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Grade Guide: AISI 4140 Steel


AISI 4140 steel is a formidable and versatile material, celebrated for its exceptional properties that cater to many different industrial applications. Let’s explore the 4140 steel properties that make this alloy an engineering marvel.

What is 4140 Steel?

AISI 4140 steel is a low alloy steel containing chromium, molybdenum, and manganese. It is widely used across numerous industries and is an excellent material choice due to its toughness, high fatigue strength, and abrasion and impact resistance. Not many grades can match the versatility and usefulness of 4140.

4140 Grade Designation

When discussing AISI 4140, it is important to understand what the grade number means:

Number Meaning
4 Designates that 4140 steel is molybdenum steel, indicating that it possesses higher amounts of molybdenum than other steels, such as the 1xxx series.
1 Designates that 4140 steel has additions of chromium, more so than 46xx steel, for example.
40 Used to differentiate 4140 Steel from other steels in the 41xx series.

How is 4140 Steel Made?

AISI 4140 is made by placing iron, carbon, and other alloying elements into an electric furnace or oxygen furnace. The major alloying elements added to AISI 4140 are:

Chromium – When added to 4140 steel, chromium enhances its corrosion resistance and hardenability. It forms chromium carbides during heat treatment, improving wear resistance and overall strength. This makes 4140 steel suitable for applications in harsh environments and exposed to corrosive substances.

Manganese – Manganese serves as a deoxidizer and improves the steel’s strength and toughness. It also facilitates grain refinement during heat treatment, enhancing its machinability. This addition helps achieve uniform hardness and structural integrity in 4140 steel components.

Molybdenum – Adding molybdenum to 4140 steel enhances its hardenability and high-temperature strength. It forms hard, heat-resistant carbides, making the steel suitable for applications involving elevated temperatures and high-stress conditions, such as aerospace components and industrial machinery.

Once the iron, carbon, and other alloying elements have been mixed together in liquid form, it can cool. The steel may then be annealed, possibly several times.

After the annealing, the steel is heated to a molten phase again so that it can be poured into the desired form and either be hot worked or cold worked through rollers or other tools to reach the desired thickness. Other special operations can be added to reduce mill scale or improve mechanical properties.

Mechanical Properties of 4140 Steel

AISI 4140 is a low alloy steel. Low alloy steels rely on elements besides iron and carbon to enhance their mechanical properties. In AISI 4140, additions of chromium, molybdenum, and manganese are used to increase the strength and hardenability of the steel. The additions of chromium and molybdenum are why AISI 4140 is considered a “chromoly” steel.

There are several important mechanical properties of AISI 4140, including:

Tensile Strength – AISI 4140 steel typically has a target ultimate tensile strength of around 95,000 psi, making it exceptionally strong and suitable for applications demanding robust structural integrity and load-bearing capabilities.

Toughness – 4140 steel exhibits remarkable toughness, enabling it to withstand high-stress conditions without fracturing. This property is vital in applications like automotive components and industrial machinery where impact resistance is crucial.

Ductility – While being exceptionally strong, 4140 steel also offers good ductility. It can be readily formed and shaped, making it suitable for various manufacturing processes, including forging and machining.

Hardenability – One of the key 4140 steel properties is excellent hardenability, making it ideal for heat treatment processes like quenching and tempering. This enhances its durability and wear resistance, making it valuable in applications such as gears and shafts.

Chemical Properties – 4140 steel has a well-balanced chemical composition, primarily consisting of iron, chromium, and molybdenum. These elements contribute to its exceptional strength, corrosion resistance, and ability to withstand extreme conditions, further expanding its range of industrial applications.

The table below highlights the chemical composition of AISI 4140:

C Cr Mn Si Mo S P Fe
0.38-.43% 0.80-1.10% 0.75-1.0% 0.15-0.30% 0.15-0.25% 0.040% max 0.035% max Balance

The addition of chromium and molybdenum promotes corrosion resistance. The molybdenum can be particularly useful when resisting corrosion due to chlorides. The manganese in AISI 4140 increases hardenability and is a deoxidizer. Manganese can also combine with sulfur in alloy steels to improve machinability and make the carburizing process more effective.

Applications for 4140 Steel

The exceptional 4140 steel properties make it a prized material across various industries.:

Automotive Industry – 4140 steel finds extensive use in automotive components, such as axles, crankshafts, and gears. Its high tensile strength, toughness, and wear resistance enhance vehicle durability and reliability.

Aerospace – In aerospace applications, 4140 steel is utilized for critical components like landing gear parts and structural elements. Its combination of strength and heat resistance makes it ideal for withstanding extreme conditions.

Oil and Gas Exploration – The demanding conditions of the oil and gas industry benefit from 4140 steel’s ability to maintain its structural integrity at high temperatures and pressures. It is commonly used in drill collars, pipelines, and wellhead equipment.

Manufacturing and Machinery – The manufacturing sector relies on 4140 steel for various machinery components, including shafts, gears, and machine tool parts. Its ease of machining and excellent strength ensure efficient operation and long-lasting performance.

Construction – Structural applications, such as connecting rods, anchor bolts, and crane hooks, benefit from 4140 steel’s high tensile strength, providing the structural integrity needed in large construction projects.

Defense and Firearms – Firearms and military equipment utilize 4140 steel for its strength and durability. It is commonly found in gun barrels, rifle receivers, and tactical gear components.

Power Generation – Power generation equipment, including turbine components and generator shafts, utilizes the high-temperature and high-stress capabilities of 4140 steel to ensure reliable operation.

Agricultural Equipment – Farm machinery components like tractor axles, plowshares, and cultivator tines benefit from 4140 steel’s toughness, helping withstand the rigors of agricultural operations.

Mining Industry – 4140 steel is used in mining equipment such as drilling bits and conveyor components, where its strength and wear resistance help maximize efficiency and longevity.

4140 Steel Grade Guide Video

Learn more about 4140 Steel with our Grade Guide video blog:

From its high tensile strength and toughness to its exceptional hardenability, 4140 steel properties continue to redefine the standards of durability, reliability, and versatility in construction, manufacturing, and engineering. Shop AISI 4140 Steel to see how it can be helpful for your industrial project.


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