4140 vs 4340 Steel
4140 and 4340 are both 0.40% carbon chromium-molybdenum steels that quench and temper to similar hardness. The difference is the 1.65 to 2.0% nickel in 4340. Nickel raises hardenability, so 4340 hardens fully through much larger sections, and it raises toughness at a given strength, which is why 4340 shows up in aircraft landing gear, crankshafts and heavy shafts.
For parts under roughly 50 mm (2 in) thick that need 28-45 HRC, 4140 does the job at lower cost and is stocked pre-hardened everywhere. Go to 4340 when the section is thick, the part is fatigue- or impact-critical, or the spec calls for strength above about 1400 MPa with real toughness.
| Property | 4140 | 4340 |
|---|---|---|
| Type | Cr-Mo alloy steel | Ni-Cr-Mo alloy steel |
| Carbon | 0.38-0.43% | 0.38-0.43% |
| Nickel | None specified | 1.65-2.0% |
| Cr / Mo | 0.8-1.1% / 0.15-0.25% | 0.7-0.9% / 0.2-0.3% |
| Yield (annealed) | ~415 MPa (60 ksi) | ~470 MPa (68 ksi) |
| Yield (Q&T, ~30 HRC) | ~900 MPa (130 ksi) | ~950 MPa (138 ksi) |
| Max practical Q&T strength | ~1500 MPa tensile at ~45 HRC | ~1900 MPa tensile at ~52 HRC |
| Hardenability (through-hardens) | To roughly 50-75 mm | To roughly 150 mm and beyond |
| Density | 7.85 g/cm³ | 7.85 g/cm³ |
| Machinability (annealed) | ~65% | ~50% |
| Typical uses | Shafts, gears, bolts, tooling | Landing gear, crankshafts, heavy shafts, high-strength bolts |
Typical values for the stated conditions. Confirm against the mill certificate and your spec.
Need a 4140 or 4340 part made?
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Attach a drawing (PDF or STEP) or describe the part; a U.S. shop that fits quotes it.
Decision matrix
A check marks the side with the edge on that dimension.
| Dimension | 4140 | 4340 |
|---|---|---|
| Machining | ✓ Good annealed and at 28-32 HRC pre-hard | Fair; tougher chips, more tool wear at the same hardness |
| Welding | ✓ Fair; preheat and post-weld temper | Poor; higher hardenability makes the HAZ crack-prone, rarely welded |
| Forming | Fair annealed; hot forging common | Fair annealed; hot forging common |
| Strength | Up to ~1500 MPa tensile Q&T | ✓ Up to ~1900 MPa tensile Q&T with better toughness |
| Corrosion resistance | Poor; rusts, protect it | Poor; rusts, protect it |
| Finishing | Nitrides, plates, blackens, phosphates | Same finishes; hydrogen embrittlement bake required after plating at high strength |
| Material cost | ✓ Baseline alloy steel | About 1.3-1.6x, nickel content |
| Availability | ✓ Bar and plate, annealed and pre-hard, everywhere | Bar common, plate less so; aerospace grades on longer lead |
Choose 4140 when
- Sections under about 50 mm that need 28-45 HRC through the part
- Pre-hardened bar machined to size with no heat treat afterward
- General shafts, gears, fixtures, bolts and hydraulic parts
- Cost, stock availability or some welding are part of the job
Choose 4340 when
- Thick sections that must harden fully to the core
- Fatigue- or impact-critical parts: crankshafts, landing gear, drive shafts, gear blanks
- Strength above about 1400 MPa tensile with acceptable toughness
- Aerospace work calling out AMS 6414 or a vacuum-melted 4340 variant
Frequently asked questions
- What is the difference between 4140 and 4340?
- Nickel. 4340 adds 1.65 to 2.0% nickel to the same carbon-chromium-molybdenum base as 4140. That raises hardenability, so thick parts harden through, and it improves toughness at high strength. In thin sections at moderate hardness the two behave similarly.
- Is 4340 stronger than 4140?
- At the same tempered hardness the strengths are close. 4340 can be tempered to a higher useful strength, about 1900 MPa tensile versus about 1500 MPa for 4140, while keeping toughness. Its advantage grows with section thickness, where 4140 may not harden all the way through.
- Can 4340 be welded?
- Only with difficulty and usually avoided. Its high hardenability means the heat-affected zone hardens and cracks unless you preheat, control interpass temperature and temper after welding. 4140 is easier but still needs preheat and post-heat. Design 4340 parts to be machined, bolted or forged, not welded.
- Which is easier to machine?
- 4140. In the annealed state its machinability rating is around 65% of 1212 steel against roughly 50% for 4340. Both are routinely machined pre-hardened at 28-32 HRC, but 4340 wears tools faster and demands more rigid setups.
