H11 TOOL STEEL | 1.2343 | skd6

4. H11 Tool Steel Heat Treatment

Precise H11 tool steel heat treatment is crucial for achieving its optimal performance as a 5% chromium hot-work die steel and ultrahigh-strength steel, similar to H11 Modified and H13. This process tunes the microstructure for demanding factory tooling. 

4.1 Forging

This steel is easily forged, with a suitable forging temperature range of 1120 to 1150°C (2050 to 2100°F). We recommend preheating the billet to 790 to 815°C (1450 to 1500°F), then uniformly heating it to the forging temperature. The forging temperature must NOT be lower than 925°C (1700°F). If the temperature drops near this level, it must be reheated before continuing the forging process.

4.2 Austenitizing(Hardening)

Austenitizing, the primary hardening step, involves heating to transform its structure into austenite. This allows uniform conversion and dissolution of alloying elements and carbides.

• Preheating: Preheating H11 to 760-815 °C (1400-1500 °F) before austenitizing is recommended to minimize thermal shock and cracking, especially for this high-alloy material.

• Austenitizing Temperatures and Soaking Times: The Austenitizing Temperature is 995 to 1025 °C (1825 to 1875 °F). The soaking time is 20 minutes + 5 minutes per 25 mm (1 inch) of thickness.

Holding at the austenitizing temperature ensures uniform transformation and dissolution.

4.3 Quenching

After austenitizing, H11 steel is rapidly quenched to form hard martensite. H11 is an air-hardening steel, providing uniform hardening through large sections with minimal residual stress and dimensional change. Air cooling is the standard and preferred method for H11 tool steel. While air cooling is standard, oil quenching from 995 °C (1825 °F) is a possible alternative. H11 and other hot work tool steels must never be water quenched, as this can lead to cracking.

4.4 Tempering

After hardening, tempering H11 tool steel by reheating to a lower temperature is crucial for improving toughness, relieving stress, and stabilizing properties. Steel H11 is a secondary hardening steel. It achieves its best performance when tempered at temperatures above 510°C (950°F). We recommend performing multiple tempering treatments on it to achieve optimal toughness and extend tool life. Allow parts to cool to room temperature between each tempering cycle to minimize residual austenite. 

4.5 Annealing 

The annealing temperature is 845-900°C (1550-1650°F). The cooling rate is 22-40°C/h (40-75°F/h). The hardness after annealing is 192-229 HB.

4.6 Other Important H11 Heat Treatment Considerations

• Normalizing: This process is generally not recommended for H11 or H13 tool steels due to the high risk of cracking, especially with surface decarburization.

• Stress-Relief: After H11 has undergone processing, such as grinding, welding, or EDM (electrical discharge machining), we strongly recommend that it undergo stress relief treatment at 650°C. This treatment should be carried out at a temperature 14–28°C (25–50°F) lower than the previous tempering temperature. After stress relief, cool slowly at a maximum rate of 300°C/h to prevent new stresses from forming.

• Cryogenic Treatment: Sub-zero treatment (e.g., -73°C / -100°F) can reduce retained austenite, but H11’s high tempering temperatures usually make it unnecessary for achieving maximum secondary hardening response.

• Surface Protection: Protecting its surfaces from oxidation and decarburization during high-temperature treatments, such as austenitizing and annealing, is vital. This is often achieved using controlled atmospheres, vacuum furnaces, or salt baths.