p20 tool steel | 1.2311

3. P20 Tool Steel Heat Treatment

Achieving optimal performance and longevity from your P20 tool steel components hinges on precise heat treatment.

3.1 Understanding P20 Tool Steel’s As-Delivered Condition

Typically, P20 tool steel is supplied in a prehardened state. This means it has already undergone a specific heat treatment by the mill, resulting in a hardness generally between 28 and 40 HRC (Hardness Rockwell C), though sometimes reaching up to 60 HRC. This pre-hardened condition offers excellent machinability. For many applications, such as die casting of low-melting-point alloys, you can machine cavities directly into this prehardened P20 and put the mold into service without requiring further high-temperature heat treatment. This approach offers the significant advantages of minimizing distortion and dimensional changes.

3.1.1 Preheating

Preheating is a critical preparatory step for nearly all tool steels, including P20.

• Objective: To minimize thermal shock when the steel is introduced to higher hardening temperatures, thereby reducing the risk of distortion or cracking. It also helps to alleviate stresses induced during machining.
• Procedure: Heat the P20 component slowly and uniformly to approximately 650°C (1200°F). Ensure the entire cross-section reaches this temperature. A holding time of 10-15 minutes at the temperature is typical once uniformity is achieved.

3.1.2 Austenitizing (Hardening)

This is the high-temperature stage where the steel’s microstructure is transformed.

• Objective: To heat the P20 steel to a temperature where its structure converts to austenite and carbides dissolve, which is essential for achieving full hardness upon quenching.
• Procedure: For P20 tool steel, the hardening temperature is typically between 830°C (1525°F) and 845°C (1550°F). The soaking time at this temperature is crucial: allow 45-60 minutes for components up to 1 inch (25mm) in thickness. For thicker sections, maintain the soak for an additional 45-60 minutes for each inch of thickness.

3.1.3 Quenching

Rapid cooling is necessary to achieve the desired hardness.

• Objective: To cool the austenitized P20 steel quickly enough to transform the austenite into martensite, a very hard microstructure.
• Procedure: P20 is a medium-hardenability steel and is normally quenched in oil. Oil provides a cooling rate that is slower and less severe than water, reducing thermal shock and internal stresses. After quenching, the steel should be cooled to 66-93°C (150-200°F) before proceeding to tempering.

3.1.4 Tempering

Tempering is performed immediately after hardening to refine the steel’s properties.

• Objective: To relieve internal stresses developed during quenching, significantly increase toughness, reduce brittleness, and transform any retained austenite.
• Procedure: Heat the quenched P20 component slowly and uniformly to the selected tempering temperature. For P20, this typically ranges from 200°C to 650°C (390°F to 1200°F), with common practice falling between 400-550°C (750-1020°F), depending on the final hardness required. Hold at the tempering temperature for approximately 2 hours per inch of cross-section. Multiple tempering cycles (e.g., two cycles of 2 hours each) are highly recommended for P20 to ensure optimal stress relief and microstructural stability.

3.2 Specialized Heat Treatment Processes for P20 Steel

Beyond the standard hardening and tempering cycle, other treatments can be beneficial for P20 tool steel.

3.2.1 Stress Relieving P20 Prior to Hardening

If extensive machining has been performed on the P20 component, stress relieving before the final hardening process is advisable.

• Purpose: To minimize the risk of distortion and cracking during the subsequent austenitizing and quenching stages.
• Procedure: Heat the steel slowly to 705-730°C (1300-1350°F). Hold at this temperature for a minimum of 2 hours, or 30 minutes per inch of the largest cross-section. Follow this with slow cooling in still air.

3.2.2 Carburizing P20 for Enhanced Surface Hardness

For applications requiring superior surface wear resistance, such as in plastic molding, P20 tool steel is frequently carburized.

• Purpose: To introduce carbon into the surface layer of the steel, significantly increasing its surface hardness.
• Outcome: A carburized P20 surface can achieve hardness levels up to 65 HRC in the as-quenched condition. It’s important to note that carburizing temperatures exceeding 870°C (1600°F) can negatively impact the polishability of P20. The carburized case will also soften more readily during tempering compared to the core material.

3.3 Managing Dimensional Changes in P20 Heat Treatment

P20, being an oil-hardening steel, generally exhibits less dimensional change during heat treatment compared to water-hardening steels. An expected growth of approximately +0.0015 inches per inch of dimension can be anticipated. Implementing proper stress relieving before hardening and adhering to correct tempering procedures are key to controlling and minimizing distortion.