Overview
JIS SUP7 is a silicon-manganese alloy spring steel standardized under JIS G4801, with a silicon content of 1.80–2.20% — the highest among the Si-Mn JIS spring grades. It is most commonly used in heavy-duty leaf springs and suspension coil springs for trucks and buses, where high fatigue life and sag resistance are required. Its nearest international equivalent is SAE/AISI 9255.
SUP7 is the upper-tier Si-Mn spring steel in the JIS system, positioned above SUP6 (SAE 9260 nearest equivalent) in terms of silicon content. The additional silicon raises the elastic limit and improves sag resistance under sustained loading. When even higher performance is needed, the Si-Cr grade SUP12 is the next step up.
Quick Comparison: SUP7 vs. Global Equivalents
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4801 | SUP7 | Japan | Reference |
| SAE/AISI | 9255 | USA | Nearest Equivalent |
| EN 10089 | 55Si7 | Europe | Nearest Equivalent |
| DIN | 55Si7 | Germany | Nearest Equivalent |
| ISO 683-14 | 55Si7 | International | Nearest Equivalent |
SAE 9255 (Si: 1.80–2.20%) closely matches SUP7 in silicon range, but the carbon range differs slightly. EN 55Si7 uses a narrower carbon band. Confirm composition from mill certificates before substitution in structural spring applications.
Chemical Composition
| Element | SUP7 % (JIS G4801) | SAE 9255 % | EN 55Si7 % |
|---|---|---|---|
| C | 0.56–0.64 | 0.51–0.59 | 0.52–0.59 |
| Si | 1.80–2.20 | 1.80–2.20 | 1.60–2.00 |
| Mn | 0.70–1.00 | 0.70–1.00 | 0.70–1.00 |
| Cr | — | — | — |
| P | ≤0.035 | ≤0.035 | ≤0.025 |
| S | ≤0.035 | ≤0.040 | ≤0.025 |
Source: JIS G4801, SAE J404, EN 10089
SUP6 vs. SUP7: SUP6 has Si 1.50–2.00%; SUP7 has Si 1.80–2.20%. The overlap region (1.80–2.00% Si) means some heats of SUP6 and SUP7 are compositionally similar. Always specify the JIS grade explicitly on drawings.
Mechanical Properties
| Property | Value | Imperial |
|---|---|---|
| Tensile Strength | ≥1,910 MPa | ≥277 ksi |
| Yield Strength (0.2%) | ≥1,670 MPa | ≥242 ksi |
| Elongation (GL=50mm) | ≥8% | ≥8% |
| Reduction of Area | ≥35% | ≥35% |
| Hardness (after HT) | 52–60 HRC (typical) | — |
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 | 790–820°C (1454–1508°F) | Furnace cool ≤25°C/hr |
High silicon content accelerates surface decarburization during austenitizing. Protect the surface with a controlled atmosphere furnace or salt bath. Even a shallow decarburized layer (0.1–0.2 mm / 0.004–0.008 in) measurably reduces fatigue life in spring applications.
Practical Advice
Machinability
SUP7 in annealed condition has lower machinability than plain carbon steels due to the high Si content (~55–60% of AISI 1212 baseline). Silicon promotes built-up edge on cutting tools and causes abrasive wear on grinding wheels. Use sharp, fine-grain grinding wheels and dress frequently. Carbide tooling with positive rake angles recommended for turning operations.
Heat Treatment Guide
Austenitize at 850–880°C (1562–1616°F) for 20–30 minutes. Oil quench immediately. Temper within 1 hour of quenching to prevent quench cracking. Tempering at 450°C (842°F) is the typical target for automotive leaf springs, giving a good balance between tensile strength and ductility.
Welding
SUP7 is not suitable for welding in finished spring form. High carbon equivalent (typically >0.7) and high Si promote cold cracking and heat-affected zone embrittlement. Raw stock welding requires preheat to 350–400°C (662–752°F) and immediate post-weld stress relief annealing.
Common Mistakes
Both are Si-Mn grades and share overlapping silicon ranges. The distinction matters when maximum Si content must be controlled — for example, when silicon content affects surface coating adhesion or welding procedures on adjacent components. Specify the JIS grade explicitly.
Shot peening introduces compressive residual stresses on the spring surface, significantly extending fatigue life. Most automotive leaf and coil springs made from SUP7 are shot-peened as a standard process step. Omitting it can reduce fatigue life by 30–50%.
When to Choose SUP7
- Heavy truck and bus leaf springs requiring high fatigue life
- Suspension coil springs with sustained loading (sag resistance critical)
- When SUP6 shows early fatigue or sag in service
- Applications without the budget for Cr-V or Si-Cr grades
FAQ
Q: Is SUP7 the same as SAE 9255?
A: Nearest equivalent only. Silicon ranges match (1.80–2.20%), but carbon ranges differ: SUP7 (0.56–0.64%) vs. SAE 9255 (0.51–0.59%). The higher carbon in SUP7 raises strength but marginally reduces weldability. Confirm composition from the mill certificate before substituting.
Q: How does SUP7 compare to SUP6?
A: SUP6 (SAE 9260 nearest equivalent) has a lower silicon ceiling (2.00% vs. 2.20% for SUP7). In practice, heats near the upper Si limit of SUP6 overlap with heats near the lower Si limit of SUP7. For critical fatigue applications, specify the grade explicitly and verify the actual Si content from the heat certificate.
Q: Can SUP7 be shot-peened?
A: Yes — and it should be. Shot peening after heat treatment is standard practice for SUP7 automotive springs. It introduces beneficial compressive residual stresses that significantly extend fatigue life.
Q: What is the maximum cross-section for SUP7?
A: SUP7 (without Cr) has moderate hardenability. Cross-sections up to approximately 30 mm (1.18 in) can be through-hardened with oil quenching. For larger sections, consider SUP12 (Si-Cr) or SUP11A (Mn-Cr-B) which offer better hardenability.
Summary
- JIS SUP7 = Si-Mn alloy spring steel, JIS G4801
- Nearest equivalent: SAE 9255 (C range differs slightly)
- Tensile strength ≥1,910 MPa (≥277 ksi) after Q&T
- Heat treat: quench 850–880°C oil / temper 430–480°C
- Best for: heavy-duty leaf springs and truck suspension coil springs
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