SUP6 (JIS G4801) is Japan’s silicon-manganese spring steel, equivalent to SAE 9260. Every heavy truck that passes you on the highway is riding on leaf springs made from grades in this family. The high silicon content (1.50–1.80%) gives SUP6 its exceptional elastic limit and resistance to permanent set — properties that define its role in heavy vehicle suspension.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SUP6
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4801 | SUP6 | Japan | Reference |
| SAE J404 | SAE 9260 | USA | ✅ Nearest Exact — slightly higher Si minimum (1.80–2.20%) |
| EN 10089 | 60Si7 / 1.5026 | Europe | ✅ Nearest Exact |
| DIN 17221 | 60Si7 / 1.5026 | Germany | ✅ Nearest Exact |
2. Chemical Composition
| Element | JIS SUP6 | SAE 9260 | EN 60Si7 |
|---|---|---|---|
| C | 0.56–0.64% | 0.56–0.64% | 0.55–0.65% |
| Si | 1.50–1.80% | 1.80–2.20% | 1.50–2.00% |
| Mn | 0.70–1.00% | 0.75–1.00% | 0.60–1.00% |
| P | ≤ 0.035% | ≤ 0.035% | ≤ 0.025% |
| S | ≤ 0.035% | ≤ 0.040% | ≤ 0.025% |
Sources: JIS G4801:2021, SAE J404, EN 10089:2002
3. Mechanical Properties
After Quench and Temper — Typical Leaf Spring Condition
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength | 1180–1420 MPa | 171–206 ksi |
| Yield point (0.2% proof) | ≥ 1030 MPa | ≥ 149 ksi |
| Elongation | ≥ 9% | ≥ 9% |
| Reduction of area | ≥ 35% | ≥ 35% |
| Hardness | HRC 38–47 | HRC 38–47 |
Fatigue Performance
| Condition | Fatigue Limit (rotating bending, R = −1) |
|---|---|
| Un-peened | ~280–350 MPa (41–51 ksi) |
| Shot-peened | ~500–600 MPa (72–87 ksi) |
4. Physical Properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Young’s modulus | 206 GPa | 29,900 ksi |
| Thermal conductivity | 44 W/(m·K) | 305 BTU·in/(hr·ft²·°F) |
| Thermal expansion (20–100°C) | 11.7 × 10⁻⁶ /°C | 6.5 × 10⁻⁶ /°F |
| Specific heat | ~486 J/(kg·K) | 0.116 BTU/(lb·°F) |
5. Heat Treatment Conditions
| Process | Temperature | Cooling | Purpose |
|---|---|---|---|
| Austenitize + quench | 850–900°C (1562–1652°F) | Oil quench | Full hardening of Si-Mn alloy |
| Temper | 400–500°C (752–932°F) | Air cool | Target HRC 38–47; optimize strength/toughness balance |
| Shot peening | Room temperature | — | Induce compressive residual stress; mandatory for fatigue life |
6. Machinability
- Machinability approximately 40–50% relative to AISI 1212 — poor due to high C and Si content.
- High Si makes the steel abrasive to cutting tools, accelerating insert wear.
- Spring steel is typically hot-formed (roll-formed or press-formed from heated bar) and ground — not conventionally machined.
- If any machining is required (e.g., eye forming, hole drilling), use carbide tooling at moderate cutting speeds (50–80 m/min / 165–260 ft/min) with adequate coolant.
- Perform any machining operations before heat treatment — after Q+T to HRC 38–47, only grinding is practical.
7. Weldability
- Rating: Severely restricted — welding is essentially prohibited for spring applications.
- Carbon equivalent (Ceq) is approximately 0.75–0.90 for SUP6 — among the highest of common engineering steels.
- Even with preheat, weld heat input creates a HAZ with extreme hardness gradient and high cracking risk.
- Any weld in a spring steel component will create a stress concentration and fatigue initiation site, invalidating the spring design.
