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ResourcesMaterials4340 Carbon Steel: Uses, Composition, Properties
4340 rolled steel. Image Credit: Shutterstock.com/kasarp studio

4340 Carbon Steel: Uses, Composition, Properties

Xomety X
By Team Xometry
July 13, 2023
 8 min read
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4340 steel is carbon steel, well known for its high strength. 4340 is produced the same way as other steels by melting the alloying elements, cooling them, and working them. Due to its strength, 4340 steel is used mostly in structural applications in aviation and automotive applications. 4340 is not known for its corrosion resistance, however. 

What Is 4340 Carbon Steel?

4340 is an American standard carbon steel that is sought after for its high-strength properties. 4340 steel is ferromagnetic which means its magnetic properties will depend on the phase of the steel. However, the lack of substantial alloying elements results in the iron creating very magnetic steel. 

What Is 4340 Carbon Steel Used For?

4340 carbon steel is a strong material, and so it has uses in structural and high-strength applications. These can include but are not limited to transmissions, gears and shafts, landing gear, hydraulic systems, automotive frames, and fasteners. It finds uses in heavy-duty applications including landing gear, transmissions, and other structural components.  

How Is 4340 Carbon Steel Made?

Creating 4340 carbon steel involves melting the elements and then cooling them in a controlled manner. The steel is heat treated, and then cut into its final form. The iron and all alloying elements are weighed and added to an electric furnace which takes up to 12 hours to melt all the elements. A Vacuum Oxygen Decarburization (VOD) system can control the amount of carbon in the steel. The amount of carbon removed will affect the hardness and tensile strength of the material. Before cooling, the steel will be stirred to ensure the elements are uniformly mixed. As the metal is cooled into billets it may be hot rolled while still above its crystallization temperature to form the steel into shape. If precise dimensions are required the steel may be cooled and rolled below its crystallization temperature. Once cooled, the steel may be heat treated to relieve its internal stresses. Finally, the 4340 steel will be cut to its final shape and surface finish, which will depend on the application of the steel but may include grinding. 

What Is the Chemical Composition of 4340 Carbon Steel?

The main element of 4340 steel is iron which makes up approximately 95.5% of the total composition. The next most-common elements in 4340 steel are nickel, which makes up 1.85%, and chromium, which makes up 0.8% of the total composition. Table 1 below shows the full composition of 4340 steel:

Table 1: The Chemical Composition of 4340 Steel
ElementContent
Element
Iron
Content
95%–96%
Element
Nickel
Content
1.65%–2%
Element
Chromium
Content
0.7%–0.9%
Element
Manganese
Content
0.6%–0.8%
Element
Carbon
Content
0.37%–0.43%
Element
Molybdenum
Content
0.2%–0.3%
Element
Silicon
Content
0.15%–0.3%
Element
Sulfur
Content
0.04%
Element
Phosphorus
Content
0.03%

What Is the Carbon Content of 4340 Carbon Steel?

The carbon content of 4340 steel is between 0.37% and 0.43%. This means it is not a low-carbon steel as low-carbon steels have a carbon level of less than 0.3%. As the carbon content of steel increases so does its hardness, meaning that 4340 carbon steel is harder than any low-carbon steel.

What Are the Properties of 4340 Carbon Steel?

Below is Table 2 which contains various properties of a typical 4340 steel variant:


Table 2: Properties of 4340 Steel
PropertyValue
Property
Density
Value
0.284 lb/in3
Property
Yield Strength
Value
125,000 psi
Property
Hardness (Rockwell B)
Value
100
Property
Magnetism
Value
Ferromagnetic

Machinability Rating of 4340 Carbon Steel

4340 carbon steel has a machinability rating of 55% in the annealed condition. The recommended speed for turning is between 710–950 SFM. The recommended speed for milling is 430–590 SFM. 

Is 4340 Stainless Steel?

No, 4340 steel is not stainless steel. 4340 steel is classified as a medium-carbon low-alloy steel. For steel to be classified as stainless steel it needs to have less than 1.2% carbon (which 4340 does) and more than 10.5% chromium—which 4340 does not. 

What Are the Thermal Properties of 4340 Carbon Steel?

The melting point of 4340 steel is 2,600 °F. 4340 carbon steel also has a thermal conductivity of 309 Btu-in/hr-ft2°F, a specific heat capacity of 0.114 Btu/lb-°F, and a rate of linear expansion of 6.83 µin.in-°F.

4340 Steel Heat Treatment

4340 steel is heat treated using an annealing process in which internal stresses are relieved with the application of heat. To anneal 4340, the steel must first be heated to 1,525 °F and then cooled slowly. Initially, it must be cooled to 1,350 °F and then furnace-cooled to 1,130 °F. Cooling between 1350–1130 °F must happen at a rate of 20 °F an hour. Finally, the steel must be air-cooled to room temperature. 

