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. Chemical composition of 4140 alloy steel

  • Carbon (C): Ranges from 0.38% to 0.43%. The higher carbon content compared to 4130 steel contributes to its greater hardenability and strength.
  • Manganese (Mn): Usually within the range of 0.75% to 1.00%.
  • Silicon (Si): Typically between 0.15% and 0.35%.
  • Chromium (Cr): Ranges from 0.80% to 1.10%. Some standards may show a range of 0.90% to 1.20%.
  • Molybdenum (Mo): Generally 0.15% to 0.25%. Other standards might list 0.15% to 0.30%. Molybdenum helps yield a desirable fine microstructure after hot working and heat treatment.
  • Phosphorus (P): Maximum of 0.035%.
  • Sulfur (S): Maximum of 0.040%. Some specific standards might list a maximum of 0.035%.

2. Physical properties

Physical Properties of 4140 Alloy Steel (42CrMo4) vs. Temperature

Temperature (°C)

Temperature (°F)

Thermal Conductivity (W/m·K)

Mean Specific Heat Capacity (kJ/kg·K)

Average Coefficient of Linear Expansion (x 10⁻⁶ K⁻¹)

Density (kg/m³)

Young’s Modulus (x 10⁵ MPa)

20

68

46.71

472.91

11.28

7848.2

211.6

100

212

46.06

486.33

11.67

7820.7

203.1

200

392

45.59

499.21

12.32

7790.2

197.5

300

572

43.47

519.18

12.85

7757.4

193.7

400

752

40.7

543.45

13.37

7722.3

188.6

500

932

37.67

570.3

13.9

7684.7

180.2

600

1112

34.63

599.31

14.42

7644.5

167.0

3. Mechanical Properties

The mechanical properties of 4140 alloy steel can be significantly tailored through heat treatment. The most common process, quenching and tempering, is widely used for machinery parts, pressure vessels, and structural applications to enhance yield strength, tensile strength, and notch toughness beyond what is achievable in as-rolled, annealed, or normalized states.

3.1 Quenched and Tempered 4140 Steel

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

Table 1: Typical Mechanical Properties of Oil-Quenched and Tempered 4140 Alloy Steel

Tempering Temperature

Tensile Strength (MPa)

Yield Strength (MPa)

Elongation (%)

Reduction in Area (%)

Hardness (HB)

205 °C (400 °F)

1965 – 1980

1740 – 1860

11

39 – 42

520 – 578

260 °C (500 °F)

1860

1650

11

44

534

315 °C (600 °F)

1720 – 1760

1570 – 1620

11.5 – 12

44 – 46

490 – 495

425 °C (800 °F)

1450 – 1500

1340 – 1365

14 – 15

48 – 50

429 – 440

540 °C (1000 °F)

1150 – 1240

1050 – 1160

17 – 17.5

53 – 55

341 – 360

595 °C (1100 °F)

1020

910

19

58

311

650 °C (1200 °F)

900 – 1020

790 – 860

20 – 21

60 – 61

277 – 290

705 °C (1300 °F)

810 – 860

690 – 740

23

63 – 65

235 – 250

Note: Values may vary based on specific mill practices and slight compositional differences. HB denotes Brinell Hardness.

3.2 Influence of Section Size (Mass Effect) on 4140 Steel 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, meaning larger sections may not achieve the same through-hardness or strength as smaller sections when subjected to the same heat treatment.

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

Table 2: Effect of Bar Diameter on Mechanical Properties of 4140 Steel (Tempered at 540 °C / 1000 °F)

Bar Diameter

Tensile Strength (MPa)

Yield Strength (MPa)

Elongation (%)

Reduction in Area (%)

Surface Hardness (HB)

25 mm (1 in.)

1140

985

15

50

335

50 mm (2 in.)

920

750

18

55

202

75 mm (3 in.)

860

655

19

55

293

Understanding this mass effect is crucial for designing components that will perform reliably.

3.3 Mechanical Properties of 4140 Steel in Other Conditions

Besides quenching and tempering, 4140 alloy steel can be supplied or processed in other conditions:

3.3.1 Annealed 4140 Steel

Annealing, which involves heating the steel to between 830 °C and 870 °C (1525 °F to 1600 °F) followed by slow furnace cooling, results in a softer, more ductile material. This condition is often preferred for subsequent machining operations.

  • Hot Rolled, Annealed 4140: Typically exhibits a yield strength of around 434 MPa (63 ksi) and a tensile strength of 620 MPa (90 ksi), with approximately 27% elongation.
  • Cold Drawn, Annealed 4140: Generally shows higher strength, with a typical yield strength of 620 MPa (90 ksi) and a tensile strength of 703 MPa (102 ksi), and around 18% elongation.

