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Steel 4340

Steel 4340 is a medium-carbon nickel-chromium-molybdenum steel alloy with excellent tensile strength, toughness, and fatigue resistance. Since the material usually comes in the annealed condition, additional heat treatments (oil quenching and tempering, or normalizing and tempering) can produce parts with even higher strength and fatigue resistance.

Furthermore, Steel 4340 is a wear-resistant material with especially good abrasion resistance, shock- and -impact resistance, and excellent atmospheric corrosion resistance. The alloy also possesses very good ductility, especially in the annealed condition, where it can be easily formed and machined.

Thanks primarily to its toughness, Steel 4340 finds use in heavy machinery, specifically for the manufacturing of transmission parts, such as heavy-duty shafts, gears, and axles.

Other uses include the military industry – both for ground vehicles and military aircraft, commercial aircraft industry (landing gear), various automotive uses, rocket motor cases, pressure vessels, fasteners, ordnance hardware, etc.

Tech Steel & Materials offers Steel 4340 in seven specifications and multiple shapes/forms:

Steel 4340 Chemical Composition

Element Min (%) Max (%)
Carbon, C 0.370 0.430
Manganese, Mn 0.600 0.800
Silicon, Si 0.150 0.300
Phosphorous 0.0350
Chromium, Cr 0.700 0.900
Nickel, Ni 1.65 2.00
Molybdenum, Mo 0.200 0.300
Sulfur, S 0.0400
Iron, Fe *Balance

*Not exclusively to the element mentioned, but that one predominates other elements that are used in smaller quantities.

Fabrication and Working Instructions

Steel 4340 is readily machinable using standard methods, including sawing, turning, drilling, cutting, boring, etc. A coarse lamellar pearlitic structure or a spheroidized structure are recommended for the best results.

Nonetheless, it’s strongly recommended to machine the material in the annealed condition since machinability significantly decreases after heat treatments since the strength of the material increases. If necessary, a spheroidized structure can improve machinability in the heat-treated condition.

Steel 4340 also possesses excellent ductility in the annealed condition.

For the best mechanical properties, the material should be welded in the annealed condition. Furthermore, welding should be outright avoided in the hardened and tempered condition since there is a danger of quench cracking.

If welding in the hardened condition is absolutely necessary, then the material should be preheated from 400-580 °F to (≈200-300 °C), followed by after-welding stress relief at 1020-1200 °F (≈550-650 °F). For tempered-condition material, the stress relief should be 60 °F (15 °C) lower than the tempering temperature.

Steel 4340 Physical Properties

Property Imperial Metric
Mean Coefficient of Thermal Expansion 68 °F to 212 °F (20 °C to 100 °C) oil-hardened; tempered @ 1100 °F (600 °C) 6.83 [10-6/°F] 12.3 [10-6/K]
Mean Coefficient of Thermal Expansion 68 °F to 390 °F (20 °C to 200 °C) oil-hardened; tempered @ 1100 (600 °C) 7.06 [in/in/°F·106] 12.7 [10-6/K]
Mean Coefficient of Thermal Expansion 72 °F to 500 °F (22 °C to 260 °C) normalized & tempered 7.0 [10-6/°F] 12.6 [10-6/K]
Mean Coefficient of Thermal Expansion 68 °F to 750 °F (20 °C to 400 °C) oil hardened; tempered @ 1100 (600 °C) 7.61 [10-6/°F] 13.7 [10-6/K]
Thermal Conductivity (@ 212 °F) (@ 100 °C) 309 Btu/(hr/ft²/in/°F) 44.5 [W/m-K]

Steel 4340 Mechanical Properties

Property Imperial Metric
Ultimate Tensile Strength 161000 psi 1100 MPa
Yield Strength 103000 psi 710 MPa
Elongation at Break (in 4D) 13.2 % 13.2 %
Rockwell Hardness C 35 35
Rockwell Hardness B 99 99
Brinell Hardness 321 321
Knoop Hardness 348 348
Vickers Hardness 339 339
Reduction of Area 36 % 36 %
Machinability (annealed and cold-drawn condition) 50 % 50 %
Modulus of Elasticity 29700 ksi 205
Shear Modulus 11600 ksi 80 GPa
Poisson’s Ratio 0.29 0.29
Bulk Modulus 20300 ksi 140 GPa

Heat Treatment

Dramatically improved tensile strength and toughness properties of Steel 4340 can be obtained depending on the heat treatment used. The alloy usually comes in the annealed condition unless specified otherwise.

The standard annealing process for Steel 4340 starts at 1525 °F (830 °C), followed by cooling to 1350 °F (730 °C), and furnace cooling to 1130 °F (610 °C) at 11 °F (20 °C) per hour, followed by air cooling to room temperature.

Steel 4340 is usually normalized at 1500 °F (815 °C) +/- 50 °F (10 °C). Furthermore, the standard hardening temperature is 1500-1550 °F (815-845 °C), followed by tempering for stress relief.

After tempering, flame or induction hardening will produce material with Rc 50, while nitriding will produce Rc 60.