51CrV4 (1.8159) spring steel: properties, composition, equivalents
51CrV4 (1.8159) is the chromium-vanadium spring steel of EN 10089: 1350–1650 MPa tensile at 42–48 HRC, hardening through heavy sections. Composition, heat treatment and 6150/SUP10 equivalents.
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What is 51CrV4?
51CrV4 (1.8159) is the workhorse of EN 10089 and the most widely specified spring steel in Europe. Half a per cent of carbon with 1 % chromium takes it to 44–48 HRC and 1450–1650 MPa tensile after an oil quench and a 450 °C temper; the chromium lets it harden right through a 40 mm leaf or a 25 mm torsion bar, where a plain carbon spring steel would be hard only at the surface.
The vanadium is the part that is usually overlooked. At 0.10–0.25 % it forms fine carbides that pin the grain boundaries, so the steel tolerates being heated for hot forming or coiling without coarsening, and it comes out of an overheated furnace cycle far better than 5160 or a silicon grade would. That is why 51CrV4 is also the steel of choice for hand tools — chisels, wrenches, pry bars, mower blades — where the part is hot-formed, hardened and then used in shock.
The third thing worth understanding is that EN 10089 does not give you a tensile table for your spring. It fixes the analysis, the delivery hardness, the cleanliness and — unusually — a limit on surface decarburisation, because a spring fails from its surface and a decarburised skin has no fatigue strength. Everything else is set by the spring maker: austenitising, quench, temper, and above all shot peening, which routinely doubles the fatigue life of the same bar. A certificate showing 1550 MPa tells you nothing about how long the spring will last.
SAE 6150, JIS SUP10, GB 50CrVA, GOST 50KhFA and BS 735A50 (EN47) are the same steel. The older German name 50CrV4 is still in daily use and refers to the identical grade.
51CrV4 chemical composition
| Element | Mass % | Note |
|---|---|---|
| C | 0.47 – 0.55 | |
| Si | ≤ 0.4 | |
| Mn | 0.7 – 1.1 | |
| P | ≤ 0.025 | |
| S | ≤ 0.025 | |
| Cr | 0.9 – 1.2 | |
| V | 0.1 – 0.25 |
EN 10089:2002 Table 1 (51CrV4, 1.8159). The phosphorus and sulphur limits of 0.025 % are tighter than in an ordinary engineering steel, and deliberately so: a spring fails from a surface or subsurface defect, and sulphide inclusions are defects. SAE 6150 allows C 0.48–0.53 with P 0.035 and S 0.040 — the analysis is the same, the cleanliness requirement is not.
51CrV4 mechanical properties
| Thickness / condition | Yield strength ReH / Rp0.2 MPa (ksi) | Tensile strength Rm MPa (ksi) | Elongation A % | Hardness |
|---|---|---|---|---|
| +A annealed (delivery for forming) | — | ≤ 850 (≤ 123.3) | — | ≤ 248 HB |
| +QT, 850 °C oil + 450 °C temper | 1250–1400 (181.3–203.1) | 1450–1650 (210.3–239.3) | 6 | 44–48 HRC |
| +QT, 850 °C oil + 500 °C temper | 1100–1250 (159.5–181.3) | 1300–1450 (188.5–210.3) | 7 | 40–44 HRC |
| +QT, 850 °C oil + 550 °C temper | 950–1100 (137.8–159.5) | 1150–1300 (166.8–188.5) | 9 | 36–40 HRC |
| Hardened 850 °C oil, as-quenched | — | — | — | 55–58 HRC |
| Fatigue limit, shot-peened spring at 1500 MPa (typical) | ≈ 550 (±, bending) (≈ 79.8 (±, bending)) | — | — | — |
EN 10089 specifies the chemistry, the annealed or +QT delivery hardness and the decarburisation limit; it does **not** standardise a tensile table for the finished spring, because the properties are set by the spring maker's own heat treatment after forming. The figures above are the usual results for a 20 mm section. The last row is the number that actually decides a spring's life, and it depends far more on the surface — peening, decarburisation, scale pits — than on the tensile strength.
