080M40
EN8 · BS 970
080M40 (EN8) is an engineering steel (Plain Carbon family) to BS 970. It has a specified 0.2 % proof stress of 385 MPa and a typical tensile strength of about 620 MPa, with a uniform elongation of 15 %. Nominal carbon content is around 0.40 %. For nonlinear FEA, a bilinear model uses E = 210 GPa up to yield and a tangent modulus of 1608 MPa above it. A plain-carbon steel: economical and predictable, strengthened by carbon content and heat treatment rather than alloying.
Bilinear material model — nonlinear FEA
Uniform elongation εu = 15.0% · conservative analysis limit ½εu = 7.5%
Bilinear (BISO) approximation. Elastic modulus E up to the 0.2% proof stress R_p0.2, then a single tangent slope Eₜ to the ultimate tensile strength: Eₜ = (Rₘ − R_p0.2) / (εu − R_p0.2/E − 0.002), where εu is the uniform elongation and Rₘ is a typical (nominal) UTS within the specified band. The half-εu figure is a conservative strain limit for the analysis. Treat these as guidance for first-pass models; for critical work use a measured true stress–strain curve for the actual product and condition.
Ratings — engineering judgement
Weldabilitymoderate
Weldable with precautions. 0.40% C means preheat (100–200 °C) is advisable over ~15 mm; hydrogen-controlled consumables recommended. HAZ hardening risk grows with section size — prefer low-hydrogen processes (MIG/MAG, TIG).
Formabilitymoderate
Reasonable cold formability when normalised — bending and forming possible but more limited than mild steel. Hot forming straightforward.
Machinabilitygood
Machines well, particularly bright-drawn — good chip formation and surface finish. A key reason it's so popular for general engineering.
Corrosion resistancepoor
No corrosion resistance — plain carbon steel. Must be protected in any exposed application.
Hardenabilitymoderate
Through-hardens by water quench in small sections (<25 mm) or oil quench with limited depth. Surface hardening (flame, induction) very effective — hard wearing surface on a tough core. Not in the league of alloy steels.
Toughnessmoderate
Moderate — better than higher-carbon steels but well below Ni-Cr-Mo grades like EN24. Adequate for most general engineering at ambient temperature.
Expert notes
EN8 is the steel that everything gets made from when there's no strong reason to use anything else — the engineering default. Handed a mystery shaft or pin? Your first guess should probably be EN8.
Bright-drawn bar is the most commonly used form — decent surface finish straight off the shelf and machines cleanly. For anything that sees wear (bearing journals, camshaft lobes, gear teeth), induction-harden the surface to 50–55 HRC while keeping the core tough.
Don't try to through-harden EN8 in anything larger than about 25 mm diameter — it's plain carbon so hardenability is limited. For genuine through-hardness, step up to EN19 (709M40) or EN24.
For bolted joints: commonly used for studs and set-screws in non-critical applications — roughly equivalent to Grade 5.6 or 6.8 depending on heat treatment. For proper high-tensile bolting you need EN24 or a commercial bolt grade.
One practical trap: EN8 and EN3 (mild steel) look identical on the shelf. If material ID matters, mark or segregate — a hardness test is the quickest tell (EN8 normalised ~170 HB vs ~130 HB for mild steel).
Mechanical properties
| Condition | Section | Yield (MPa) |
|---|---|---|
| 080M40-N · normalised | — | 280 |
| 080M40-HT · hardened & tempered | RS ≤ 19 mm | 385 |
| 080M40-HT | RS 19–63 mm | 340 |
UTS 625–775 MPa · Impact 20 J @ 20 °C (N) · Hardness ~170 HB (N); 179–229 HB (HT)
Physical properties
| Density | 7845 | kg/m³ |
| Elastic modulus | 210 | GPa |
| Shear modulus | 81 | GPa |
| Poisson's ratio | 0.30 | |
| Thermal expansion | 11.3 | ×10⁻⁶/°C |
| Thermal conductivity | 49 | W/(m·K) |
| Specific heat | 480 | J/(kg·K) |
Chemical composition (wt %)
Available forms
Equivalent grades
| Standard | Grade | Note |
|---|---|---|
| AISI/SAE | 1040 | |
| EN 10083-2 | C40 / 1.0511 | |
| DIN 17200 | C40 / Ck40 | |
| AFNOR | AF42 / C40 | |
| JIS | S40C | |
| GB | 40 | |
| IS 1570 (Pt.2) | C40 / 40C8 | ~0.40% C, 0.60–0.90% Mn — very close to EN8. |
| UNI | C40 |
Historical designations
| Standard | Grade | Note |
|---|---|---|
| BS 970:1955 | EN8 | Original British designation. Still universally used — ask for 'EN8' and any UK shop knows exactly what you mean. |
| BS 970:1970 | 080M40 | 0 = carbon/carbon-manganese, 80 = Mn variant, M = wrought, 40 = 0.40% C. |
Common applications
- General-purpose shafts and axles
- Studs, bolts and fasteners (non-critical)
- Gears and pinions (light to moderate duty)
- Spindles and collets
- Connecting rods (lower-duty)
- Hydraulic cylinder rods
- Agricultural machinery components
- Keys and dowels
- Induction-hardened wear surfaces (journals, camshafts)
History
EN8 is a plain medium-carbon steel that has been the backbone of British general engineering since the 1940s — a meaningful step up from mild steel in strength and wear resistance at minimal cost premium.
- 1942EN8 defined in BSI War Emergency BS 970 as a standard medium-carbon engineering steel
- 1970BS 970 overhaul — EN8 redesignated as 080M40
- 1991BS 970:1991 revision — 080M40 maintained