4140 Alloy steel | 42CrMo4 | SCM440

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4140 alloy steel is a versatile Cr-Mo alloy steel offering good strength, toughness, and hardenability, making it suitable for moderately severe service conditions after appropriate heat treatment. It’s classified with Chromium and Molybdenum as its primary alloying elements.

1. Applications

• Machinery Components: Shafts (a primary use), Axles, Gears, Spindles, Couplings, Crankshafts, Connecting Rods, Valve Bonnets, Chuck Bodies, Collets
• Tooling & Fixtures: Jigs, Fixtures, Tool holders, Drill collars, Bolts, Studs, Conveyor parts
• Automotive & Aerospace: Axles, Crankshafts, Steering knuckles, High-strength structural parts
• Oil & Gas Industry: Downhole drilling tools, Tool joints, Pump shafts
• Surface Hardened Parts: Components requiring enhanced surface durability

2. 4140 Steel Composition¹

3. Physical properties

Physical Properties of 4140 Alloy Steel under Different Temperatures

4. Mechanical Properties

4140 steel properties can be significantly tailored through heat treatment. Quenching and tempering can improve the yield strength, tensile strength, and notch toughness of 4140 metal.

4.1 Quenched and Tempered 4140 Metal

When 4140 steel is oil-quenched and then tempered at various temperatures, its mechanical properties change predictably. This enables precise control over the final characteristics of the steel to meet specific operational requirements. Below is a summary of typical mechanical properties achieved at different tempering temperatures:

4.2 Influence of Section Size (Mass Effect) on 4140 Material Properties

It’s important to consider the section size, or mass, of the 4140 steel component during heat treatment specification, especially when aiming for high strength levels. AISI 4140 is not a deep-hardening steel, and under the same heat treatment conditions, larger cross-sections may not achieve the same overall hardness or strength as smaller cross-sections.

The following table illustrates the effect of 4140 bar diameter on the mechanical properties of 4140 steel oil-quenched from 845 °C (1550 °F) and tempered at 540 °C (1000 °F):

Effect of Bar Diameter on Mechanical Properties of 4140 Steel (Tempered at 540 °C / 1000 °F)

4.3 Annealed 4140 Steel Mechanical Properties

Annealing is also divided into two states: hot rolling and cold drawing.

4.4 Welding 4140 steel²

4140 steel has high hardenability. When welding 4140, the heat-affected zone (HAZ) and weld metal can cool rapidly, leading to the formation of hard, brittle martensite. This martensite is susceptible to hydrogen cracking, which can lead to high internal stresses and reduced ductility, making the welding process more difficult. We recommend welding in the 4140 annealed condition whenever possible and performing heat treatment after welding to mitigate the risks associated with its high hardenability.

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5. Heat Treatment

5.1 Normalizing

Normalizing is a heat treatment process used to refine grain size, achieve a uniform structure, and improve machinability to obtain the required hardness. 

The normalizing temperature range is 870-900°C (1600-1650°F). Hold at this temperature for at least 1 hour, or 15 to 20 minutes per 25 mm (1 inch) of maximum section thickness. After holding, cool the material in the air to room temperature. 4140 hardness after normalizing is 150-200 HB.

5.2 Annealing

Annealing is primarily used to soften the steel and relieve stresses, thereby preparing it for subsequent processing, such as machining.

The annealing temperature is 830-870 °C (1525-1600 °F). The holding time depends on the section thickness or furnace load. Based on Aobo Steel’s more than 20 years of industry experience, the holding time is: 1 hour for each 25 mm (1 inch) of section thickness, with an additional 0.5 hours added for each additional 25 mm (1 inch) of thickness. After holding, the material is furnace-cooled at a rate of approximately 15 °C/h (30 °F/h) down to 480 °C (900 °F) and then air-cooled. The hardness of 4140 after annealing is 150-200 HB.

5.3 Quenching (Hardening)

Hardening involves heating the steel to form austenite, followed by rapid cooling to transform it into martensite, thereby increasing 4140 hardness and strength. 

The austenitizing temperature ranges from 860 to 885 °C (1550 to 1660 °F). Based on our experience, we recommend a temperature of 855 °C (1575 °F). Avoid using excessively high temperatures, as this may result in excessive austenite grain size and martensite brittleness. The soaking time is 5 minutes per inch of the smallest cross-section, or until the part is thoroughly soaked. The quenching medium is oil.

5.4 Tempering

Tempering must be performed immediately when the temperature of 4140 steel reaches 52-65°C (125-150°F).

The tempering temperature range is 200-700°C (400-1300°F). The temperature we commonly use is 175°C (350°F). The holding time determines the steel 4140 hardness. Our standard holding time is 2 hours per inch (25 mm) of cross-section. It is important to avoid under-tempering.
4140 steel generally does not require secondary tempering, but multiple tempering cycles can refine the grain structure to enhance toughness. The temperature for the second tempering should be 14°C (25°F) lower than the first tempering temperature to maintain the original 4140 material hardness.
We do NOT suggest tempering within the temperature range of 230–370°C (450–700°F), as this temperature range can cause “blue brittleness.” The microstructure of quenched and tempered 4140 steel is typically tempered martensite.

5.5 Spheroidizing

Spheroidizing produces a microstructure of globular carbides in a ferritic matrix, which improves machinability. Spheroidizing temperature is 760-775°C (1400-1425°F), with a hold time of 4 to 12 hours, followed by slow cooling.

Spheroidization can also be achieved by prolonged holding just below the Ae1 temperature, by heating and cooling alternately between temperatures just above Ac1 and just below Ar1, or by heating just above Ac1 followed by very slow furnace cooling or holding. For full spheroidizing, austenitizing temperatures are slightly above the Ac1 temperature, or about midway between Ac1 and Ac3.

6. Equivalent

• Europe (EN/DIN): 42CrMo4 or 1.7225
• Japan (JIS): SCM440
• China (GB/T): 42CrMo
• Great Britain (BS): EN19 (or 708M40/709M40)

1. ASM International. (1991). ASM Handbook, Volume 4: Heat Treating (p. 496). ASM International. ↩︎
2. Jenney, C. L., & O’Brien, A. (Eds.). (2000). Welding Handbook, Ninth Edition, Volume 1: Welding Science and Technology (p. 141). American Welding Society. ↩︎

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