high elastic limit in bearing steel. The requirements for the uniformity of the chemical composition of bearing steel, the content and distribution of non-metallic inclusions, and the distribution of carbides are very strict, making it one of the most demanding steel types in all steel production. In 1976, the International Organization for Standardization (ISO) incorporated some general bearing steel grades into the international standard, categorizing bearing steel into four types: fully quenched bearing steel, surface hardened bearing steel, stainless bearing steel, and high-temperature bearing steel, totaling 17 grades.
Certain countries have added a category for bearing steel or alloys with special applications. China's standardized classification method for bearing steel, similar to ISO, includes four main categories: high-carbon chromium bearing steel, carburized bearing steel, stainless corrosion-resistant bearing steel, and high-temperature bearing steel. Over the past five decades, China has made significant advancements in bearing steel grades and materials used in bearings, such as chrome-free bearing steel, medium-carbon bearing steel, special-purpose bearing steel and alloys, and metal ceramics.
Based on the basic requirements for bearing steel mentioned above, the following basic requirements for the metallurgical quality of bearing steel are proposed.
Strict chemical composition requirements.
General bearing steel mainly consists of high carbon-chromium bearing steel, with approximately 1% carbon content, 1.5% chromium, and a small amount of manganese and silicon elements in the hypoeutectic steel. Chromium can improve the heat treatment properties, increase hardenability, microstructure uniformity, and tempering stability, as well as enhance the steel's corrosion resistance and abrasive properties.
When the chromium content exceeds 1.65%, quenching increases the residual austenite in the steel, reducing hardness and dimensional stability, increasing the non-uniformity of carbides, and lowering the steel's impact toughness and fatigue strength. Consequently, the chromium content in high-carbon chromium bearing steel is typically controlled below 1.65%. Only by strictly controlling the chemical composition of the bearing steel can the desired microstructure and hardness for bearing performance be achieved through the heat treatment process.
Higher dimensional accuracy requirements should be applied to hot-rolled and annealed billets used on high-speed upsetting machines, as they demand a greater precision in size.
Steel requirements for roller bearings demand high dimensional accuracy, as most bearing components undergo pressure forming. To conserve materials and enhance labor productivity, the vast majority of bearing rings are forged into shape, steel balls are cold headed or hot rolled, and small-sized rollers are also cold headed. If the steel's dimensional accuracy is insufficient, it becomes impossible to calculate cutting dimensions and weights, compromising the quality of bearing components and potentially damaging equipment and molds.
③ Rigorous purity requirements.
The purity of steel refers to the amount of non-metallic inclusions present within it; the higher the purity, the fewer the non-metallic inclusions in the steel. Oxides and silicates, among other harmful inclusions, in bearing steel are the primary causes of early fatigue spalling and significantly reduced bearing life. Particularly, brittle inclusions pose the greatest harm, as they are prone to flake off from the metallic matrix during the processing, severely affecting the surface quality of the finished bearing components. Therefore, to enhance the service life and reliability of bearings, it is essential to reduce the content of inclusions in bearing steel.
④ Strict low-power and microscopic (high-power) tissue requirements.
The low-magnification microstructure of bearing steel refers to general porosity, central porosity, and segregation. The high-magnification microstructure includes the steel's annealing structure, carbide network, strip, and liquid segregation. Carbide liquid segregation is hard and brittle, with the same harmfulness as brittle inclusions. Networked carbides reduce the steel's impact toughness and cause uneven structure, making it prone to deformation and cracking during quenching. Strip-like carbides affect the annealing and quenching tempering structure as well as the contact fatigue strength. The quality of low and high magnification microstructures greatly influences the performance and service life of rolling bearings, hence strict requirements are placed on them in the bearing material standards.
⑤ Strict surface and internal defect requirements.
Surface defects in bearing steel include cracks, inclusions, burrs, scaling, and oxide skin, while internal defects include shrinkage cavities, bubbles, white spots, severe porosity, and segregation. These defects greatly affect the processing, performance, and lifespan of bearings, and are explicitly prohibited in bearing material standards.
⑥ Strict requirement for uniformity in carbide distribution.
In bearing steel, severe inhomogeneity in carbide distribution can easily lead to uneven microstructure and hardness during the heat treatment process. The inhomogeneity of the steel's microstructure has a significant impact on contact fatigue strength. Additionally, severe inhomogeneity in carbides can easily cause cracks in bearing components during quenching cooling. The inhomogeneity of carbides can also reduce the service life of bearings. Therefore, specific requirements are clearly outlined for different specifications of steel in bearing material standards.
⑦ Strict surface decarburization depth requirements.
Stringent regulations are in place for the decarburization layer on steel surfaces in bearing material standards. If the decarburization layer exceeds the standard's specified range and is not fully removed during the machining process before heat treatment, quenching cracks are more likely to occur during the heat treatment, leading to the scrapping of parts.
Other requirements.
The standard for bearing steel materials also imposes strict requirements on the smelting method, oxygen content, annealing hardness, fracture, residual elements, spark test, delivery condition, and marking of bearing steel.
Performance Requirements
To meet the above performance requirements for rolling element bearings, the following basic performance requirements for bearing steel materials have been proposed:
High contact fatigue strength
2) The heat treatment should result in high hardness or a hardness that meets the performance requirements for bearing use.
High wear resistance, low coefficient of friction.
4) High elasticity limit
5) Excellent impact and fracture toughness
6) Excellent dimensional stability,
7) Excellent rust-proof performance,
8) Excellent cold and hot working properties.
Manufacturing Requirements
The content of inclusions is closely related to the oxygen content in steel; the higher the oxygen content, the more inclusions there are, and the shorter the lifespan.
The larger and more uneven the particle size and distribution of inclusions and carbides, the shorter the service life. Their size and distribution are closely related to the smelting process and quality used. The main processes for producing bearing steel are continuous casting, electric furnace smelting plus electroslag remelting, and a small amount use double vacuum or multiple vacuum consumable processes, such as vacuum induction plus vacuum consumable, to improve the quality of bearing steel.
High-quality steelmaking is required for bearing steel, necessitating strict control over the contents of sulfur, phosphorus, hydrogen, as well as the quantity, size, and distribution of non-metallic inclusions and carbides. This is because the quantity, size, and distribution of these non-metallic inclusions and carbides greatly affect the service life of bearing steel, often leading to failure due to the propagation of microcracks around larger inclusions or carbides.