- If a joining operation is required, mechanical fastening (clips, clamps, center bolts) is the correct engineering solution for spring assemblies.
8. Common Mistakes
Si-Mn spring steels are significantly more sensitive to surface decarburization during heating than chromium spring grades (SUP9, SUP10). Processing SUP6 in a conventional air atmosphere furnace without protective measures will produce a soft, decarburized surface layer that becomes the fatigue crack initiation site. Even if shot peening follows, the compressive stress layer cannot fully compensate for a decarburized surface. The correct approach is to use a controlled atmosphere (endothermic gas, nitrogen, or vacuum), salt bath, or a protective fluidized bed furnace. If decarburization has occurred, centerless grind the surface before shot peening.
Shot peening is not optional for leaf spring fatigue life — it is the primary process that doubles the fatigue endurance limit. Un-peened SUP6 leaf springs will fail prematurely in service. An under-specified peening process (too low Almen intensity, insufficient coverage) provides partial improvement but leaves fatigue life well below the design target. For heavy truck leaf springs, the minimum acceptable Almen intensity is 0.30 A with 100% coverage verification. Do not reduce peening intensity based on cost pressure — the consequence is warranty returns and field failures.
9. When to Choose SUP6
✅ Choose SUP6 when:
- ✅ Heavy truck and bus leaf spring blades (the primary application for this grade)
- ✅ Parabolic and tapered leaf spring assemblies for commercial vehicles
- ✅ Applications where SAE 9260 or EN 60Si7 (1.5026) is the design standard
- ✅ Large cross-section spring bars where Cr addition (SUP9) provides no additional advantage
- ✅ Cost-sensitive leaf spring programs where Si-Mn strengthening is sufficient
❌ Avoid SUP6 when:
- ❌ Coil springs — specify SUP9 (Cr provides better fatigue life in wound spring forms)
- ❌ Valve springs or high-cycle fatigue applications — specify SUP10 (Si-Cr) or SUP12 (Cr-V)
- ❌ Any application requiring welding — spring steel is not weldable
- ❌ Service above ~200°C (392°F) — spring will take permanent set as the yield strength reduces
10. FAQ
Q: What is the difference between SUP6 and SUP9?
The key difference is alloying element: SUP6 uses Si (1.50–1.80%) as the primary strengthener in a Si-Mn system; SUP9 adds Cr (0.65–0.95%) while reducing Si to 0.15–0.35%. Chromium in SUP9 improves hardenability, fatigue life in thinner sections, and surface stability during austenitizing (lower decarburization sensitivity). SUP6 is primarily specified for heavy flat leaf spring blades; SUP9 is used for coil springs and lighter leaf springs. For very thick leaf spring blades (above 20 mm / 0.8 in), both grades perform similarly, and SUP6 is typically preferred on cost grounds.
Q: Why is shot peening mandatory for spring steel?
Spring fatigue failures initiate at or near the surface, where tensile stresses from bending are maximum. Shot peening bombards the surface with hardened steel shot, plastically deforming the surface layer and creating a compressive residual stress zone 0.1–0.3 mm (0.004–0.012 in) deep. This compressive layer resists crack opening under the tensile component of the fatigue cycle, raising the effective fatigue endurance limit by 30–100% depending on peening intensity and spring geometry. For automotive and commercial vehicle leaf springs, shot peening increases the design fatigue life from approximately 200,000 cycles to over 500,000 cycles — the difference between an acceptable warranty and a field recall.
Q: Can SAE 9260 be substituted for SUP6?
Yes, with one important consideration: SAE 9260 specifies Si at 1.80–2.20%, while SUP6 specifies 1.50–1.80%. The higher Si minimum in 9260 increases resistance to permanent set (favorable) but also raises decarburization sensitivity during austenitizing (unfavorable if atmosphere control is not in place). If substituting 9260 for SUP6, verify that the heat treatment process — specifically the furnace atmosphere or salt bath — is compatible with the higher Si content of 9260. Mechanical properties after Q+T will be essentially equivalent for both grades.
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