51CrV4 equivalent grades
| Grade | System | Match | Why / differences |
|---|---|---|---|
| 6150 | AISI / SAE | Identical | SAE 6150 (UNS G61500): C 0.48–0.53, Mn 0.70–0.90, Cr 0.80–1.10, V ≥ 0.15. The same chromium-vanadium steel. The difference is in the delivery requirements rather than the analysis — SAE allows P 0.035 and S 0.040 against 0.025 for EN 10089, and does not impose a decarburisation limit. |
| 735A50 | BS (superseded British) | Identical | BS 970 735A50, the former EN47: C 0.45–0.55, Mn 0.70–1.00, Cr 0.80–1.20, V 0.15 min. Still quoted on British drawings for torsion bars and hand tools. |
| 50CrVA | GB/T (China) | Identical | GB/T 1222 50CrVA: C 0.47–0.54, Mn 0.50–0.80, Cr 0.80–1.10, V 0.10–0.20. The Chinese spring steel standard mirrors EN 10089 closely, including the decarburisation requirement. |
| 50KhFA | GOST (Russia/CIS) | Identical | GOST 14959 50KhFA: C 0.46–0.54, Mn 0.50–0.80, Cr 0.80–1.10, V 0.10–0.20. |
| SUP10 | JIS (Japan) | Identical | JIS G4801 SUP10: C 0.47–0.55, Si 0.15–0.35, Mn 0.65–0.95, Cr 0.80–1.10, V 0.15–0.25 — effectively the EN analysis with a silicon minimum. |
| 5160 | AISI / SAE | Near | The chromium leaf-spring steel without vanadium: C 0.56–0.64, Cr 0.70–0.90. Higher carbon gives similar hardness more cheaply; the missing vanadium means a coarser grain and less tolerance of overheating during forming. |
| 52CrMoV4 | EN (European) | Near | 1.7701: the same steel with 0.50 % molybdenum added. Hardens through heavier bars and resists relaxation better at temperature; used for very heavy leaf and parabolic springs. |
| 9260 | AISI / SAE | Functional | 1.8159: chromium-vanadium, deeper hardening and far more tolerant of hot forming because the vanadium pins the grain. Chosen where section size or process robustness governs rather than elastic limit. |
| 60SiCr7 | EN (European) | Functional | 1.7108: the silicon-chromium coil-spring steel. Higher elastic limit and better sag resistance at the same hardness, but shallower hardening and more prone to decarburisation — the usual choice for valve and suspension coil springs where 51CrV4 goes into leaf springs and torsion bars. |
| 42CrMo4 | EN (European) | Functional | 1.7225: the general engineering quenched and tempered steel. Overlaps in strength up to about 1100 MPa but has 0.40 % carbon, so it cannot reach spring hardness, and it is not supplied against a decarburisation limit. |
| 60Si7 | IS (India) | 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 51CrV4 →
Old and superseded designations
| Old name | Standard | Note |
|---|---|---|
| 50CrV4 | DIN 17221 (withdrawn) | Same number 1.8159. EN 10089 renamed it 51CrV4 when the carbon window was re-centred; the two names are used interchangeably in the trade and on mill certificates. |
| 735A50 | BS 970 Part 2 / BS 970:1955 EN47 | The British spring and tool grade, also written EN47 in the pre-1970 scheme. |
| Z50CV4 | NF A 35-571 | The French designation. |
| 50CrV4 | UNI 3545 | The Italian standard used the German short name directly. |
Product forms and tolerance standards
Product forms: hot-rolled flat bar for leaf springs (annealed or +QT), round bar for torsion bars and coil springs, cold-rolled strip (EN 10132-4), square and profiled spring bar, peeled and ground bar for machined parts.
