4140 and SCM440 properties
A chromium-molybdenum low-alloy steel at roughly 0.40 % carbon. The chromium and molybdenum give it hardenability, so it hardens through a thick section where plain carbon steel won't.
- TypeCr-Mo low-alloy steel, roughly 0.38–0.43 % C with about 1 % Cr and 0.2 % Mo
- StandardsJIS G4105 SCM440 · AISI / SAE 4140 · UNS G41400 · EN 42CrMo4 (1.7225) · GB 42CrMo
- Density7.85 g/cm³
- AnnealedAround 197 HB. The softest supply condition, used where the part is hardened afterwards
- Pre-hardSupplied quenched and tempered, commonly 28–34 HRC. Machined to finish with no further heat treatment
- HT on printsUS prints often write only HT. On 4140 that usually means quench and temper to 28–34 HRC
- TensileAround 950–1080 MPa at 28–32 HRC, rising as the tempering temperature falls
- NitridingResponds well, giving a hard case of roughly 50–58 HRC with almost no distortion
- CorrosionNone. It rusts like any alloy steel and needs a coating or oil for export
- CertificateMill test certificate to EN 10204 3.1 on request
Pre-hard or harden afterwards is the real decision. Pre-hardened bar at 28–34 HRC saves a heat treatment cycle and avoids quench distortion, and the part finishes to tolerance in one route. Machining soft and hardening afterwards gets you higher hardness, but the part moves in the quench and needs grinding to bring it back. Tell us which one the design needs, and we quote that route.
4140 and SCM440 grade equivalents
| Grade | JIS (Japan) | US (ASTM / AISI / UNS) | GB/T (China) | EN / DIN (Europe) | Cross-reference |
|---|---|---|---|---|---|
| 4140 the AISI designation | SCM440 (JIS G4105) | 4140 · UNS G41400 · ASTM A29 | 42CrMo (GB/T 3077) | 42CrMo4 · EN 1.7225 | Standard equivalent |
| 4140H guaranteed hardenability | SCM440H | 4140H · UNS H41400 | 42CrMoH | 42CrMo4 with H band | Standard equivalent |
| 4340 nickel added, deeper hardening | SNCM439 (JIS G4103) | 4340 · UNS G43400 | 40CrNiMoA | 34CrNiMo6 · EN 1.6582 | Check the MTC |
| S45C plain carbon, cheaper | S45C (JIS G4051) | 1045 · UNS G10450 | 45 | C45 · EN 1.0503 | Standard equivalent |
Designations taken from JIS G4105 and G4103, ASTM A29, GB/T 3077 and EN 10083-3. SCM440, 4140 and 42CrMo4 cross-reference closely enough for most work. The molybdenum bands differ slightly, and when a drawing is written to a specific standard with a hardenability requirement we buy to that standard, not the nearest local equivalent. 4340 is a different, more heavily alloyed steel. Last reviewed 13 September 2026.
When 4140 is the right steel
Loaded shafts, spindles and couplings
Any part that carries torque or bending where the section is too thick for plain carbon steel to harden through. That's the main job 4140 does.
Fixture bodies and tooling that must stay stiff
Pre-hardened 4140 at 28–34 HRC machines to a good finish, holds a thread, resists denting and needs no heat treatment afterwards. For jigs and die shoes it often comes out cheaper overall than a soft steel that gets hardened later.
Parts that will be nitrided
4140 nitrides well. You get a hard wear case of roughly 50–58 HRC and almost no dimensional movement, so when a part needs a hard surface and cannot take distortion, nitriding usually beats quench hardening.
Not where hardness above about 45 HRC is needed everywhere
4140 can be tempered harder, but toughness drops off quickly and the part gets brittle. Above that range you want a tool steel.
Not for corrosion service
Its chromium content is nowhere near stainless, and 4140 rusts. If a part is both loaded and exposed, the drawing needs to specify a coating, or the material should be 17-4PH.
Expect movement if it's quenched after machining
A long or asymmetric part will distort in the quench, and you cannot always predict by how much. When a part is hardened after machining we leave grinding stock on the critical features and grind them afterwards. That grinding is in the quotation, and it's a large part of why pre-hard often works out cheaper.
Machining process
Cutting 4140 is routine. The planning is where the attention goes, mostly around what heat treatment is going to do to the part.
