080M40 steel: properties, composition, equivalents
080M40 — the steel still universally called EN8 — is the British medium-carbon engineering steel of BS 970: 0.36–0.44 % C, supplied normalised or hardened and tempered for shafts, studs and general machined parts.
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What is 080M40?
080M40 is the medium-carbon engineering steel that British machine shops have run on for seventy years, and almost nobody calls it that. Its name is EN8 — a designation from BS 970:1955 that was formally withdrawn in 1970 and is still what the bar is ordered, stocked and drawn as from Birmingham to Bangalore.
The modern designation is readable once the pattern is known. 080 places the steel in the carbon-manganese group, M means it is supplied to *mechanical property* requirements, and 40 is the nominal carbon in hundredths of a per cent. The same steel ordered to chemical analysis alone is 080A40, with no guaranteed strength — a distinction that matters on a purchase order and is routinely lost.
A warning about the name: the EN in EN8 has nothing to do with the European EN standards. It stands for *Emergency Number*, from the wartime British specification series. EN8 is not an EN 10025 or EN 10083 grade, and the coincidence has caused a great many mis-specified orders.
What the steel actually offers is a good balance for general machined parts: 550–700 MPa tensile normalised, rising to 700–850 MPa hardened and tempered, with clean machining and straightforward through-hardening in small sections. What it does not offer is hardenability in thick sections or guaranteed toughness. Both of those are the reason 709M40 (EN19) and 817M40 (EN24) exist above it.
080M40 chemical composition
| Element | Mass % | Note |
|---|---|---|
| C | 0.36 – 0.44 | |
| Si | 0.1 – 0.4 | |
| Mn | 0.6 – 1 | |
| P | ≤ 0.05 | |
| S | ≤ 0.05 | 080A40 is the same analysis ordered to chemical composition only; 212M36 is the free-cutting variant with added sulphur |
BS 970-1:1991 ladle analysis. The designation encodes it: **080** places the steel in the carbon-manganese group with manganese around 1 %, **M** means the bar is supplied to mechanical property requirements, and **40** is the nominal carbon in hundredths of a per cent. Change the letter and the meaning changes with it — **080A40** is the same chemistry supplied to analysis only, with no guaranteed mechanical properties.
080M40 mechanical properties
| Thickness / condition | Yield strength ReH / Rp0.2 MPa (ksi) | Tensile strength Rm MPa (ksi) | Elongation A % | Hardness |
|---|---|---|---|---|
| Condition R — normalised, ≤ 63 mm ruling section | 280 (40.6) | 550–700 (79.8–101.5) | 16 | 152–207 HB |
| Condition S — hardened and tempered, ≤ 63 mm | 355 (51.5) | 625–775 (90.6–112.4) | 16 | 179–229 HB |
| Condition T — hardened and tempered, ≤ 63 mm | 385 (55.8) | 700–850 (101.5–123.3) | 15 | 201–255 HB |
| Condition U — hardened and tempered, ≤ 30 mm | 450 (65.3) | 775–925 (112.4–134.2) | 13 | 223–277 HB |
| As-supplied bright drawn bar | — | typically 600–750 (typically 87.0–108.8) | — | ≈ 180–230 HB |
BS 970 guarantees properties against a **limiting ruling section** — the largest diameter for which the stated band applies. The values above are for the common small and medium sections; the same condition letter on a 100 mm bar guarantees less, because the core of a thick bar cannot be cooled fast enough. Always read the condition letter and the ruling section together, and note that bright drawn bar is cold-worked rather than heat-treated, so its strength comes from the drawing and not from a specified condition.
Impact toughness
080M40 requires a minimum Charpy V-notch energy of J at °C (not a standard requirement). BS 970 does not require a Charpy test on 080M40 in the normalised or bright drawn conditions. Impact values can be specified on hardened and tempered material by agreement, and are then reported at room temperature. This is an engineering steel for machined parts, not a structural steel — where toughness governs the design, the alloy grades 709M40 (EN19, ≈ 42CrMo4) or 817M40 (EN24, ≈ 34CrNiMo6) are the correct step up.
