O1 TOOL STEEL | 1.2510 | SKS3

4. O1 Heat treatment

The heat treating O1 steel generally involves four primary steps: preheating, austenitizing (hardening), quenching, and tempering.

4.1 Preheating

Preheating is an essential step for virtually all tool steels, including O1. Metallurgically, it doesn’t directly contribute to the hardening reaction but serves several crucial functions:

1. Reduces Thermal Shock: Placing a cold tool into a hot furnace can cause thermal shock, leading to excessive distortion or cracking. Preheating minimizes this risk.
2. Relieves Stress: It helps relieve some stress developed during machining or forming processes.
3. Increases Equipment Productivity: By bringing the tool to a uniform temperature before the high-heat furnace, it decreases the time required in the austenitizing furnace.
4. Reduces Surface Degradation: If the high-heat furnace isn’t neutral, preheating reduces carburization and decarburization.

The preheat temperature is 1200°F (650°C). The steel should be held at this temperature for 10 to 15 minutes, or until it is uniformly heated throughout its cross-section. DO NOT soak the tool steel for too long at this temperature, as it can upset the molecular structure. Based on our experience, if placing the part in a preheated furnace (up to 1200°F/650°C), first place it at the top of the furnace to remove any chill, which helps reduce thermal shock and lower the risk of cracking.

4.2 Austenitizing (Hardening)

After preheating, set the furnace temperature to the austenitizing temperature, which is 1500°F (815°C). We must heat the material until it fully reaches this temperature, which can be confirmed by observing its color matching the furnace. Once the temperature is consistent, begin calculating the soaking time. Soaking for an additional 5 minutes per inch of the smallest cross-section. While O1 steel may be damaged by over-soaking, its tolerance is generally higher compared to high-alloy steels.

4.3 Quenching

O1 steel is classified as an O-type steel because it is steel that uses oil as a quenching medium. Oil as a quenching medium is slower than water but safer, reducing internal stress and the tendency for cracking and deformation. After quenching, it should be cooled to 125 to 150°F (52 to 65°C). Once this temperature range is reached, tempering should be performed immediately.

[Reference: Bryson, W. E. (2007). Heat Treatment, Selection, and Application of Tool Steels (p. 91). Hanser Publications.]

4.4 Tempering

After quenching, the steel is in a highly stressed state and susceptible to cracking. Tempering serves to:

1. Relieve Internal Stresses: It reduces the internal stresses set up during hardening, which can cause brittleness.
2. Increase Toughness: Tempering significantly increases the steel’s toughness.
3. Transform Retained Austenite: It transforms retained austenite from the quenching step into fresh martensite.

This steel typically requires only one tempering process, but in some cases, two tempering processes may be necessary. Two tempering processes can refine the grain structure to enhance toughness, making it suitable for parts with complex details or those requiring high strength.

The single tempering temperature is 350°F (175°C), with a soaking time of 2 hours per inch (25mm) of cross-section.

If a second tempering is performed, the temperature should be slightly lower, typically 325°F (160°C), and the part should be allowed to cool to room temperature between the two tempering processes.

The as-quenched O1 tool steel hardness is 64-65 HRC. After tempering at 350°F (177°C), the hardness is 62-63 HRC, and at 400°F (204°C), it is 62 HRC.