AISI 8740 and AISI 4340 are both low-alloy steels used in the quenched and tempered condition — neither is stainless. Their carbon contents are the same (0.38–0.43 %). The difference lies in the quantity of alloying elements: 4340 contains far more nickel and chromium.
The difference in composition
8740 is an 87xx series steel with modest chromium, nickel and molybdenum. 4340 raises nickel by more than threefold and chromium by roughly half again. Molybdenum is at the same level in both.
The practical consequence is hardenability. Earle M. Jorgensen states that 4340 has “much deeper hardenability than the 4100 series”; voestalpine BÖHLER calls its hardenability good, with higher tensile and yield strength than 4140. Service Steel Aerospace reports through-hardening up to about 89 mm (3.5 inches) diameter for 4340.
Compared in figures
The values are cast analysis limits.
| PROPERTY | AISI 8740 | AISI 4340 |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 % | 0.38 – 0.43 % |
| Manganese (Mn) | 0.75 – 1.00 % | 0.60 – 0.80 % |
| Chromium (Cr) | 0.40 – 0.60 % | 0.70 – 0.90 % |
| Nickel (Ni) | 0.40 – 0.70 % | 1.65 – 2.00 % |
| Molybdenum (Mo) | 0.20 – 0.30 % | 0.20 – 0.30 % |
| Silicon (Si) | 0.15 – 0.35 % | 0.15 – 0.35 % |
| Phosphorus (P) | 0.035 % max | — |
| Sulphur (S) | 0.040 % max | — |
| Aerospace specification | AMS 6322 | AMS 6414 (VAR) / AMS 6415 (air melt) |
| Stainless? | no | no |
Note on molybdenum: TimkenSteel/Metallus publishes 0.15–0.25 % for 8740, while Aircraft Materials, AMS Resources, MW Components and Titanium Industries give 0.20–0.30 %. The majority agree on the second figure, but we note the difference.
The mechanical properties of AISI 8740
Publishing a single “mechanical properties table” for 8740 would be wrong: the values depend on the supply condition, and even in the normalized condition the sources do not agree with each other. What could be found, with attribution:
| NORMALIZED CONDITION | VALUE | SOURCE |
|---|---|---|
| Hardness | 269 HB / 27 HRC | MW Components, Alloys International, Titanium Industries |
| Elongation | 16 % | MW Components, Alloys International, Titanium Industries |
| Modulus of elasticity | 205 GPa (29,700 ksi) | MW Components, Alloys International, Titanium Industries |
| Tensile strength (Rm) | 930 MPa (135,000 psi) | MW Components (normalized at 870 °C, 13 mm round) |
| Tensile strength (Rm) | 938 MPa | Alloys International |
| Tensile strength (Rm) | 855 MPa (120 ksi) | ASTM A519 (tubing, “typical”) |
| Proof strength (Rp0.2) | 550 MPa (79,800 psi) | MW Components |
| Proof strength (Rp0.2) | 620 MPa | Alloys International |
| Proof strength (Rp0.2) | 621 MPa (90 ksi) | ASTM A519 (tubing, “typical”) |
The 70 MPa gap in proof strength is real and cannot be averaged away. If you are designing, state which source you are working to, or ask for the mill certificate of the actual heat.
Quenched and tempered condition
The only independent and complete measurement set we could find is NASA’s 1973 report TM X-64791 (J. W. Montano). Bar of approximately 25 mm diameter, oil quenched from 899 °C:
| TEMPERING | Hardness | Rm | Rp0.2 | A (50 mm) | Reduction of area |
|---|---|---|---|---|---|
| 635 °C (1175 °F) | 28 – 29 HRC | 906 MPa (131.4 ksi) | 822 MPa (119.2 ksi) | 23.0 % | 65.1 % |
| 454 °C (850 °F) | 40 HRC | 1271 MPa (184.3 ksi) | 1205 MPa (174.8 ksi) | 16.7 % | 57.5 % |
Source: NASA TM X-64791. Beyond these two points no multi-temperature tempering curve could be verified; the curves circulating online all trace back to a single data aggregator.
Heat treatment
The quench medium is oil. The only complete cycle found comes from SECO/WARWICK’s Heat Treating Data Book: normalizing 816–871 °C, annealing 816–871 °C, hardening 829–857 °C, tempering “to desired hardness”. Note: NASA austenitised the same steel from 899 °C; both figures should be quoted with their source and not averaged.
4340 is typically oil quenched from about 800 to 860 °C and tempered between 204 and 649 °C according to the strength level required; voestalpine BÖHLER recommends 540–680 °C for general engineering use.
8740H: the hardenability-guaranteed version
8740 also has an H version (UNS H87400) for which the hardenability band is guaranteed, covered by ASTM A304 and SAE J1268. Its composition limits differ slightly from standard 8740:
| ELEMENT | 8740H (ASTM A304 / SAE J1268) |
|---|---|
| Carbon (C) | 0.37 – 0.44 % |
| Manganese (Mn) | 0.70 – 1.05 % |
| Silicon (Si) | 0.15 – 0.35 % |
| Nickel (Ni) | 0.35 – 0.75 % |
| Chromium (Cr) | 0.35 – 0.65 % |
| Molybdenum (Mo) | 0.20 – 0.30 % |
The numerical Jominy values of the hardenability band are given only as a graph in both standards; no numerical table is published. Do not trust the 8740H Jominy tables you will find online.
