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4340 is a heat treatable, low alloy steel containing nickel, chromium and molybdenum. It is known for its toughness and capability of developing high strength in the heat treated condition while retaining good fatigue strength. 4340 has a favorable response to heat treatment (usually oil quenching followed by tempering) and exhibits a good combination of ductility and strength when treated thusly.

Typical applications are for structural use, such as aircraft landing gear, power transmission gears and shafts, rocket motor cases, ordnance hardware, aircraft structural components, piston pins, bearings, ordnance, dies, transmission gears, components, fasteners, and pressure vessels.

Chemical Properties

Element min max
Carbon 0.38 0.43
Manganese 0.65 0.85
Silicon 0.15 0.35
Phosphorus 0.025
Sulfur 0.025
Chromium 0.70 0.90
Nickel 1.65 2.00
Molybdenum 0.20 0.30
Copper 0.35

Mechanical Properties, Normalized (AMS 6414)

Property Value
Tensile Strength 260 ksi
Yield Strength at 0.2% Offset 217 ksi
Elongation in 4D 10%
Reduction of Area 30%