The two most frequently compared alloy steels in aerospace, defence and energy projects are AISI 4140 and AISI 4340. Both are quenched and tempered steels, both contain chromium and molybdenum, and both are supplied in the same forms. The real difference between them comes down to a single element: nickel.
What nickel does
Looking at the compositions in the Defence Metal datasheets, AISI 4140 is chromium–molybdenum based: roughly 1.00% chromium, 0.25% molybdenum and 0.40% carbon, with no nickel at all. AISI 4340 adds 1.65–2.00% nickel on top of that same chromium–molybdenum backbone.
Nickel increases the steel’s hardenability. That means not only the surface of the part hardens, but the core of the section as well. For a thick shaft, a gear body or a landing gear component, that difference translates directly into the strength of the part. Nickel also raises toughness (impact resistance) after tempering; the part reaches high strength without becoming brittle.
Comparison in numbers
| PROPERTY | AISI 4140 | AISI 4340 |
|---|---|---|
| Carbon (C) | 0.40% | 0.38–0.43% |
| Chromium (Cr) | 1.00% | 0.70–0.90% |
| Molybdenum (Mo) | 0.25% | 0.20–0.30% |
| Nickel (Ni) | — | 1.65–2.00% |
| Quenched + tempered | Rm 930 MPa (50 mm section) | Rm 1793 MPa |
| Yield (Rp0.2) | 770 MPa (50 mm section) | 1496 MPa |
| UNS | G41400 | G43406 |
| AMS | 6349 · 6382 | 6414 · 6415 · 6484 |
Values are taken from our product datasheets. The AISI 4140 values are for a 50 mm section and the AISI 4340 values are for the quenched + tempered condition; section thickness and the heat treatment regime change the result.
Which one, and when?
AISI 4140 is sufficient — medium-section shafts, fasteners, die bodies, hydraulic components, general machine manufacturing. It is easier to source, it machines well, and you do not pay for more alloy than you need.
AISI 4340 is required — when through-hardness to the core is needed in a thick section, when toughness is required together with high strength, and when the specification calls out AMS 6414 / AMS 6415 directly. If those two AMS numbers appear on an aerospace drawing, 4140 is not an acceptable substitute.
A step up
Where even 4340 is not enough, the next step is higher-strength alloys such as 300M and AerMet 100. On the case-hardening (carburising) side of the same family you will find AISI 8620 and AISI 9310.
Technical datasheets
AISI 4140 — chemical composition, mechanical values, AMS equivalents
AISI 4340 — chemical composition, mechanical values, AMS equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.