GCr15 vs SUJ2 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
GCr15 and SUJ2 are two widely used high-carbon chromium bearing steels specified under different national standards. Engineers and procurement managers often face the choice between them when specifying rolling-element bearings, shafts, or wear components where high contact fatigue strength and dimensional stability are required. Typical selection trade-offs include cost and local availability versus specification traceability and slight differences in chemical ranges or processing history that can affect cleanliness, decarburization tolerance, and recommended heat treatment.
The central practical difference is that GCr15 and SUJ2 are nominally equivalent bearing steels in Chinese and Japanese standards, respectively, but their standard compositions, permitted tolerances, and typical production/quality-control practices differ enough that they are not strictly interchangeable without verification. This is why designers compare them: they deliver comparable performance for bearings but can vary in sulfur/phosphorus limits, chromium and silicon windows, and in how suppliers control microcleanliness and heat treatment.
1. Standards and Designations
- GCr15: Chinese standard GB/T (commonly GB/T 18254 for bearing steel designations). Equivalent in practice to AISI/SAE 52100 in many applications, but specified within the GB framework.
- SUJ2: Japanese standard JIS G4805 (SUJ2 is the JIS designation for high-carbon chromium bearing steel). Also considered a counterpart to AISI/SAE 52100.
- AISI/SAE 52100: Often cited in international supply chains as the American designation for the same functional material class.
Material classification: both GCr15 and SUJ2 are high-carbon, chromium alloyed bearing steels — technically high-carbon alloy steels optimized for bearing applications (not stainless, not HSLA, not tool steel in the conventional sense).
2. Chemical Composition and Alloying Strategy
Table: Typical standard composition ranges (wt%). Note: ranges are those commonly specified in national standards; exact limits vary by revision and procurement spec. Consult the current standard or supplier certificate for shop-floor chemistry.
| Element | GCr15 (typical GB range) | SUJ2 (typical JIS range) |
|---|---|---|
| C | 0.95 – 1.05 | 0.95 – 1.03 |
| Mn | 0.25 – 0.45 | 0.25 – 0.45 |
| Si | 0.17 – 0.37 | 0.15 – 0.35 |
| P | ≤ 0.035 (max) | ≤ 0.035 (max) |
| S | ≤ 0.035 (max) | ≤ 0.035 (max) |
| Cr | 1.40 – 1.65 | 1.30 – 1.60 |
| Ni | ≤ 0.30 (trace) | ≤ 0.30 (trace) |
| Mo | ≤ 0.10 (trace) | ≤ 0.10 (trace) |
| V, Nb, Ti, B, N | typically ≤ trace (not specified as alloying) | typically ≤ trace (not specified as alloying) |
How alloying affects properties - Carbon (C): Primary hardenability and carbide-forming element; high C allows high hardened hardness and contact fatigue resistance but reduces weldability and cold-formability. - Chromium (Cr): Promotes hardenability and forms chromium carbides for wear resistance and rolling contact fatigue performance. - Manganese and Silicon (Mn, Si): Deoxidation and strength; contribute modestly to hardenability. - S and P: Impurities—higher levels can improve machinability but reduce fatigue and fracture toughness. Standards limit S and P for bearing steels to protect fatigue life.
3. Microstructure and Heat Treatment Response
Typical microstructures: - In the annealed condition: pearlitic or spheroidized carbides in a ferritic matrix to enable machining/forming. - After quenching and tempering (bearing heat treatment): tempered martensite with dispersed chromium carbides; the exact carbide size, distribution, and matrix retained austenite content depend strongly on heating rates, quench severity, and tempering temperature.
Processing routes and effects: - Normalizing: refines prior austenite grain size; useful prior to final quench to improve toughness marginally. - Quenching & tempering: standard route to deliver high hardness and rolling contact fatigue life. Typical treatments: austenitize in the range appropriate to the composition (careful control to avoid excessive grain growth) followed by oil or polymer quenches to obtain martensitic structure, then temper to target hardness/ toughness balance. - Induction hardening or case hardening: for components needing a hard surface and tough core, but note GCr15/SUJ2 are through-hardening grades by chemistry; induction hardening is commonly used for localized surface hardness. - Thermo-mechanical treatments: fine grain and controlled carbide distribution can be achieved with modern controlled rolling and accelerated cooling; differences in mill practice between suppliers may influence microcleanliness and inclusion morphology.
Differences between GCr15 and SUJ2 in microstructure response are subtle and primarily arise from slight composition window differences and supplier heat-treatment practice rather than from fundamentally different alloy systems.
4. Mechanical Properties
Table: Typical properties (representative ranges after typical heat treatments). Values are indicative; actual properties depend on exact chemistry, thermal cycle, part geometry, and quench/temper.
| Property | GCr15 (typical, quenched & tempered / through-hardened) | SUJ2 (typical, quenched & tempered / through-hardened) |
|---|---|---|
| Tensile strength (MPa) | ~1200 – 2100 | ~1200 – 2100 |
| Yield strength (MPa) | Not always specified in hardened condition; typically high and material-condition dependent | Similar to GCr15 |
| Elongation (A%) | ~4 – 18 (depending on hardness & temper) | ~4 – 18 (depending on hardness & temper) |
| Impact toughness (Charpy) | Low to moderate when hardened; improves with tempering | Comparable to GCr15; supplier processing affects result |
| Hardness (HRC) | Common bearing hardness range 58 – 65 HRC (surface/through) | Common bearing hardness range 58 – 65 HRC |
Interpretation - Strength and hardness: Both grades are engineered to reach high hardness and contact fatigue strength when appropriately heat-treated; nominal tensile/hardness ranges are similar. - Toughness/ductility: Both exhibit reduced toughness at high hardness; tempering to a lower HRC will improve toughness at the expense of some contact fatigue resistance. - Any small observed differences in mechanical properties between GCr15 and SUJ2 typically stem from differences in exact carbon or chromium content, inclusion cleanliness, and supplier heat-treatment practice rather than from radical alloy chemistry differences.
