4140 vs 4340: differences explained
4140 vs 4340: chromium-molybdenum against nickel-chromium-molybdenum — the same carbon, and nickel buys depth of hardening and toughness.
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Overview
Both carry 0.40 % carbon, both are quenched and tempered, and both are bought for shafts and highly stressed parts. The difference is a single alloying element with a large consequence.
4140 is chromium-molybdenum: around 1 % chromium and 0.2 % molybdenum, the workhorse engineering steel of every machine shop, stocked in every size and priced as a commodity.
4340 adds 1.65–2.00 % nickel on top of a similar chromium-molybdenum base. Nickel does two things that nothing else in this price bracket does: it pushes hardenability deeper into the section, and it raises toughness at a given strength — including at low temperature, where 4140 starts to lose it.
The practical question is therefore almost always about section size. In a 40 mm shaft the two behave close enough that 4140 wins on price. In a 150 mm shaft, 4140 does not harden to the core and 4340 does, and no amount of tempering closes that gap.
4140 vs 4340 side by side
| Property | 4140 | 4340 |
|---|---|---|
| Standard | SAE J404 / ASTM A29 (alloy) | SAE J404 / ASTM A29 (alloy) |
| Type | Chromium-molybdenum | Nickel-chromium-molybdenum |
| Carbon | 0.38–0.43 % | 0.38–0.43 % |
| Nickel | Not specified | 1.65–2.00 % |
| Chromium | 0.80–1.10 % | 0.70–0.90 % |
| Molybdenum | 0.15–0.25 % | 0.20–0.30 % |
| Manganese | 0.75–1.00 % | 0.60–0.80 % |
| EN equivalent | 42CrMo4 / 1.7225 | 34CrNiMo6 / 1.6582 (near) |
| Tensile, Q&T typical | 850–1 000 MPa | 1 000–1 300 MPa |
| Yield, Q&T typical | 700–900 MPa | 900–1 200 MPa |
| Hardness range, Q&T | 28–34 HRC typical, up to ~50 HRC | 30–40 HRC typical, up to ~53 HRC |
| Through-hardening section | Roughly to 65–75 mm | Roughly to 150 mm and beyond |
| Toughness at strength | Good | Better — the reason to pay for it |
| Low-temperature toughness | Falls away | Retained; nickel is what does it |
| Weldability | Poor without preheat and PWHT | Poorer still; higher carbon equivalent |
| Machinability | Good in the annealed condition | Slightly harder work at the same hardness |
| Nitriding | Excellent; the standard nitriding base | Also good |
| Availability | Stocked everywhere, every form | Order item in most sizes outside aerospace supply |
| Typical uses | Shafts, gears, spindles, tooling, hydraulic parts | Landing gear, heavy crankshafts, high-integrity shafts, large gears |
| Typical price | Commodity alloy steel | Roughly 1.5–2× 4140, moving with nickel |
Which should you choose?
4140 until the section is too big for it, then 4340. For parts up to roughly 65–75 mm ruling section, 4140 reaches the required properties through the wall and costs a fraction as much; it is also stocked, which for a repair or a one-off shaft matters more than a data sheet.
Move to 4340 when the section is heavier than that, when the design needs high strength *and* toughness together rather than a trade between them, when the part will be loaded at low temperature, or when a failure would be expensive enough to justify the metallurgy. Aerospace and heavy machinery specify it for exactly those reasons.
Neither is a welding grade. If the part has to be welded, that decision should push the design toward a quenched and tempered structural plate such as S690QL instead.
- Shafts, spindles and gears up to roughly 65–75 mm ruling section
- Anything that has to be bought off the shelf tomorrow
- Nitrided components — it is the classic nitriding base
- Tooling, dies and holders where hardness matters more than toughness
- Cost-sensitive production where 4340's properties are not being used
- Heavy sections that must be hardened through, beyond about 100 mm
- Parts needing high strength and high toughness at the same time
- Low-temperature service, where 4140's toughness falls away
- Fatigue-critical and safety-critical components — landing gear, crankshafts, large gears
- Applications where a failure costs far more than the material premium
Frequently asked questions
What is the difference between 4140 and 4340 steel?
Nickel. Both carry about 0.40 % carbon with chromium and molybdenum, but 4340 adds 1.65–2.00 % nickel, which deepens hardenability and raises toughness at a given strength. The chemistry difference translates almost entirely into how large a section can be hardened through.
Is 4340 stronger than 4140?
In practice yes, though the headline reason is depth rather than peak strength. Quenched and tempered, 4140 typically runs 850–1 000 MPa tensile and 4340 1 000–1 300 MPa — but the decisive difference is that 4340 reaches those numbers at the centre of a heavy section where 4140 does not.
Which should be used for a shaft?
4140 up to roughly 65–75 mm diameter, where it hardens through and costs far less; 4340 above that, and for any shaft that is fatigue-critical or works at low temperature. Section size decides this more often than load does.
What is the European equivalent of 4140 and 4340?
4140 corresponds to 42CrMo4 (1.7225) and 4340 to 34CrNiMo6 (1.6582), the second being a near equivalence rather than an identity — 34CrNiMo6 runs slightly lower carbon and higher chromium. Compare the certificate rather than assuming the substitution.
Can 4140 and 4340 be welded?
Both can be, neither should be casually. They need preheat, controlled interpass temperature, low-hydrogen consumables and post-weld heat treatment, and 4340 is the more difficult of the two because of its higher carbon equivalent. A design that requires welding is usually better served by a quenched and tempered structural grade.
Why is 4340 so much more expensive?
Nickel at 1.65–2.00 %, and a smaller, more specialised market. The price runs roughly 1.5 to 2 times 4140 and moves with the nickel market, which is why the substitution question is asked so often — and why the honest answer is to check the ruling section before paying it.
Related comparisons
Verified against SAE J404 · ASTM A29/A29M. Last checked: November 2026.