60Si7 spring steel to IS 3431: properties, composition, equivalents
60Si7 to IS 3431 is the main Indian leaf-spring steel: 0.60 % C with about 1.75 % Si, finished at 46–50 HRC. Composition, how the designation is read, and its 9260/SUP7/56Si7 equivalents.
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What is 60Si7?
60Si7 is the steel most leaf springs made in India are rolled from. Its specification is IS 3431, the Bureau of Indian Standards document for hot-rolled bars and flats used to make volute, helical and laminated springs for automotive suspension; the other four grades in the same standard are 55Si7, 65Si7, 50Cr4V2 and 60Cr4V2. The analysis — 0.55–0.65 % carbon with 1.5–2.0 % silicon and no chromium — makes it the same silicon spring steel the rest of the world knows as SAE 9260, JIS SUP7, EN 56Si7 or GB 60Si2MnA.
The designation is readable once the convention is known, and it is the same convention EN uses. The leading number is carbon in hundredths of a per cent; the number after an element symbol is that element's content multiplied by four. So 60Si7 is 0.60 % carbon with 7 ÷ 4 = 1.75 % silicon, 65Si7 is the same steel at 0.65 % carbon, and 50Cr4V2 reads as 0.50 % C with 1.0 % Cr and 0.5 % V — the Indian counterpart of 51CrV4.
The question that causes the most trouble in enquiries is a boundary between documents rather than a metallurgical one. IS 3431 specifies the steel; IS 7906 specifies the spring. A mill certificate to IS 3431 reports the analysis, the hardenability and the bar's freedom from defects, and says nothing about the hardness, camber, load rating or fatigue life of the finished leaf — those belong to the spring standard and to the spring maker's heat treatment. "What is the hardness of 60Si7" is a question IS 3431 does not answer.
Two features of the standard are worth knowing because they are unusual. It requires a minimum 16:1 reduction ratio from ingot to finished product, which is a soundness requirement aimed at closing centreline porosity in a component that must not contain internal defects. And it lists hardenability among the mandatory tests, which matters precisely because a silicon steel with no chromium does not harden deep.
60Si7 chemical composition
| Element | Mass % | Note |
|---|---|---|
| C | 0.55 – 0.65 | |
| Si | 1.5 – 2 | |
| Mn | 0.6 – 0.9 |
IS 3431:1982, grade 60Si7. The standard also sets ceilings on phosphorus and sulphur; they are looser than the 0.025 % of EN 10089, so the heat analysis on the certificate is what should be read rather than assumed. The other four grades in the same standard are 55Si7 and 65Si7 (the same steel at lower and higher carbon) and the chromium-vanadium grades 50Cr4V2 and 60Cr4V2.
60Si7 mechanical properties
| Thickness / condition | Yield strength ReH / Rp0.2 MPa (ksi) | Tensile strength Rm MPa (ksi) | Elongation A % | Hardness |
|---|---|---|---|---|
| Annealed (delivery for forming) | — | ≈ 780–900 (≈ 113.1–130.5) | ≈ 15 | ≤ 269 HB |
| Hot rolled, as supplied | ≈ 650 (≈ 94.3) | ≈ 1050 (≈ 152.3) | ≈ 11 | 280–340 HB |
| 830–870 °C oil + 420 °C temper | 1550–1700 (224.8–246.6) | 1750–1900 (253.8–275.6) | 5 | 50–54 HRC |
| 830–870 °C oil + 460 °C temper | 1400–1550 (203.1–224.8) | 1600–1750 (232.1–253.8) | 6 | 46–50 HRC |
| 830–870 °C oil + 500 °C temper | 1250–1400 (181.3–203.1) | 1420–1570 (206.0–227.7) | 7 | 42–46 HRC |
| Hardened 850 °C oil, as-quenched | — | — | — | 60–63 HRC |
IS 3431 covers the bar and flat: composition, hardenability, mechanical properties, freedom from defects, straightness and tolerances. What it does not fix is the hardness of your spring, because that is produced by the spring maker after forming and is accepted against **IS 7906** and the vehicle maker's drawing. The rows above are the usual results for a 12–20 mm leaf.
