H13 TOOL STEEL | 1.2344 | skd61

4. Heat Treatment

The H13 steel heat treatment process is a multi-stage procedure designed to develop the desired microstructure and mechanical properties. Each step plays a vital role in the final performance of the tool.

4.1 Preheating: The Essential First Step for H13 Steel

Before the main hardening phase, preheating H13 steel is crucial. We recommend a preheat temperature of approximately 815°C (1500°F). This step serves two primary purposes:

• Ensuring uniform temperature distribution throughout the component as it approaches the higher austenitizing temperature.

• Minimizing thermal shock, which can be detrimental to the integrity of the steel.

4.2 Austenitizing (Hardening): Achieving the Optimal Microstructure

Austenitizing is the core of the hardening process, where the H13 steel is heated to transform its microstructure into austenite. For effective H13 steel heat treatment:

• Recommended Austenitizing Temperature: Target a range between 1020°C and 1065°C (approximately 1875°F to 1950°F).

• Holding Time: Maintain the steel at the austenitizing temperature for approximately 1 hour for every 25mm (1 inch) of material thickness. It is critical to adhere to the correct austenitizing temperature and holding time. Temperatures that are too low (e.g., 890°C) or too high (e.g., 1150°C), or insufficient soaking, can negatively impact vital properties such as toughness. The as-quenched hardness of H13 steel is directly influenced by both the austenitizing conditions and the subsequent cooling rate.

4.3 Quenching: The Significance of Air Cooling in H13 Steel Heat Treatment

H13 is specifically designed as an air-hardening steel. Therefore, air quenching from the austenitizing temperature is the specified method. This controlled cooling process offers distinct advantages:

• Achieves through-hardening, even in larger cross-sections.

• Results in minimal residual stresses compared to more aggressive liquid quenching methods, which is beneficial for dimensional stability and service life.

4.3 Tempering: Unlocking Peak Performance and Durability in H13 Steel

Tempering is arguably the most critical stage in the H13 steel heat treatment cycle, especially because H13 is a secondary-hardening steel. This means it develops its optimal properties, particularly hardness and strength at elevated service temperatures, when tempered at temperatures above its secondary hardening peak, which typically occurs around 510°C (950°F).

Key aspects of tempering H13 steel include:

• Secondary Hardening: Achieved through the precipitation of fine, dispersed alloy carbides (primarily V8C7, along with M2C, M6C, and M7C3 types). These vanadium-rich MC carbides are the main contributors to the steel’s strength.

• Benefits of High-Temperature Tempering (above ~510°C / 950°F):

  • Provides substantial stress relief after hardening.

  • Stabilizes the microstructure and mechanical properties for reliable performance at elevated operational temperatures.

  • Allows for preheating components for subsequent operations like welding or warm working at temperatures up to 55°C (100°F) below the prior tempering temperature without significantly affecting hardness.

• Recommended Practice: For optimal results, H13 steel should be subjected to two tempering treatments at a high temperature after hardening (e.g., following hardening at 1020°C).

• Hardness Achievement: The final hardness is determined by the chosen tempering temperature. For instance, tempering at approximately 610°C can yield a hardness of around 45 HRC.

• Caution: Tempering at lower temperatures (e.g., 250°C) should be avoided as it can lead to a reduction in toughness and impair the steel’s resistance to further tempering.

• Multistage Tempering: In some cases, employing a multistage tempering approach can offer further benefits over a single tempering cycle.

• Dimensional Changes: Be aware that the tempering temperature will influence the final dimensions of the H13 steel component.

Even bainitic microstructures, which can form during slower cooling of larger H13 sections, will exhibit significant secondary hardening upon appropriate tempering, achieving hardness levels comparable to tempered martensite due to this carbide precipitation.

4.4 Critical Considerations for Successful H13 Steel Heat Treatment

Beyond the primary stages, several factors require careful attention to ensure the best outcomes from your H13 steel heat treatment:

• Surface Decarburization: During the high-temperature processes involved in H13 steel heat treatment, there is a risk of surface decarburization if the furnace atmosphere is not adequately controlled. This can lead to a soft surface layer with reduced wear resistance and fatigue strength.

• Surface Preparation: To enhance resistance to thermal cracking, especially in demanding hot work applications, consider surface preparation techniques such as polishing or mechanical abrasion on the finished tool.

• Special Homogenization Anneal (Note: Not Standard Normalizing): Standard normalizing is generally not recommended for H13 steel. However, a specific thermal cycle can be employed to improve microstructural homogeneity. This involves:

  1. Preheating to around 790°C (1450°F).

  2. Slow, uniform heating to an austenitizing temperature range of 1040°C to 1065°C (1900°F to 1950°F).

  3. Holding for approximately 1 hour per 25mm (1 inch) of thickness.

  4. Air cooling. It is imperative that this specific homogenization treatment is immediately followed by a full spheroidizing anneal as the steel approaches or reaches room temperature. This is a specialized procedure and carries a risk of cracking, particularly if the furnace atmosphere does not prevent surface decarburization.

4.5 H13 Steel Heat Treatment: Recommended Parameters Summary

For quick reference, the typical parameters for H13 steel heat treatment are summarized below. Note that these are general guidelines, and precise parameters may need adjustment based on specific component geometry, equipment, and desired final properties.

By carefully controlling each stage of the H13 steel heat treatment process, manufacturers can consistently produce tools with the high strength, toughness, and thermal fatigue resistance required for demanding hot work environments.