SUP9 Spring Steel: Composition, Heat Treatment & 5155 / 55Cr3 Equivalent
SUP9 is a manganese-chromium spring steel in JIS G 4801 with C 0.52–0.60%, Mn 0.65–0.95% and Cr 0.65–0.95%, close to AISI/SAE 5155 and EN 55Cr3. After quenching and tempering it reaches tensile strength ≥ 1230 MPa and HBW 363–429, and is mainly used for automotive leaf springs and stabilizer bars.
Overview
SUP9 adds chromium and manganese to a roughly 0.55% carbon base. Chromium raises hardenability so thicker leaves still harden through in an oil quench; tempering at 460–510°C then gives a high elastic limit with enough toughness — the combination a leaf spring needs under repeated bending.
Compared with the silicon-manganese grades SUP6 and SUP7, SUP9 is less prone to decarburization and tougher. For thicker leaves or more hardenability, the step up is SUP9A (slightly higher C, Mn and Cr) or the vanadium-bearing SUP10. Double Steel's alloy range lists SUP9 and SUP9M, and our flat bar range serves leaf spring applications. Round bar is supplied in Ø 13–100 mm; both round and flat bar can be produced to order — please inquire for stock.
Chemical Composition (JIS G 4801)
| Element | Spec Range (%) |
|---|---|
| C (Carbon) | 0.52 – 0.60 |
| Si (Silicon) | 0.15 – 0.35 |
| Mn (Manganese) | 0.65 – 0.95 |
| P (Phosphorus) | ≤ 0.030 |
| S (Sulfur) | ≤ 0.030 |
| Cr (Chromium) | 0.65 – 0.95 |
| Cu (Copper) | ≤ 0.30 |
Mechanical Properties After Q+T (JIS G 4801 Specified)
| Condition | Yield (MPa) | Tensile (MPa) | Elongation (%) | Reduction of Area (%) | Hardness (HBW) |
|---|---|---|---|---|---|
| Quench 830–860°C oil + temper 460–510°C | ≥ 1080 | ≥ 1230 | ≥ 9 | ≥ 20 | 363 – 429 |
Values are the JIS G 4801 requirements for heat-treated test pieces, not properties of as-rolled or annealed bars; finished spring properties depend on section, heat treatment and shot peening. Hardness conversion (ASTM E140): HBW 363 ≈ HRC 39, HBW 429 ≈ HRC 46.
Heat Treatment Characteristics
Hardening
JIS reference hardening is 830–860°C with oil quench. Chromium raises hardenability so thicker leaves harden to the core; beyond that thickness, move to a higher-hardenability grade such as SUP9A or SUP11A.
Tempering
Tempering at 460–510°C after quenching gives the HBW 363–429 spring hardness window. Lower tempering raises hardness at the cost of toughness; higher tempering leaves the elastic limit too low.
Decarburization Control
Spring fatigue cracks usually start at the surface, so decarburization during rolling and heat treatment cuts fatigue life noticeably. Without high silicon, SUP9 decarburizes less than SUP6 and SUP7.
Shot Peening
Leaf springs are usually shot peened after heat treatment to introduce compressive surface stress and extend fatigue life; stress peening (peening under load) is even more effective.
International Standard Equivalents
| Standard | Equivalent Grade | Correspondence |
|---|---|---|
| JIS (Japan) | SUP9 (G 4801) | Reference standard |
| AISI/SAE (USA) | 5155 | Near equivalent |
| EN (Europe) | 55Cr3 (1.7176) | Near equivalent |
| GB (China) | 55CrMnA | Near equivalent |
AISI/SAE 5155 is C 0.51–0.59%, Mn 0.70–0.90%, Cr 0.70–0.90% — slightly narrower ranges; EN 10089 55Cr3 and GB/T 1222 55CrMnA are also close to but not identical with SUP9. The sister grade SUP9A is C 0.56–0.64%, Mn 0.70–1.00%, Cr 0.70–1.00% with higher hardenability. Where a drawing names a specific standard, follow that standard.
Typical Applications
Commercial Vehicle Leaf Springs
Multi-leaf and parabolic leaf springs for truck, bus and trailer suspensions — the main use of SUP9.
Stabilizer & Torsion Bars
Anti-roll bars and torsion bars in automotive suspensions that need a high elastic limit and fatigue strength.
Agricultural & Construction Machinery
Suspension, seat and clutch springs and other elastic parts under repeated loading.
Industrial Spring Parts
Large disc springs, retaining rings and spring clamps that need quench-and-temper elasticity.
Supply Specifications
| Form | Size Range | Stock |
|---|---|---|
| Round bar | Ø 13 – 100 mm | Please inquire |
| Flat bar | Made to order | Please inquire |
Length 5–12 m; cut-to-length available on request. Both round bar and flat bar can be produced to order; please inquire for stock availability.
Material Selection Guide
SUP9 suits medium-thickness springs loaded mainly in bending fatigue. Choose by section thickness (is hardenability enough?), fatigue requirement and cost.
SUP9A / SUP11A
Thicker leaves needing more hardenability
SUP9A has slightly higher C, Mn and Cr; boron-bearing SUP11A hardens through even thick sections.
SUP10
High stress and longer fatigue life
Chromium-vanadium spring steel with finer grain and better toughness for heavily loaded springs.
Small elastic parts where cost comes first
Chromium-free high-carbon steel with lower hardenability — only for thin-section spring parts.
Forged parts needing impact toughness, such as hand tools
Chromium-vanadium steel with good Q+T toughness, used for wrenches and sockets rather than springs.
For technical consultation or a quotation, contact the Double Steel sales team via the inquiry form below or LINE. MOQ, port of loading and Incoterms are agreed case by case.
Related Grades
For cross-standard composition and mechanical property tables, see the technical reference tables.
FAQ
- What is SUP9 steel?
- SUP9 is a manganese-chromium spring steel in JIS G 4801 with C 0.52–0.60%, Mn 0.65–0.95% and Cr 0.65–0.95%, used quenched and tempered for leaf springs, stabilizer bars and other elastic parts.
- What is SUP9 equivalent to?
- It is usually compared with AISI/SAE 5155, EN 55Cr3 (1.7176) and GB 55CrMnA, but composition ranges differ slightly; where certification is required, follow the standard on the drawing.
- What is the hardness of SUP9?
- JIS G 4801 specifies HBW 363–429 for heat-treated test pieces (oil quench from 830–860°C, temper 460–510°C), roughly HRC 39–46.
- What is the tensile strength of SUP9?
- JIS G 4801 requires tensile strength ≥ 1230 MPa, yield ≥ 1080 MPa, elongation ≥ 9% and reduction of area ≥ 20% after quenching and tempering.
- What is the difference between SUP9 and SUP9A?
- SUP9A has higher carbon (0.56–0.64%) and higher manganese and chromium (0.70–1.00% each), giving better hardenability for thicker leaf springs.
- SUP9 vs SUP6: which should I choose?
- SUP6 is a silicon-manganese grade with more tendency to decarburize; SUP9 uses chromium for hardenability with better toughness and decarburization control, and most leaf springs today use the SUP9 family.
- How do I request a SUP9 quotation from Double Steel?
- Round bar is supplied in Ø 13–100 mm, length 5–12 m with cut-to-length available; both round and flat bar can be produced to order, and stock is by inquiry. Send the grade, form, size, quantity and application through the inquiry form and our sales team will reply.
Or call +886-424078309 or email [email protected]