AISI 4140 Steel

AISI 4140 is a popular low-alloy steel that belongs to the chromium-molybdenum steel group. It is known for its high strength, toughness, and wear resistance. The “AISI” in AISI 4140 stands for the American Iron and Steel Institute, which is responsible for classifying and standardizing various types of steel.

AISI 4140 steel is Renowned for its exceptional strength, toughness, and versatility, AISI 4140 steel has become a go-to choice for a wide range of applications. In this article, we will learn the 4140 material specifications, chemical compositions, mechanical properties, heat treatment, applications, and welding considerations of AISI 4140 steel.

Material Specification:

AISI 4140 steel is a low-alloy steel that falls under the category of chromium-molybdenum (Cr-Mo) steels. The UNS Number of 4140 steel is UNS G41400. It is designated by the American Iron and Steel Institute (AISI) with the number 4140.

AISI 4140 steel has several equivalent grades in different international standards. Some of the commonly used equivalent grades are:

  • ASTM A29/A29M: 4140
  • DIN EN 10083-3: 42CrMo4
  • JIS G4105: SCM440
  • GB/T 3077: 42CrMo
  • ISO 683-1: 42CrMo4
  • BS EN 10083-1: 42CrMo4
  • UNI 7845: 42CrMo4
  • NF EN 10083-1: 42CrMo4
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Chemical Compositions:

The chemical composition of AISI 4140 steel plays a vital role in determining its mechanical properties. The typical chemical composition of AISI 4140 steel is as follows:

  • Carbon (C): 0.38-0.43%
  • Chromium (Cr): 0.80-1.10%
  • Manganese (Mn): 0.75-1.00%
  • Silicon (Si): 0.15-0.35%
  • Molybdenum (Mo): 0.15-0.25%
  • Phosphorus (P): 0.035% (maximum)
  • Sulfur (S): 0.040% (maximum)

Mechanical Properties:

AISI 4140 steel exhibits excellent mechanical properties, making it suitable for various demanding applications. The typical mechanical properties of AISI 4140 steel are as follows:

  • Tensile Strength: 850-1000 MPa (123-145 ksi)
  • Yield Strength: 680-900 MPa (98-131 ksi)
  • Elongation: 12-20%
  • Hardness: 22-35 HRC (Rockwell C hardness)

Heat Treatment:

AISI 4140 steel can be heat-treated to enhance its mechanical properties. The most common heat treatment process for AISI 4140 steel is quenching and tempering. This involves heating the steel to a specific temperature, followed by rapid cooling (quenching) in oil or water. Subsequently, the steel is reheated to a lower temperature and then cooled slowly (tempering). This process helps achieve the desired hardness, strength, and toughness for specific applications.

Applications:

AISI 4140 steel finds extensive use in various industries due to its exceptional properties. Some common applications of AISI 4140 steel include:

  1. Automotive Industry: AISI 4140 steel is used in the manufacturing of crankshafts, axle shafts, gears, and other critical components in automobiles.
  2. Oil and Gas Industry: This steel is employed in the production of drill collars, drill rods, and other components used in oil exploration and extraction.
  3. Aerospace Industry: AISI 4140 steel is utilized in the construction of aircraft landing gears, engine components, and structural parts.
  4. Construction and Heavy Machinery: It is used in the manufacturing of high-strength bolts, gears, shafts, and other components in construction and heavy machinery.
  5. Tool and Die Making: AISI 4140 steel is commonly used for making molds, dies, and tooling components due to its excellent hardness and wear resistance.
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Welding Considerations:

Forming

AISI 4140 alloy steel is a process that involves shaping the material using conventional techniques while it is in the annealed condition. This steel has high ductility, but due to its toughness compared to plain carbon steels, it requires more pressure or force during the forming process.

Welding

When it comes to welding AISI 4140 steel, preheating and post-weld heat treatment are crucial to prevent cracking and ensure the desired mechanical properties. It is recommended to preheat the steel to a specific temperature range before welding and then perform a post-weld heat treatment to relieve residual stresses and improve toughness.

AISI 4140 alloy steel can be welded using various conventional techniques. However, it’s important to note that if the steel is welded in the heat-treated condition, its mechanical properties will be affected. To mitigate this, post-weld heat treatment should be performed.

Heat Treatment

The heat treatment process for AISI 4140 alloy steel involves heating it to 845°C (1550°F) and then quenching it in oil. Prior to hardening, the steel can be normalized by heating it at 913°C (1675°F) for an extended period of time, followed by air cooling.

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Forging

AISI 4140 alloy steel is forged within a temperature range of 926 to 1205°C (1700 to 2200°F).

Hot Working

AISI 4140 alloy steel can be hot worked within a temperature range of 816 to 1038°C (1500 to 1900°F).

Cold Working

AISI 4140 alloy steel can be cold worked using conventional methods while it is in the annealed condition.

Annealing

Annealing of AISI 4140 alloy steel involves heating it to 872°C (1600°F) and then slowly cooling it in a furnace.

Tempering

Tempering of AISI 4140 alloy steel is performed at temperatures ranging from 205 to 649°C (400 to 1200°F), depending on the desired hardness level. Lower tempering temperatures result in increased hardness. For example, tempering at 316°C (600°F) can achieve a tensile strength of 225 ksi, while tempering at 538°C (1000°F) can achieve a tensile strength of 130 ksi.

Hardening

AISI 4140 alloy steel can be hardened through cold working or by heating and quenching it.