Overview
JIS SUP3 is a plain high-carbon spring steel standardized under JIS G4801, with a carbon range of 0.75–0.90%. It is the simplest of the JIS spring steel grades — no alloying elements beyond carbon, silicon, and manganese — making it cost-effective for thin-section springs, clips, and flat springs where cross-section size does not demand high hardenability. Its nearest international equivalents are SAE/AISI 1075 and 1080.
The absence of chromium, vanadium, or boron means SUP3 relies entirely on carbon for its hardening response. This limits its use to thin cross-sections (generally under 15 mm / 0.59 in) where water or oil quenching can achieve full hardness through the section.
Quick Comparison: SUP3 vs. Global Equivalents
| Standard | Grade | Region | Match Type |
|---|---|---|---|
| JIS G4801 | SUP3 | Japan | Reference |
| SAE/AISI | 1075 | USA | Nearest Equivalent |
| SAE/AISI | 1080 | USA | Nearest Equivalent |
| EN 10089 | C75S | Europe | Nearest Equivalent |
| DIN | Ck75 | Germany | Nearest Equivalent |
| ISO 683-14 | C75 | International | Nearest Equivalent |
SAE 1075 (C: 0.70–0.80%) and SAE 1080 (C: 0.75–0.88%) bracket the SUP3 carbon range (0.75–0.90%). Neither is an exact match. Always verify carbon content from the mill certificate before substitution.
Chemical Composition
| Element | SUP3 % (JIS G4801) | SAE 1080 % | EN C75S % |
|---|---|---|---|
| C | 0.75–0.90 | 0.75–0.88 | 0.72–0.80 |
| Si | 0.15–0.35 | 0.15–0.35 | 0.15–0.35 |
| Mn | 0.30–0.60 | 0.60–0.90 | 0.60–0.90 |
| P | ≤0.035 | ≤0.040 | ≤0.025 |
| S | ≤0.035 | ≤0.050 | ≤0.025 |
Source: JIS G4801, SAE J403, EN 10089
Mechanical Properties
Values for quenched and tempered condition. Actual values depend on cross-section size and heat treatment parameters.
| Property | Value | Imperial |
|---|---|---|
| Tensile Strength | ≥1,370 MPa (≤10 mm dia.) | ≥199 ksi |
| Tensile Strength | ≥1,270 MPa (10–25 mm dia.) | ≥184 ksi |
| Yield Strength (0.2%) | ≥1,175 MPa | ≥170 ksi |
| Elongation | ≥10% | ≥10% |
| Reduction of Area | ≥40% | ≥40% |
| Hardness (after HT) | 40–52 HRC (typical) | — |
Physical Properties
| Property | Value (Metric) | Value (Imperial) |
|---|---|---|
| Density | 7.85 g/cm³ | 0.284 lb/in³ |
| Thermal Conductivity | ~46 W/(m·K) | ~319 BTU·in/(hr·ft²·°F) |
| Thermal Expansion (20–200°C) | ~11.9 × 10⁻⁶ /°C | ~6.6 × 10⁻⁶ /°F |
| Young’s Modulus | ~206 GPa | ~29,900 ksi |
Heat Treatment Conditions
| Process | Temperature | Cooling |
|---|---|---|
| Austenitizing (Quench) | 790–830°C (1454–1526°F) | Water or oil quench |
| Tempering | 300–400°C (572–752°F) | Air cool |
| Annealing | 750–790°C (1382–1454°F) | Furnace cool ≤20°C/hr |
Water quenching of SUP3 is effective but increases quench-cracking risk compared to oil quenching. For cross-sections above 10 mm (0.39 in), oil quenching is preferred to reduce distortion and cracking.
Practical Advice
Machinability
In annealed condition, SUP3 machines at roughly 65% of AISI 1212 free-machining baseline. The high carbon content promotes work hardening and tool wear. Use sharp carbide tooling, low feeds, and flood coolant. Avoid interrupted cuts on hardened material.
Heat Treatment Guide
Austenitize at 790–830°C (1454–1526°F) for 15–30 minutes depending on section size. Oil quench for sections above 8 mm (0.31 in). Temper promptly after quenching to reduce quench stress. Tempering at 350°C (662°F) gives a good balance of hardness and spring-back resistance.
Welding
With carbon content up to 0.90%, the carbon equivalent of SUP3 far exceeds the 0.45 threshold for weldability. Welding is not recommended. Any welding on raw bar stock requires preheating above 350°C (662°F), which is impractical for most spring applications.
Common Mistakes
Without alloying elements, SUP3 has poor hardenability. Cross-sections above 15 mm (0.59 in) will have a soft, unhardened core after quenching, resulting in inadequate spring performance and early fatigue failure. Use SUP6, SUP9, or SUP10 for thicker sections.
As-quenched SUP3 is extremely brittle (>60 HRC). Tempering is mandatory to restore adequate ductility and impact resistance for spring service. Never use as-quenched material in a finished spring.
When to Choose SUP3
- Thin flat springs, clips, and snap rings (cross-section <15 mm / 0.59 in)
- Cost-sensitive applications where alloy grades are over-specified
- Low-stress, high-volume stamped spring components
- Applications without high fatigue-cycle requirements (<10⁶ cycles)
FAQ
Q: Is SUP3 equivalent to SAE 1080?
A: They are nearest equivalents. SUP3 (C: 0.75–0.90%) and SAE 1080 (C: 0.75–0.88%) have nearly identical carbon ranges, but SAE 1080 specifies higher Mn (0.60–0.90%) versus SUP3 (0.30–0.60%). This affects hardenability slightly. Confirm with the mill certificate before substituting.
Q: Can SUP3 replace SUP6 or SUP7?
A: Generally no. SUP6 and SUP7 contain Si (1.50–2.00%) for superior fatigue life and sag resistance. SUP3 is limited to thin, lower-stress applications. Substituting SUP3 for SUP6/7 in a suspension spring would likely lead to early failure.
Q: What cross-section size is practical for SUP3?
A: Up to approximately 10–15 mm (0.39–0.59 in) diameter or thickness for oil quenching, or 8–10 mm (0.31–0.39 in) for water quenching. Beyond this, core hardness becomes insufficient for spring performance.
Q: Why is SUP3 less common than SUP6 or SUP9?
A: The automotive industry has largely moved to alloy spring steels (Si-Mn, Cr-V) for better fatigue life and reliability. SUP3 remains in use for simple, low-cost stamped springs and clips where alloy grades are unnecessary.
Summary
- JIS SUP3 = plain high-carbon spring steel, JIS G4801
- Nearest equivalents: SAE 1075 / 1080, EN C75S
- Tensile strength ≥1,370 MPa (≥199 ksi) for sections ≤10 mm
- Heat treat: quench 790–830°C / temper 300–400°C
- Best for: thin-section, low-cost, low-to-medium stress spring applications
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