SUP9 (JIS G4801) is Japan’s chromium spring steel — known as SAE 5160 in North America and 55Cr3 in Europe. Three names, one spring steel, and a shot peening step that determines whether your suspension survives 500,000 cycles. The Cr addition gives SUP9 better hardenability, fatigue resistance, and decarburization stability than its Si-Mn counterpart SUP6.
- International Equivalent Grades
- Chemical Composition
- Mechanical Properties
- Physical Properties
- Heat Treatment Conditions
- Machinability
- Weldability
- Common Mistakes
- When to Choose SUP9
- FAQ
1. International Equivalent Grades
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4801 | SUP9 | Japan | Reference |
| SAE J404 | SAE 5160 | USA | ✅ Nearest Exact — C 0.56–0.64%, Mn 0.75–1.00%, Cr 0.70–0.90% |
| EN 10089 | 55Cr3 / 1.7176 | Europe | ✅ Nearest Exact |
| DIN 17221 | 55Cr3 / 1.7176 | Germany | ✅ Nearest Exact |
2. Chemical Composition
| Element | JIS SUP9 | SAE 5160 | DIN 55Cr3 |
|---|---|---|---|
| C | 0.52–0.60% | 0.56–0.64% | 0.52–0.60% |
| Si | 0.15–0.35% | 0.15–0.35% | 0.15–0.35% |
| Mn | 0.65–0.95% | 0.75–1.00% | 0.70–1.00% |
| P | ≤ 0.035% | ≤ 0.035% | ≤ 0.025% |
| S | ≤ 0.035% | ≤ 0.040% | ≤ 0.025% |
| Cr | 0.65–0.95% | 0.70–0.90% | 0.60–0.90% |
Sources: JIS G4801:2021, SAE J404, EN 10089:2002
3. Mechanical Properties
After Quench and Temper — Typical 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 | ≥ 40% | ≥ 40% |
| Hardness | HRC 38–47 | HRC 38–47 |
Fatigue Performance
| Condition | Fatigue Limit (rotating bending, shot-peened) |
|---|---|
| Standard shot peened | ~530–650 MPa (77–94 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 | 43 W/(m·K) | 298 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 | 840–880°C (1544–1616°F) | Oil quench | Full hardening of Cr-Mn alloy |
| Temper | 400–500°C (752–932°F) | Air cool | Target HRC 38–47; optimize strength/toughness balance |
| Shot peening | Room temperature | — | Induce surface compressive stress; mandatory for fatigue life |
| Stress relief / presetting (coil springs) | 200–250°C (392–482°F) | Air cool | Stabilize spring rate after coiling; reduce subsequent relaxation |
6. Machinability
- Machinability approximately 45% relative to AISI 1212 — difficult due to high C and Cr content.
- In annealed condition (HB 190–240), bar stock can be turned and drilled with sharp carbide tooling at moderate speeds (60–90 m/min / 200–295 ft/min).
- After Q+T to HRC 38–47, only grinding is practical for dimensional finishing.
- Anneal before rough machining if the incoming bar is in the as-rolled condition: 820–850°C (1508–1562°F), furnace cool to below 600°C (1112°F), then air cool.
- Spring steel coil forms are typically hot-wound (bar heated to 850–950°C / 1562–1742°F) and then quenched directly from the coiling heat — this avoids separate austenitizing.
7. Weldability
- Rating: Severely restricted — spring steel must not be welded.
- Carbon equivalent (Ceq) is approximately 0.72–0.85 — among the highest of engineering steels in common use.
- Any weld introduces a hard, brittle HAZ and a geometric stress concentration — both are fatigue initiation sites that will cause premature spring failure.
- Spring end eyes and attachment features must be formed mechanically (press-eye, wrapped eye, Berlin eye) — never by welding.
- No repair welding is acceptable on spring components in service.