3.3.2 Normalized 4140 Steel

Normalizing involves heating 4140 steel to between 845 °C and 925 °C (1550 °F to 1700 °F) followed by air cooling. This process refines the grain structure and improves uniformity. For instance, a valve-bonnet forging made of 4140 steel, when normalized at 870 °C (1600 °F) and subsequently tempered, can achieve a hardness in the range of 220 to 240 HB.

3.4 Hardenability of 4140 Alloy Steel

A key feature of 4140 alloy steel is its good hardenability, which refers to its capacity to be hardened by heat treatment. This is a measure of the depth to which the steel will harden when quenched. For specialized applications, 4140H steel grades offer guaranteed hardenability characteristics, often verified using the Jominy end-quench test, with requirements such as a minimum hardness of 49 HRC at specific Jominy distances.

4140 alloy steel
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4. Heat Treatment

Achieving peak performance from 4140 alloy steel applications hinges on precise and appropriate heat treatment. As a versatile chromium-molybdenum alloy steel, 4140 offers an excellent combination of strength, toughness, and wear resistance when correctly processed. 

4.1 Heat Treatment Processes for 4140 Alloy Steel

Several standard heat treatments can be applied to 4140 alloy steel to achieve a wide range of mechanical properties. Selecting the correct process is crucial for the end-use application.

4.1.1 Normalizing 4140 Steel

Normalizing is often employed as a conditioning treatment prior to final heat treatment or to refine the grain structure of components that have undergone non-uniform cooling from high temperatures. This process improves the homogeneity and mechanical properties of the 4140 steel.

  • Heating Temperature: 845°C to 925°C (1550°F to 1700°F). A common specific range is 870°C to 900°C (1600°F to 1650°F).
  • Holding Time: Minimum 1 hour, or 15-20 minutes per 25mm (1 inch) of maximum section thickness.
  • Cooling: Air cool.

4.1.2 Annealing 4140 Steel

Annealing is primarily used to soften 4140 alloy steel, improving its machinability and relieving internal stresses. This treatment results in a more ductile material.

  • Heating Temperature: Typically 830°C to 870°C (1525°F to 1600°F). An alternative range for achieving a maximum hardness of 197 HB is 790°C to 845°C (1450°F to 1550°F).
  • Holding Time: Dependent on section thickness or furnace load.
  • Cooling: Furnace cool at a controlled rate of approximately 15°C/h (30°F/h) down to 480°C (900°F), followed by air cooling.
  • Expected Hardness: Approximately 150-200 HB in the annealed condition.

4.1.3 Spheroidizing 4140 Steel for Enhanced Machinability

Spheroidizing creates a microstructure of globular carbides within a ferritic matrix, which is optimal for the machinability of 4140 steel.

  • Heating Temperature: 760°C to 775°C (1400°F to 1425°F).
  • Holding Time: 4 to 12 hours.
  • Cooling: Slow cool. Note: Full spheroidizing may involve austenitizing at temperatures slightly above Ac1 or midway between Ac1 and Ac3, followed by very slow cooling or holding.

4.1.4 Hardening 4140 Alloy Steel: Quenching and Tempering

The most common method to achieve high strength and toughness in 4140 alloy steel is through quenching and tempering. This two-stage 4140 alloy steel heat treatment is critical for demanding applications.

a. Austenitizing (Heating for Hardening). 

  • Furnace Heating Temperature: 830°C to 855°C (1525°F to 1575°F).
  • Induction Heating Temperature: 870°C to 900°C (1600°F to 1650°F) is recommended for induction hardening.
  • General Hardening Range: 860°C to 885°C (1575°F to 1625°F).
  • Soak Time: A general guideline is 5 minutes per inch (25mm) of the smallest cross-section once the part reaches temperature.

b. Quenching. 

  • Quenching Medium: Oil is the preferred medium for 4140 steel. Water quenching is possible but increases the risk of cracking and distortion due to the steel’s carbon content; if used, extreme caution is necessary.
  • Critical Post-Quench Step: Tempering must be initiated as soon as parts cool to 52°C to 65°C (125°F to 150°F) to prevent cracking.
  • Distortion Control: For flat or tubular sections, vertical immersion during quenching can help minimize stress and distortion.

c. Tempering 4140 Steel.

  • Process: Heat the quenched part to a specific temperature below the critical point (Ac1), hold for a sufficient duration, and then cool.
  • Holding Time: Typically 2 hours per inch (25mm) of cross-section.
  • “Blue Brittleness” Range: Avoid tempering 4140 steel between 230°C and 370°C (450°F to 700°F) as this can result in reduced toughness.

