SUJ2 (JIS G4805) is Japan’s 52100-equivalent bearing steel — the most precisely specified tool steel in everyday industrial use. Its 1.0% C and 1.5% Cr composition, combined with strict cleanliness requirements for oxide and sulfide inclusions, produces a microstructure capable of sustaining 10⁹ to 10¹⁰ contact fatigue cycles under Hertzian contact stresses exceeding 3,000 MPa (435 ksi). Over 90% of JIS bearing steel production is SUJ2. The remaining grades (SUJ1, SUJ3, SUJ4, SUJ5) serve specific needs — smaller sections, larger sections requiring Mo-enhanced hardenability, or ultra-high-cleanliness applications. This guide covers the heat treatment protocol that achieves the correct microstructure for maximum rolling contact fatigue life, the role of cryogenic processing in eliminating dimensional instability, and the three failure modes that determine whether a bearing fails in service or meets its L₁₀ life rating.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Heat Treatment
- Cryogenic Processing for Dimensional Stability
- Rolling Contact Fatigue: Inclusions as Crack Origins
- Common Mistakes
- When to Choose SUJ2
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4805 | SUJ2 | Japan | Reference |
| ASTM A295 | AISI 52100 | USA | ✅ Nearest Exact |
| EN ISO 683-17 | 1.3505 / 100Cr6 | Europe | ✅ Nearest Exact |
| DIN 17230 | 100Cr6 | Germany | ✅ Nearest Exact |
| BS 970 | 534A99 | UK | ✅ Nearest Exact |
| GB/T 18254 | GCr15 | China | ✅ Nearest Exact |
| ГОСТ 801 | ШХ15 | Russia | ✅ Nearest Exact |
SUJ2 and AISI 52100 are the same material. The JIS and ASTM specifications have nearly identical composition ranges and similar cleanliness requirements. EN 100Cr6 (1.3505) has the tightest composition tolerances of the three, reflecting Europe’s precision bearing manufacturing tradition. Material certified to any of these standards is interchangeable in bearing applications.
2. Chemical Composition
| Element | SUJ2 (JIS G4805) | AISI 52100 (ASTM A295) | 100Cr6 (EN ISO 683-17) |
|---|---|---|---|
| C | 0.95–1.10% | 0.93–1.05% | 0.93–1.05% |
| Si | 0.15–0.35% | 0.15–0.35% | 0.15–0.35% |
| Mn | ≤ 0.50% | 0.25–0.45% | 0.25–0.45% |
| Cr | 1.30–1.60% | 1.35–1.60% | 1.35–1.65% |
| Mo | ≤ 0.08% | ≤ 0.10% | ≤ 0.10% |
| P | ≤ 0.025% | ≤ 0.025% | ≤ 0.025% |
| S | ≤ 0.025% | ≤ 0.015% | ≤ 0.015% |
| Cu | ≤ 0.30% | ≤ 0.30% | ≤ 0.30% |
| Ni | ≤ 0.25% | ≤ 0.25% | ≤ 0.30% |
Source: JIS G4805:2019, ASTM A295-14, EN ISO 683-17:2014. The 1.0% C / 1.5% Cr combination produces fine (Cr,Fe)₃C carbides that dissolve partially during austenitizing and reprecipitate as fine secondary carbides during tempering. Undissolved primary carbides serve as hard wear-resistant particles. The strict P/S limits and oxygen content requirements (typically <10 ppm O for premium grades) minimize oxide and sulfide inclusions — the primary crack initiation sites in rolling contact fatigue.
3. Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Working hardness | 62–65HRC | Standard bearing condition; 64–65HRC for precision races |
| Compressive yield strength | ~2200–2500 MPa (319–363 ksi) | Supports high Hertzian contact stress |
| Tensile strength | ~2000–2300 MPa (290–334 ksi) | Rings and races — bending resistance |
| Charpy Impact (unnotched) | ~10–20 J (7–15 ft·lbf) | Not for impact applications |
| Modulus of elasticity | 210 GPa (30,500 ksi) | — |
| Density | 7.83 g/cm³ (0.283 lb/in³) | — |
| Thermal conductivity | 46.6 W/(m·K) (annealed) / ~34 W/(m·K) (hardened) | — |
| Coefficient of thermal expansion | 11.9 μm/(m·K) (20–200°C) | Important for dimensional stability calculations |
4. Heat Treatment
Spheroidize Annealing (for machining)
Heat to 750–800°C (1382–1472°F), hold 4–6 hours, furnace cool at ≤ 20°C/hr to 600°C, then air cool. The goal is full spheroidization of carbides — converting lamellar pearlite carbides to spherical particles dispersed in a ferrite matrix. Target: ≤ 207HBW. This is the standard supply condition for bearing rings; spheroidized steel machines freely and provides the best starting carbide morphology for subsequent hardening.
Hardening
Preheat to 400–500°C (752–932°F). Austenitize at 830–870°C (1526–1598°F) for 20–40 min (section-dependent). The austenitizing temperature window is narrow and critical: below 820°C, insufficient Cr and C dissolve into austenite — resulting hardness will be below specification; above 880°C, grain growth and excessive retained austenite content increase. Oil quench at 60–80°C (140–176°F). As-quenched hardness: 65–67HRC; retained austenite: 10–20%.
Tempering
| Temper Temperature | Final Hardness | Application |
|---|---|---|
| 150–160°C (302–320°F) | 64–65HRC | Precision bearings, ball screw nuts — maximum hardness, minimum retained austenite |
| 160–180°C (320–356°F) | 62–64HRC | Standard industrial bearings, bearing rings |
| 180–200°C (356–392°F) | 60–62HRC | Better toughness for larger rings, vibration-loaded applications |
| 230–250°C (446–482°F) | 57–59HRC | Stabilizing temper for high-temperature service (to ~120°C operating temp) |
Standard bearings are tempered at 160–180°C (1–2 hours), producing 62–64HRC with ~8–12% retained austenite. This retained austenite level provides some toughness contribution but creates a dimensional instability risk — it transforms slowly to martensite in service, causing a small but measurable dimensional growth that can affect precision bearing preload.
5. Cryogenic Processing for Dimensional Stability
When dimensional stability is critical — precision machine tool spindles, ball screws, coordinate measuring machine components, optical instrument bearings — the 10–15% retained austenite in standard-tempered SUJ2 becomes a specification problem. Retained austenite is a metastable phase that converts to martensite over time at room temperature, driven by thermal cycling and mechanical stress.
The solution is cryogenic treatment immediately after oil quenching:
| Step | Condition | Effect |
|---|---|---|
| Oil quench | 830–870°C → oil at 60–80°C | Martensite forms; 10–20% retained austenite remains |
| Cryogenic treatment | −80°C (dry ice) or −196°C (liquid nitrogen), 1–4 hours | Retained austenite continues transforming to martensite below Mf; RA reduced to <3% |
| Return to room temperature | Slowly, in ambient air | — |
| Temper | 150–180°C, 1–2 hours | Tempers both original martensite and cryogenically-induced martensite |
After −196°C cryogenic treatment and subsequent temper at 160°C, retained austenite content is typically <2%, and dimensional change over time at room temperature is less than 1 μm per 25 mm (1 μin/in) — acceptable for precision bearing applications.