8. Common Mistakes
Some engineers push the temper temperature below 380°C (716°F) to achieve hardness at the upper end of HRC 38–47, reasoning that higher hardness equals better fatigue performance. This is incorrect and dangerous. Spring steel tempered below 380°C (716°F) retains high levels of residual hydrogen from the quench and is susceptible to hydrogen embrittlement — a delayed, brittle fracture mode that can occur hours or days after manufacturing, often during proof load testing or early field use. The temper must be above 380°C (716°F). The fatigue performance target is achieved through shot peening, not through pushing hardness at the expense of temper temperature.
There is no such thing as a non-critical spring in automotive and commercial vehicle suspension. All SUP9 coil springs should be shot peened to a minimum Almen intensity of 0.20 A with ≥ 98% coverage. Reducing peening intensity — or skipping peening entirely on springs deemed “non-safety-related” — eliminates the primary fatigue life enhancement and can cut spring life by 40–60%. The consequence is warranty claims, suspension noise, and in severe cases, spring fracture during vehicle operation. Peening intensity and coverage should be verified by Almen strip measurements for each production batch.
9. When to Choose SUP9
✅ Choose SUP9 when:
- ✅ Automotive coil springs — front and rear suspension, shock absorber springs (primary application)
- ✅ Light-to-medium leaf springs where Cr hardenability improves consistency in thinner blade sections
- ✅ Applications where SAE 5160 or DIN 55Cr3 (1.7176) is the referenced design standard
- ✅ When better decarburization resistance than Si-Mn grades (SUP6) is required
- ✅ Stabilizer bars and torsion bar applications in passenger vehicle chassis
❌ Avoid SUP9 when:
- ❌ Heavy truck multi-leaf springs with blade thickness > 20 mm (0.8 in) — SUP6 is standard and more economical
- ❌ High-cycle valve springs — specify SUP10 (Si-Cr) or SUP12 (Cr-V) for better fatigue limit at very high cycles
- ❌ Service temperatures above 200°C (392°F) — spring will take permanent set as yield strength reduces
- ❌ Any application requiring welded joints — spring steel cannot be welded
10. FAQ
Q: Is SAE 5160 the same as SUP9?
Very close, with minor C range differences. SAE 5160 specifies C at 0.56–0.64%; SUP9 specifies 0.52–0.60%. The SUP9 lower carbon minimum (0.52%) allows slightly better toughness at the lower end of the range. Chromium ranges are essentially identical (5160: 0.70–0.90%; SUP9: 0.65–0.95%). For all practical automotive and commercial vehicle spring applications, 5160 and SUP9 are fully interchangeable. When formally substituting on an engineering drawing, note the C range difference and verify the mill certificate for the actual heat analysis before approving the substitution.
Q: What shot peening parameters are standard for SUP9 coil springs?
Typical automotive production parameters: steel shot S230 (0.58 mm / 0.023 in nominal diameter), Almen intensity 0.20–0.45 A (measured per SAE J442), coverage ≥ 98% (verified per SAE J2277). After shot peening, coil springs undergo presetting — compressed to solid height one or more times to induce a favorable permanent set that raises the elastic limit and stabilizes the spring rate in service. Exact parameters depend on spring wire diameter, coil geometry, and application duty cycle. Always reference the spring manufacturer’s process specification for the authoritative peening parameters for a specific spring design.
Q: Can SUP9 replace SUP6 for leaf spring applications?
Yes, SUP9 can replace SUP6 in most leaf spring applications. SUP9’s Cr content (0.65–0.95%) improves hardenability for thinner blade sections and provides better surface stability during austenitizing, reducing decarburization risk. For thin blades below 12 mm (0.47 in) where decarburization is a concern, SUP9 is the superior choice. SUP6 remains preferred for very thick blades above 20 mm (0.8 in) in heavy truck multi-leaf assemblies where the Si-Mn strengthening mechanism is equally effective and the material cost is lower. When replacing SUP6 with SUP9, no heat treatment parameter changes are required — austenitizing and tempering temperatures are essentially the same.
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