AISI 4340 Steel (UNS G43400)


4340 alloy steel is a medium carbon nickel-chromium molybdenum alloy steel that creates an excellent combination of ductility, toughness, and strength after heat treating.  It has good shock and impact resistance as well as wear and abrasion resistance in the hardened condition.  It is often utilized where other alloy steels do not have adequate hardenability to attain high strength levels.  Typical applications for 4340 bar include heavy duty shafting, machine tool arbors, trunnion shafts, axle shafts, and mine drilling components.

Chemical Composition

Chemical Composition (wt%) limits of AISI 4340 per ASTM A29*

Carbon 0.38-0.43 Chromium 0.70-0.90
Manganese 0.60-0.80 Molybdenum 0.20-0.30
Silicon 0.15-0.35 Phosphorus 0.035
Nickel 1.65-2.00 Sulfur 0.040

*Maximum, unless range is indicated

Physical Properties

Physical properties for AISI 4340 Steel

Density, lb/in3 0.284
Modulus of Elasticity, psi 29 x 106
Coefficient of Thermal Expansion, 68-212˚F, /˚F 6.83 x 10-6
Thermal Conductivity, Btu/ft hr ˚F 30.0
Specific Heat, Btu/lb ˚F 0.114
Electrical Resistivity, Microhm-in 6.42


Typical standards for AISI 4340 Steel