4340 Steel Technical Overview

4340 Steel Technical Overview: 4340 steel is a popular medium-carbon, low-alloy steel. It is known for its high strength, deep hardenability, and toughness, achieved through heat treatment processes like quenching and tempering. This steel typically contains alloying elements such as chromium, nickel, and molybdenum and is often used in applications requiring moderately severe service conditions, including components like gears, splines, and shafts.

1. Chemical composition

Carbon (C)	Chromium (Cr)	Nickel (Ni)	Molybdenum (Mo)	Manganese (Mn)	Silicon (Si)	Phosphorus (P)	Sulfur (S)
0.38 – 0.43%	0.70 – 0.90%	1.65 – 2.00%	0.20 – 0.30%	0.60 – 0.80%	0.15 – 0.35%	≤ 0.035% (max)	≤ 0.040% (max)

2. Mechanical Properties of 4340 Steel

The typical mechanical properties of 4340 steel after oil quenching from 845 °C (1550 °F) and tempering at various temperatures

Tempering Temperature	Tensile Strength	Yield Strength	Elongation in 50mm (2 in.)	Reduction in Area	Hardness	Izod Impact Energy
205 °C (400 °F)	Approx. 1980 MPa (287 ksi)	Approx. 1860 MPa (270 ksi)	Around 11%	Around 39%	Approx. 520 HB (≈ 53 HRC)	Around 20 J (15 ft·lb)
425 °C (800 °F)	Approx. 1500 MPa (217 ksi)	Approx. 1365 MPa (198 ksi)	Around 14%	Around 48%	Approx. 440 HB (≈ 46 HRC)	Around 16 J (12 ft·lb)
650 °C (1200 °F)	Approx. 1020 MPa (148 ksi)	Approx. 860 MPa (125 ksi)	Around 20%	Around 60%	Approx. 290 HB (≈ 31 HRC)	Around 100 J (74 ft·lb)

3. 4340 steel applications

• Gears, Splines, and Shafts: The high strength and toughness of 4340 steel make it widely used to manufacture gears and splines that must endure large amounts of torque and impact loads. It’s also employed to make shafts for other machinery where resistance to fatigue and torsional stress are essential.
• Aircraft Components: Heat-treated 4340 steel’s ability to achieve high strength-to-weight ratios makes it used in specific aircraft structures, such as landing gear components and parts of the airframe, where failure under extreme conditions is not an option. It’s important to note that we must require rigorous testing and quality control, such as transverse mechanical-property testing, for aircraft.
• Automotive Parts: Some applications for 4340 steel may include high-demand automobile parts where strength and durability are important, such as certain types of axles and suspension components.
• Tooling and Dies: Although 4340 is not typically a primary “tool steel” like high-carbon or high-speed steels, its combination of strength and toughness can make it useful for some tooling applications, such as gripper dies and drop-forging dies, primarily for shorter production runs or less severe service where extreme wear resistance is not paramount.
• Oil & Gas Industry Components: The good strength and toughness of 4340 steel also make it suitable for some of the oil and gas industry components that are in difficult environments and high-stress parts. For example, it could be used in piping and valves for petroleum gathering and refining
• Bearing Applications: 52100 is frequently used for bearings, but for some tighter tolerance or lower performance applications, 4340, especially in a carburized condition, would still be considered, where a balance of strength and toughness is needed.
• High-Strength Bolting and Fasteners: Due to its capacity for high strength levels attainable via heat treatment, 4340 steel is a candidate for high-strength bolts and fasteners in critical or high-stress scenarios.
• Machinery and Structural Components: 4340 is commonly used as a high-strength machinery steel, though it may also be used in a wide variety of structural and mechanical components where moderate to high strength and good toughness are required. Examples could be parts for heavy equipment, industrial machinery, or other high-demand engineering.

4. Heat treatment

4.1 Preheating

This step is useful to limit thermal shock to steel in the following high-temperature austenitizing stage. It improves the overall uniformity of the heating and reduces the chances of cracking, particularly in areas of the component with complex geometries or varying cross-sections. For 4340 steel, a common preheat temperature would be 1200°F (650°C) for a short period of time to bring the part up to uniform temperature. This step can also help relax some stresses induced during prior manufacturing processes such as machining.

4.2 Austenitizing

Austenitizing is the key process, and it consists of heating the steel to a selected high temperature, which is usually in the range of 1500 to 1575°F (815 to 855°C) and holding at this temperature for a sufficient soak time to allow the microstructure to transform 100% to austenite. The thickness of the sections will determine the specific austenitizing temperature and hold period. The goal is to achieve a homogeneous austenitic structure before the next stage.

4.3 Quenching

After austenitizing, the steel has to be cooled quickly enough to convert the austenite to martensite, a high-strength and frequently brittle phase. Oil quenching is the most common method for 4340 steel to achieve a good balance of hardness and minimize distortion and cracking. For larger sections, though (e.g., greater than 75 mm (3 in) diameter), water quenching may be used to obtain through hardening, although this greatly increases the likelihood of cracking. It is important to mention that straightening of the part, if needed due to distortion during quenching, can often be carried out while the steel temperature is still above about 400°F (205°C).

4.4 Tempering

As-quenched 4340 steel has a totally martensitic structure, which is typically too brittle for most engineering applications. After quenching, the hardened steel must be tempered, usually by re-heating it to a lower temperature, generally between 300 to 1300°F (150 to 705°C), holding it at that temp for a specific length of time (normally at least 2 hours per inch of cross-section), before cooling to room temperature. As mentioned earlier, the tempering temperature directly influences the mechanical properties achieved. Generally, lower tempering temperatures produce higher strength and hardness, while higher temperatures produce greater ductility and toughness. For some applications, a double temper could sometimes be used.

4.5 Stress Relieving (Optional, Recommended)

To reduce residual stresses from machining, forming, or welding, consider a stress-relieving treatment before the hardening process (to avoid excessive distortion during hardening) or after hardening (but below the tempering temperature to avoid affecting the tempered hardness). For steel, a common stress relief temperature range would be 650 to 675°C (1200 to 1250°F), with a holding time depending on section thickness.

4.6 Subzero Treatment (Optional)

If dimensional stability is of utmost importance, components may be subjected to a subzero treatment (refrigerated to -87 to -60°C or -125 to -75°F) after quenching and prior to tempering to convert remaining austenite into martensite. This is followed by tempering to obtain the final desired properties and lessen the brittleness of the newly formed martensite.

5. What’s the difference between 4140 and 4340 steel?

The key difference between 4140 and 4340 steel lies in the addition of nickel in 4340, which, along with a slightly higher molybdenum content in some specifications, leads to enhanced hardenability, higher strength, and improved toughness compared to 4140. This makes 4340 more suitable for higher-stress applications and larger component sizes. When selecting between the two, it is vital to consider the specific mechanical property requirements and the dimensions of the part in relation to the hardenability characteristics of each steel grade.