Material Overview
4340 steel (also referenced as E4340, 4340V, and 4340AQ) is a chrome nickel-molybdenum alloy steel (UNS G43406) that is highly versatile and widely used for high-stress parts across aerospace, defense, automotive, oil & gas, and heavy industrial applications. It exhibits exceptional properties including high fatigue strength in large cross sections and excellent transverse ductility and toughness at high strength levels. These characteristics make it a preferred material for demanding applications where strength, durability, and resistance to wear and impact are essential.
4340 steel is available via two primary melt methods: VAR (Vacuum Arc Remelted / CEVM – Consumable Electrode Vacuum Melted) and Electric Furnace with Vacuum Degassing (VD). The VAR process produces superior cleanliness, reduced non-metallic inclusions, and enhanced fatigue performance—making it the preferred choice for the most demanding aerospace and defense applications such as aircraft landing gear and primary structural components. Aero-Vac stocks both standard and VAR grades in multiple forms with full traceability documentation.
Available Product Forms
All product forms are available to the applicable AMS specification (AMS 6414, AMS 6415, AMS 6484, AMS 6409, AMS-S-5000, BMS7-28, SS 9702). For plate and sheet, AMS 6454 (VAR plate) and AMS 6359 apply. Custom sizes, lengths, and pre-machined forms are available upon request. Contact our materials specialists for non-standard dimensions.
Nominal Chemical Composition
| Element |
Symbol |
Range % |
Notes |
| Carbon |
C |
0.38–0.43% |
Primary hardening element |
| Manganese |
Mn |
0.60–0.80% |
Hardenability, strength |
| Silicon |
Si |
0.15–0.30% |
Deoxidizer |
| Nickel |
Ni |
1.65–2.00% |
Toughness and impact resistance |
| Chromium |
Cr |
0.70–0.90% |
Hardenability, wear resistance |
| Molybdenum |
Mo |
0.20–0.30% |
High-temperature strength, hardenability |
| Phosphorus |
P |
0.035% max |
Residual element |
| Sulfur |
S |
0.040% max |
Residual element |
| Iron |
Fe |
Balance |
Base metal |
Chemical composition ranges are nominal. Consult the current issue of the applicable specification (AMS 6414, AMS 6415, AMS 6484, or AMS 6409) for precise minimum/maximum requirements. Aero-Vac provides full mill test reports (MTRs) with certified chemistry for every order.
Key Properties & Specification Details
| Property / Attribute |
Details |
| Primary Specifications |
AMS 6414, AMS 6415, AMS 6484, AMS 6409, AMS-S-5000, BMS7-28, SS 9702 |
| UNS Number |
G43406 |
| Also Known As |
E4340, 4340V, 4340AQ, 4340 VAR |
| Issuing Organization |
SAE International (AMS specs), Boeing (BMS7-28) |
| Material Type |
Chrome-Nickel-Molybdenum Alloy Steel |
| Melt Methods |
VAR (CEVM – Consumable Electrode Vacuum Melted) / Electric Furnace with VD |
| Available Forms |
Round Bar, Flat Bar, Rectangular Bar, Forged Bar, Forged Block, Machine Ready Blanks, Machined Block, Forgings, Rings, Forged Disks, Hollows, Plate and Sheet |
| Tensile Strength |
Up to 280,000 psi (depending on heat treatment condition) |
| Hardness |
Up to Rc 60 (surface hardened via nitriding or flame/induction hardening) |
| Fatigue Strength |
High – excellent fatigue performance in large cross sections |
| Machinability |
Best in annealed or normalized & tempered condition; reduced in high-strength states |
| Weldability |
Fair – preheat to 200–300°C required; not recommended in hardened/tempered condition |
| Certification Available |
AS9100D / ISO 9001, Certificate of Conformance, MTR |
Typical Applications
Aircraft Landing Gear: Its high strength and toughness make 4340 steel ideal for withstanding the extreme stresses and loads experienced during takeoff, landing, and ground operations. It is one of the most widely specified materials for aerospace landing gear components.
Automotive Systems: Where strength, durability, and impact resistance are crucial, 4340 steel is employed in components such as crankshafts, axle shafts, gears, and connecting rods in high-performance and heavy-duty automotive applications.
Oil and Gas Drilling: 4340 steel finds widespread application in the oil and gas industry, particularly in drilling equipment and tools, due to its high strength and resistance to wear and fatigue in harsh downhole environments.
Forging & Machine Building: Utilized in the production of forged components such as shafts, gears, and other critical parts requiring high strength and durability. Also used in machine frames, chucks, molds, heavy-duty pins, and couplings.
Structural & Power Transmission: Construction of heavy-duty structures including bridges, cranes, and industrial equipment. Well-suited for power transmission gears, shafts, and components that transmit power and torque under high loads and harsh conditions.
Key Specifications & Similar Grades