A silicon-chromium valve-spring steel (JIS SUP12 / AISI 9254, China 55CrSiA). The Si-Cr chemistry lifts the elastic limit and fatigue strength, so it is specified for high-stress engine valve springs and performance suspension.
What Is SUP12 / 9254 Spring Steel?
SUP12 (and its US twin AISI 9254) is a silicon-chromium spring steel. The chromium plus high silicon give it a higher elastic limit and much better fatigue life than plain carbon or Si-Mn grades, which is exactly what valve and high-stress coil springs need.
It is used for engine valve springs, performance suspension and other high-cycle springs running at elevated temperature. It is not stainless and must be protected against corrosion.
Supply Forms and Sizes
Every form is available soft-annealed (for further forming and machining) or heat-treated to your hardness window.
| Form | Condition | Size Range | Typical Use |
|---|---|---|---|
| Cold-drawn wire | Annealed / oil-tempered | 0.2 – 12 mm | Valve springs, coil springs |
| Hardened & tempered strip | Q&T, edge dressed | 0.2 – 6 mm | Clutch springs, washers |
| Hot-rolled bar | Hot rolled / normalized | 8 – 80 mm | Suspension leaves, forgings |
| Flat spring steel | Q&T | 3 – 30 mm | Brackets, springs |
Chemical Composition (JIS G4801 (SUP12) / AISI 9254)
| C | Si | Mn | Cr | P max | S max |
|---|---|---|---|---|---|
| 0.50 – 0.60 | 1.20 – 1.60 | 0.50 – 0.80 | 0.50 – 0.80 | 0.035 | 0.035 |
Heat analysis is reported on the mill certificate for every lot. If your drawing calls for tighter limits than the standard, flag it at enquiry — ladle-refined heats usually meet it without a surcharge.
Physical Properties
| Property | Value |
|---|---|
| Density | 7.85 g/cm³ |
| Elastic modulus (E) | ~200 GPa |
| Shear modulus (G) | ~79 GPa |
| Thermal conductivity | ~40 W/m·K at room temperature |
| Coefficient of thermal expansion | ~12 ×10⁻₆ /K (20 – 200 °C) |
Mechanical Properties
Typical values for SUP12 / 9254 after oil quenching and tempering near 480 °C. The mill certificate is the binding document.
| Condition | Tensile Strength | Yield Strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | ≥ 1000 MPa | ≥ 850 MPa | ≥ 10 % | ≤ 321 HB |
| Q&T | 1350 – 1550 MPa | ≥ 1200 MPa | ≥ 8 % | 48 – 52 HRC |
| Oil-tempered wire | 1600 – 2000 MPa | — | — | per size class |
Equivalent Grades
| Standard | Designation |
|---|---|
| Japan JIS G4801 | SUP12 |
| USA AISI / SAE | 9254 |
| China GB/T 1222 | 55CrSiA (similar) |
| Europe EN | 54SiCr6 (similar) |
Heat Treatment and Processing
Preheat slowly, then raise to the grade forging range. Work above the lower limit and cool slowly after forging to avoid cracks.
820 – 860 °C, soak, furnace cool. Soft condition for forming and coiling.
860 – 900 °C, oil. Full through-harden in wire and bar.
430 – 540 °C by required hardness; ~480 °C gives the usual 48 – 52 HRC valve-spring window.
After machining or forming, 200 – 350 °C for 1 – 2 hours, air cool, to stabilise dimensions and relieve residual stress.
Heat-treated bars and plates are supplied with the hardness and test certificate you specify. For critical parts we can control decarburisation depth and supply a spheroidized or soft-annealed structure on request.
Applications of SUP12 / 9254 Spring Steel
The classic use — high-cycle life at elevated temperature.
Compressors, hydraulic and performance springs.
Fatigue resistance under continuous load.
Where relaxation could fail the assembly.
Springs that must keep their set.
Critical automotive springs.
SUP12 / 9254 Spring Steel vs Alternative Grades
| Grade | vs SUP12 / 9254 Spring Steel | Choose It When |
|---|---|---|
| SUP12 / 9254 | — | High-stress valve/coil springs needing Si-Cr fatigue life. |
| 50CrV4 | Cr+V chemistry, same role | When EN 51CrV4 / AISI 6150 is specified |
| 65Mn | Cheaper, lower alloy | Light, low-cost strip and wire springs |
| 9260 (Si-Mn) | Higher Si but no Cr | Large leaf springs at lower cost |
Quality Control and Mill Certificates
Every shipment ships with an EN 10204 3.1 mill certificate covering heat chemical analysis, hardness and the mechanical test on record. On request we add: ultrasonic test on heavy bar and plate, magnetic particle inspection on machined surfaces, and third-party inspection (SGS / BV) before loading. Dimensional tolerances follow the relevant EN / ASTM / JIS standard or your drawing.
Frequently Asked Questions
What is SUP12 equivalent to?
AISI 9254 and China 55CrSiA are the same silicon-chromium spring steel; European 54SiCr6 is similar. Confirm against the certificate for critical parts.
Is SUP12 good for valve springs?
Yes — its silicon-chromium chemistry gives the fatigue strength and elastic limit valve springs require.
What hardness does SUP12 reach?
Typically 48 – 52 HRC in the quenched-and-tempered spring condition.
SUP12 vs 50CrV4?
Very close roles; 50CrV4 (Cr+V) and SUP12 (Si+Cr) both serve high-stress springs — pick per the spec called out.
What is the minimum order quantity?
Mill lots typically start at 500 kg per item; trial wire and bar are often from stock.
Related Products
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