4340 Alloy steel | 34CrNiMo6 | 1.6582

3. Heat Treatment

The following are standard heat treatment procedures applicable to 4340 alloy steel, designed to modify its microstructure and mechanical properties:

3.1 Annealing 4340 Steel for Enhanced Machinability

Annealing is primarily employed to soften 4340 alloy steel, thereby improving its machinability for subsequent processing.

• Process: Heat the steel to a temperature range of 790 to 845 °C (1450 to 1550 °F).
• Holding: Maintain this temperature for a duration 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.
• Outcome: Results in a machinability rating of about 45% (compared to B1112 cold-rolled steel at 100%). Yield strength will be lower than in hardened and tempered states. This 4340 alloy steel heat treatment step is crucial for preparing the material for machining operations.

3.2 Normalizing 4340 Steel for Structural Uniformity

Normalizing serves to refine the grain structure of 4340 forgings or rolled products, promoting a more uniform microstructure and potentially enhancing machinability. It is also a common conditioning treatment prior to final heat treatment.

• Process: Heat the 4340 steel to a temperature above its upper transformation point, typically 870 °C (1600 °F), or within the range of 845 to 925 °C (1550 to 1700 °F).
• Holding: Hold for a minimum of 1 hour, or 15 to 20 minutes per 25 mm (1 inch) of maximum section thickness.
• Cooling: Air cool.
• Outcome: For optimal machinability, a structure obtained by normalizing followed by tempering at 650 °C (1200 °F) is often recommended. This 4340 alloy steel heat treatment ensures a consistent starting point for further processing.

3.3 Hardening 4340 Steel: Austenitizing and Quenching

The hardening process is fundamental to achieving the high hardness and strength levels characteristic of properly treated 4340 alloy steel. This involves forming a martensitic microstructure.

• Austenitizing: Heat to 800 to 845 °C (1475 to 1550 °F), typically targeting 845 °C (1550 °F).
• Holding: Soak for approximately 15 minutes per 25 mm (1 inch) of section size, with a minimum hold time of 150 minutes.
• Quenching: Due to its excellent deep hardenability, 4340 steel is normally oil quenched for round sections up to about 75 mm (3 inches) in diameter. For larger sections, water quenching may be necessary to achieve through-hardening, though this significantly increases the risk of cracking at this carbon level. Thin sections of 4340 can even achieve hardness through air cooling.
• Outcome: The as-quenched microstructure is primarily martensite, resulting in high hardness, potentially around 601 HB or a surface hardness of 58 HRC. This stage of 4340 alloy steel heat treatment is critical for developing its strength.

3.4 Tempering 4340 Steel for Toughness and Ductility

Immediately after quenching, the hardened 4340 steel, with its brittle martensitic structure, must be tempered. Tempering reduces hardness slightly but significantly improves toughness, ductility, and impact strength, achieving the desired balance of mechanical properties.

a. Timing: Initiate tempering as soon as the quenched parts have cooled to approximately 52 to 65 °C (125 to 150 °F). This is crucial to prevent cracking.

b. Temperature Range: Tempering temperatures for 4340 typically range from 200 to 650 °C (400 to 1200 °F). The specific temperature selected directly dictates the final mechanical properties.

c. Holding Time: A standard tempering time is 1 to 2 hours, or 2 hours per 25mm (1 inch) of cross-section.

d. Property Impact:

• Tempering at 205 °C (400 °F) yields high tensile strength (approx. 1980 MPa) and hardness (approx. 520 HB / 53 HRC), but with relatively low impact energy (approx. 20 J).
• Tempering at 650 °C (1200 °F) results in lower strength (approx. 1020 MPa) and hardness (approx. 290 HB / 31 HRC), but significantly higher impact energy (approx. 100 J).

e. Caution: It is generally recommended to avoid tempering 4340 alloy steel within the “blue brittleness” range of 230 to 370 °C (450 to 700 °F).

f. Considerations: While 4340 is relatively immune to temper embrittlement, achieving very high strength levels (above approx. 1400 MPa or 200 ksi) makes it susceptible to hydrogen embrittlement. In such cases, baking after processes like pickling or plating is necessary. Furthermore, 4340 exhibits poor resistance to stress-corrosion cracking when tempered to extremely high strength levels (1500 to 1950 MPa or 220 to 280 ksi). Careful selection of tempering parameters in the 4340 alloy steel heat treatment cycle is paramount.

3.5 Stress Relieving 4340 Steel

Stress relieving is performed to reduce internal stresses induced by prior manufacturing operations such as straightening, forming, or heavy machining, especially before hardening.

• Temperature: A typical stress relief temperature is 650 to 675 °C (1200 to 1250 °F).
• Note: If stress relieving an already hardened and tempered part, the stress-relieving temperature must not exceed the original tempering temperature to avoid altering the desired mechanical properties.
• Welding: For welded 4340 components, stress relieving immediately after welding, preferably before the part cools to room temperature and at a temperature below A1 (the lower critical temperature), is often recommended.

3.6 Nitriding 4340 Steel for Surface Hardness

4340 alloy steel is well-suited for nitriding, a surface hardening process that produces a very hard, wear-resistant case and can improve fatigue life.

• Prerequisite: Nitriding is typically performed on 4340 steel that has already been hardened and tempered to establish the desired core properties.
• Optimizing Case Hardness: To achieve maximum case hardness from nitriding, the core hardness should be maximized. This means tempering the steel at the lowest acceptable temperature (consistent with desired core toughness) prior to the nitriding process.
• Outcome: Typical case hardness for nitrided 4340 can exceed 60 HRC. This specialized 4340 alloy steel heat treatment is ideal for wear-critical applications.

3.7 Summary of 4340 Alloy Steel Heat Treatment Parameters

To provide a quick overview, the table below summarizes the key parameters for common heat treatments of 4340 alloy steel: