D2 Tool Steel | 1.2379 | SKD11

3. D2 Steel Heat Treatment

As William E. Bryson describes in his book Heat Treatment, Selection, and Application of Tool Steels, the heat treating D2 tool steel is often compared to cooking, where precise control of time and temperature is critical to avoid undercooking (leading to lack of hardness) or overcooking (destroying molecular structure and causing brittleness). The following steps outline the process.

3.1 Material Preparation and Preheat

Before heat treatment, the material should be thoroughly degreased and preferably wrapped in stainless steel foil to protect its surface. Because of the high chromium content and low thermal conductivity of this steel, it should be slowly and evenly preheated to the target temperature to minimize the risk of cracking during heating. The target temperature is 1200°F (650°C), and the heating time is 10 to 15 minutes.

The purpose of the entire preheating process is to ensure that heat is distributed uniformly throughout the material, allowing internal stresses to be released before the material becomes too soft and its plasticity increases, thereby preventing deformation.

3.2 Austenitizing (Hardening)

This is the second step in heat treatment, during which the material’s structure changes from ferrite-pearlite to austenite, and various complex alloy carbides are dissolved. The heating temperature for this step is 1850°F (1010°C), with a soak time of 1 hour per 1 inch (25 mm) of cross-section. This soak time ensures that the austenitization process occurs uniformly. However, it is important to note that an excessively long soak time, even just a few minutes, may have a negative impact on the steel.

3.3 Quenching

D2 tool steel is an air-hardening steel, whose advantage lies in minimizing deformation and dimensional changes during the formation of martensite. The process involves the following steps: After soaking, the material is rapidly cooled to approximately 150°F (65°C). During this process, when the temperature reaches 1050°F (565°C) and before the material transforms into a hardened structure at 400°F (205°C), the workpiece can be removed from the foil packaging and placed on a cooling rack. Caution must be exercised to avoid putting the material directly on a cold table surface, as this can cause localized temperature fluctuations and result in deformation. From a microstructural perspective, this process transforms the steel’s internal structure into finer-grained martensite, imparting excellent wear resistance to D2.

After proper quenching, the material still contains a certain proportion of “residual austenite,” with the optimal martensite content ranging from 95% to 96%.

3.4 Tempering

Tempering improves the toughness of steel, reduces internal stress, and provides secondary hardening. Tempering must be performed immediately when the material temperature drops to 125°F to 150°F (52°C to 65°C).

If D2 is only tempered once, the tempering temperature is 400°F (205°C) to achieve a Rockwell hardness of 62HRC.

We recommend using a secondary tempering process for D2, which can improve its wear resistance by 20-30%.

In the secondary tempering process, the first tempering temperature is 960°F (515°C) for 2 hours per inch (25 mm) of cross-section. Before the second tempering, i.e., during the interval between the first and second tempering, the material must be allowed to cool to room temperature before the second tempering. This period may last several hours. It may also take several days, but the key point is that the second tempering must NOT begin at 150°F (65°C), which is a completely different temperature from the single tempering mentioned earlier. The second tempering temperature is 900°F (480°C) for 2 hours per inch of cross-section. The second tempering achieves a Rockwell hardness of 58 HRC.

While D2 has weak secondary hardness, tempering at higher temperatures (e.g., ~550°C/1020°F) can be used to achieve a hardness of 60 HRC, thereby improving stability during nitriding or other surface hardening methods. However, this can lead to increased retained austenite and grain growth, potentially decreasing toughness and causing microstructural instabilities.

3.5  Cryogenic/Sub-Zero Treatment (Optional)

This process is designed to eliminate or reduce residual austenite and enhance material dimensional stability. Since D2 tool steel can retain a significant amount of austenite (up to 20%) after standard heat treatment, this can lead to dimensional instability as retained austenite spontaneously transforms into untempered martensite over time at room temperature.

The process involves the following steps: After stress relief treatment (approximately 150°C), the material is cooled to an extremely low temperature (approximately -300°F/-184°C), approaching or below the final Mf temperature. Subsequent tempering is still required to prevent brittleness caused by the newly formed fresh martensite.

This process creates a more compact molecular structure within the material (reducing friction, heat, and wear), reduces residual stress, and enhances tensile strength, toughness, and dimensional stability, significantly improving material performance.

[References: Reardon, A. C. (Ed.). (2011). Metallurgy for the Non-Metallurgist (2nd ed., p. 231). ASM International. ]

3.6 Potential Issues

1. Residual austenite can cause dimensional instability in materials, especially at higher austenitizing temperatures. Controlled quenching, precise hold times, and double and triple tempering are employed to manage this process.
2. Factors such as non-uniform heating and cooling, phase transformations (especially martensite formation), and residual stress during heat treatment can cause deformation and cracking in materials. Therefore, it is important to ensure slow and uniform heating, proper quenching media, and stress relief treatment.
3. D2 steel is susceptible to decarburization. We recommend heating D2 materials in a controlled neutral atmosphere, a vacuum, or a neutral salt furnace environment to prevent decarburization.