H10 Tool Steel | 1.2365 | SKD7

3. H10 Tool Steel Heat Treatment

H10 tool steel, a robust chromium-based hot-work tool steel from the H-series, is engineered for demanding applications such as hot forging, extrusion, and die casting. Its performance under extreme operational temperatures, typically 315°C to 650°C (600°F to 1200°F), relies heavily on precise heat treatment. 

3.1 The Critical Steps in H10 Heat Treatment

Achieving peak performance from H10 tool steel involves several distinct and vital stages. Each step must be carefully controlled to develop the desired microstructure and mechanical properties.

3.2 Annealing

Annealing is typically the first step for H10 tool steel. It’s crucial for softening the steel for easier machining and creating a uniform microstructure for subsequent heat treatments.

• Purpose: To soften the steel for machinability and create microstructural uniformity.
• Recommended Temperature: 845°C to 900°C (1550°F to 1650°F).
• Cooling Procedure: A slow cool at a rate of approximately 22°C per hour (40°F per hour) is advised.
• Target Hardness: The aim is to achieve an annealed hardness between 192 and 229 HB.
• Resulting Microstructure: A properly annealed H10 will exhibit spheroidized carbides dispersed within a ferrite matrix, ideal for effective austenite formation and grain refinement during hardening.

3.3 Preheating

Preheating is a vital stage before hardening. It significantly reduces thermal shock, minimizing distortion or cracking when the tool enters the high-temperature furnace.

• Purpose: To reduce thermal shock, lessen distortion/cracking risks, relieve machining stresses, and decrease time in the high-heat austenitizing furnace.
• Typical Temperature: Around 815°C (1500°F) is common. Staged preheating can also be beneficial.
• Duration: Ensure the entire cross-section of the tool is uniformly heated.

3.4 Austenitizing (Hardening)

Austenitizing (or hardening) involves heating H10 tool steel to a precise high temperature. This transforms its microstructure to austenite and dissolves essential alloy carbides, crucial for hardness and secondary hardening characteristics during tempering.

• Purpose: To transform the steel’s structure to austenite and dissolve necessary alloying elements (like Chromium, Molybdenum, and Vanadium) into solid solution.
• Recommended Temperature: Generally between 1010°C and 1040°C (1850°F and 1900°F).
• Holding Time: Hold at temperature only long enough to heat completely through (typically 15-40 minutes after reaching temp). Avoid prolonged soaking for H10. Austenitizing temperature affects dissolved carbides, Ms temperature, and retained austenite.

3.5 Quenching

After austenitizing, quenching rapidly cools the steel, transforming austenite into martensite – the hard, desirable microstructure.

• Purpose: To rapidly cool the steel, transforming austenite into a hard martensitic structure.
• Recommended Quench Medium: Air quenching is preferred for H10 (an air-hardening grade), minimizing residual stresses and dimensional changes.
• Alternatives for Large Sections: For very large sections, an air blast or oil quench may be necessary.
• Critical Note: H10 tool steel should never be water quenched.
• Optional Method: A salt bath quench (around 540-595°C or 1000-1100°F), followed by air cooling, is also viable.
• Resulting Microstructure: Primarily martensite with some retained austenite.

3..6 Tempering

Tempering is the final, indispensable step after quenching. Hardened H10 is stressed and can be brittle; tempering relieves these stresses and significantly improves toughness.

• Purpose: To relieve internal stresses, transform retained austenite, precipitate fine alloy carbides for secondary hardening (enhancing hot strength), and improve toughness.
• Timing: Temper as soon as possible after quenching (ideally before reaching room temperature) to prevent cracking.
• Typical Temperature Range: 550°C to 650°C (1020°F to 1200°F). H10 shows a secondary hardening peak around 540°C (1000°F).
• Multiple Tempers: A Minimum of two (often up to four) tempering cycles are essential for H10 to maximize retained austenite transformation and optimize toughness.
• Soak Time: Each temper should involve a soak time of 2 to 4 hours.
• Target Working Hardness: Typically 38 to 54 HRC, depending on the application.

3.7 Summary of H10 Tool Steel Heat Treatment Parameters

For quick reference, the key parameters for H10 tool steel heat treatment are:

3.8 Enhancing H10 Tool Steel with Surface Treatments

To further boost wear resistance, surface treatments like nitriding can be applied to H10 tool steel.

• Nitriding: This process introduces nitrogen into the steel’s surface, creating an extremely hard outer layer (often exceeding 1000 HV).
• Benefits: Significantly improves wear resistance and anti-galling properties.
• Core Properties: Nitriding is typically performed around 540°C, often below or within H10’s tempering range. This hardens the surface without negatively affecting the core toughness and strength established by prior heat treatment.