817M40
EN24 · BS 970
817M40 (EN24) is an engineering steel (Nickel-Chromium-Molybdenum family) to BS 970. It has a specified 0.2 % proof stress of 835 MPa and a typical tensile strength of about 1000 MPa, with a uniform elongation of 13 %. 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 1330 MPa above it. The Ni–Cr–Mo alloying gives deep hardenability with an excellent strength-to-toughness balance.
Bilinear material model — nonlinear FEA
Uniform elongation εu = 13.0% · conservative analysis limit ½εu = 6.5%
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
Weldabilitypoor
Weldable only with significant precautions. High CE means preheat (200–300 °C) and controlled interpass are essential; PWHT usually required; hydrogen-controlled electrodes mandatory. Avoid if possible — consider mechanical fastening.
Formabilitypoor
Very limited cold formability when heat-treated. Hot forming (900–1200 °C) possible but re-heat-treatment is mandatory afterward.
Machinabilitygood
Machines well in the T condition (pre-hardened & tempered) — EN24T is stocked for this purpose. Good chip formation; use carbide tooling.
Corrosion resistancepoor
No corrosion resistance. Must be protected (plating, paint, nitriding) in exposed applications.
Hardenabilityexcellent
Excellent through-hardenability from the Ni-Cr-Mo combination — through-hardens in sections up to ~100 mm. Typically oil-quenched and tempered; nitriding also possible.
Toughnessexcellent
Outstanding for its strength level. The Ni-Cr-Mo balance provides excellent impact resistance even at elevated hardness — a key reason for its enduring popularity.
Expert notes
EN24 is the first steel most British engineers reach for when they need serious strength and toughness. 'I need a high-strength shaft' — EN24T is almost always the starting point.
The 'T' condition (pre-hardened & tempered, 850–1000 MPa UTS) is how the vast majority is sold and used. Stocked in this condition by virtually every UK stockholder — buy it, machine it, done. A huge practical advantage.
Be careful with the AISI 4340 equivalence — close but not exact, particularly on Si and Mn. Interchangeable for most purposes, but use the right one if certifying to a specific standard.
Welding EN24 is not casual. If your design requires welding, seriously consider whether EN8 or a lower-alloy grade could do the job. If you must weld it, preheat is non-negotiable and PWHT will almost certainly be required.
For bolted joints: EN24 makes excellent high-tensile studs and bolts when properly heat-treated — often used for special fasteners where standard Grade 12.9 isn't available in the size needed.
Mechanical properties
| Condition | Section | Yield (MPa) |
|---|---|---|
| 817M40-T · quenched & tempered | RS ≤ 63 mm | 835 |
| 817M40-T | RS 63–100 mm | 755 |
| 817M40-U · lower strength | all RS | 680 |
UTS 1000–1160 MPa · Impact 40 J @ 20 °C (T) · Hardness 269–331 HB (T)
Physical properties
| Density | 7850 | kg/m³ |
| Elastic modulus | 205 | GPa |
| Shear modulus | 80 | GPa |
| Poisson's ratio | 0.30 | |
| Thermal expansion | 11.2 | ×10⁻⁶/°C |
| Thermal conductivity | 38 | W/(m·K) |
| Specific heat | 460 | J/(kg·K) |
Chemical composition (wt %)
Available forms
Equivalent grades
| Standard | Grade | Note |
|---|---|---|
| AISI/SAE | 4340 | |
| EN 10083-3 | 34CrNiMo6 | Close but not identical — check composition limits. |
| DIN | 1.6582 / 34CrNiMo6 | |
| AFNOR | 35NCD6 | |
| JIS | SNCM439 | |
| GB | 40CrNiMoA | |
| IS 1570 (Pt.4) | 40Ni4Cr4Mo3 | Approximate; verify against IS spec for critical use. |
| UNI | 40NiCrMo7 |
Historical designations
| Standard | Grade | Note |
|---|---|---|
| BS 970:1955 | EN24 | Original British designation — still universally used. Most engineers know it as EN24, not 817M40. |
| BS 970:1970 | 817M40 | 8 = Ni-Cr-Mo group, 17 = variant, M = wrought, 40 = 0.40% C. |
| BS 970:1942 | EN24 | First appeared in the War Emergency edition; Mo could originally be omitted by agreement. |
Common applications
- Shafts and axles — transmission, propeller, drive shafts
- Gears and pinions — power transmission, gearboxes
- Connecting rods and crankshafts
- High-tensile studs and bolts
- Aircraft landing gear components
- Rock drilling equipment
- Die holders and heavy-duty tooling
- Hydraulic cylinders and rams
History
EN24 traces to the Sheffield mills of the late 19th century, where Ni-Cr alloy steels were first developed. It was formally standardised in the 1942 wartime BS 970 and has been a cornerstone of British engineering ever since.
- 1900Sheffield mills producing proprietary Ni-Cr alloy steels under trade names
- 1942EN24 formally defined in BSI War Emergency BS 970
- 1970BS 970 overhaul — EN24 redesignated as 817M40
- 1991BS 970:1991 revision — 817M40 maintained with minor updates