AISI 4340 Alloy Steel (UNS G43400)

Topics Covered

Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
     Fabrication and Heat Treatment
     Heat Treatment
     Cold Working


Alloy steels are designated by AISI four-digit numbers. They comprise different kinds of steels having composition exceeding the limitations of B, C, Mn, Mo, Ni, Si, Cr, and Va set for carbon steels.

AISI 4340 alloy steel is a heat treatable and low alloy steel containing chromium, nickel and molybdenum. It has high toughness and strength in the heat treated condition.

Chemical Composition

The following table shows the chemical composition of AISI 4340 alloy steel.

Element Content (%)
Iron, Fe 95.195 - 96.33
Nickel, Ni 1.65 - 2.00
Chromium, Cr 0.700 - 0.900
Manganese, Mn 0.600 - 0.800
Carbon, C 0.370 - 0.430
Molybdenum, Mo 0.200 - 0.300
Silicon, Si 0.150 - 0.300
Sulfur, S  0.0400
Phosphorous, P  0.0350

Physical Properties

The physical properties of AISI 4340 alloy steel are given in the following table.

Properties Metric Imperial
Density 7.85 g/cm3 0.284 lb/in³
Melting point 1427°C 2600°F

Mechanical Properties

The mechanical properties of annealed AISI 4340 alloy steel are displayed in the following table.

Properties Metric Imperial
Tensile strength 745 MPa 108000 psi
Yield strength 470 MPa 68200 psi
Bulk modulus (typical for steel) 140 GPa 20300 ksi
Shear modulus (typical for steel) 80 GPa 11600 ksi
Elastic modulus 190-210 GPa 27557-30458 ksi
Poisson's ratio 0.27-0.30 0.27-0.30
Elongation at break 22% 22%
Reduction of area 50% 50%
Hardness, Brinell 217 217
Hardness, Knoop (converted from Brinell hardness) 240 240
Hardness, Rockwell B (converted from Brinell hardness) 95 95
Hardness, Rockwell C (converted from Brinell hardness. Value below normal HRC range, for comparison purposes only) 17 17
Hardness, Vickers (converted from Brinell hardness) 228 228
Machinability (annealed and cold drawn. Based on 100 machinability for AISI 1212 steel.) 50 50

Thermal Properties

The following table outlines the thermal properties of AISI 4340 alloy steel.

Properties Metric Imperial
Thermal expansion co-efficient (20°C/68°F, specimen oil hardened, 600°C (1110°F) temper 12.3 µm/m°C 6.83 µin/in°F
Thermal conductivity (typical steel) 44.5 W/mK 309 BTU in/hr.ft².°F

Other Designations

Other designations that are equivalent to AISI 4340 alloy steel include the following:

AMS 6359 AMS 6454 ASTM A331 (4340) ASTM A711 SAE J1397 (4340)
AMS 6409 ASTM A29 (4340) ASTM A506 (4340) ASTM A752 (4340) SAE J404 (4340)
AMS 6414 ASTM A320 (L43) ASTM A519 (4340) ASTM A829 SAE J412 (4340)
AMS 6415 ASTM A322 (4340) ASTM A646 (4340-7) MIL S-5000

Fabrication and Heat Treatment


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AISI 4340 alloy steel can be machined using all conventional techniques. Machining process can be performed in the annealed or normalized and tempered conditions.


AISI 4340 alloy steel has good ductility and formability in the annealed condition. It can be bent or formed by pressing or spinning in the annealed state.


AISI 4340 alloy steel can be welded using fusion or resistance welding methods. Preheat and post heat weld procedures are followed while welding this steel by established methods.

Heat Treatment

AISI 4340 alloy steel is heat treated at 830°C (1525°F) followed by quenching in oil.


AISI 4340 alloy steel is forged at 427 to 1233°C (1800 to 2250°F).

Cold Working

AISI 4340 alloy steel can be cold worked using all conventional methods in the annealed condition. It has high ductility.


AISI 4340 alloy steel is annealed at 844°C (1550°F) followed by cooling the furnace.


AISI 4340 alloy steel should be in the heat treated or normalized and heat treated condition before tempering. Tempering temperatures depend upon the desired strength level.


AISI 4340 alloy steel can be hardened by cold working or heat treatment.