4.2 Hardenability in 4140 Steel

Hardenability refers to the depth to which steel can be hardened during quenching. 4140 alloy steel’s chromium and molybdenum significantly contribute to its hardenability. It’s essential to acknowledge that the mechanical properties, particularly in larger cross-sections, can vary from the surface to the core due to differences in cooling rates. 

4.3 Specialized Heat Treatments for 4140 Alloy Steel

Beyond the primary processes, other treatments can be applied to 4140 steel for specific requirements:

a. Stress Relieving:

  • Purpose: To reduce internal stresses induced by machining, forming, or straightening, especially before hardening.
  • Temperature: 650°C to 675°C (1200°F to 1250°F). If applied to hardened material, the stress-relieving temperature must not exceed the prior tempering temperature.

b. Induction Hardening:

  • Purpose: A surface hardening technique to significantly improve wear resistance and surface yield strength, creating a hard case over a tougher core. Often used for pins, shafts, and other high-duty components after a primary quench and temper.
  • Austenitizing Temperature (Induction): 870°C to 900°C (1600°F to 1650°F).
  • Tempering (Induction): Involves very short cycles; time and temperature are critical.

c. Nitriding:

  • Purpose: A surface hardening process that introduces nitrogen into the steel’s surface, enhancing wear resistance and often improving fatigue life. 4140 steel is readily nitridable.
  • Temperature: Typically around 520°C to 550°C (970°F to 1020°F) for extended periods in a nitrogen-rich atmosphere.

5. Applications of 4140 Alloy Steel

4140 alloy steel applications fully benefit from heat treatment. Processes like quenching and tempering develop a wide range of mechanical properties. 

5.1 Industrial Uses of 4140 Alloy Steel

The inherent properties of 4140 make it exceptionally well-suited for a diverse range of components across various industries. Here’s a breakdown of typical 4140 alloy steel applications:

Application Category

Specific Examples

Key 4140 Properties Utilized

Machinery Components

Shafts (a primary use), Axles, Gears, Spindles, Couplings, Crankshafts, Connecting Rods, Valve Bonnets, Chuck Bodies, Collets

High tensile & yield strength, Good toughness, Wear resistance, Moderate hardenability

Tooling & Fixtures

Jigs, Fixtures, Tool holders, Drill collars, Bolts, Studs, Conveyor parts

Good strength, Durability, Machinability (especially prehardened or modified grades)

Automotive & Aerospace

Axles, Crankshafts, Steering knuckles, High-strength structural parts

Strength-to-weight ratio, Fatigue resistance, Toughness

Oil & Gas Industry

Downhole drilling tools, Tool joints, Pump shafts

Strength under stress, Resistance to wear and moderate corrosion

Surface Hardened Parts

Components requiring enhanced surface durability

Excellent for nitriding (better than 4337, 4340, 8640), Suitable for induction & flame hardening

5.2 Tailoring 4140 Steel for Specific Applications

Heat Treatment and Hardenability:

Good hardenability is critical for many 4140 alloy steel applications, ensuring sufficient hardness throughout a part’s cross-section. While not as deep-hardening as alloys like 4340, it offers an excellent balance for many component sizes. Prehardened 4140 (typically 262-321 HB) is popular for machine parts and tooling due to its ready-to-use high tensile and yield strengths, often eliminating further heat treatment by the end-user.

Surface Hardening Capabilities:

For superior surface wear resistance, 4140 alloy steel offers excellent characteristics for surface hardening. It is particularly well-suited for nitriding, often outperforming other chromium-containing steels due to its higher chromium content and nickel-free composition. Induction and flame surface hardening are also common methods to enhance 4140 component surfaces.

Machinability Considerations:

While standard 4140 has reasonable machinability, for applications prioritizing ease of machining, modified grades are available. Resulfurized versions (e.g., 4140+S) and leaded variants (e.g., 41L40) are free-cutting low-alloy steels, offering improved machining for high-volume production.

Weldability:

4140 alloy steel is weldable, but its air-hardening nature and higher carbon equivalent necessitate specific procedures like preheating, post-weld stress relief (e.g., annealing or normalizing and tempering), and suitable filler materials to prevent cracking and ensure sound welds. Its design focused on high strength through heat treatment, not optimal weldability.

Available Forms & Specifications:

To serve diverse 4140 alloy steel applications, this grade is available in various forms:

  • Billets
  • Bars (round, square, flat)
  • Rods
  • Forgings
  • Sheet
  • Tubing
  • Light Plate

Specific AMS (Aerospace Material Specifications) include:

  • AMS 6349B: Normalized bars
  • AMS 6381E, AMS 6390C: Mechanical tubing
  • AMS 6382K: Bars, forgings, and rings

6. Equivalent grades for 4140 alloy steel 

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

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