1. Introduction
4140 steel is a low-alloy chromium-molybdenum steel.
It offers an excellent combination of strength, toughness, and wear resistance, making it an ideal candidate for demanding industrial applications.
This article explores 4140 alloy steel from multiple technical perspectives, including its chemical composition, mechanical behavior, heat treatment response, machinability, corrosion performance, and common uses.
2. Chemical Composition of 4140 Alloy Steel
The unique performance of 4140 alloy steel originates from its carefully controlled chemical composition:
| Element | Weight % | Role in Steel Properties |
|---|---|---|
| Carbon (C) | 0.38–0.43 | Enhances strength and hardenability |
| Chromium (Cr) | 0.8–1.1 | Improves hardness, wear resistance, and corrosion |
| Manganese (Mn) | 0.75–1.0 | Boosts toughness and deoxidation |
| Molybdenum (Mo) | 0.15–0.25 | Enhances creep resistance and depth of hardening |
| Silicon (Si) | 0.15–0.35 | Increases strength, improves toughness slightly |
| Phosphorus (P) | ≤ 0.035 | Typically minimized to reduce embrittlement |
| Sulfur (S) | ≤ 0.04 | Added for machinability but can reduce toughness |
Compared to similar alloys like 4130 (lower carbon) and 4340 (higher nickel), 4140 balances strength and machinability, making it a practical and cost-effective solution for many structural applications.
3. Physical Properties of 4140 Steel
| Property | Value | Unit | Notes |
|---|---|---|---|
| Density | 7.85 | g/cm³ | Typical for low-alloy steels |
| Modulus of Elasticity (E) | ~205 | GPa | Stiffness in tension and compression |
| Shear Modulus (G) | ~80 | GPa | Useful for torsional applications |
| Poisson’s Ratio | 0.27–0.30 | – | Ratio of transverse strain to axial strain |
| Thermal Conductivity | 42.6 | W/m·K | At 100 °C; decreases slightly with higher temperatures |
| Specific Heat Capacity | 475 | J/kg·K | Approximate at room temperature |
| Electrical Resistivity | 205 | nΩ·m (nano-ohm meters) | Higher than pure iron; low conductivity compared to copper |
| Thermal Expansion Coefficient | ~12.0 | µm/m·K (20–100°C range) | Important in designing for thermal cycling or dimensional stability |
| Melting Point | 1416–1471 | °C | Narrower range due to alloying elements |
4. Mechanical Properties of 4140 Steel
AISI 4140 is a versatile chromium-molybdenum alloy steel known for its excellent mechanical strength, toughness, and fatigue resistance.
These 4140 steel properties can vary significantly depending on their heat treatment condition (e.g., annealed, normalized, quenched, or tempered).
Mechanical Properties Table
| Property | Annealed | Quenched & Tempered (Q&T) | Unit | Notes |
|---|---|---|---|---|
| Yield Strength | ~655 MPa | Up to 1,600 MPa | MPa (megapascals) | Q&T improves strength significantly |
| ~95 ksi | ~232 ksi | ksi (imperial) | ||
| Tensile Strength | 850–1,000 MPa | 1,000–1,100 MPa | MPa | Typical range after various heat treatments |
| 123–145 ksi | 145–160 ksi | ksi | ||
| Elongation at Break | 25–30% | 12–18% | % | Higher ductility in annealed state |
| Reduction in Area | ~50% | ~45% | % | Indicator of ductility and formability |
| Hardness (Rockwell C) | 18–28 HRC | Up to 50–55 HRC | HRC | Highly responsive to quenching and tempering |
| Charpy V-Notch Toughness | >54 J (annealed) | 20–35 J (Q&T at high hardness) | Joules | Performance in impact loading applications |
| Fatigue Strength (Endurance Limit) | ~420 MPa | Up to 700 MPa | MPa | Dependent on surface finish and loading cycles |
| Modulus of Elasticity (E) | ~205 GPa | – | GPa | Stiffness remains constant across conditions |
5. Heat Treatment Behavior of 4140 Alloy Steel
AISI 4140 alloy steel is highly responsive to a variety of heat treatment processes, allowing it to achieve a broad spectrum of mechanical properties tailored to specific engineering applications.
Its chromium and molybdenum content enhance its hardenability, making it especially well-suited for quenching and tempering operations.
Common Heat Treatment Processes
| Process | Typical Temperature Range (°C) | Purpose |
|---|---|---|
| Annealing | 760–790°C | Refines grain structure, softens steel, improves machinability |
| Normalizing | 870–900°C | Increases uniformity, refines structure, enhances mechanical consistency |
| Quenching | ~845–875°C, followed by oil/water/polymer quench | Produces martensitic structure for high hardness and strength |
| Tempering | 400–650°C (post-quench) | Adjusts hardness, relieves internal stress, improves ductility & toughness |
| Austempering | Quench to 260–400°C, hold until transformation | Produces bainitic structure, reduces distortion, balances strength-toughness |
6. Machinability and Fabrication of 4140 Steel
Machinability
Material 4140 steel exhibits moderate machinability in its annealed state and becomes more challenging as hardness increases.
In the annealed condition (typically around 18–22 HRC), it can be machined with high-speed steel or carbide tools, yielding good surface finishes and acceptable tool life.
However, once the steel is quenched and tempered to higher hardness levels (such as 30–50 HRC), its machinability decreases.
At this stage, carbide tooling, lower cutting speeds, and rigid machine setups become essential to avoid tool wear and part distortion.
