Bearing Steel
CNC Machining

AISI 52100 (also known as SUJ2 in Japan and GCr15 in China) is a high-carbon, chromium-alloyed bearing steel specifically engineered for rolling contact fatigue resistance. Its composition — approximately 1.0% carbon and 1.5% chromium — produces a fine, uniform distribution of chromium carbides after heat treatment that gives exceptional hardness (HRC 60–64), wear resistance, and contact fatigue life. These properties make it the standard material for precision ball bearings, roller bearings, bearing races, precision ball screws, and other components subjected to repeated rolling contact stress.

52100 (AISI), SUJ2 (JIS G4805), and GCr15 (GB/T 18254) are national standard designations for essentially the same bearing steel composition — high carbon (approximately 0.95–1.10%), chromium (1.30–1.65%), and small additions of manganese and silicon. Minor compositional differences exist between standards, but all three are functionally interchangeable for precision bearing and wear part applications. 52100 is the American designation used internationally; SUJ2 is specified by Japanese OEMs and in JIS supply chains; GCr15 is the Chinese GB standard and is the most commonly stocked grade in mainland China.

52100 bearing steel achieves HRC 60–64 after standard quench and temper heat treatment — significantly harder than 440C stainless (HRC 58–60) and comparable to high-speed tool steels. The high hardness combined with fine chromium carbide distribution gives 52100 exceptional rolling contact fatigue life. Sub-zero treatment after quenching converts retained austenite to martensite, further increasing hardness and improving dimensional stability in service.

52100 is typically machined in the annealed or spheroidized-annealed condition (approximately HRC 20–25) using conventional CNC turning and milling. After machining, parts are heat-treated to HRC 60–64, then precision-ground on all critical surfaces to recover final tolerance after heat treatment distortion. The annealed machinability is similar to medium-carbon alloy steel, though the high carbide content can cause more rapid tool wear than plain carbon steels. Sub-zero treatment is routinely applied after quench before tempering to maximize hardness and dimensional stability.

Sub-zero treatment (also called cryogenic processing) is the cooling of hardened bearing steel to temperatures between −75°C and −196°C immediately after quenching, before tempering. This low-temperature treatment converts retained austenite — a softer, metastable phase that remains after quenching — into martensite, the hard phase that gives bearing steel its properties. Without sub-zero treatment, retained austenite can transform to martensite in service under stress or temperature changes, causing dimensional growth and bearing failure. For precision bearing applications, sub-zero treatment is routinely specified to ensure dimensional stability and maximum hardness.

52100 bearing steel and tool steels (D2, H13) both achieve high hardness, but are engineered for different stress types. 52100 is specifically optimized for rolling contact fatigue — the repeated hertzian contact stress in bearings — through its fine, uniformly distributed chromium carbide microstructure and extremely high cleanliness. Tool steels like D2 are optimized for wear resistance in sliding contact and impact. 52100 has higher fatigue life under rolling contact but lower toughness and shock resistance compared to D2. For bearing components and precision rolling elements, 52100/GCr15 is the correct choice; for dies, punches, and cutting tools, D2 or H13 is more appropriate.

After heat treatment and precision cylindrical grinding, we hold tolerances of ±0.002–0.005 mm on critical bearing seat diameters and bore dimensions. Roundness (out-of-roundness) can be controlled to 0.001–0.002 mm on precision ground bearing races. Surface roughness Ra 0.2–0.4 μm is standard on ground bearing surfaces; Ra 0.05–0.1 μm is achievable with superfinishing. All critical dimensions are CMM-verified before release.

Black oxide is the most common surface treatment for 52100 bearing steel parts — it provides light corrosion protection with essentially zero dimensional change, and is compatible with the hardened surface. Phosphate coating is also used for similar reasons. Hard chrome plating and electroless nickel can be applied for enhanced corrosion protection but must be applied carefully to avoid hydrogen embrittlement in high-hardness steel — post-plate baking at 190°C for 4 hours is required. PVD coatings (TiN, DLC) are used on precision bearing components to further reduce friction and improve wear life without affecting base steel hardness.