- Route APre-hardened bar at 28–34 HRC · machine to finish · no heat treatment · ship
- Route BAnnealed bar · rough · quench and temper · grind critical features to size
- TurningMori Seiki and Okuma lathes to Ø 310 × 800 mm, at ±0.005 mm typical
- MillingDoosan, Haas, Hision and FANUC RoboDrill machining centres, 3, 4 and 5-axis
- GrindingOkamoto and Chevalier surface and NC grinding to 500 × 200 × 250 mm, ±0.005 mm typical
- Wire EDMSodick wire cut for hardened features and sharp internal corners, to ±0.003 mm
- ToolingRigid setups and coated carbide. From 34 HRC up, feed and depth come down, cycle time goes up, and the price shows it
- HardnessEvery heat treated batch checked on our Mitutoyo QMH 14BX before packing, and the reading reported
Typical figures from normal production, not a blanket specification. On Route B they apply to the features ground after heat treatment, so mark those features on the drawing and we'll build the sequence around them. General tolerance follows ISO 2768 fine or medium class, whichever your print calls out.
Hardness and protection
- Q and TQuench and temper, commonly to 28–34 HRC, higher on request. Hardness checked before shipping
- NitridingFor a hard case with minimal distortion where the drawing calls for it
- InductionInduction hardening of a running band where only part of the surface needs to be hard
- Black oxideMild protection and negligible dimensional change, but it needs oil to stay effective
- Zinc platingClear, blue, yellow, black chromate and zinc-nickel to ASTM B633 / ISO 4042
- ChromeThin dense chrome, where wear resistance is needed on a finished dimension
- PackingVCI paper and film as standard on unplated steel, ISPM-15 heat-treated pallets for sea freight
Plating a hardened part needs a bake after it. Zinc plating a part above roughly 32 HRC carries a hydrogen embrittlement risk, and the drawing should call for a post-plate bake. If the bake is missing we raise it with you before the part goes to plating. Full finishing and heat treatment list →
Stock and lead time
- Pre-hard barRound bar at 28–34 HRC available from Vietnamese stockists in common diameters
- Annealed barAvailable where the part is hardened after machining
- PlateCommon thicknesses available; thick and oversized plate confirmed per job
- QuoteWithin 24 business hours of a complete RFQ
- Lead timeStandard 14 days after PO on Route A. Route B adds a heat treatment cycle plus grinding
- CertificateEN 10204 3.1 mill certificate on request, plus our own hardness reading on heat treated batches
If a drawing names 42CrMo4 or SCM440 specifically instead of 4140, tell us. The grades cross-reference closely, but they're written to different standards. When your quality system needs the certificate to name the standard on your print, we buy the material differently. Stock and section size are confirmed at quotation.
Common questions
Is SCM440 the same as 4140?
Close enough for most work, and they cross-reference as standard equivalents. SCM440 (JIS G4105), 4140 (AISI) and 42CrMo4 (EN 1.7225) differ slightly in the molybdenum band. If your drawing is written to one standard and your quality system needs the certificate to match, tell us and we buy to that standard.
What does HT mean on my drawing?
On 4140 it almost always means quench and temper, which in practice is 28–34 HRC unless a range is given. If the hardness matters, put the range on the drawing and do not rely on the abbreviation.
Should I machine from pre-hardened bar or harden afterwards?
Pre-hard, if 28–34 HRC is enough. It's one route, there's no quench distortion, and it's usually cheaper. If you need more hardness, harden afterwards and expect grinding on the critical features to bring the part back to size.
How much will the part distort in the quench?
On anything long or asymmetric, enough to matter, and you cannot always predict it. That's why we leave grinding stock on critical features when a part is hardened after machining.
Can 4140 be nitrided?
Yes, it nitrides well. You get roughly 50–58 HRC in a thin case with almost no dimensional change, so it suits parts that cannot tolerate quench distortion.
Can you plate hardened 4140?
Yes. One condition: zinc plating above about 32 HRC needs a post-plate bake against hydrogen embrittlement. Put the bake on the drawing.
Do you check the hardness before shipping?
Yes, every heat treated batch, on our Mitutoyo QMH 14BX. The reading goes out with the inspection documents.
Related information
S45C / 1045 carbon steel
The cheaper carbon steel, for sections thin enough that plain carbon does the job.
17-4PH stainless steel
When the loaded part also has to resist corrosion, and still needs hardness.
Finishing and heat treatment
Quench and temper, nitriding, induction hardening and plating, with the conditions we specify for each.