60Si7 equivalent grades
| Grade | System | Match | Why / differences |
|---|---|---|---|
| 9260 | AISI / SAE | Identical | SAE 9260 (UNS G92600): C 0.56–0.64, Si 1.80–2.20, Mn 0.75–1.00. The same steel for all practical purposes. 9260 sits at the top of the silicon range where 60Si7 spans the whole of it, so a 60Si7 heat at 1.55 % Si is a shade leaner than any 9260 heat. |
| 60Si2MnA | GB/T (China) | Identical | GB/T 1222 60Si2MnA: C 0.56–0.64, Si 1.60–2.00, Mn 0.60–0.90 — the grade most often offered against a 60Si7 enquiry, and a genuine match. |
| 60S2A | GOST (Russia/CIS) | Identical | GOST 14959 60S2A: C 0.57–0.65, Si 1.60–2.00, Mn 0.60–0.90. |
| SUP7 | JIS (Japan) | Identical | JIS G4801 SUP7: C 0.56–0.64, Si 1.80–2.20, Mn 0.70–1.00. SUP6 corresponds more closely to the leaner end of the 60Si7 window. |
| 56Si7 | EN (European) | Near | EN 10089 56Si7 (1.5026): C 0.52–0.60, Si 1.60–2.00, Mn 0.60–0.90. Almost the same analysis, bought against phosphorus and sulphur limits of 0.025 % and a contractual surface decarburisation limit — for an importer the difference to examine is the delivery condition, not the chemistry. |
| 65Si7 | IS (India) | Near | The next grade up in IS 3431, with carbon at 0.60–0.70 % and the same silicon. Harder and stronger at the same temper, marginally less tough; Indian spring makers select between the two by leaf thickness. |
| 5160 | AISI / SAE | Functional | The chromium leaf-spring steel: deeper hardening and less decarburisation-sensitive, with a lower elastic limit at the same hardness. |
| 51CrV4 | EN (European) | Functional | 1.8159: chromium-vanadium instead of silicon. Hardens through roughly twice the section and tolerates hot forming far better, at a higher price. Within IS 3431 itself the equivalent step is to 50Cr4V2. |
Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for 60Si7 →
Old and superseded designations
| Old name | Standard | Note |
|---|---|---|
| 60Si7 | IS 3431:1965 | The designation has not changed across editions; the 1982 third revision reworked the grade list and the test requirements around it. |
| 55Si2Mn90 | older Indian practice | The silicon-manganese spring grade, closer to SAE 9255, still written on some older drawings. |
| SUP7 | JIS G4801 | Not an Indian designation, but the cross-reference most often printed beside 60Si7 on export drawings. |
Product forms and tolerance standards
Product forms: hot-rolled spring flats for laminated leaf springs, round and square bar for coiled and volute springs, tapered and parabolic rolled leaf sections, annealed flat bar for forming before heat treatment.
Dimensional tolerances: hot-rolled bars and flats: dimensions, tolerances and straightness are specified inside IS 3431 itself; general delivery conditions: IS 8910; finished spring: IS 7906 — the spring, not the steel, is what that standard governs.
Key property values
How to read the designation 60Si7
60 = 0.60 % carbon, written in hundredths of a per cent. Si7 = silicon at 7 ÷ 4 = 1.75 %, four being the standard divisor for silicon, manganese, chromium and nickel in this scheme. The same rule reads 65Si7 as 0.65 % C with 1.75 % Si, and 50Cr4V2 as 0.50 % C with 1.0 % Cr and 0.5 % V. It is the convention behind the European names 56Si7 and 51CrV4 as well, which is why the Indian and EN grade names look so similar — and why a 60Si7 drawing can be read by a European mill without a conversion table.
