Overview
JIS SUP12 is a silicon-chromium alloy spring steel standardized under JIS G4801, designed for high-fatigue, high-stress spring applications such as automotive suspension coil springs and heavy-duty leaf springs. Among JIS spring grades, it offers superior fatigue life and sag resistance compared to Si-Mn grades (SUP6, SUP7), making it the preferred choice when long-term elastic performance is critical.
The chromium addition in SUP12 improves hardenability over Si-only grades, ensuring uniform hardness across larger cross-sections. Its closest international equivalent is SAE/AISI 9262, though minor differences in composition ranges mean a direct substitution requires engineering confirmation.
Quick Comparison: SUP12 vs. Global Equivalents
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4801 | SUP12 | Japan | Reference |
| SAE/AISI | 9262 | USA | Nearest Equivalent |
| EN 10089 | 60SiCr7 | Europe | Nearest Equivalent |
| DIN | 60SiCr7 | Germany | Nearest Equivalent |
| ISO 683-14 | — | International | No direct match |
SAE 9262, EN 60SiCr7, and JIS SUP12 have overlapping but not identical composition ranges. Always verify the mill certificate before substitution in safety-critical spring applications.
Chemical Composition
| Element | SUP12 % (JIS G4801) | SAE 9262 % |
|---|---|---|
| C | 0.56–0.64 | 0.56–0.64 |
| Si | 1.50–2.00 | 1.80–2.20 |
| Mn | 0.70–1.00 | 0.75–1.00 |
| Cr | 0.60–1.00 | 0.25–0.45 |
| P | ≤0.035 | ≤0.035 |
| S | ≤0.035 | ≤0.040 |
Source: JIS G4801 (spring steels), SAE J404
Key difference: SUP12 has a wider and higher Cr range (0.60–1.00%) versus SAE 9262 (0.25–0.45%), giving SUP12 significantly better hardenability for large-section springs.
Mechanical Properties
Values apply to quenched and tempered condition per JIS G4801.
| Property | Value | Imperial |
|---|---|---|
| Tensile Strength | ≥1,910 MPa | ≥277 ksi |
| Yield Strength (0.2% proof) | ≥1,670 MPa | ≥242 ksi |
| Elongation (GL=50mm) | ≥8% | ≥8% |
| Reduction of Area | ≥35% | ≥35% |
| Hardness (after HT) | 56–62 HRC | — |
| Charpy Impact (20°C) | Reference only | — |
Physical Properties
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Thermal Conductivity | ~38 W/(m·K) | ~264 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–200°C) | ~11.5 × 10⁻⁶ /°C | ~6.4 × 10⁻⁶ /°F |
| Young’s Modulus | ~206 GPa | ~29,900 ksi |
Heat Treatment Conditions
| Process | Temperature | Cooling |
|---|---|---|
| Austenitizing (Quench) | 850–880°C (1562–1616°F) | Oil quench |
| Tempering | 430–480°C (806–896°F) | Air cool |
| Annealing (softening) | 780–810°C (1436–1490°F) | Furnace cool ≤25°C/hr |
Si-Cr spring steels are prone to decarburization during austenitizing. Use a controlled atmosphere or salt bath to protect the surface. Surface decarburization reduces fatigue life significantly.
Practical Advice
Machinability
SUP12 in annealed condition has moderate machinability (roughly 60% of AISI 1212 free-machining steel). High silicon content accelerates tool wear and promotes built-up edge. Carbide tooling and lower cutting speeds are recommended. Always machine before final heat treatment.
Heat Treatment Guide
Oil quenching from 850–880°C achieves full hardness. Tempering at 430–480°C yields the best balance of strength and ductility for spring service. Tempering below 400°C risks tempered martensite embrittlement; tempering above 500°C reduces fatigue strength excessively.
Welding
SUP12 has a carbon equivalent well above 0.6, making it highly susceptible to heat-affected zone cracking. Welding of finished spring components is not advised. Any repair welding on raw stock requires preheating to 300–400°C (572–752°F) and immediate post-weld stress relief.
Common Mistakes
SUP7 (Si-Mn) and SUP12 (Si-Cr) look similar in Si content but differ critically in Cr. Specifying SUP7 when SUP12 is required leads to lower hardenability in large cross-sections, causing inadequate core hardness and premature fatigue failure.
SAE 9262 has a lower Cr ceiling (0.45% max) versus SUP12 (1.00% max). Substituting SAE 9262 for SUP12 in a thick cross-section spring may result in insufficient through-hardening.
When to Choose SUP12
- High-cycle fatigue applications (>10⁷ cycles) in automotive suspension
- Large cross-section coil springs where through-hardening is critical
- When SUP6 or SUP7 shows premature sag or fatigue cracks in service
- Temperature range: −40°C to +120°C (−40°F to +248°F)
FAQ
Q: Is SUP12 the same as SAE 9262?
A: They are nearest equivalents, not identical. The main difference is the Cr content: SUP12 allows up to 1.00% Cr, while SAE 9262 limits Cr to 0.45%. For most spring applications they are interchangeable, but confirm hardenability requirements for cross-sections above 30 mm (1.18 in).
Q: How does SUP12 compare to SUP7?
A: Both are high-Si grades with similar tensile requirements, but SUP12 contains Cr (0.60–1.00%) which SUP7 lacks. Cr improves hardenability and slightly improves corrosion resistance. Choose SUP12 over SUP7 for thicker sections or higher fatigue-life requirements.
Q: What is the typical hardness of SUP12 after quench and temper?
A: 56–62 HRC, depending on section size and exact tempering temperature. Tempering at 430°C yields the upper end; 480°C yields the lower end with improved toughness.
Q: Can SUP12 be used at low temperatures?
A: Si-Cr spring steels generally maintain adequate impact toughness down to −40°C (−40°F), but verify Charpy impact values with the mill if cryogenic conditions are involved.
Summary
- JIS SUP12 = Si-Cr alloy spring steel per JIS G4801
- Nearest equivalent: SAE 9262 (Cr range differs — confirm for thick sections)
- Tensile strength ≥1,910 MPa (≥277 ksi) after Q&T
- Heat treat: quench 850–880°C oil / temper 430–480°C
- Best for: high-fatigue, large-section automotive and industrial springs
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