AISI 4340 alloy steel is mainly used in power transmission gears and shafts, aircraft landing gear, and other structural parts.


  1. Akhtar Aziz Akhtar Aziz Netherlands says:

    Thanks for all the information. I have a question for you:

    We applied cadmium cyanide plating to avoid rusting of this material but after certain time, the material started to rust. We need to know what type of coating is compatible with the Cadmium Cyanide coating for a stronger rust protection system. Not Electro plating.

  2. Özgür Albayrak Özgür Albayrak Turkey says:

    Chrome coating will be economic and efecctive solution with high corrosion resistance

    • David Goldstein David Goldstein United States says:

      ArmorGalv Thermal Diffusion Coating should be considered. Zero chance of Hydrogen Embrittlement. Over 1,000 hours of salt spray resistance, high lubricity (replacement for cadmium plating), high ductility, various sealers either stand-alone coating or apply additional coatings such as paint, Teflon, etc. for specific applications. Over 60 world-wide applicators. Process requires 600-750 degrees F so only need to watch tempering temperatures, though consult with applicators.

  3. Ako Samuel Stinger Ojonuba Ako Samuel Stinger Ojonuba Korea says:

    Please why is the same shear modulus assumed for both forged carbon steel and alloy steel ?

  4. karol bosak karol bosak Poland says:

    Looking for characteristics magnetization curve for 4340 steel.

  5. karol bosak karol bosak Poland says:

    I'm looking for a B-H magnetization curve for 4340 steel

  6. Emre Uygun Emre Uygun Turkey says:

    is it possible to weld grade 304stainless steel on AISI 4340 Alloy Steel

    • Joshua Williams Joshua Williams United States says:

      Yes, however the 4340 material needs to be purchased in the annealed or normalized condition. AISI 309L weld filler should be used.

      The material MUST be preheated to around 315°C and temperature in the welds zone must not be permitted to drop below 300°C during welding operations. Welded areas should be slow cooled over several hours. Typically this is done by reheating to 350-400°C then wrapping parts in insulation, or placing in a preheated furnace and furnace cooling.

      For complex parts with large amounts of welding, it's recommended to heat treat the assembly immediately after welding as finished and welds have been visually inspected.

      In general because if the very high strength of this material, welds must be of the utmost possible quality. Weld Undercutting, lack of fusion defects, cracks, porosity, and incomplete root penetration cannot be tolerated.

      It's strongly recommend to inspect welds for HAZ cracks and other defects by a variety of nondestructive testing methods. 100%  of assemblies should be examined for weld defects.

  7. Phan Pham Thi Phan Pham Thi Vietnam says:

    I want to reference the Mill test of alloy steel 4340 (plate) (34CrNiMo-6). Who have it,  please give me? Thank you so much. My email: [email protected].

  8. pradeep Nagaraj pradeep Nagaraj India says:

    is this possible to do case hardening and which type?

  9. Tim Hogue Tim Hogue United States says:

    Can this steel be hot-dip galvanized?  I will be using it as a structural pin in an outdoor industrial environment.

    • Joshua Williams Joshua Williams United States says:

      Yes. 4340 lends itself well to HDG in the quenched and tempered state because it has moderate tempering resistance. For the same reason it's sometimes bern used for hot-work tooling. Holding in the zinc bath while it heats to the galvanizing temperature will generally result in slight  reduction in hardness and yield strength. However keep in mind that parts should be double  tempered at slightly above the expected zinc bath temperature beforehand.

      Parts containing high residual stresses from quenching and high overall hardness may be subject to liquid metal embrittlement during HDG. Therefore consideration needs to be taken with regards to the tempering step.

      Lastly keep in mind that very high strength Q&T alloy steels are prone to hydrogen embrittlement. Certain corrosion conditions can lead to hydrogen production inside joint surfaces or crevices due to sacrificial corrosion of the thick zinc layer. This has been known to cause unexpected failures of hot-dipped fasteners, for example.

  10. Quý Tăng Quý Tăng Vietnam says:

    Heat treatment process of AISI4340 steel

  11. Satesh Bangar Satesh Bangar India says:

    does it required stress reliving/ Normalizing process after friction welding process.
    if yes then what temperature would require for Normalizing process

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of

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