They don’t call it “O1” for nothing. O1 tool steel is among the most popular steels for knife makers and other hobbyists, thanks to excellent wear resistance, toughness, and edge retention.

But before you can make the most of these key properties, it’s important to understand the ins and outs of heat treating O1 tool steel. In this beginner-friendly guide, we’ll cover the basics you need to know to get good results in heat treatment for high-quality, durable products.

What Is O1 Tool Steel?

O1 tool steel is a premium, oil-hardening alloy known for its ability to keep its shape even when subjected to high temperatures.

The “O1” actually isn’t a reference to the steel’s popularity, but rather to its carbon content: around 1%. The O, meanwhile (not a zero!), refers to its oil hardening quality. Being a high carbon steel makes O1 hard and strong, but also more brittle and prone to cracking without proper heat treatment.

Important Properties

O1 steel offers versatility based on several strengths:

• Wear and abrasion resistance – Well-suited for cutting tools that need to maintain a sharp edge
• Toughness – Withstands impacts and stresses without cracking or breaking
• Machinability – Easy to machine and grind
• Corrosion resistance – Somewhat protective, although not as corrosion-resistant as fully stainless steels (such as 416 stainless steel, for example)

Composition

O1 tool steel is an alloy steel designed for high performance in various applications. It usually includes the following elements:

Common Applications

O1 tool steel is a popular choice for:

• Lots of different dies – Blanking, bending, molding, forming and more
• Gauges
• Shims
• Punches
• Knives, scissors and other cutting tools

Heat Treatments for O1 Steel

Careful heat treatment of knives and other products is essential to get the hardness and toughness of your O1 tool steel just right. Here’s a rundown of the different processes you may need to apply. (Note: Below is just a guideline. Your optimal recipe will depend on multiple factors including your application, the size and shape of your workpiece, the quality of your furnace/oven, and other factors.)

Annealing

Soften the steel to make it more workable.

1. Heat slowly to 1140°F.
2. Cool gradually to prevent cracking and warping: Turn off your furnace and allow the steel to cool slowly (40°F per hour) inside until below 1000ºF—several hours or even overnight.
3. Once the workpiece has cooled to 1000ºF, remove the steel from the furnace and allow it to air cool, bringing the temperature gradually down to room temperature. This may take several days depending on the thickness of the piece.

Stress Relieving

If you’re doing a lot of machining after annealing, it helps to relieve stresses in the metal afterward.

1. Heat to 1200 -1250°F.
2. Hold for one hour.
3. Do not quench. Remove the steel from the furnace and air cool for at least 1 hour per inch of thickness.

Hardening

After shaping and machining, hardening provides the hardness (up to Rockwell C 65) and wear resistance you need.

1. Preheat your furnace to 1200 - 1250°F.
2. Place the steel in the furnace and allow it to reach the preheat temperature.
3. Raise the furnace temperature and heat the steel up evenly to 1450 - 1500°F (opt for the high end for thick sections).
4. Once the steel is heated through, immediate oil quenching is key. Use warmed oil with low viscosity in a circulating bath if you have one. If not, agitate the workpiece in the oil to prevent soft spots. Be sure to wear all your safety gear.
5. Remove the steel when it reaches 125°F—well before room temperature—to avoid quench cracking.
6. Move immediately to tempering.

Tempering

O1 tool steel can become brittle in hardening, so immediate tempering is key.

1. For most applications, heat back up to 400 - 450°F for at least two hours (plus 1 hour per inch of thickness beyond 2 inches). For knives and other cutting tools, heat to the lower end of the temperature range—300 - 375ºF—holding for the same duration.
2. Remove from the furnace and allow it to air cool to room temperature.
3. Minimum tempering duration at the low end of the temperature range should yield a hardness of Rockwell C 63 - 64. On the higher end of the temperature range, look for C 60 - 62. Verify with a hardness tester.

Key Considerations & Best Practices

To keep you on track when working with O1 tool steel, here are some helpful reminders:

Preheat. Before heat treating, it’s important to preheat the steel to minimize the risk of cracking or distortion. Don’t skip this step!

Don’t delay. Immediate quenching after heating, as well as tempering after quenching, is crucial to minimizing brittleness and cracking.

Use shaping quality tools. High hardness makes O1 harder to sharpen. You may need specialized grinding equipment to get the shape you want.

Keep it clean and dry. O1 tool steel has a low chromium content, which makes it prone to rust and corrosion. Consider applying a protective coating or oil.

Control quality. Keep a close eye on temperature and cooling rate during quenching and verify the final properties of the steel through testing.

Stay safe. Keep yourself protected when dealing with hot furnaces and metal workpieces. Wear proper safety gear at all times. Choose safe equipment such as Hot Shot heat treating ovens with Cool-Touch technology.

All in all, O1 tool steel is a solid choice in making a wide range of durable metal products. Got questions as you get to work? Contact us for help!