Chances are pretty good that you’ve come across a tool made of D2 steel. This versatile metal material has been around a long time and used in a tremendous variety of cutting and other tools.
Interested in using D2 in one of your creations? Heat treating is essential to optimizing the properties. Even if you plan to outsource heat treating to a shop, it’s helpful to know how it all works. So get started with this hobbyists’ overview of how to heat treat D2 steel.
What Is D2 Steel?
D2 tool steel is a high-carbon, high-chromium tool steel widely used for cutting tools, molds, and dies (represented by the D in D2). You might also see it referred to as SKD11, Hitachi SLD, and others.
You’ll often hear D2 referred to as “semi-stainless steel” because it contains 12% chromium. This provides some corrosion resistance—more than similarly tough steels like o1 tool steel, but not as much as with full stainless steels like 416 stainless steel or 410 stainless steel, which contain more chromium.
Known for its ability to hold a sharp edge at high temperatures, D2 tool steel also has good toughness and is relatively easy to machine and grind.
Patented back in 1927, D2 tool steel is a tough workhorse steel that’s seen plenty of research and field use over the years. Key properties include:
- High hardness – As high as 62 HRC; suitable for high-wear applications.
- Good wear resistance – Ideal for cutting tools and cold-work / forming applications.
- Good toughness – Resists fracture and cracking under high stress or impact.
- Good corrosion resistance – High chromium content.
- Excellent dimensional stability – Retains shape and size even under high-stress conditions.
- Good machinability – Shaped and formed into complex geometries using standard machining processes; for higher machinability requirements, experts recommend a lower-chromium tool steel such as A2.
D2 usually includes the following ingredients (percentages will vary somewhat by supplier):
Hardness and strength
Hardness and wear resistance
Wear and corrosion resistance
Toughness and high-temp strength
Wear resistance and toughness
D2 is a popular choice for:
- A wide variety of dies, including blanking, forming, stamping, thread rolling, bending, trim, and more
- Wear parts
- Machine parts (e.g., injection screws)
- Cutting tools such as knives, slitting cutters, rotary cutters, shear blades, tire shredders, and scrap choppers
D2 Steel Heat Treating Processes
With consideration and care, you can heat treat D2 tool steel to create a good balance of hardness and toughness. The following are key steps for each process.
Soften the steel to make it more workable.
- Heat slowly to 1550°F. Be sure to heat the entire workpiece through.
- Cool slowly (40°F per hour) in your furnace until around 1000ºF.
- After that point, cool in the ambient air.
You should perform this step after any rigorous machining processes.
- Heat slowly to 1050°-1250°F.
- Once the metal is heated evenly, hold at that temperature for 1 hour per inch of thickness.
- Cool in the furnace at reduced temperature, or remove and allow to cool in the ambient air.
Maximize the hardness (up to Rockwell C 652) and wear resistance you need.
- Preheat. For complex tools, do this in two steps. Heat no faster than 400°F per hour up to 1200°F. Allow the workpiece to equalize. Then heat up to 1450°F. For basic tools, you can do this preheat all in one step up to 1450°F.
- After preheating, raise your furnace temperature to 1800° - 1850°F. Hold the metal at that temperature until heated through.
- Remove your workpiece from the furnace and allow to cool in still air. Or, if possible, cool in a vacuum furnace if you have one. If not, experts recommend wrapping the metal in stainless steel foil, or using an inert gas (argon) to displace oxygen in your kiln, to protect from scaling.
- Once your piece has cooled to 150°F, move immediately to tempering.
Prompt tempering is essential to reduce internal stresses. Temperatures will depend on your desired balance of hardness / wear resistance and toughness.
Heat the metal back up to your chosen temperature for two hours, with an additional hour for every inch of thickness over 2 inches.
- For the highest hardness (such as for cutting tools), use 300-350°F (149-177°C) for a hardness of Rockwell C 62 - 64.
- To split the difference between hardness and toughness, go with 500 - 550°F, for Rockwell C 58 - 60.
- To max out toughness, crank the temperature up to 950°F and do a double temper: 2 hours, then another 2 hours at a lower temperature.
After that, in all cases, let the metal cool in the air to room temperature.
Tips, Advice & Best Practices
Heat treating D2 tool steel does take some attention. Here are some key considerations and cautions to keep in mind:
Don’t skip the preheat. It helps prevent cracking and distortion, so be sure to follow the guidelines.
Avoid overheating. Don’t go above 1850°F for austenitizing / hardening. Go higher and you risk grain growth in the steel.
Oil quench if necessary. Although D2 is an air hardening steel, you might encounter issues when air cooling thick sections. Quench in a warmed oil instead.
Temper promptly. As discussed above, consider your tempering temperature carefully, as well.
Protect the surface. Because D2 steel can develop a decarburized layer on its surface during heat treating, take preventive measures such as a stainless steel foil wrap. Another option is using an inert gas such as argon (available in a convenient argon gas kit from Hot Shot), which can provide an oxygen-free environment for all kinds of heat treating processes.
Protect yourself. The heat treatment process for D2 steel involves high temperatures and in some cases flammable oils. Always wear personal protective gear and choose high-quality heat treating equipment, such as a Hot Shot heat treating oven with Cool-Touch technology.
With these considerations in mind, heat treating D2 can create a product with a valuable combination of wear and corrosion resistance plus toughness. A century of application use shows that D2 has a lot going for it. Do your due diligence, and you’re sure to create some great D2 tools!