D2 Tool Steel Properties and Applications

What is D2 Tool Steel? It is a high-carbon, high-chromium, cold-work tool steel. It is particularly known for its good wear resistance and deep hardening capabilities. When we discuss D2 steel specifications, we will talk about the D2 steel composition, heat treatment, and hardness, applications and performance, comparison of D2 steels with other D-series steels, common situations where D2 steel needs to be considered for substitution, and disadvantages of D2 steel.

1. D2 Tool Steel Chemical Composition

2. Heat Treatment And Hardness Specifications for D2 Tool Steel

2.1 Preheating

Preheating is crucial for high-chromium steels, such as D2, due to their lower thermal conductivity. This step prevents cracking during the main heating phase by ensuring even temperature distribution and relieving internal stresses.

• Recommended Temperature: Heat slowly and uniformly to 1200°F (650°C). A range of 1200–1250°F (649–677 °C) is sometimes used to stress-relieve unhardened material.
• Soak Time: Hold at temperature for approximately 10-15 minutes for a 3″ (75mm) section. Larger sections require longer soaking. For stress relieving, a guideline is 2 hours per inch (4.7 min/mm) of thickness, followed by slow furnace cooling.

2.2 Austenitizing (Hardening)

Austenitizing transforms the steel’s microstructure to austenite and dissolves alloy carbides, which is necessary for hardening.

• Hardening Temperature: Heat the D2 steel to 1850°F (1010°C).
• Soak Time: Hold at the austenitizing temperature for 1 hour per inch (25 mm) of cross-section to ensure a complete and uniform transformation.
• Atmosphere Control: To prevent decarburization (loss of surface carbon), austenitizing is performed in a controlled neutral atmosphere, vacuum, or neutral salt furnace. Alternatively, wrap the part securely in stainless steel foil (Type 309, 0.002-in./0.05-mm thick is suitable for temperatures up to 2240°F /1227° C).

2.3 Quenching

D2 is an air-hardening steel, which simplifies quenching and minimizes distortion compared to oil or water quenching methods.

• Method: Cool the steel in still air after removing it from the furnace.
• Target Temperature: Allow the steel to cool to approximately 150°F (65°C) before tempering.
• Straightening: If needed, straightening can be done after the steel cools to about 1050° F (565° C) but before it transforms to martensite (around 400° F / 205° C). To prevent stress and distortion, avoid rapid or uneven cooling below this transformation point.

2.4 Tempering

Tempering is essential to reduce the brittleness inherent in as-quenched steel, relieve stresses, and increase toughness while maintaining necessary hardness. Double tempering is recommended for D2 steel.

• First Temper: Heat to 960°F (515°C). Hold for 2 hours per inch (25mm) of cross-section.
• Cooling: Allow the part to cool completely to room temperature after the first temper.
• Second Temper: Reheat to 900°F (480°C). Hold for 2 hours per inch (25mm) of cross-section.
• Expected Hardness: After the second tempering and cooling, a typical hardness of 58 HRC is achieved. This double tempering process refines the grain structure, potentially improving wear resistance.
• Alternative (Single Temper): A single temper around 400°F (205°C) yields higher hardness (approx. 62 HRC) but generally offers less toughness and wear resistance compared to double tempering.

2.5 Stress Relieving (After Hardening)

If hardened D2 steel undergoes significant grinding, welding, or electrical discharge machining (EDM), a stress-relief temper is highly recommended to prevent cracking or failure caused by residual stresses.

• Temperature: Heat slowly to a temperature 25 to 50°F (14 to 28°C) below the final tempering temperature used. An alternative approach for general stress relief involves heating to 1200–1250°F (649–677 °C).
• Soak Time: Hold for 1–2 hours per inch (25.4 mm) of thickness.
• Cooling: Cool slowly in the furnace to room temperature.

2.6 Important Considerations for D2 Heat Treatment

• Distortion: While D2 offers good stability, distortion can occur due to uneven heating or cooling or complex part geometry. Careful preheating, uniform austenitizing, and controlled air quenching minimize this risk.
• Retained Austenite: Some austenite may remain untransformed after quenching. Double tempering helps convert this retained austenite to martensite, improving dimensional stability. Subzero (cryogenic) treatments can further reduce retained austenite for demanding applications.
• Decarburization: Protecting the steel surface during heating, using a controlled atmosphere or foil wrap, is vital to prevent a soft outer layer and maintain full hardness.

3. Specification of D2 Tool Steel Applications And Performance

Firstly, as we mentioned, D2 steel is a high-carbon, high-chromium steel. These alloy elements render D2 steel highly hard and wear-resistant.

Secondly, the high chromium content improves the hardenability of D2 steel. Hardenability refers to a steel’s ability to be hardened through heat treatment. This uniform deep hardening provides wear and deformation resistance in demanding applications.

Based on these properties, D2 Steel is utilized widely in cold work tooling applications.

3.1 Blanking Dies

D2 steel is renowned for its exceptional wear resistance and hardness, making it ideal for dies that cut or punch shapes out of metal sheets. Blanking die for different materials from thinner gauge materials (t≤3mm) to thicker sheets. Moreover, it is particularly suitable for blanking dies used with abrasive materials, such as silicon steel sheets and aluminum sheets.

3.2 Piercing Dies

Like blanking, D2 steel’s high hardness and wear resistance make it a good choice for piercing holes in sheet metal.

3.3 Cold Forming Dies

D2 steel’s ability to withstand high compressive stresses and resist wear makes it suitable for cold forming operations at room temperature. This includes applications such as cold extrusion and cold heading, which require maintaining their shape and resisting wear under high pressure.

3.4 Drawing Dies

Dies are involved in the sheet metal drawing process. D2 steel is well-suited to resist wear associated with the friction experienced during drawing.

3.5 Thread Rolling Dies

Dies are involved in the sheet metal drawing process, and D2 steel is well-suited to resist wear associated with the friction experienced during drawing.

3.6 Forming Rolls

In rolling operations where metal is shaped by passing, D2 steel’s hardness ensures the rolls maintain their profile and resist wear over long run production

3.7 Gauges

When applying measuring tools, D2 steel’s dimensional stability after heat treatment, along with its high wear resistance capabilities, is crucial for maintaining accuracy over an extended period.

3.8 Slitters and Shear Blades

The sharp edges of slitting and shearing tools made from D2 steel retain their sharpness for long periods due to the material’s high hardness and wear resistance. This applies to various thicknesses of materials, but we have to consider both balance and toughness for thicker materials.

We must remember that while D2 steel offers excellent wear resistance and hardness for cold work, its toughness should be considered for applications involving very high impact loads or significant bending stresses.