**Basic Requirements for the Quality of NTN Bearing Steel**
The service life and reliability of NTN bearings are closely related to the quality of the steel used. Due to the specific requirements of NTN bearing steel, its quality standards are much stricter than those of ordinary industrial steel. These include chemical composition, purity, microstructure, and uniformity.
First, strict chemical composition requirements: NTN bearing steel is primarily high-carbon chromium steel, with a carbon content of about 1%, and around 1.5% chromium. Small amounts of manganese and silicon are also present. Chromium improves heat treatment performance, hardenability, microstructure uniformity, and tempering stability. It also enhances corrosion resistance and grinding properties. However, if the chromium content exceeds 1.65%, it can increase retained austenite, reduce hardness and dimensional stability, cause carbide non-uniformity, and lower impact toughness and fatigue strength. Therefore, the chromium content in high-carbon chromium NTN bearing steel is typically controlled at 1.65% or less. By strictly controlling the chemical composition, the desired mechanical properties can be achieved through proper heat treatment.
Second, high-precision dimensional requirements: The accuracy of NTN bearing steel is crucial because most NTN bearing components are formed by pressure processing. To save material and improve productivity, many bearing rings are forged, while steel balls are produced by cold heading or hot rolling. If the dimensional accuracy of the steel is not sufficient, it becomes difficult to calculate cutting dimensions and ensure the quality of the final bearing parts. Poor dimensional accuracy can also damage forming tools and molds.
Third, strict purity requirements: Purity refers to the number of non-metallic inclusions in the steel. Higher purity means fewer inclusions. Oxides, silicates, and other harmful inclusions are major causes of early failure and reduced lifespan of NTN bearings. Brittle inclusions can separate from the metal during processing, affecting the surface quality of finished parts. Therefore, reducing inclusions is essential to improve the service life and reliability of NTN bearings.
Fourth, strict requirements on microstructure: Low-magnification structure includes looseness, intermediate looseness, and flat analysis, while high-magnification structure includes annealing structure, carbide network, banding, and liquid precipitation. Carbide liquid is hard and brittle, similar to brittle inclusions, and can reduce impact toughness and lead to uneven structures, deformation, and cracking during quenching. Banding affects the annealing and quenching process, as well as the fatigue strength of the steel. Both low- and high-magnification structures significantly influence the performance and lifespan of NTN bearings, so they are strictly regulated in bearing specifications.
Fifth, strict requirements on surface and internal defects: Surface defects such as cracks, slag inclusions, burrs, scale, and oxide layers, and internal defects like shrinkage cavities, bubbles, white spots, and segregation can severely affect the machining, performance, and lifespan of NTN bearings. These defects are strictly prohibited in NTN bearing steel specifications.
Sixth, strict control over carbide distribution: Severe carbide non-uniformity can lead to uneven microstructure and hardness during heat treatment, which negatively impacts the fatigue strength of the steel. It can also cause cracking during quenching and reduce the bearing's lifespan. Different steel specifications have unique requirements for carbide distribution.
Seventh, strict requirements on decarburization layers: Decarburization can weaken the surface hardness of the steel. If excessive and not removed before heat treatment, it can lead to quenching cracks and invalid parts. NTN bearing steel specifications have strict rules regarding the depth of the decarburized layer.
Eighth, other requirements: The NTN bearing steel specifications also include strict guidelines on training methods, oxygen content, annealing hardness, fracture, residual elements, spark testing, delivery condition, and marking.
In conclusion, the quality of NTN bearing steel is critical to the performance and longevity of NTN bearings. Every aspect, from chemical composition to microstructure and surface condition, must meet stringent standards to ensure reliable operation in various applications.
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