D3 TOOL STEEL | 1.2080 | skd1

3. Heat Treatment

The comprehensive D3 tool steel heat treatment protocol involves four primary stages: preheating, austenitizing (hardening), quenching, and tempering. Each step plays a vital role in achieving the desired metallurgical structure and mechanical properties.

3.1 Preheating

Preheating is a crucial preparatory stage in the D3 tool steel heat treatment cycle, especially for D-type steels like D3, which have relatively low thermal conductivity.

• Purpose: Its main functions are to minimize thermal shock when the tool is introduced to high austenitizing temperatures, thereby reducing the risk of distortion or cracking. Preheating also helps alleviate some residual stresses from prior machining operations.
• Recommendation for D3 Steel: We recommend preheating D3 tool steel to a temperature range of 650-705°C (1200-1300°F). A common practice is to hold at 650°C (1200°F) for approximately 10 to 15 minutes, or until the part is uniformly heated.
• Heating Rate: A controlled, slow heating rate is generally preferable. Rapid heating can create significant temperature differences between the surface and the core of the tool, potentially leading to distortion or cracks.

3.2 Austenitizing (Hardening)

Austenitizing is the core hardening phase of D3 tool steel heat treatment. During this stage, the annealed microstructure (primarily ferrite and carbides) is transformed into austenite, and critical alloy carbides are dissolved to develop the steel’s hardness and wear characteristics.

• Process: For D3 steel, which has a high concentration of chromium carbides, austenitizing is conducted in the austenite-carbide two-phase field.
• Temperature for D3 Steel: D3 tool steel is typically austenitized at a slightly lower temperature than D2, generally around 960°C (1760°F). D2, by comparison, is often treated at 1020°C (1870°F).
• Holding Time: The steel must be held at the austenitizing temperature long enough for the core to reach the target temperature and for sufficient carbide dissolution to occur, ensuring uniform heating throughout the cross-section.
• Critical Considerations: Proper austenitizing is vital for achieving high as-quenched hardness. Overheating must be avoided, as excessive temperatures can lead to coarse grain size, coarsening of carbides, and an increase in retained austenite, all of which negatively impact toughness and dimensional stability. Employing neutral atmospheres, protective foil wrapping, or salt baths during austenitizing is important to prevent surface decarburization or scaling.

3.3 Quenching

Following austenitizing, quenching is the controlled cooling process in D3 tool steel heat treatment designed to transform the austenite into hard martensite.

• Quenching Medium for D3 Steel: Unlike many other D-type steels that are air-hardening, D3 tool steel is typically oil quenched. Oil quenching provides a cooling rate that is less severe than water or brine, which helps to minimize distortion and the risk of cracking, though D3 remains more susceptible to distortion than air-hardening grades.
• Cooling Procedure: The part should be cooled through the martensite formation range. It is critical to cool the component to near ambient temperature, ideally between 66-93°C (150-200°F), before proceeding to tempering.
• Timing is Key: Do not allow the quenched D3 steel part to cool completely to room temperature and sit for an extended period (generally, more than 2 hours is discouraged) before tempering. Delaying tempering can make the steel extremely brittle and susceptible to cracking and instability.

3.4 Tempering

Tempering is an indispensable final stage in the D3 tool steel heat treatment process. As-quenched D3 steel is extremely hard but also very brittle and carries high internal stresses.

• Purpose: Tempering relieves these internal stresses, significantly increases toughness, and allows for the fine-tuning of the final hardness.
• Multiple Tempers for D3 Steel: For high-carbon, high-chromium steels like D3, conducting multiple tempering cycles (double or even triple tempering) is strongly recommended. This practice helps to refine the grain structure, further improves wear resistance, and facilitates the transformation of any retained austenite.
• Temperatures and Hardness: To achieve a typical target hardness of approximately 60 HRC, D3 is often tempered at lower temperatures, around 200°C (390°F).
• Secondary Hardening Range: Tempering in the secondary hardening range (typically 400-600°C for alloyed steels) can also be beneficial. This promotes the transformation of retained austenite and the precipitation of fine alloy carbides, which can enhance hot hardness and tempering resistance, although D3’s tempering resistance is more limited compared to steels with higher molybdenum (Mo) and vanadium (V) content.
• Holding Time: A standard tempering duration is 2 hours per inch (25mm) of the tool’s thickness at the specified temperature. Parts should be cooled to room temperature between each tempering cycle. Effective tempering is absolutely vital for the service life and performance of D3 tool steel components.

3.5 Key Considerations for D3 Tool Steel Heat Treatment

• Retained Austenite: Due to its high carbon content, D3 steel can have retained austenite after quenching. Multiple tempering cycles are the primary method to transform this. For applications demanding maximum dimensional stability or the absolute minimum of retained austenite, subzero or cryogenic treatments can be employed after quenching (and typically before the final temper). These treatments must be followed immediately by tempering.
• Dimensional Stability: D3 tool steel has good dimensional stability, with expected growth typically around ±0.0005 in./in. (or ±0.05%). Proper preheating, controlled heating and cooling rates, and thorough multiple tempering are essential to minimize dimensional changes.
• Toughness: It’s important to note that D3 generally has lower toughness compared to D2 steel, primarily due to its higher carbon content. The D3 tool steel heat treatment parameters must be carefully chosen to balance wear resistance with adequate toughness for the intended application.
• Surface Treatments: For enhanced surface hardness and wear resistance, D3 tool steel components can be subjected to surface treatments such as nitriding or Thermal Diffusion (TD) processing after the primary heat treatment.