60Si7 hardness in a finished spring
Typical acceptance for a laminated leaf spring is 46–50 HRC, from an oil quench at 830–870 °C and a temper near 440–470 °C. As-quenched the steel reaches 60–63 HRC, so the temper is what sets the part. Above roughly 52 HRC the leaf cracks under impact instead of flexing; below about 43 HRC it takes a permanent set and the spring loses camber in service.
IS 3431 against IS 7906 and IS 3195
Three documents that get confused in enquiries. IS 3431 is the automotive spring *steel*: bars and flats, five grades, composition, hardenability, defects, tolerances. IS 7906 is the *spring*: helical compression springs and automobile leaf-spring assemblies, with the dimensions, load, camber and testing that a vehicle maker accepts against. IS 3195 is the parallel steel standard for railway rolling-stock volute and helical springs — a different application and a different grade list, and not the standard for an automotive leaf. If the question is about material, look at IS 3431; if it is about what the spring must do, look at IS 7906.
Why silicon and not chromium
Silicon raises the elastic limit disproportionately — it strengthens the ferrite in solid solution and slows tempering — so a 60Si7 spring resists taking a set and relaxes less in service than a chromium steel at the same hardness. What it does not improve is hardenability. With no chromium, full hardness reaches only about 15–20 mm in oil, which is why IS 3431 makes hardenability a mandatory test and why the core of a heavy leaf will read lower than the surface. Where that becomes the limit, the step within the same standard is to 50Cr4V2 or 60Cr4V2.
Decarburisation and the 16:1 reduction ratio
A leaf spring carries its highest stress at the surface, and 2 % silicon accelerates surface carbon loss during rolling, forming and austenitising. A decarburised skin has lost its strength exactly where it is needed — the result is a spring that passes a hardness check and fails on fatigue. IS 3431 attacks internal soundness by requiring a minimum 16:1 reduction from ingot to product and by requiring the bar to be free of pipe, heavy scale, laminations, seams and cracked edges; the surface carbon question is handled by the spring plant through controlled furnace atmospheres, short cycle times and shot peening after forming and grinding.
Physical properties
Density 7.80 g/cm³, E = 200 GPa, shear modulus G = 78 GPa — leaf deflection scales with E and coil deflection with G, so both matter depending on the spring type. Thermal conductivity ≈ 38 W/m·K, expansion 11.9 × 10⁻⁶/K (20–200 °C). Usable to roughly 250 °C before relaxation becomes measurable, better than a plain carbon spring steel, which is another consequence of the silicon.
Weldability
Not weldable in the sprung length. Silicon at around 1.75 % makes the heat-affected zone more crack-sensitive than a plain spring steel, on top of a carbon equivalent near 0.9. Spring eyes, centre bolts, clips and brackets are formed or attached before heat treatment, or fixed mechanically. Where a weld cannot be avoided it needs 300–400 °C preheat, hydrogen-controlled consumables, a high interpass temperature and an immediate temper at 480–520 °C — and the welded zone will still lack the quenched and tempered structure the leaf depends on.
Machining, forming and heat treatment
Machined annealed at ≤ 269 HB, where machinability is around 55–60 % relative to 1 % C free-cutting steel; silicon stiffens the ferrite so the cut is harder than the hardness number suggests. Centre-bolt holes are punched or drilled before hardening, never after. Hot forming of leaves, eyes and tapered sections runs at 1150–850 °C and is followed by a full austenitise and quench, not by quenching from the forming heat, because the grain is coarse at that temperature; furnace time is kept short to limit decarburisation. After tempering to 48 HRC, grinding only — and grind wet, since a burnt surface is a locally tempered, weakened surface. Shot peen last.
