AISI 4340 Ultra-High-Strength Ni-Cr-Mo Alloy Steel
ASTM A29/A322 / AMS 6415 · Updated: 2026-05-25
AISI 4340 is a nickel-chromium-molybdenum ultra-high-strength alloy steel (0.38-0.43% C, 1.65-2.0% Ni, 0.7-0.9% Cr, 0.2-0.3% Mo) providing the best combination of deep hardenability, high strength, and low-temperature toughness among standard engineering alloy steels. Through-hardens in sections up to 150 mm. In the Q&T condition at 200°C temper, achieves >1,500 MPa tensile with adequate toughness. Standard material for aircraft landing gear struts, helicopter rotor shafts, heavy-duty gear trains, and military armor plate. Vacuum arc remelting (VAR) produces 4340M (E4340) with superior cleanliness for aerospace critical parts.
Quick Facts
| Category | Alloy Steel |
| Standard | ASTM A29/A322 / AMS 6415 |
| Density | 7.85 g/cm³ |
| Yield Strength | 1,240 MPa (180 ksi) Q&T |
| Tensile Strength | 1,470 MPa (213 ksi) Q&T |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN 36CrNiMo4 | Compare |
| DIN 1.6511 | Compare |
| SNCM439 | Compare |
| 40CrNiMoA | Compare |
| BS 817M40 | Compare |
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Frequently Asked Questions
What is the difference between 4340 and 300M?
300M is a modified 4340 with added silicon (1.6%) and vanadium (0.08%) for improved temper resistance and higher strength capability. 300M achieves 1,860-2,070 MPa tensile vs 4340's 1,470-1,720 MPa. 300M is the standard material for commercial aircraft landing gear (Boeing 777, Airbus A380). 4340 is preferred when lower strength but better machinability and lower cost are required.