080M40 equivalent grades
| Grade | System | Match | Why / differences |
|---|---|---|---|
| C45 | EN (European) | Near | C45 (1.0503) is 0.42–0.50 % C against 080M40's 0.36–0.44 %, so it sits slightly above — closer to 080M46. For general shafting the substitution is routine, but C45 will through-harden marginally deeper and machine marginally harder. |
| 1045 | AISI / SAE | Functional | AISI 1045 is 0.43–0.50 % C — the American counterpart of 080M46. Against 080M40 it is one carbon step higher; AISI 1040 is the true match, at 0.37–0.44 % C. |
| 42CrMo4 | EN (European) | Functional | The alloy step up, equivalent to 709M40 (EN19). Substitute where hardenability in sections above about 60 mm or genuine toughness is required — 080M40 cannot deliver either. |
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 080M40 →
Old and superseded designations
| Old name | Standard | Note |
|---|---|---|
| En8 | BS 970:1955 (withdrawn) | The name almost everyone still uses. En8 became 080M40 in the 1970 revision, and more than half a century later the old name has not gone away — suppliers, drawings and stock lists across Britain, Ireland, India, Australia and the Gulf all still say EN8. |
| En8D | BS 970:1955 (withdrawn) | The letter suffix marked a tighter sulphur and phosphorus limit for better ductility; closest modern equivalent is 080A40 ordered to analysis. |
| 080M46 | BS 970-1 | The next grade up in the same family: 0.42–0.50 % carbon, the British counterpart of AISI 1045 rather than 1040. Frequently confused with 080M40 in cross-reference tables. |
Product forms and tolerance standards
Product forms: bright drawn and turned round bar, hot-rolled black bar and billet, flat, square and hexagon bar, forging stock, hollow bar and tube for machining.
Dimensional tolerances: bright bar: BS EN 10278 (h9, h11); hot-rolled round: BS EN 10060; hot-rolled flat and square: BS EN 10058 / 10059; straightness and decarburisation per BS 970-1 Annex.
Key property values
080M40 mechanical properties
Normalised (condition R): 550–700 MPa tensile, 280 MPa yield, 152–207 HB. Hardened and tempered: 625–775 MPa (condition S), 700–850 MPa (T) or 775–925 MPa (U), with hardness rising to 277 HB. Each band is guaranteed only up to its limiting ruling section.
What the limiting ruling section means
BS 970 does not guarantee a strength for a grade; it guarantees a strength for a grade in a stated maximum diameter. A 25 mm bar in condition T reaches 700–850 MPa through its section; a 100 mm bar in the same condition does not, because the quench cannot cool its core fast enough. Ordering condition T without stating the section is ordering something the standard cannot supply.
EN8 does not mean European EN
The En series came from the British wartime *Emergency Number* specifications and was made permanent by BS 970:1955. EN8, EN19, EN24 and EN31 are British legacy names, unrelated to EN 10025, EN 10083 or any CEN document. A drawing calling for "EN8" wants 080M40; a drawing calling for "EN 10083-2 C40E" wants something else.
080M40 heat treatment
Normalise at 830–860 °C, air cool. Harden from 830–860 °C, quench in oil or water depending on section — oil for thin, water for thicker, with the crack risk rising accordingly. Temper between 550 and 660 °C to reach conditions S, T or U; the higher the tempering temperature, the lower the strength and the higher the ductility. Stress-relieve machined parts at 550–600 °C.
080M40 against C45
C45 (1.0503) is the grade most often offered against an EN8 enquiry in Europe, and for general shafting it works. The difference is one carbon step: C45 is 0.42–0.50 % against 0.36–0.44 %, so it hardens a little deeper, machines a little harder and sits closer to 080M46 than to 080M40. Where the part is heat-treated to a specified band, check which grade the band was written for.
Where 080M40 runs out
Three limits. Section size: above roughly 60 mm the quench does not reach the core and the properties fall away. Toughness: no impact requirement, and a plain carbon steel at 0.40 % C has little to offer at low temperature. Fatigue under shock: shafts with keyways and shoulders in duty cycles belong in 709M40 (EN19) or 817M40 (EN24). 080M40 is a general-purpose steel, not a highly stressed one.