Strength classes in aerospace bolting
This is what makes 8740 important. It is used at three standard strength classes:
| CLASS | Minimum tensile strength | Standard | Hardness |
|---|---|---|---|
| 125 ksi | 862 MPa (125,000 psi) | AN bolts (Jet-Tek, Aircraft Spruce) | — |
| 160 ksi | 1103 MPa (160,000 psi) | NAS6203 – NAS6220 | 36 – 40 HRC (Military Fasteners) |
| 180 ksi | 1241 MPa (180,000 psi) | NAS624 – NAS644 (twelve point) | 39 – 43 HRC (FastenerDimensions) |
The text of NAS6203–6220 lists the permitted materials as 4140 (AMS 6349), 4340 (AMS 6415 / 6484) and 8740 (AMS 6322 or AMS 6325/6327), with heat treatment to AMS 2759. SPS Technologies gives the 180 ksi class in AISI 8740 in its own catalogue, with a maximum operating temperature of 232 °C (450 °F).
We do not publish numerical mechanical requirements for AMS 6322.
SAE’s full text is paywalled, and the three sets of “AMS 6322 values” circulating online contradict each other; in one of them the hardness and tensile strength are metallurgically incompatible. If you need the specification values, the current edition of SAE AMS 6322 must be purchased.
How to read this table. Rows without a source name have been confirmed by at least four independent organisations. Rows with an organisation named in brackets were found in fewer sources and are therefore given with attribution. No figure we could not verify has been published.
What to watch when ordering — the most common error
AMS 6349 is not 8740 or 4340 — it is 4140.
The title of AMS 6349 is “Steel Bars, 0.95Cr – 0.20Mo (0.38 to 0.43C) (SAE 4140), Normalized”. That specification has nothing to do with either material in this pair. The aerospace specification for 8740 is AMS 6322.
The difference between AMS 6414 and AMS 6415 is melting practice, not chemistry.
Both are SAE 4340. AMS 6414 is vacuum arc remelted (VAR/CEVM) and AMS 6415 is air melted. Supplying 6415 where 6414 is called out is a non-conformance — and that single line on the certificate is what distinguishes them.
Two metallurgical warnings
Hydrogen embrittlement. When these high-strength steels are plated (cadmium, zinc-nickel) they pick up hydrogen and can embrittle. Embrittlement relief baking is governed by SAE AMS 2759/9. Do not use the temperatures and times circulating online; consult the standard itself.
Temper embrittlement. voestalpine states that the molybdenum addition protects 4340 from classic temper embrittlement. Separately, the literature describes a tempered martensite embrittlement specific to 4340 in roughly the 250–400 °C tempering region. These are two different phenomena and should not be conflated.
Neither is stainless
Their chromium contents are between 0.40 and 0.90 %. EN 10088-1 requires at least 10.5 % chromium for a steel to be called stainless. Both materials therefore cannot be used without a protective coating.
Where are they used?
4340: heavy-duty and aircraft shafting, crankshafts, axle shafts, gears, landing gear components, high-tensile bolts and studs, downhole tools, dies.
8740: axles, tool joints, drill collars, piston rods, aircraft engine bolts.
Designations, standards and AMS equivalents
The same steel is called different things depending on which system you are speaking in. Most ordering and certification confusion starts here, so a short summary:
- AISI — the American Iron and Steel Institute’s grade naming (410, 316L and so on). It is the most widely used name in everyday use, but on its own it is not a purchasing specification: composition and mechanical requirements are defined in the ASTM standards.
- UNS — the Unified Numbering System operated jointly by ASTM and SAE (S41000 and so on). It is the number used for ordering and certification in North America, and it is more precise than the AISI name.
- W.Nr / EN — the European material number (1.4006 and so on) and EN name (X12Cr13 and so on). This is the governing designation in Europe. It is usually only an approximate equivalent of the AISI grade; carbon, sulphur or molybdenum limits frequently differ.
- AMS — SAE’s Aerospace Material Specifications. An AMS specification binds not only the composition but also the melting practice, the product form, the heat treatment condition and the inspection requirements. In aerospace and defence orders this is the designation that governs.
| DESIGNATION | AISI 8740 | AISI 4340 |
|---|---|---|
| AISI / trade name | AISI 8740 | AISI 4340 |
| EN material no. (W.Nr) | — | — |
| UNS number | — | G43406 |
| AMS specifications | AMS 6322 · AMS 6325 | AMS 6414 · AMS 6415 |
| Product page | AISI 8740 technical page | AISI 4340 technical page |
Two reminders. First: an AMS number is specific to a product form and condition. The same grade may have separate AMS specifications for bar, sheet and forgings, so establish which product form you need before calling up an AMS number.
Second: the AISI name, material number, UNS and AMS data in the table above are the designations shown on Defence Metal’s product pages. If you are working to a different edition of a standard or to a different product form, state the material number and the specification edition explicitly on your order.