708M40
EN19 · BS 970
708M40 (EN19) is an engineering steel (Chromium-Molybdenum family) to BS 970. It has a specified 0.2 % proof stress of 710 MPa and a typical tensile strength of about 850 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 1123 MPa above it. The Cr–Mo alloying provides good hardenability and creep/temper resistance.
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
Weldabilitymoderate
Weldable with preheat (200–300 °C) and hydrogen-controlled consumables; PWHT usually advisable. Easier than the Ni-bearing EN24 but still a hardenable alloy steel.
Formabilitypoor
Limited cold formability, especially once heat treated. Hot-form then re-heat-treat.
Machinabilitygood
Machines well in the annealed or pre-tempered (T) condition — the usual supply state. Carbide tooling at higher hardness.
Corrosion resistancepoor
No corrosion resistance — protect by plating, paint or nitriding.
Hardenabilityexcellent
Excellent through-hardenability from the 1 %Cr–Mo pair; oil-quenches and tempers reliably and nitrides well.
Toughnessgood
Good strength-to-toughness balance when properly tempered; Mo resists temper embrittlement.
Expert notes
708M40 is EN19 — the UK’s 1 %Cr–Mo through-hardening steel, and for practical purposes the same material as 42CrMo4 and AISI 4140. If a drawing calls for EN19, 4140 or 42CrMo4, you are looking at one steel under three standards.
It sits one rung below EN24 (817M40): no nickel, so slightly less hardenability and toughness in the largest sections, but cheaper and perfectly capable for the vast majority of shafts, gears and high-tensile bolts. Bought pre-tempered (the “T” condition) it machines to finish with no further heat treatment.
Design to the ruling-section strength, not the headline figure — like all Q&T alloy steels the yield falls as the bar gets thicker because the quench can’t reach the core.
Weldable with preheat and (usually) PWHT; it nitrides well if you want a hard fatigue-resistant surface over a tough core.
Mechanical properties
| Condition | Section | Yield (MPa) |
|---|---|---|
| 708M40-T · quenched & tempered | RS ≤ 63 mm | 710 |
| 63 < RS ≤ 100 mm | 650 | |
| 708M40-U | all RS | 570 |
UTS 850–1000 MPa · Impact ≥ 50 J @ 20 °C (T) · Hardness 248–302 HB (T)
Physical properties
| Density | 7850 | kg/m³ |
| Elastic modulus | 210 | GPa |
| Shear modulus | 81 | GPa |
| Poisson's ratio | 0.30 | |
| Thermal expansion | 11.1 | ×10⁻⁶/°C |
| Thermal conductivity | 42 | W/(m·K) |
| Specific heat | 460 | J/(kg·K) |
Chemical composition (wt %)
Available forms
Equivalent grades
| Standard | Grade | Note |
|---|---|---|
| EN 10083-3 | 42CrMo4 / 1.7225 | Near-identical Cr-Mo grade; check Mn/Mo limits. |
| AISI/SAE | 4140 | |
| JIS | SCM440 | |
| GB | 42CrMo |
Historical designations
| Standard | Grade | Note |
|---|---|---|
| BS 970:1955 | EN19 | The designation most UK engineers still use for this 1 %Cr–Mo steel. |
| BS 970:1970 | 708M40 | 7 = Cr-Mo group, M = wrought, 40 = 0.40 % C. |
Common applications
- Shafts, axles and spindles
- Gears, pinions and couplings
- High-tensile bolts and studs
- Connecting rods and machine parts
- Forged components for general engineering
History
708M40 (EN19) is the BS 970 chromium-molybdenum through-hardening steel — the British sibling of 42CrMo4 / AISI 4140, standardised in the 1942 BS 970.
- 1942EN19 defined in the War Emergency BS 970
- 1970BS 970 overhaul — EN19 redesignated 708M40
- 1991BS 970:1991 revision maintained the grade