For CNC turning, milling, or drilling operations, using proper cooling methods—particularly flood coolant—helps dissipate heat and improve chip evacuation.
Drilling harder 4140 sections often requires cobalt or carbide-tipped tools, while tapping hardened parts may benefit from thread milling or forming taps rather than conventional cutting taps.
Welding
Welding 4140 steel requires caution due to its high hardenability and risk of cracking.
To mitigate these risks, preheating the workpiece—typically to 200–400°C depending on the thickness—is strongly recommended.
Maintaining an interpass temperature around 200–300°C helps prevent thermal shock and hydrogen-induced cracking.
After welding, stress-relieving the component at approximately 600–650°C helps restore ductility and reduce residual stresses.
Low-hydrogen electrodes such as E8018-B2 or ER80S-D2 are typically used for filler material to ensure compatibility and reduce porosity.
In critical applications, post-weld heat treatment (PWHT) is necessary to maintain the integrity and toughness of the welded zone.
Cold and Hot Forming
4140 alloy steel can be cold worked in its annealed condition, although its higher strength compared to low-carbon steels limits its ductility.
Cold forming processes such as drawing and swaging are possible but require higher forces and may induce residual stresses that necessitate subsequent heat treatment.
Hot working, including forging and hot rolling, is more favorable for steel 4140.
The ideal forging temperature range is between 900°C and 1200°C, with the material typically finished above 850°C.
After hot forming, normalizing or annealing is recommended to refine the grain structure and prepare the steel for final machining or heat treatment.
7. Corrosion Resistance of 4140 Steel
While 4140 alloy steel excels in mechanical strength, it lacks inherent corrosion resistance.
In humid or marine environments, it readily oxidizes unless protected. To counter this, surface treatments such as:
- Nitriding for surface hardening and oxidation resistance
- Black oxide coating for light corrosion protection
- Electroplating or painting in high-humidity environments
8. Common Forms and Standards
4140 alloy steel is available in a wide variety of commercial forms to accommodate diverse industrial applications.
Its availability in different shapes, combined with its excellent mechanical properties and heat treatment versatility, makes it a popular choice in both standard and custom-fabricated components.
Common Forms of 4140 Steel
Manufacturers and fabricators can obtain 4140 steel in numerous forms, depending on the intended use and required processing:
- Round Bars: Commonly used for shafts, pins, gears, and fasteners, round bars are one of the most frequently supplied forms of steel 4140 due to their versatility in machining and heat treatment.
- Flat Bars and Plates: Ideal for tooling, wear components, and structural parts requiring large surface contact areas.
These forms are also suitable for flame cutting or water jet processing. - Forged Rings and Discs: Used in high-strength rotating machinery such as bearing races, couplings, and flanges.
- Hollow Bars and Tubes: Preferred in applications demanding weight reduction while maintaining strength, such as hydraulic cylinders and pressure-containing parts.
- Blocks and Billets: Suitable for custom machining and large forged components. These are typically used in die-making and heavy industrial equipment.
Industry Standards and Designations for 4140 Steel
| Standard Organization | Designation | Region/Country | Description |
|---|---|---|---|
| ASTM | ASTM A29 | United States | General specification for hot-wrought bars of carbon and alloy steels |
| ASTM | ASTM A322 | United States | Specification for alloy steel bars used in mechanical applications |
| ASTM | ASTM A519 | United States | Specification for seamless carbon and alloy steel mechanical tubing |
SAE |
SAE 4140 | United States | Chromium-molybdenum low-alloy steel for automotive and engineering applications |
| AISI | AISI 4140 | United States | Commonly used designation aligned with SAE 4140 |
| EN / DIN | 1.7225 / 42CrMo4 | Europe / Germany | European equivalent under EN 10083 for quench and temper steels |
| JIS | SCM440 | Japan | Japanese equivalent for high-strength alloy steel |
| GB | 42CrMo | China | Chinese equivalent with similar mechanical properties |
9. Applications of 4140 Alloy Steel
Steel 4140 is a go-to material in applications requiring strength, toughness, and wear resistance under fatigue and shock loading:
- Automotive: gears, crankshafts, tie rods, axles
- Aerospace: landing gear components, actuators
- Oil & Gas: drill collars, hydraulic fracturing parts
- Manufacturing: mandrels, dies, molds, tool holders
Case study: In a comparative fatigue test, a steel 4140 Q&T gear shaft demonstrated 10x the lifespan of a similar design made from mild steel, highlighting its long-term value.
10. Advantages and Limitations of 4140 Alloy Steel
Advantages:
- High strength-to-weight ratio for structural applications
- Excellent wear resistance after hardening
- Versatile heat treatment response
- Readily available in multiple forms and standards
Limitations:
- Not suitable for corrosive environments without surface protection
- Requires careful welding practices to avoid cracking
- Higher cost and complexity than plain carbon steels
11. Conclusion
4140 alloy steel offers a compelling blend of mechanical strength, toughness, and wear resistance, making it indispensable in performance-critical engineering applications.
When properly heat-treated and protected, it delivers exceptional service life under demanding operating conditions.
Whether for aerospace, energy, or tooling components, material 4140 steel remains one of the most trusted and capable materials in modern manufacturing.
Engineers who understand its behavior and processing requirements can fully harness its potential.
DEZE is the perfect choice for your manufacturing needs if you need high-quality 4140 steel parts.
