O1 Steel Heat Treatment Guide

O1 tool steel is a popular general-purpose, oil-hardening tool steel known for its good abrasion resistance and sufficient toughness across a wide range of tooling applications. The correct heat treatment process is key to achieving the performance of O1 tool steel. Improper heat treatment can cause O1 tools to fail prematurely. The heat treatment process for O1 steel, like all tool steels, follows three fundamental stages: austenitizing, quenching, and tempering. It is crucial to adhere to specific time and temperature parameters, as improper heat treatment is a frequent cause of premature tool failure.

1. Preheating

Preheating is an essential step for almost all tool steels. It helps reduce thermal shock in O1 steel during heat treatment, thereby reducing the likelihood of cracking.

The preheating temperature for O1 tool steel is 1200°F (650°C). Hold at this temperature for 10 to 15 minutes, or until it is uniformly heated.

O1 steel is oil-quenched steel, which can be heated faster than air-quenched steel and can be placed directly into a furnace preheated to the required temperature. If you choose to put it directly into a preheated furnace, it is recommended first to heat the O1 material on the furnace top.

2. Austenitizing

Following the preheat, heat the steel to the austenitizing temperature to transform its structure into austenite.

The austenitizing temperature for O1 tool steel is 1500°F (815°C). After heating to the austenite temperature, observe whether the color of the O1 material matches the color of the furnace chamber to determine whether the material has been heated uniformly to the predetermined temperature. Then start calculating the insulation time. The general rule of thumb for O1 is to soak for 5 minutes per each inch (25mm) of the smallest cross-section, or until you are certain the part is thoroughly soaked.

O1 tool steel is susceptible to decarburization (loss of carbon from the surface) during heat treatment. To prevent this, it should be annealed and/or hardened in a controlled neutral atmosphere, vacuum, or neutral-salt furnace.

3. Quenching

Quenching rapidly cools the steel from the austenitizing temperature to transform the austenite into martensite, a very hard phase. O1 is an oil-hardening steel, and thus requires oil quenching. 

During the quenching process of O1 steel, when the temperature drops to 125-150°F (52-65°C), tempering should be performed immediately. Delayed tempering may cause cracking, which is a common problem with oil-quenched O1 steel.

When oil quenched from the proper hardening temperature, O1 is expected to expand approximately 0.0015 in./in. (0.0015 mm/mm). 

4. Tempering

Tempering is a crucial reheating process performed after quenching to reduce brittleness and improve toughness by modifying the martensitic structure. It exchanges some strength for greater ductility.

The most commonly used tempering temperature for O1 tool steel is 350°F (175°C). The part must be held at the tempering temperature for 2 hours per inch (25mm) of cross-section.

While O1 tool steel requires typically only a single temper, double tempering may sometimes be preferred. A second temper, typically at a slightly lower temperature (e.g., 325°F/160°C), can be beneficial, particularly for parts with intricate details or those subjected to hard service applications. This additional temper refines the grain structure, leading to improved toughness, wear resistance, and stress relief. Air cooling to room temperature should occur between tempers.

As the tempering temperature increases, the room-temperature hardness and strength of the steel generally decrease, while ductility increases. For O1 tool steel, typical hardness values after tempering are¹:

1. Leed, R. M. (Year of publication). Tool and Die Making TROUBLESHOOTER (p. 244). Society of Manufacturing Engineers. ↩︎