SUJ2 is the standard bearing steel while 2A and UL represent SUJ2 treated with carbonitriding and ultra-clean process separately.

Carbonitriding is a metallurgical enhancement technique that used to increase the surface hardness of a metal. During the process, atoms of carbon and nitrogen diffuse into the metal through interstices between lattices, creating barriers to slip, and eventually increase the hardness near the surface. This kind of heat treatment is commonly used in bearing industry to increase the bearing life.

Production of ultra-clean steel was a patented process in the past. Nowadays, this process has been used when it comes to high hardness and strength levels steel, such as bearing steel, aerospace steel, etc. Bearing with ultra-clean steel has the oxygen mostly removed within, which avoid the inclusions while adding alloying elements, such as manganese and chromium. Inclusions of oxygen and alloying elements cause weak spots, and when it comes to bearings, it cannot be tolerated since considerable pressures are exerted on very small areas (e.g. area between ball surface and ring raceways). Consequently, bearing life and wear resistance have been increased while toughness and heat stabilization were maintained by using ultra clean steel.

Although bearing life mostly depends on the operating condition, life ratios of standard, carbonitriding and ultra-clean bearing steel can generally be referred to in the diagram below.


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