Heat Treatment of H13 Tool Steel: Procedures and Guidelines

H13 tool steel is a versatile and widely used material known for its excellent combination of hardness, toughness, and resistance to thermal fatigue cracking.

In this blog post, we will explore the recommended heat treatment process for H13 tool steel, including preheating, austenitizing, quenching, tempering, and annealing.

Understanding these heat treatment procedures will help maximize the performance and longevity of H13 tool steel in various applications.

Preheating

To minimize distortion, especially in complex tools, a double preheating method is recommended for H13 tool steel. The process involves heating the steel at a controlled rate not exceeding 400°F per hour (222°C per hour) to a range of 1150-1250°F (621-677°C) for equalization.

Subsequently, the temperature is raised to 1500-1600°F (816-871°C) for equalization. For normal tools, a single preheating treatment using only the second temperature range is sufficient.

Austenitizing (High Heat)

After preheating, the next step is austenitizing, which involves heating the H13 tool steel rapidly. The recommended temperature range for austenitizing is 1800-1890°F (982-1032°C). For maximum toughness, a temperature of 1800°F (982°C) is ideal.

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On the other hand, for maximum hardness, resistance to thermal fatigue cracking, and wear resistance, a temperature of 1890°F (1032°C) is recommended. The steel should be soaked at the austenitizing temperature for 30 to 90 minutes.

Quenching

Various quenching methods can be employed for H13 tool steel, including air cooling, pressurized gas quenching, or warm oil quenching. Typically, section thicknesses up to 5 inches (127 mm) will fully harden when cooled in still air from the austenitizing temperature.

For sections greater than 5 inches (127 mm) in thickness, accelerated cooling methods such as forced air, pressurized gas, or interrupted oil quenching are necessary to achieve maximum hardness, toughness, and resistance to thermal fatigue cracking.

For pressurized gas quenching, a minimum quench rate of approximately 50°F per minute (28°C per minute) to below 1000°F (538°C) is required for optimal steel properties. When using oil quenching, the steel should be quenched until black, around 900°F (482°C), and then cooled in still air to 150-125°F (66-51°C).

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Tempering

Immediately after quenching, tempering is performed to relieve stresses and enhance the toughness of the H13 tool steel. The typical tempering range for H13 is 1000-1150°F (538-621°C).

The steel should be held at the tempering temperature for 1 hour per inch (25.4mm) of thickness, with a minimum of 2 hours, followed by air cooling to ambient temperature. Double tempering is required to achieve the desired properties.

Additionally, a third tempering process can be employed as a stress relief step after all finish machining, grinding, and EDM work are completed on the tool. This extra tempering helps maximize toughness and tool performance.

Annealing

Annealing is necessary after hot working and before re-hardening of H13 tool steel. The annealing process involves heating the steel at a controlled rate not exceeding 400°F per hour (222°C per hour) to a range of 1575-1625°F (857-885°C).

The steel should be held at this temperature for 1 hour per inch of maximum thickness, with a minimum of 2 hours. Slow cooling with the furnace at a rate not exceeding 50°F per hour (28°C per hour) down to 1000°F (538°C) is recommended, followed by further cooling to ambient temperature either in the furnace or in air. The resulting hardness after annealing should be a maximum of 235 HBS.

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