42CrMo4
4140 · EN 10083-3 · 1.7225
42CrMo4 (4140) is an engineering steel (Chromium-Molybdenum family) to EN 10083-3 (1.7225). It has a specified 0.2 % proof stress of 900 MPa and a typical tensile strength of about 1100 MPa, with a uniform elongation of 10 %. Nominal carbon content is around 0.42 %. For nonlinear FEA, a bilinear model uses E = 210 GPa up to yield and a tangent modulus of 2134 MPa above it. The Cr–Mo alloying provides good hardenability and creep/temper resistance.
Bilinear material model — nonlinear FEA
Uniform elongation εu = 10.0% · conservative analysis limit ½εu = 5.0%
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 EN24 but still not casual — weld before final heat treatment where possible.
Formabilitypoor
Limited cold formability, especially once hardened. Hot form at 850–1150 °C then re-heat-treat.
Machinabilitygood
Machines well in the annealed or pre-tempered (T) condition — the usual supply state for machined parts. Use carbide tooling at high hardness.
Corrosion resistancepoor
No corrosion resistance — protect by plating, paint or nitriding.
Hardenabilityexcellent
Excellent through-hardenability from the Cr–Mo pair; oil-quenches and tempers reliably in medium sections, and takes nitriding well.
Toughnessgood
Good strength-to-toughness balance when properly tempered; the molybdenum resists temper embrittlement. Not quite the Ni-bearing EN24, but excellent for its cost.
Expert notes
42CrMo4 is the everyday high-strength engineering steel — the one you reach for when EN8 isn’t strong enough and EN24 is more (and more expensive) than you need. Shafts, gears, arms, high-tensile bolts: it’s the sensible default across a huge range of machined and forged parts.
It’s usually bought pre-hardened (the +QT / “T” condition, ~900 MPa yield in ≤16 mm), so you machine it to finish with no further heat treatment — a big practical win. Remember the ruling-section effect, though: yield falls with diameter, so design to the value for the actual bar size, not the headline 900 MPa.
4140, EN19 and 42CrMo4 are effectively the same steel from three standards; they’re interchangeable for most purposes, but if you’re certifying to a specification use the right one — the Mn and Mo ranges differ slightly.
It welds noticeably more easily than EN24 (no nickel, lower alloy content), but it is still a hardenable medium-carbon steel: preheat, low-hydrogen consumables and usually PWHT. If a part must be welded a lot, ask whether a leaner grade would do.
For fasteners, 42CrMo4 is the classic property-class 10.9/12.9 bolt steel — good preload capacity and fatigue strength when correctly tempered. It also nitrides well if you want a hard, fatigue-resistant surface on a tough core.
Mechanical properties
| Condition | Section | Yield (MPa) |
|---|---|---|
| 42CrMo4 · quenched & tempered | d ≤ 16 mm | 900 |
| 16 < d ≤ 40 mm | 750 | |
| 40 < d ≤ 100 mm | 650 | |
| 100 < d ≤ 160 mm | 550 |
UTS 1100–1300 MPa · Impact ≥ 35 J @ 20 °C (Q&T) · Hardness 325–380 HB (Q&T, ≤16 mm)
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 |
|---|---|---|
| AISI/SAE | 4140 | |
| BS 970 | 708M40 | EN19 — the nearest UK grade; composition close but not identical (check Mn/Mo). |
| JIS | SCM440 | |
| GB | 42CrMo | |
| UNS | G41400 | |
| Werkstoff Nr. | 1.7225 |
Historical designations
| Standard | Grade | Note |
|---|---|---|
| BS 970:1955 | EN19 | The UK designation most engineers still use for the 1 %Cr–Mo grade. |
| BS 970:1970 | 708M40 | 7 = Cr-Mo group, M = wrought, 40 = 0.40 % C. |
| SAE J404 | 4140 | The US grade — effectively interchangeable for most work. |
Common applications
- Shafts, axles and spindles
- Gears, pinions and sprockets
- Crankshafts and connecting rods
- High-tensile bolts and studs (property class 10.9 / 12.9)
- Couplings, arbors and machine components
- Rock drill rods and mining tooling
History
42CrMo4 (AISI 4140, EN19) grew out of the early-20th-century chromium-molybdenum alloy steels developed for automotive and aircraft duty, and has become the world’s default medium-carbon through-hardening engineering steel.
- 1920Cr–Mo alloy steels adopted for automotive shafts and aircraft parts
- 1941SAE 4140 standardised in the US
- 1942EN19 defined in BS 970 (War Emergency)
- 1991EN 10083 harmonises 42CrMo4 (1.7225) across Europe