Weldability
080M40 is weldable with care and is not a welding grade. At 0.36–0.44 % carbon the carbon equivalent runs around 0.55–0.65, well above the 0.45 threshold at which cold cracking becomes a real risk, so welding requires preheat of 150–250 °C by section, low-hydrogen consumables, and a post-weld stress relief at 550–660 °C wherever the joint is restrained. Weld the material in the normalised condition where possible; welding onto hardened and tempered bar destroys the temper locally and leaves a hard, untempered zone at the fusion boundary. For assemblies that must be welded rather than machined from solid, a low-carbon structural grade or a weldable alloy steel is the right specification from the start.
Machining, forming and heat treatment
This is the grade's strength. In the normalised condition 080M40 machines cleanly and predictably with a machinability index around 60 % of free-cutting steel, giving good surface finish and manageable tool wear — it is the default choice for turned shafts, studs, pins and spacers precisely because it behaves. Bright drawn bar machines better still but carries residual stress from the drawing, so parts machined unevenly from it can move; where dimensional stability matters, use turned-and-polished bar or stress-relieve before finish machining. Where chip control dominates, 212M36 is the free-cutting version with added sulphur, at the cost of transverse ductility.
Typical applications
- Turned shafts, spindles, axles and drive pins
- Studs, bolts, keys and general machined fasteners
- Gears and sprockets in light duty, often flame or induction hardened on the teeth
- Machine tool components, jigs, fixtures and tooling bodies
- Hydraulic rams and rods in the hardened and tempered conditions
- General engineering bar stock across Britain, Ireland, India, Australia and the Gulf
Frequently asked questions
What is EN8 steel?
EN8 is the common name for 080M40, the British medium-carbon engineering steel of BS 970 with 0.36–0.44 % carbon. It is supplied normalised at 550–700 MPa tensile or hardened and tempered up to 925 MPa, and it is the default bar stock for turned shafts and general machined parts.
Is EN8 the same as 080M40?
Yes. EN8 is the 1955 designation and 080M40 the one that replaced it in 1970. The old name never stopped being used, so suppliers, drawings and stock lists carry both, and they refer to the same steel.
Does the EN in EN8 mean European standard?
No, and this is a common and expensive confusion. EN in EN8 stands for Emergency Number, from the British wartime specification series. It has no connection to EN 10025, EN 10083 or any other CEN standard.
What is the equivalent of EN8 in European standards?
C45 (1.0503) is the grade normally offered, though it is one carbon step higher at 0.42–0.50 % and is strictly closer to 080M46. C40E is the nearer match on carbon. In the American system, AISI 1040 matches 080M40 and 1045 matches 080M46.
What is the difference between 080M40 and 080A40?
The letter. M means the bar is supplied to guaranteed mechanical properties in a stated condition and ruling section; A means it is supplied to chemical analysis only, with no guaranteed strength. Ordering A when the drawing assumed M is a real and recurring problem.
Can EN8 be hardened?
Yes, by quench and temper: austenitise at 830–860 °C, quench in oil or water by section, temper at 550–660 °C to the required condition. It through-hardens reliably only in small sections — above roughly 60 mm diameter the core will not reach the specified band and an alloy grade such as EN19 is needed.
What is the difference between EN8 and EN19?
Alloy content and therefore hardenability. EN19 is 709M40, a chromium-molybdenum steel equivalent to 42CrMo4, which hardens right through much larger sections and holds far better toughness after tempering. EN8 is the plain carbon grade for general work; EN19 is for highly stressed shafts and gears.
Can EN8 be welded?
With preheat, low-hydrogen consumables and post-weld stress relief — and preferably not at all. The carbon equivalent is around 0.55–0.65, well into the cold-cracking range. Weld in the normalised condition if you must; welding hardened and tempered bar leaves an untempered zone at the fusion line.
Related grades
Verified against BS 970-1:1991; BS 970-1:1991 — Wrought steels for mechanical and allied engineering purposes, Part 1 (BSI); BS 970:1955 — for the En-series concordance. Last checked: September 2026.