Alloy Steel 4340


Principal Design Features

AISI 4340 is a heat treatable, low alloy steel containing nickel, chromium, and molybdenum. The inclusion of nickel improves hardenability. Regarded industry-wide for its toughness, 4340 supplies high strength in the heat treated condition while still maintaining excellent fatigue strength over time.


Typical applications include aircraft landing gears, power transmission gears, axle shafts, chucks, molds, drilling parts, sprockets, and other structural parts.


Machining is best accomplished with material in an annealed or normalized and tempered condition. 4340 can be machined by all conventional methods. However, in high strength conditions (200 KSI or greater), the machinability typically ranges from 10%-25% of material in the annealed condition.


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


4340 can be fusion welded or resistance welded. Preheat and post heat weld procedures should be followed when welding.

Heat Treatment

Standard heat treatment is performed 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 1800° F to 2250° F (max).

Hot Working

If working with 4340 not in an annealed condition, hot working can have a deleterious effect on tensile strength. Consult the alloy supplier prior to hot working.

Cold Working

4340 has excellent cold forming capabilities and may be cold worked in the annealed condition by conventional methods and tooling.


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


Not applicable to this alloy.


Tempering temperature depends upon the strength level desired. Prior to tempering, 4340 should be in the heat treated or normalized & heat treated condition - see "Heat Treatment".

To achieve 260 - 280 KSI tensile strength, temper at 450° F

To achieve 125 - 200 KSI tensile strength, temper at 950° F

Do NOT temper 4340 material in the 220 - 260 KSI tensile strength range (tempering can result in degradation of impact resistance).

Other Comments

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

Chemistry Data

Carbon 0.38/0.43

Chromium 0.70/0.90

Manganese 0.65/ 0.85

Molybdenum 0.20/ 0.30

Nickel 1.65/2.00

Phosphorus 0.035 max

Sulphur 0.04 max

Physical Data

Density (lb / cu. in.): 0.28

Specific Gravity: 7.80

Specific Heat (Btu/lb/Deg F - [32-212° F]): 0.116

Melting Point (Deg F): 2600°

Thermal Conductivity: 21

Mean Coefficient of Thermal Expansion: 6.6

Modulus of Elasticity in Tension: 33

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