4. Heat Treatment

To ensure your O1 steel parts deliver the durability and reliability, we’ve outlined the essential stages of the O1 tool steel heat treatment cycle.

4.1 Preheating: The Essential First Step

Before any high-temperature operations, preheating is a non-negotiable first step in the O1 tool steel heat treatment process. This stage serves two vital functions:

• It prepares the steel’s microstructure for the upcoming hardening phase.

• It helps relieve internal stresses induced during machining.

Procedure:

• Heat the O1 steel parts uniformly to approximately 650°C (1200°F).

• Hold at this temperature for 10 to 15 minutes, or until the entire part has reached a consistent temperature.

• Caution: Avoid prolonged exposure at the preheat temperature, as this can negatively affect the steel’s structure.

4.2 Austenitizing (Hardening): Transforming the Steel

Following preheating, the next phase is austenitizing. This involves heating the O1 steel above its critical transformation point to form austenite, the key to successful hardening.

Procedure:

• Heat the steel to an austenitizing temperature range of 790°C to 815°C (1450°F to 1500°F).

• Visually confirm that the part is the same color as the furnace interior to ensure uniform heating.

• Once uniformly heated, begin the soak time. For O1 steel, soak for approximately 5 minutes per inch (25mm) of the smallest cross-section.

• Caution: Oversoaking at austenitizing temperatures can lead to an undesirable amount of retained austenite, impacting final properties.

4.3 Quenching: The Path to Hardness

O1 is specifically an oil-hardening tool steel. The quench is where the steel rapidly cools to form the hard martensitic structure.

Procedure:

• Quench the parts in a suitable commercial quenching oil immediately after the austenitizing soak.

• Critical Technique:

  • Immerse flat sections vertically to ensure both sides contact the oil simultaneously, minimizing distortion.

  • Tubular parts should also be quenched vertically.

• Remove parts from the quench once they have cooled to 66°C to 93°C (150°F to 200°F).

• Important: After quenching, O1 steel is in a highly stressed condition and is very susceptible to cracking. Proceed to tempering without delay.

4.4 Tempering: The Key to Toughness and Stability

Tempering is arguably the most critical stage in the O1 tool steel heat treatment process. It must be performed promptly after quenching to prevent cracking and to develop the desired mechanical properties.

Procedure:

• Timing is Crucial: Transfer parts to the tempering furnace as soon as they reach 52°C to 65°C (125°F to 150°F) after quenching. Any delay significantly increases the risk of cracking.

• Tempering involves reheating the hardened steel to a temperature below its lower critical point (Ac1). This allows carbon atoms to precipitate as carbides, relieving internal stresses and significantly enhancing toughness and ductility.

• Standard Tempering Temperature for O1: Typically 175°C (350°F).

• Soaking Time: Hold at the tempering temperature for 2 hours per inch (4.7 minutes per mm) of cross-section for each tempering cycle. Do not shorten this duration.

Achievable Hardness:

• After quenching: Typically 64-65 HRC.

• After tempering at 177°C (350°F): Approximately 62-63 HRC.

• Low-temperature tempering (150°C – 260°C / 300°F – 500°F) is standard for O-series steels to maintain high hardness.

4.4.1 Double Tempering: When to Consider It

While a single temper is often sufficient for O1 steel, a second temper can offer benefits for:

• Parts with intricate geometries.

• Applications involving severe service conditions.

• Enhancing grain refinement for improved toughness.

• Minimizing retained austenite and improving dimensional stability.

If performing a second temper:

• Set the temperature approximately 14°C (25°F) lower than the first tempering temperature to help preserve the initial hardness.

4.5 Dimensional Stability in O1 Tool Steel Heat Treatment

O1 tool steel generally exhibits good dimensional stability during O1 tool steel heat treatment. However, some changes are expected:

• Expansion: When oil quenched from the correct hardening temperature, O1 typically expands by approximately 0.0015 inches per inch (0.0015 mm/mm).

• Tempering Effects:

  • Tempering up to 200°C (390°F) may result in a slight contraction.

  • At higher temperatures (peaking around 300°C or 570°F), dimensions can increase due to the transformation of retained austenite to bainite.

  • By 400°C (750°F), dimensions tend to return closer to the original size.

• Part Geometry: The actual shape and any pre-existing distortion will influence final dimensional changes.

• Straightening: It may be possible to straighten parts if their temperature is still above 205°C (400°F).

4.6 Post-Heat Treatment Considerations

• Stress Relief: After operations like grinding, welding, or Electrical Discharge Machining (EDM) on hardened material, a stress-relief temper is highly recommended.

  • Heat to a temperature of 14°C to 28°C (25°F to 50°F) below the final tempering temperature.

  • Cool slowly to avoid introducing new stresses.

• Decarburization: O1 tool steel can be prone to decarburization (loss of surface carbon) during annealing or hardening.

  • To minimize this, use a controlled neutral atmosphere, vacuum furnace, or a neutral-salt furnace during O1 tool steel heat treatment.

  • Decarburization can lead to soft surfaces and an increased risk of cracking if not properly managed.