D2 Tool Steel | 1.2379 | SKD11

6. Properties

D2 tool steel offers a compelling set of mechanical characteristics vital for tooling applications.

6.1 Hardness

Hardness is a defining feature among D2 steel properties.

• As-Quenched: Depending on the austenitizing temperature and quenching method (air or oil), D2 typically achieves a hardness in the 60-65 HRC range.

• After Tempering: Hardness levels adjust with tempering temperature. For instance:

  • Tempering at 205°C (400°F) can result in approximately 61 HRC.

  • Tempering at 425°C (800°F) may yield around 55 HRC.

  • Tempering at 650°C (1200°F) typically reduces hardness to about 40 HRC.

    A common working hardness range is 60–62 HRC. Initial hardness for D2 tool steel die sections can be around 255 HB (Brinell Hardness).

• Surface Hardening: D2 responds well to ion nitriding, capable of achieving surface hardness levels of 750-1200 HV (Vickers Hardness) with a core hardness between 61-64 HRC, at a shallow depth (5-8 micrometers).

6.2 Strength

D2 steel exhibits robust strength characteristics:

• Tensile Strength: Studies have shown an Ultimate Tensile Strength (UTS) of approximately 758 MPa.

• Yield Strength: A 0.2% Offset Yield Strength of around 411 MPa and a standard Yield Strength of 350 MPa have been recorded.

• Compressive Strength: Hardened D-type steels like D2 possess high compressive strength, especially when tempered at lower temperatures. This strength is directly correlated with the hardness level; as tempering temperature increases, both hardness and compressive strength tend to decrease.

6.3 Ductility and Toughness

When considering D2 STEEL properties, ductility and toughness are important aspects:

• D2 is generally considered to have moderate toughness, which is superior to grades like D3 (which has higher carbon and more retained carbides). The carbide content in D2 is balanced to offer a good combination of wear resistance and toughness compared to other D-series steels with higher carbon.

• Tensile tests often show a ductile fracture mode, characterized by dimple-like structures. However, specimens might exhibit a flat fracture surface with minimal necking and low area reduction (e.g., around 1.3% in some tests).

• The Modulus of Toughness has been measured at 81 MPa, with a fracture strain of 1.97%.

• It’s important to note that mechanical D2 steel properties, including strength and ductility, can be anisotropic (direction-dependent) due to the elongation of primary alloy carbides during the hot working process. Maximum strength and ductility are typically found parallel to the rolling direction.

6.4 Dimensional Stability

One of the most valued D2 steel properties is its excellent dimensional stability during heat treatment.

• It exhibits minimal distortion compared to many other tool steels.

• When air quenched from the correct hardening temperature, users can expect an expansion or contraction of approximately 0.0005 inches per inch (or mm/mm).

• Factors like part geometry and existing distortions can influence the total movement.

• A stress-relief temper is highly recommended after significant processing (grinding, welding, EDM), typically at a temperature 14-28°C (25-50°F) lower than the last tempering temperature.

6.5 Wear Resistance

Exceptional wear resistance is a hallmark of D2 steel properties.

• It offers excellent abrasion resistance, often serving as a benchmark for other tool steels.

• This high wear resistance is directly attributed to the substantial volume of hard, chromium-rich carbides in its microstructure. This makes D2 a preferred material for tooling subjected to abrasive conditions and long production runs. Its wear resistance is noted to be about 30-40% improved over A2 tool steel.

6.6 Processing Considerations for D2 Steel

While offering excellent performance, certain processing D2 STEEL properties should be noted:

• Machinability: D2 is considered to have relatively poor machinability. In its annealed condition, its machinability rating is around 45 (compared to a 1% carbon steel rated at 100). It can be difficult to work and grind.

• Weldability: Welding D2 using conventional methods is generally challenging and often not recommended due to its high carbon content and the presence of significant carbides. However, newer techniques like thixowelding have shown some promise.

6.7 Optimizing D2 properties through Heat Treatment

The final D2 STEEL properties are critically dependent on a precise heat treatment process. This typically involves several key stages:

• Stress Relieving (Unhardened Material): Before hardening, especially for complex parts or those heavily machined, stress relieving is vital. This usually involves heating slowly and uniformly to 649–677°C (1200–1250°F), soaking for 1-2 hours per inch of thickness, and cooling slowly (ideally in the furnace).

• Preheating: Due to D2’s relatively low thermal conductivity, preheating to around 650°C (1200°F) is strongly recommended before austenitizing. This minimizes thermal shock, reducing cracking risk and distortion.

• Austenitizing: The steel is heated to approximately 1010°C (1850°F). At this temperature, the structure transforms to austenite, and a significant portion of the carbides dissolves. Soak time is typically 1 hour per inch of cross-section. Proper temperature control is crucial to avoid issues like excess retained austenite.

• Quenching: D2 is an air-hardening steel. Air quenching from the austenitizing temperature transforms austenite into martensite (the hard structure) and helps minimize distortion. The steel should be cooled to about 65°C (150°F) before tempering.

• Tempering: This step is crucial for increasing toughness and relieving internal stresses in the as-quenched martensite. For D2, tempering also provides secondary hardness due to the precipitation of specific alloy carbides.

  • Double Tempering: Standard practice for D2 to ensure complete tempering and microstructural stability. A first temper might be around 515°C (960°F), followed by a second temper around 480°C (900°F). Soak times are generally 2 hours per inch of cross-section.

• Managing Retained Austenite: D2 can retain significant austenite (up to 20%) after standard heat treatment. This unstable austenite can transform over time, causing dimensional changes.

  • Subzero Treatments: Deep cryogenic treatments (e.g., near -184°C / -300°F) can help transform retained austenite, potentially improving dimensional stability, tensile strength, toughness, and reducing wear.

  • Post-Cryo Tempering: If a subzero treatment is performed, an additional tempering step is necessary to temper the newly formed martensite and prevent brittleness.

• Stress-Relief Tempering (Hardened Material): Recommended after operations like grinding or EDM on hardened D2, typically at a temperature slightly below the final tempering temperature.