Typical applications
- Laminated and parabolic leaf springs for trucks, buses, trailers and tractors
- Spring flats, tapered leaves and helper leaves
- Volute and hot-coiled helical springs for suspension
- Agricultural spring tines, cultivator shanks and seed-drill springs
- Spring clips, washers and retaining springs
- Flat and spiral springs in general engineering
- Forged hand tools where flex matters more than edge retention
Frequently asked questions
What is the equivalent of 60Si7?
SAE 9260 and JIS SUP7 are the closest matches, with GB 60Si2MnA and GOST 60S2A equally close; EN 10089 56Si7 (1.5026) is the European counterpart. All are 0.55–0.65 % carbon silicon spring steels with no chromium. The difference between them is not chemistry but delivery condition — EN 10089 caps phosphorus and sulphur at 0.025 % and limits surface decarburisation, where IS 3431 sets looser ceilings and leaves the surface carbon question to the spring plant.
What does the 7 in 60Si7 mean?
Silicon content multiplied by four: 7 ÷ 4 = 1.75 % nominal, with IS 3431 allowing 1.5–2.0 %. The leading 60 is carbon in hundredths of a per cent, so 60Si7 is 0.60 % C with 1.75 % Si. The same convention gives 65Si7 at 0.65 % carbon and 50Cr4V2 at 0.50 % C with 1.0 % Cr and 0.5 % V.
What is the hardness of 60Si7?
It depends entirely on the heat treatment, and IS 3431 does not specify one for the finished part — it is a steel standard. A laminated leaf spring is normally accepted at 46–50 HRC, from an oil quench at 830–870 °C and a temper near 450 °C. As-quenched the steel reaches 60–63 HRC, far too hard for service.
Which standard covers 60Si7?
IS 3431 — steel for the manufacture of volute, helical and laminated springs for automotive suspension — which lists 55Si7, 60Si7, 65Si7, 50Cr4V2 and 60Cr4V2. IS 3195 is the parallel standard for railway rolling-stock springs and is a different grade list; IS 7906 covers the finished springs. General delivery conditions come from IS 8910.
What is the difference between 60Si7 and 65Si7?
Carbon only: 0.55–0.65 % against 0.60–0.70 %, with the same silicon. 65Si7 is harder and stronger at the same tempering temperature and marginally less tough, so it tends to go into thinner leaves and higher-stressed springs. Spring makers treat them as near-interchangeable and choose by leaf thickness.
How thick a leaf will 60Si7 harden through?
About 15–20 mm in oil. There is no chromium to slow the transformation, so hardness falls away in thicker sections and the middle of a heavy leaf reads lower than the surface — which is why IS 3431 includes hardenability among its mandatory tests. For thicker springs the choices are 50Cr4V2 or 60Cr4V2 within the same standard, or 51CrV4 and 5160 from the EN and SAE systems.
Why do 60Si7 springs lose camber in service?
Because they have taken a permanent set, which is a yield problem rather than a fracture problem. The usual causes are an over-temper that left the leaf below about 43 HRC, a core that never hardened because the section was too thick for a chromium-free grade, or a decarburised surface that yields at a lower stress than the certificate implies. Checking hardness at mid-thickness rather than only at the surface normally identifies which one it is.
Why does IS 3431 require a 16:1 reduction ratio?
To close internal defects. A cast ingot or billet carries centreline porosity and segregation, and a spring is a component in which an internal defect becomes a fatigue origin. Requiring the cross-section to be reduced at least sixteenfold from ingot to finished bar forces enough hot work to weld that porosity shut. It is a soundness requirement, not a strength requirement, and it sits alongside the standard's demand that the bar be free of pipe, laminations, seams and cracked edges.
Related grades
Verified against IS 3431:1982 (grade list and scope); IS 3431:1982 — steel for volute, helical and laminated springs for automotive suspension; IS 8910 — general technical delivery requirements for steel; SAE J404 / ASTM A29 (9260 comparison); EN 10089:2002 (56Si7 comparison); JIS G4801:2011 (SUP7 comparison). Last checked: October 2026.