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Alloy Steel 4340

4340 - Highly alloyed steel, nominally 1.8% Ni, .80% chromium and .25% molybdenum. High strength characteristics. Used for heavily stressed parts operating in fatigue and other duty conditions.

AISI 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. Uses include piston pins, bearings, ordnance, gears, dies, and pressure vessels.

AISI 4340 is a Standard grade Alloy Steel. It is commonly called AISI 4340 Nickel-chromium-molybdenum steel. It is composed of (in weight percentage) 0.38-0.43% Carbon (C), 0.60-0.80% Manganese (Mn), 0.035%(max) Phosphorus (P), 0.04%(max) Sulfur (S), 0.15-0.30% Silicon (Si), 0.70-0.90% Chromium (Cr), 1.65-2.00% Nickel (Ni), 0.20-0.30% Molybdenum (Mo), and the base metal Iron (Fe). Other designations of AISI 4340 alloy steel include UNS G43400 and AISI 4340.

AISI 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 treating condition while retaining good fatigue strength.

Typical applications are for structural use, such as aircraft landing gear, power transmission gears and shafts and other structural parts.

The alloy will harden by cold working or by heat treatment -- see "Heat Treatment" and "Tempering".

Machining is best done with this alloy in the annealed or normalized and tempered condition. It can be machined by all conventional methods. However in the high strength conditions of 200 ksi or greater the machinability is only from 25% to 10% that of the alloy in the annealed condition.

4340 has good ductility in the annealed condition and most forming operations are carried out in that condition. It can be bent or formed by spinning or pressing in the annealed state. Bend radii should be 3t or greater.

The alloy can be fusion or resistance welded. Preheat and post heat weld procedures should be followed when welding this alloy by established methods.

Heat Treatment
Heat treatment for strengthening is done at 1525 F followed by an oil quench. For high strength (over 200 ksi) the alloy should first be normalized at 1650 F prior to heat treatment. See "Tempering" for strength levels.

Forging may be done in the range of 2250 F max. down to 1800 F.

Hot Working
4340 has very good cold forming capability so that hot working should not be needed. Hot working in any but the annealed condition can affect the strength level. Consult the alloy supplier in regard to hot working.

Cold Working
The 4340 alloy may be cold worked, in the annealed condition, by conventional methods and tooling. It has good ductility.

A full anneal may be done at 1550 F followed by controlled (furnace) cooling at a rate not faster than 50 F per hour down to 600 F. From 600 F it may be air cooled.

The temperature for tempering depends upon the strength level desired. Before tempering the alloy should be in the heat treated or normalized & heat treated condition - see "Heat Treatment". For strength levels in the 260 - 280 ksi range temper at 450 F. For strength in the 125 - 200 ksi range temper at 950 F. Do NOT temper the alloy if it is in the 220 - 260 ksi strength range as tempering can result in degradation of impact resistance for this level of strength.

Other Comments
AISI 4340 is considered to be a "through hardening" steel such that large section sizes can still be heat treated to high strength.

Chemical Composition
Carbon 0.38 - 0.43
Chromium 0.7 - 0.9
Iron Balance
Manganese 0.6 - 0.8
Molybdenum 0.2 - 0.3
Nickel 1.65 - 2
Phosphorus 0.035 max
Silicon 0.15 - 0.3
Sulphur 0.04 max
Physical Properties
Density (lb / cu. in.) 0.28
Specific Gravity 7.8
Specific Heat (Btu/lb/Deg F - [32-212 Deg F]) 0.116
Melting Point (Deg F) 2600
Thermal Conductivity 21
Mean Coeff Thermal Expansion 6.6
Modulus of Elasticity Tension

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