What Are the Common Forms of 4340 Carbon Steel Material?

The common forms of 4340 carbon steel are listed below:

Sheet

The sheet form of 4340 steel can be found in thicknesses between 0.018” and 0.25” thick. Sheet metal is usually used for bodywork and panels for machinery. 

Bar

The bar form of 4340 carbon steel can be found in multiple geometries including tubular and circular forms. The bar form of 4340 steel will be used to create structural support for different applications, especially in aircraft and automotive applications. 

Plate

Plates of 4340 steel will have a thickness greater than 0.25” thick. 4340 plates can be used in various applications but are known for their use as steel armor. 

Hot Rolled

Hot rolling 4340 creates a steel that is renowned for its toughness. The hardened condition of hot-rolled 4340 steel also improves the impact and wear resistance of the steel. The hot rolling of steel creates these properties by imparting a strain on the microstructure of the metal causing it to work harden. The strain on the microstructure increases the dislocation density which is what hardens and strengthens the steel. 

Annealed

During annealing, 4340 carbon steel is raised to a temperature of 1,525 °F then cooled to 1,350 °F and furnace-cooled to 1,130 °F at a rate of 20 °F per hour. The annealing process is time-consuming, however, it is required to obtain a pearlite structure ready for machining. 

Cold-Drawn

Cold drawn versions of the 4340 bar have widespread use in the aerospace industry because of their high strength and high ductility. Cold-drawn steel is very similar to hot-rolling steel as it increases hardness and strength. However, cold drawing has a greater effect than hot rolling. The cold-drawn process also ends with a more dimensionally accurate metal, due to the lack of thermal expansion and contraction. The only downside is that cold-drawn steel is more expensive as it is more labor-intensive. 

What Are Equivalents of 4340 Carbon Steel?

Table 3 showing the equivalent forms of 4340 steel is given below. Note that some chemical compositions may not be an exact match but are the nearest equivalent:

Table 3: Equivalent-Grade Steels from Different Countries
CountryEquivalent Grade
Country
EU
Equivalent Grade
36CrNiMo4
Country
USA
Equivalent Grade
4340
Country
Germany
Equivalent Grade
34CRNIMO6
Country
Japan
Equivalent Grade
SNCM 439/SNCM8
Country
France
Equivalent Grade
40NCD3
Country
England
Equivalent Grade
EN24/817M40
Country
Russia
Equivalent Grade
36KH2N2MFA
Country
China
Equivalent Grade
40CrNiMoA
Country
Italy
Equivalent Grade
35NICRMO6

What Are the Advantages of Using 4340 Carbon Steel?

One of the advantages of choosing to use 4340 over other steels is it has really good fatigue resistance. Other advantages of using 4340 carbon steel are:

  1. Good shock resistance.
  2. Good impact resistance.
  3. High wear and abrasion resistance. 

What Are the Disadvantages of Using 4340 Carbon Steel?

4340 steel has its limitations. One such limitation is that it is not very corrosion-resistant. Other limitations of 4340 steel are:

  1. Not highly machinable. 
  2. At high risk of quench cracking.
  3. 4340 steel, as with every steel, is very heavy. 

What Is the Difference Between 4340 Steel and 4330 Steel?

4330 and 4340 are both medium-carbon and low-alloy steels with distinct differences. For example, 4330 has more manganese, chromium, nickel, and vanadium than 4340 steel. Due to these extra alloying elements, 4330 has a much higher tensile strength, yield strength, and hardness. Both 4340 and 4330 have found use in transmission gears and aircraft landing gear.  

What Is the Difference Between 4340 Steel and 4140 Steel?

4340 and 4140 are very similar metals. However, they do have some differences both in composition and properties. For example, 4340 has a higher carbon content while 4140 alloy steel has a higher chromium content. However, the most significant composition difference is that 4340 includes nickel which accounts for its higher strength and fracture toughness. The lack of nickel in 4140 also makes it more susceptible to fracturing. 

What Is the Difference Between 4340 Steel and 300M Steel?

300M steel and 4340 steel are similar. However, 300M steel has the addition of vanadium and copper and a higher silicon and molybdenum content. 300M is considered a modified version of 4340 steel which has better fatigue strength, ductility, impact strength, and fracture toughness. 

Summary

This article presented 4340 steel, explained what it is, and discussed its various applications. To learn more about 4340 steel, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

Disclaimer

The content appearing on this webpage is for informational purposes only. Xometry makes no representation or warranty of any kind, be it expressed or implied, as to the accuracy, completeness, or validity of the information. Any performance parameters, geometric tolerances, specific design features, quality and types of materials, or processes should not be inferred to represent what will be delivered by third-party suppliers or manufacturers through Xometry’s network. Buyers seeking quotes for parts are responsible for defining the specific requirements for those parts. Please refer to our terms and conditions for more information.


Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.

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