4340 Alloy Steel

4340 is a nickel chromium molybdenum high strength low alloy steel conforming to AMS 6414 (vacuum arc remelted) and AMS 6415 (air melt). One of the most widely used structural steels for aerospace and defence industries, 4340 gives an outstanding combination of high strength, adequate toughness and deep hardenability making it possible to use the steel for large section parts where it is necessary to have uniform mechanical properties through the thickness.

Strength is achieved using quench and temper heat treatment resulting in tensile strength of 1800-2100 MPa depending on the tempering temperature. Nickel (1.65-2.00%) gives toughness and impact resistance, chromium (0.70-0.90%) contributes hardenability and moderate corrosion resistance, and molybdenum (0.20-0.30%) gives increased strength and temper brittleness resistance. It is this addition of alloying elements that enables 4340 to obtain uniform mechanical properties in sections above 75mm thickness.

4340 is available in two different production processes that are covered by different AMS specifications. Air melt 4340 that conforms to AMS 6415 is used for general aerospace applications. Vacuum arc remelted (VAR) 4340 that conforms to AMS 6414 is characterized by improved purity and consistency and is suitable for demanding applications where any defects in the material can result in serious consequences like aircraft landing gear and structural fittings.

Alloy 4340 is the base alloy for 300M — the alloy with higher silicon and vanadium content, higher carbon and molybdenum content results in increased tensile strength.