D2 Tool Steel Heat Treatment Guide

At Aobo Steel, with our extensive experience in tool steel forging, we understand that proper heat treatment is crucial to achieving optimal performance from D2 tool steel. This guide outlines the key steps for processing D2 in your factory applications. Following these procedures carefully will help you achieve the desired hardness, wear resistance, and toughness.

Preparation and Preheating

Before starting the heat treatment, proper preparation is important.

Minimizing Decarburization

To prevent carbon loss from the steel surface during heating (decarburization), we recommend wrapping the D2 part in stainless steel foil. For general heat treating up to 1093 °C (2000°F), Type 321 foil (0.05 mm / 0.002 in. thick) is suitable. If using higher austenitizing temperatures, consider Type 309 foil.

Preheating D2 Steel

Preheating is a critical step, especially for high-chromium steels like D2, which don’t transfer heat quickly. This helps prevent thermal shock and potential cracking.

• Recommended Preheat: Heat the steel to approximately 649 °C (1,200°F).
• Hold Time: Maintain this temperature for 10-15 minutes.

Uniform heating is crucial for preventing stress buildup.

Austenitizing D2

Austenitizing transforms the steel’s structure and dissolves carbides, which is vital for achieving the final properties.

• Temperature: Heat the preheated D2 steel to the austenitizing range, typically 1010 °C (1850°F).
• Soak Time: Maintain the steel at this temperature to ensure the core reaches the target temperature and undergoes the necessary transformation. A common guideline is 1 hour per 25 mm (1 inch) of the thickest section.
• Atmosphere: It’s essential to heat D2 in a controlled atmosphere, such as a vacuum furnace or neutral salt bath, or use the stainless steel wrap method to prevent surface decarburization.

Precise temperature control during this stage is crucial.

Quenching

D2 is an air-hardening steel. This means you don’t need a liquid quench, which simplifies the process and helps minimize distortion.

• Method: Cool the steel in still air after removing it from the furnace.
• Goal: The cooling rate must be fast enough to form the hard martensitic structure but controlled enough to prevent quench cracks.
• Result: The structure after quenching will be predominantly martensite, but it will also contain some retained austenite, which can be as high as 20%. Retained austenite can be an issue for dimensional stability later on.

Tempering D2

After quenching, D2 steel is very hard but also brittle, with internal stress. Tempering relieves this stress and increases toughness.

• Process: Reheat the hardened steel to a specific temperature below its critical point, hold it at this temperature, and then cool it, typically in air.
• Double Tempering: For D2, we strongly recommend double tempering. This provides better stress relief, improves toughness, and helps transform some of the retained austenite for better stability.
  • Typical Cycle:
    1. First Temper: Heat to approximately 515 °C (960°F). Hold for 2 hours per 25 mm (1 inch) of thickness. Cool to about 65° C (150° F).
    2. Second Temper: Reheat to a slightly lower temperature, approximately 480 °C (900°F). Hold for the same duration (2 hours per 25 mm / 1 inch).
• Hardness vs. Toughness: The tempering temperature determines the final balance.
  • Lower temperatures (e.g., 200 °C / 390°F) result in higher hardness (approximately 60 HRC).
  • Higher tempering temperatures increase toughness but slightly reduce hardness. Choose the temperature based on your application’s specific needs.

Stress Relief (Post-Hardening)

If the D2 part undergoes significant grinding, EDM, or welding after hardening and tempering, an additional stress relief operation is highly recommended.

• Temperature: Heat the part slowly and uniformly to a temperature approximately 14°C to 28°C (25°F to 50°F) below the final tempering temperature.
• Hold Time: Soak for an adequate amount of time (e.g., 1-2 hours per inch of thickness).
• Cooling: Cool slowly, preferably in the furnace.

Managing Potential Issues

Even with careful processing, some issues can arise.

Distortion

D2 steel is known for having relatively good dimensional stability during heat treatment compared to other tool steels, with an expected change of around 0.0005 in./in. However, factors such as complex part geometry or uneven heating and cooling can still cause warping or twisting. Ensure uniform heating and air cooling to minimize this. Limited straightening may be possible during cooling between approximately 565 °C (1,050°F) and 205 °C (400°F).

Quench Cracks

Cracking during quenching can happen if cooling is too fast or uneven, or if the steel has internal flaws. Proper preheating and controlled air cooling are the best ways to prevent this.

Conclusion

Successfully heat treating D2 tool steel requires careful attention to preparation, temperature, time, and cooling rates throughout each stage: preheating, austenitizing, air quenching, and double tempering. Following established procedures and considering the specific demands of your application will ensure you get the optimal balance of properties D2 is known for. If you have questions about heat treating D2 supplied by Aobo Steel, feel free to contact us.