AISI H21 Tool Steel Properties

3. Heat Treatment Process for H21 Tool Steel

Achieving the optimal performance from H21 tool steel relies heavily on a precise heat treatment process. This procedure develops the necessary hardness, wear resistance, and toughness essential for demanding hot work applications. At Aobo Steel, we recognize the importance of these steps, leveraging our extensive experience with tool steels.

3.1 Preheating

Before the main hardening process, H21 steel must be carefully preheated.

• Temperature: Heat uniformly to 760–815 °C (1400–1500 °F).
• Purpose: This crucial step minimizes thermal shock and prevents cracking, especially in components with complex shapes or varying thicknesses. Slow and even heating ensures that the temperature is evenly distributed throughout the steel, preparing it for the high temperatures of austenitizing.

3.2 Austenitizing (Hardening)

Following preheating, the steel is brought to the austenitizing temperature.

• Temperature: Heat to 1095–1205 °C.
• Purpose: At this stage, the steel’s microstructure transforms into austenite. This allows the carbon and critical alloying elements like tungsten (W) and chromium (Cr) to dissolve into the structure, which is fundamental for achieving hardness upon cooling.

Soaking at Austenitizing Temperature

Holding the steel at the austenitizing temperature (soaking) is vital.

• Duration: Typically 2 to 5 hours, depending on the cross-section thickness (allow sufficient time, e.g., 1 hour per inch of thickness).
• Purpose: H21’s high content of tungsten and chromium forms stable carbides. Sufficient soaking time ensures these carbides fully dissolve and distribute evenly within the austenite, leading to a uniform structure ready for quenching.

3.3 Quenching (Cooling)

Rapid cooling transforms the austenite into hard, martensitic steel.

• Methods: H21 steel can be quenched in either air or oil.
• Considerations: Due to its high hardenability, H21 readily hardens even with slower air cooling. Air quenching is often preferred as it significantly reduces the risk of distortion and quench cracking compared to more severe oil quenching, especially for larger or intricate parts. Oil quenching can be used if maximum hardness is the primary goal. The cooling rate must be sufficiently fast to prevent the formation of softer structures, such as pearlite or bainite.

3.4 Tempering

Quenched H21 steel is extremely hard but also brittle. Tempering is necessary to refine its properties.

• Temperature: Reheat the steel to 595–675 °C.
• Purpose: Tempering reduces brittleness and significantly improves toughness and ductility while retaining substantial hardness. The specific temperature and time used will determine the final balance between hardness and toughness.
• Multiple Tempers: For high-alloy tool steels like H21, performing two or more tempering cycles is common practice. This ensures structural stability, transforms any retained austenite, and can contribute to secondary hardening, further enhancing performance at elevated temperatures.

3.5 Important Process Controls

3.5.1 Preventing Decarburization

H21 steel is prone to decarburization (loss of carbon from the surface) at high heat treatment temperatures, resulting in a soft, weak surface layer.

• Solution: Protective measures are essential during austenitizing. This includes using controlled atmosphere furnaces, salt baths, or wrapping the parts in protective material like stainless steel foil.

3.5.2 Cooling After Forging

Due to H21’s high hardenability, forgings should be cooled slowly (e.g., furnace cooling or burying in insulation) immediately after the forging operation.

• Reason: Avoid normalizing (air cooling from high temperature), as this can cause the steel to partially or fully harden, leading to brittleness and potential cracking before the intended hardening and tempering process. Slow cooling ensures a more suitable structure for subsequent heat treatment.

This structured approach to heat treating H21 tool steel ensures the material delivers the high performance required for its intended hot work tooling applications. Precise control over each parameter is key to success.