4140 Round steel

The Ultimate Guide to 4140 Round Steel Bar: Properties, Applications, and Specifications

4140 round steel bar is a versatile and widely used low-alloy steel in the manufacturing and engineering sectors. Known for its exceptional balance of toughness, strength, and wear resistance, this material is often the go-to choice for components that must withstand high-stress conditions. Often referred to as "chromoly" steel (due to its Chromium and Molybdenum content), 4140 offers superior mechanical properties compared to standard carbon steels like 1045 or 1018.

This guide provides a detailed overview of the 4140 alloy steel round bar, including its chemical composition, mechanical properties, heat treatment processes, and common industrial applications.

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What is 4140 Steel?

4140 is a low-alloy steel that includes chromium, molybdenum, and manganese as strengthening agents. This specific combination gives the steel its characteristic high fatigue strength, toughness, and resistance to abrasion and impact . It is typically supplied in the annealed condition for ease of machining, but it can be heat-treated to achieve a wide range of hardness and strength levels .

Depending on the global standard, 4140 is known by various international equivalents, including 1.7225, 42CrMo4, and SCM440 .

Mechanical Properties

The mechanical properties of 4140 round bar can vary significantly based on its heat treatment condition (annealed vs. quenched and tempered). The values below represent typical characteristics for 4140 in the annealed condition, which is common for stock material:

Property Metric Imperial

Tensile Strength (Ultimate) 655 MPa 95,000 psi

Tensile Strength (Yield) 415 MPa 60,200 psi

Hardness, Brinell 197 197

Hardness, Rockwell B 92 92

Elongation at Break 25.7% 25.7%

Machinability 65% 65%

Density 7.85 g/cm³ 0.284 lb/in³

Modulus of Elasticity 190 - 210 GPa 27,557 - 30,457 ksi

Data compiled from

When heat-treated (quenched and tempered), the tensile strength of 4140 can increase dramatically, often reaching 850–1000 MPa, making it suitable for high-performance applications .

Heat Treatment and Forging

One of the main advantages of 4140 is its response to heat treatment. It is a through-hardening steel, meaning its properties can be adjusted to meet specific design requirements . Common processes include:

• Annealing: Performed at approximately 790-840°C (1450-1540°F) followed by slow furnace cooling. This softens the steel, relieving internal stresses and improving machinability .
• Normalizing: Conducted at 840-880°C (1544-1616°F) with air cooling. This refines the grain structure and creates a more uniform material .
• Hardening (Quenching and Tempering): The steel is heated to 830-860°C (1525-1580°F) and then quenched in oil . To reduce brittleness and achieve the desired hardness, it is immediately tempered by reheating to a specific temperature (typically between 540-680°C / 1000-1200°F) depending on the required strength .

Diverse Industrial Applications

Because of its high fatigue strength and toughness, 4140 round bar is a staple in several heavy industries. It is commonly used to manufacture parts that require resistance to twisting and bending .

• Automotive Components: Crankshafts, connecting rods, axle shafts, gears, and steering knuckles .
• Industrial Machinery: Spindles, gears, sprockets, jigs, fixtures, and machine tooling .
• Oil & Gas Industry: Drill collars, tool joints, and other components requiring high strength .
• Hydraulic Systems: Hydraulic shafts, rams, and piston rods .
• General Manufacturing: Bolts, studs, pins, and forged parts .

Forms and Availability

4140 round bars are manufactured through various processes depending on the required size and finish:

• Hot Rolled / Forged: Common for larger diameters (typically > 40mm up to 1000mm). The surface is usually black or scaled .
• Cold Drawn / Turned Ground and Polished (TGP): Used for smaller diameters requiring precise tolerances and a bright, smooth surface finish. TGP bars offer enhanced straightness and surface quality ideal for shafting