Drawing No. EH–MT–003 // Engineering Materials
AISI 4340 Alloy Steel Properties
Reviewed September 2026
AISI 4340 is a nickel-chromium-molybdenum steel valued for high hardenability, toughness and fatigue resistance. The reference is explicitly tied to an oil-quenched, 650°C-tempered condition.
Representative properties
The stated basis, temperature and direction are part of the value. Switch units without changing the physical reference.
What changes these properties?
Engineering variability
- Lower tempering temperatures can raise strength substantially while reducing ductility/toughness balance.
- Low-temperature mechanical properties can differ strongly from room-temperature behavior.
- CTE and thermal values in compiled datasets may represent nearby heat-treatment conditions or typical 4000-series values.
- Use specification-specific fracture toughness, fatigue and cleanliness requirements for critical aerospace/pressure applications.
Selection note
Common for highly loaded gears, shafts, landing-gear components, pressure parts and fatigue-critical machinery where high hardenability and toughness are required.
Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.
AISI 4340 in practice
The high-toughness ultra-high-strength steel used where a failure would be unacceptable and section sizes are large.
Where it is specified
4340 (EN 34CrNiMo6, 1.6582) is the grade for aircraft landing gear, helicopter rotor shafts, heavy-duty crankshafts, gun barrels, press and forging machinery, and large power transmission shafts. It is specified where 1200–1800 MPa tensile strength must be combined with genuine fracture toughness.
Its nickel-chromium-molybdenum composition gives exceptional hardenability — sections of 150 mm and beyond can be through-hardened, which is what distinguishes it from 4140.
Chosen over what
Against 4140: substantially better hardenability and toughness at equal hardness, for perhaps twice the material cost. If 4140 will do the job, use it; 4340 is specified when section size or toughness demands it.
Against maraging steel or 300M: 4340 is far cheaper and more available. 300M is essentially 4340 with silicon and vanadium added for higher strength, and is the usual step up for landing gear.
Against titanium: 4340 is three times stiffer per unit volume and much cheaper, but 1.7 times denser and corrodes. For landing gear the density penalty is accepted because stiffness and toughness dominate.
How it fails
Hydrogen embrittlement is the dominant concern and more severe than in 4140 because the operating strength is higher. At 1500 MPa, a few parts per million of dissolved hydrogen can cause delayed brittle fracture. Plating requires immediate post-plate baking, typically 190–220 °C for 8–24 hours by section.
Stress-corrosion cracking in chloride environments at high strength levels, with threshold intensity falling as strength rises — which is why high-strength steel parts are so carefully protected and inspected.
Fatigue from surface condition. At these strengths fatigue life is dominated by surface finish and residual stress; shot peening is routinely specified and is not optional on cyclically loaded parts.
Quench distortion and cracking in complex geometries, managed with martempering or interrupted quenching.
Fabrication notes
Machinability roughly 50–60% of B1112 annealed, poorer than 4140. Rough machine before hardening and finish grind afterwards wherever possible.
Welding is generally avoided. Carbon equivalent approaches 0.9. Where welding is unavoidable, it demands preheat above 300 °C, strict hydrogen control, and full re-austenitising and tempering afterwards. Many aerospace specifications simply prohibit welding on 4340 components.
Heat treatment must be tightly controlled. Double tempering is common, and the 250–400 °C range is avoided because of tempered martensite embrittlement. Vacuum or controlled-atmosphere treatment prevents decarburisation, which would otherwise ruin fatigue performance.
Specifications and standards
SAE AMS 6414 and 6415 for aerospace bar and forgings; ASTM A29 and SAE J404 for chemistry; EN 10083-3 for 34CrNiMo6; MIL-S-5000 for aircraft-quality steel; ASTM F519 for hydrogen embrittlement testing of plating processes.
References
ASM Handbook Vol. 1 and Vol. 19 (Fatigue and Fracture); Boyer, Atlas of Fatigue Curves; MMPDS (formerly MIL-HDBK-5) for design allowables.
How to interpret this material
Stiffness
Young’s modulus describes elastic stiffness. For crystals and composites, orientation can make one scalar modulus only a directional reference.
Strength / failure
Ductile metals can use yield and tensile strength. Brittle ceramics/semiconductors and anisotropic composites require fracture, flexural or directional strength descriptions instead.
Thermal response
Conductivity, heat capacity and expansion are temperature dependent. The last column is a melting or transition descriptor only where that physical concept is appropriate.
Technical sources
The database favors manufacturer, industry, government or academic technical references and labels secondary condition-specific datasets when a primary source does not expose a complete property set.