5. Weldability
High carbon and alloy content make both grades challenging to weld in the as-quenched state. Key weldability considerations: - Carbon level: high C raises the risk of hard, brittle martensite in the heat-affected zone (HAZ) after welding, increasing cold-crack susceptibility. - Hardenability: Cr and Mn increase hardenability; the higher the combined effect, the greater the need for preheat and post-weld heat treatment.
Useful industry formulas for qualitative assessment: - Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (weld cracking propensity): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both GCr15 and SUJ2 will show relatively high $CE_{IIW}$ and $P_{cm}$ due to their carbon and chromium, indicating limited weldability without controls. - Best practice: weld in annealed condition where possible, use preheat to avoid rapid cooling to martensite, limit heat input to control HAZ width, and apply PWHT (post-weld heat treatment) when feasible. For critical bearing surfaces, prefer mechanical joining or replace welded design with alternative joining or machining.
6. Corrosion and Surface Protection
- Neither GCr15 nor SUJ2 is stainless. Chromia content (~1.3–1.6%) is insufficient to provide stainless behavior.
- Standard protection methods: painting, oiling, rust-preventive coatings, and galvanizing for parts where corrosion resistance is required. Note that galvanizing and some coatings can affect dimensional tolerances and surface treatments; post-coating grinding or lapping may be needed for precision bearing surfaces.
- PREN is not applicable: the PREN formula $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ is relevant only for stainless steels and not meaningful for high-carbon bearing steels like GCr15 or SUJ2.
7. Fabrication, Machinability, and Formability
- Machinability: Best in annealed/spheroidized conditions; both can be machined readily when annealed. Hardened conditions require grinding, honing, or abrasive machining; conventional turning of hardened surfaces is limited.
- Formability: Low ductility in hardened state; forming operations should be done before final hardening. Cold forming of annealed stock is possible but consider springback and required final dimensions.
- Surface finishing: Bearing applications often require grinding, superfinishing, or lapping to achieve required surface roughness and geometry — both grades respond similarly if microstructure and inclusion size are comparable.
- Supplier differences in cleanliness and inclusion morphology may impact polishing/honing efficiency and running-in performance.
8. Typical Applications
| GCr15 Uses | SUJ2 Uses |
|---|---|
| Rolling bearings (several types) manufactured in China and regional markets | Rolling bearings manufactured in Japan and export markets to JIS spec |
| Bearing rings, balls, rollers, shafts for machine tools, automotive, and industrial equipment | Bearing rings, balls, rollers, precision shafts and components where JIS traceability is required |
| Precision components requiring high contact fatigue life where local supply of GCr15 is economical | Precision components requiring JIS certification, documented heat-treat cycles, or when customers specify SUJ2 explicitly |
Selection rationale - Choose based on specification required by customer or international project: if drawings or procurement documents call out SUJ2 or GCr15 specifically, adhere to the designated standard. - For wear and rolling contact life, both perform similarly if chemistry and heat treatment are equivalent. For critical or high-value components, request mill certificates, inclusion analysis, and heat-treatment records.
9. Cost and Availability
- Availability: GCr15 is widely produced in China and readily available in domestic and regional markets. SUJ2 is produced under JIS control and is common in Japanese and some international supply chains. AISI/SAE 52100 is commonly available in North America and globally.
- Cost: Relative cost depends on regional production and economies of scale. GCr15 may be more cost-competitive in markets with strong Chinese production; SUJ2 may command a premium where JIS traceability, specific heat-treatment, or documentation is required.
- Product forms: Both are available as bars, rings, forged blanks, and finished parts. Lead times and minimum order quantities can vary by supplier and form.
10. Summary and Recommendation
Table: Quick comparison (qualitative)
| Criterion | GCr15 | SUJ2 |
|---|---|---|
| Weldability | Poor (high C) | Poor (high C) |
| Strength–Toughness (hardened) | High strength, lower toughness at high HRC | Comparable to GCr15 |
| Cost (typical regional) | Often lower in China/regional markets | Often higher where JIS certification is required |
| Availability (regional) | Excellent in China | Excellent in Japan / JIS supply chains |
Conclusion and practical recommendation - Choose GCr15 if you are sourcing components in China or nearby regions and cost-effectiveness is a priority, provided the purchaser accepts GB standard documentation. GCr15 is appropriate when the design calls for a high-carbon chromium bearing steel and local supply, price, and lead time are important factors. - Choose SUJ2 if the specification requires JIS material designation, tighter supplier traceability, or if the end-user explicitly mandates SUJ2. SUJ2 may be preferable when the procurement or quality system requires JIS certificates, or when previous supply history uses SUJ2 and interchangeability must be avoided.
Final note: For critical bearing components, always specify required hardness range, heat-treatment procedure, surface finish, decarburization limits, and required mill/heat-treatment certificates. While GCr15 and SUJ2 are functionally equivalent in many bearing applications, interchange should be validated through certificates of analysis and representative mechanical testing or supplier qualification.