Dimensional tolerances: hot-rolled flat spring bar: EN 10092-1 (dimensions and tolerances for spring steel flats); hot-rolled round and square: EN 10092-2 / EN 10060; cold-rolled spring strip: EN 10132-4; surface decarburisation: limited by EN 10089 Table 6 as a proportion of thickness — part of the delivery condition, not an extra.
Key property values
51CrV4 tensile strength and hardness
Delivered annealed at ≤ 248 HB for forming, or quenched and tempered. The working condition is set by the temper: 1450–1650 MPa at 44–48 HRC from a 450 °C temper, 1300–1450 MPa at 40–44 HRC from 500 °C, 1150–1300 MPa at 36–40 HRC from 550 °C. Leaf springs are normally finished at 1450–1600 MPa; hand tools at 44–50 HRC; torsion bars towards the upper end.
Heat treatment
Soft anneal 650–700 °C. Normalise 850–880 °C. Austenitise 840–870 °C (hold 10–20 min at temperature), quench in oil; water is used on heavy leaves with care. Temper 420–550 °C for 1–2 h, then air cool. Avoid tempering between 250 and 400 °C: that range crosses the tempered-martensite embrittlement trough and costs toughness without buying strength. Hot form between 1050 and 850 °C and normalise afterwards. Ac1 ≈ 750 °C, Ac3 ≈ 800 °C, Ms ≈ 270 °C.
Hardenability and section size
The chromium gives genuine through-hardness: full martensite to about 40 mm in oil, which covers virtually every leaf spring and most torsion bars. The Jominy curve runs roughly 58 HRC at J1.5, 55 HRC at J9 and 45 HRC at J25. Where a bar above 50 mm must be uniform, the molybdenum version 52CrMoV4 is the grade to move to; adding hardness by raising carbon instead costs toughness.
Decarburisation — why EN 10089 limits it
A spring is a surface-stressed component: the maximum stress is at the outer fibre, which is exactly where a decarburised layer has lost its carbon and therefore its strength. EN 10089 limits the total decarburisation depth as a proportion of thickness and makes it part of the delivery condition. The practical consequences: protect the surface during hot forming and hardening, use a controlled atmosphere or salt where possible, and if the surface has been exposed, grind or peen it. A ground bar and a hot-rolled bar from the same heat can differ by a factor of two in fatigue life.
Shot peening and fatigue
The single most effective thing done to a 51CrV4 spring. Peening puts a compressive residual stress of 500–900 MPa into the top 0.2 mm, which has to be overcome before a fatigue crack can open; measured fatigue life typically doubles or better. Stress peening (peening under load) adds more. For comparison, raising the tensile strength from 1450 to 1600 MPa gains far less and costs ductility. Pitting, scale, shot-blast damage and decarburisation all work in the opposite direction.
Physical properties
Density 7.85 g/cm³, E = 206 GPa, shear modulus G = 79.5 GPa — the two numbers a spring designer needs, since leaf deflection goes with E and coil deflection with G. Thermal conductivity ≈ 45 W/m·K, expansion 12.2 × 10⁻⁶/K (20–200 °C). Maximum service temperature about 200–250 °C before relaxation becomes significant; above that, a silicon-chromium or molybdenum grade is required.
Weldability
Weldable only with care and never in the sprung part. The carbon equivalent is roughly 0.85–0.95, so preheat 300–400 °C, hydrogen-controlled or matching filler, high interpass temperature, slow cooling and an immediate post-weld temper at 500–550 °C are all required — and the heat-affected zone will still have lost the structure the spring depends on. Standard practice is to attach brackets and eyes mechanically or by welding before heat treatment, never after. A welded repair on a finished leaf spring is not a repair; it is a planned fracture.