6. Rolling Contact Fatigue: Inclusions as Crack Origins
Rolling contact fatigue (RCF) failure — subsurface spalling where a fatigue crack initiates, propagates parallel to the surface, then turns upward to produce a spall pit — is the design failure mode of all bearings operated to their rated life. Inclusion content is the primary microstructural variable that distinguishes standard, premium, and ultra-clean bearing grades:
| Inclusion Type | Source | Effect on RCF Life |
|---|---|---|
| Oxide inclusions (Al₂O₃, TiO₂) | Deoxidation practice; refractory reversion | Hard, non-deformable — maximum stress concentration at inclusion-matrix interface → early fatigue crack initiation |
| Sulfide inclusions (MnS) | Residual S above 0.015% | Soft, deformable — less damaging than oxides but still preferential crack initiation sites under cyclic stress |
| Globular oxides | Silicate-type deoxidation | Deformable, elongated during rolling — moderate effect; better than Al₂O₃ |
Premium bearing steel (vacuum degassed, electro-slag remelted) reduces total oxygen to 5–8 ppm versus 10–15 ppm for standard grade. This roughly 2× cleanliness improvement extends L₁₀ fatigue life by 1.5–3× — the rationale for the higher cost of precision-grade bearing steels in aircraft, machine tool spindles, and high-speed applications.
7. Common Mistakes
8. When to Choose SUJ2
All ball and roller bearing applications: deep groove ball bearings, angular contact bearings, cylindrical roller bearings, tapered roller bearings, needle roller bearings, ball screw nuts and supports, linear guide rail bearings. Standard industrial and automotive bearing races and rolling elements. SUJ2 is the default unless a specific reason requires a different grade.
Dimensional stability over time is critical: machine tool spindle bearings, precision ball screws, coordinate measuring machine bearings, optical instrument bearings. Any application where bearing preload must be maintained over years of service without adjustment. Add cryogenic processing as a heat treatment specification requirement, not an option.
Maximum fatigue life is required: aircraft engine bearings, gas turbine main shaft bearings, high-speed machine tool spindles at > 1 million DN (bore mm × RPM), long-life industrial gearbox bearings. The higher cleanliness of premium SUJ2 (≤8 ppm O) recovers its cost premium through extended L₁₀ life and reduced unplanned downtime in critical applications.
9. FAQ
Q: Is SUJ2 the same as 52100?
Yes. JIS G4805 SUJ2 and ASTM A295 AISI 52100 share the same 1.0% C / 1.5% Cr composition. EN 1.3505 (100Cr6) is the European equivalent. All three meet equivalent cleanliness and mechanical performance specifications for bearing applications and are interchangeable when supplied to their respective standards.
Q: What is the maximum operating temperature for SUJ2 bearings?
Standard-tempered SUJ2 (160–180°C temper) is limited to service temperatures below ~120°C (248°F) continuous. Above this temperature, retained austenite transforms and the bearing softens below 60HRC with attendant dimensional change. For service to 200°C, specify a stabilizing temper at 230–250°C (57–59HRC). For service above 200°C, consider case-hardened bearing steels (carburized SNCM220 or equivalent) or M50 high-speed steel bearings (aircraft application).
Q: Can SUJ2 be used for applications other than bearings?
Yes — SUJ2 at 62–65HRC finds application wherever high hardness, wear resistance, and high compressive strength are needed without the elevated-temperature requirements of hot-work steels: valve seats, pump plungers, precision gauges, wear plates, cams, and precision linear shafts. Its low toughness limits use in impact applications, and it should not be confused with SKD11 in die applications (different carbide structures and compositions).
Summary
- SUJ2 = AISI 52100 = EN 100Cr6: 1.0% C / 1.5% Cr high-carbon chromium bearing steel — the global standard
- Hardens to 62–65HRC via oil quench from 830–870°C; strict cleanliness limits (P, S, O) maximize rolling contact fatigue life
- Standard temper 160–180°C; for precision applications add cryogenic treatment (−80 to −196°C) between quench and temper to reduce retained austenite below 3%
- Cryogenic sequence is critical: quench → cryo → temper (not quench → temper → cryo)
- Inclusion content is the primary microstructural driver of RCF life — specify premium/vacuum grade for critical fatigue applications
- Maximum service temperature ~120°C for standard temper; specify stabilizing temper at 230–250°C for higher-temperature service
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