4340M Alloy Steel
4340M Alloy Steel is a high-strength, low-alloy steel that contains nickel, chromium, and molybdenum as its primary alloying elements. As the “M” in the name indicates, it is a modified version of 4340 alloy steel, with disparities related to variations in composition and heat treatment. In some cases, the addition of modest amounts of vanadium and niobium may alter its properties, such that they refine its grain structure and improve its toughness.
The predominance of 4340M alloy in various applications is evident in industries like aerospace, oil and gas, and automotive, owing to its exceptional strength, fatigue resistance, and toughness. Compared to the standard 4340 alloy steel, these qualities are further optimized through thermo-mechanical processing, which results in good ductility and weldability.
4340M alloy steel is commonly used to manufacture components that are subjected to heavy loads and high stress. Examples are crankshafts, landing gears, rocket motor cases, fasteners, pressure vessels, and axles. These parts are able to thrive in the most demanding environments, withstanding extreme temperatures, pressure, and impact.
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4340M Alloy Mechanical Properties
- Fatigue strength: 690 MPa (100 x 103 psi)
- Brinell hardness: 710
- Fracture toughness: 60 MPa-m1/2 (55 x 103 psi-in1/2)
- Elastic (Young’s, Tensile) Modulus: 190 GPa (27 x 106 psi)
- Impact strength (V-notched Charpy): 18J (13 ft-lb)
- Elongation at break: 6.0%
- Rockwell C hardness: 55
- Poisson’s ratio: 0.29
- Tensile strength (Yield proof): 1240 Mpa (180 x 103 psi)
- Tensile strength (Ultimate UTS): 2340 MPa (340 x 103 psi)
- Shear modulus: 72 GPa (10 x 106 psi)
4340M Alloy Chemical Properties
| Element | min | max |
| Carbon | 0.38 | 0.43 |
| Manganese | 0.65 | 0.90 |
| Silicon | 1.50 | 1.80 |
| Phosphorus | 0 | 0.012 |
| Sulfur | 0 | 0.012 |
| Chromium | 0.7 | 1.0 |
| Nickel | 1.7 | 2.0 |
| Molybdenum | 0.35 | 0.45 |
| Vanadium | 0.050 | 0.1 |
| Iron | 93.3 | 94.8 |
4340M Alloy Thermal Properties
- Melting onset (Solidus): 1400 °C (2550 °F)
- Maximum temperature (Mechanical): 430 °C (800 °F)
- Latent heat of fusion: 280 J/g
- Melting completion (liquids): 1440 °C (2620 °F)
- Thermal expansion: 13 µm/m-K
- Thermal conductivity: 38 W/m-K (22 BTU/h-ft-°F)
- Specific heat capacity: 480 J/kg-K (0.11 BTU/lb-°F)
4340M Alloy Electric properties
- Electrical conductivity (specific equal weight): 9.1 % IACS
- Electrical conductivity (equal volume): 7.8 % IACS
Other Properties of 4340M Alloy
- Density:480 lb/ft3 (7.85 g/cm3)
- Embodied energy: 26Mj/kg (11 x 103 BTU/lb)
4340M Fabrication & Working Instructions
4340M is machinable with conventional methods like drilling, milling, and turning. Before working with the material, ensure that it is normalized to reduce the risk of cracking and use high-speed tooling with ceramic tools or carbide inserts. The machining tool should have a positive rake angle to prevent work hardening and reduce cutting forces. Use appropriate cutting speeds, feed rates, tool geometry, and coolants to prevent overheating and eliminate the risk of thermal distortion.
Since 4340M has a high hardenability, welding is not recommended because it may crack eventually. But if welding is required, use preheating and post-weld heat treatment to prevent this outcome. Ideal welding techniques include shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and gas tungsten arc welding (GTAW).
Although 4340M is highly resistant to corrosion, surface treatments like painting, plating, and powder coating can be used to improve its resistance and also enhance its appearance. Before handling this material, make sure the workspace is safe, clean, and clutter-free. Wear the right personal protective equipment like gloves, eye protection, and respiratory protection to prevent harmful exposure.
4340M Alloy Heat Treatment
Hardening and tempering is the heat treatment process used to increase the strength of 4340M alloy. First, it is heated to a temperature above its critical point of 830°C, dipped in a quenching liquid, reheated to a lower temperature, and then cooled slowly. This increases its hardness while reducing its brittleness.
Availability of 4340M Alloy
4340M alloy is supplied in more than one condition, depending on the specific application requirements. Clients often prefer it hardened and tempered with a tensile strength of 930 -1080 MPA, however, others prefer it normalized and tempered.
Hardened and Tempered
First, the steel is heated above the critical range and then quenched in oil or water to harden the microstructure. It is then tempered at a lower temperature to improve its toughness and reduce the risk of cracking. This produces the highest strength and hardness.
Normalized and Tempered
This condition involves heating the steel to a temperature above the critical range, followed by air cooling to normalize the microstructure, and then tempering at a lower temperature to improve the strength and toughness. It is often used for components that require good strength and toughness, but do not need to be as hard as those produced in the hardened and tempered condition.
Benefits of 4340M Alloy
4340M alloy offers several benefits for various applications, including:
High strength: 4340M is known for its high strength, making it ideal for heavy-duty applications.
Toughness: In addition to its high strength, it also has excellent toughness, which makes it fair well under impact without breaking or deforming.
Wear Resistance: The low-alloy material possesses good wear resistance and as such can be used in any type of application that entails sliding or abrasive wear.
Fatigue Resistance: 4340M can withstand repeated loading and unloading without bending, cracking, or breaking.
Machinability: It is relatively easy to machine compared to other high-strength steels, making it easier and more cost-effective to produce complex parts.
Weldability: When proper precautions are applied, it can be welded, which makes it useful in fabricating and repairing components.
Ductility: 4340M has good ductility, which is the ability of a material to deform without breaking under tensile stress. It can be formed into various shapes and configurations for applications that require strength and flexibility.
Why Trust Us for 4340M Delivery?
At Friend Metals Distributor, we prioritize our clients’ need for high-quality materials that meet their unique needs. 4340M alloy is a prime example.
Do you need custom sizes, shapes, or quantities? We are ready to supply them on your terms. We keep abreast of the latest tools and technologies to deliver the best solutions, some of which include precision cutting, CNC machining, heat treatment, inspection and testing, and online ordering.
So why wait? Contact us today to learn more about 4340M alloy and to place your order.