Machining, forming and heat treatment
Machined in the annealed condition at ≤ 248 HB, where machinability is about 60–65 % relative to 1 % C free-cutting steel — straightforward with carbide tooling. After quenching and tempering to 45 HRC, only grinding is practical. Hot forming of leaves and coils is done between 1050 and 850 °C and is followed by a full austenitise and quench, not by a direct quench from the forming heat, because the grain is coarse at forming temperature. Cold coiling is done on annealed or +QT wire and followed only by a stress-relief at 400–450 °C. Shot peen after all forming and grinding, never before.
Typical applications
- Leaf springs, parabolic springs and spring leaves for trucks and trailers
- Torsion bars and anti-roll bars
- Hot and cold coiled compression springs in heavy sections
- Hand tools — chisels, wrenches, pry bars, pliers, shears
- Agricultural tines, mower and cutter blades, tillage parts
- Clutch and brake springs, retaining rings
- Machine knives and scraper blades where shock matters more than wear
Frequently asked questions
Is 50CrV4 the same as 51CrV4?
Yes. Both names describe material number 1.8159. 50CrV4 is the older DIN 17221 designation; EN 10089 re-centred the carbon window at 0.47–0.55 % and renamed it 51CrV4. Mills and stockists use the two interchangeably and a certificate may carry either, so match the analysis rather than the name.
What is the AISI equivalent of 51CrV4?
SAE 6150 (UNS G61500), with C 0.48–0.53, Cr 0.80–1.10 and vanadium above 0.15 %. The analysis is effectively the same. What differs is the delivery requirement: EN 10089 caps phosphorus and sulphur at 0.025 % and limits surface decarburisation, while SAE 6150 as such does neither. For a fatigue-loaded spring that difference matters more than the chemistry.
What tensile strength does 51CrV4 reach?
1450–1650 MPa at 44–48 HRC after an oil quench and a 450 °C temper, falling to about 1150–1300 MPa if tempered at 550 °C. Leaf springs are usually finished at 1450–1600 MPa. The annealed delivery condition is 850 MPa maximum, for forming.
What does the vanadium in 51CrV4 do?
It forms fine carbides that pin the grain boundaries, so the steel resists grain coarsening when it is heated for forging, hot coiling or hardening. In practice that means a more forgiving heat treatment and better toughness at the same hardness than a plain chromium grade such as 5160. It also contributes a little secondary hardening, which is why 51CrV4 holds its properties better than a carbon spring steel near 250 °C.
How thick a section will 51CrV4 harden through?
About 40 mm in oil, which covers almost all leaf springs and torsion bars. Beyond 50 mm the core falls away and the grade to use is 52CrMoV4 (1.7701), which adds 0.50 % molybdenum for exactly this reason.
Why must 51CrV4 springs be shot peened?
Because a spring is loaded hardest at its surface, and fatigue cracks start there. Peening leaves 500–900 MPa of compressive stress in the top 0.2 mm, which must be overcome before a crack can open; it routinely doubles fatigue life or better. It also partly offsets surface damage from scale and handling. Peening is applied after all forming and grinding.
Can 51CrV4 be welded?
Not in the sprung section. The carbon equivalent is around 0.9, so welding needs 300–400 °C preheat, hydrogen control and an immediate temper, and even then the heat-affected zone no longer has the quenched and tempered structure the spring relies on. Brackets and eyes are attached before heat treatment or fixed mechanically. A weld on a finished leaf spring is where it will break.
Is 51CrV4 a good steel for tools and knives?
For impact tools, yes — it is one of the standard steels for chisels, wrenches, pry bars and mower blades, because the vanadium-refined grain gives good toughness at 45–50 HRC. For a cutting edge it is a compromise: at 0.5 % carbon it cannot hold the hardness of 1095 or a 1 % carbon tool steel, so the edge dulls sooner while the blade is far less likely to snap.
Related grades
Verified against EN 10089:2002; EN 10089:2002 Tables 1 and 6; SAE J404 / ASTM A29 (6150 comparison); JIS G4801:2011 (SUP10 comparison); GB/T 1222-2016 (50CrVA comparison). Last checked: October 2026.