M2 tool steel | 1.3343 | SKH51

3. Heat Treatment

Proper heat treatment is critical for M2 high-speed tool steel. The highest-quality M2 steel, even when made into precise tools, will not perform correctly if the heat treatment is inadequate. The primary objective is to transform the steel from its soft, annealed state (chiefly ferrite and alloy carbides) into a hardened and tempered martensitic structure, where carbides are optimally positioned for necessary cutting properties.

The typical heat treatment sequence for M2 tool steel involves the following stages:

3.1 Preheat

A preheat cycle is a necessary operation for M2 high-speed steels. Preheating is often done in stages to minimize thermal shock risk, especially if the part has significant variations in cross-section.

• The M2 steel part can sometimes be placed directly into a furnace already at about 650°C (1200°F).
• However, placing it on top of the furnace first to remove any chill can help reduce thermal shock and the potential for cracking.
• Holding at a preheat temperature, such as 650°C (1200°F), for about 10-12 minutes preconditions the steel for subsequent transformation.

3.2 Austenitizing (Hardening Heat)

This step involves heating the preheated M2 steel to a high temperature to transform its structure to austenite and dissolve essential carbon and alloying elements into this phase.

• The hardening temperature for M2 tool steel typically ranges from 1190°C to 1220°C (2175°F to 2225°F). Some references cite up to 1230°C (2250°F).
• Achieving this high temperature is necessary to ensure that carbon and alloying elements are properly dissolved in the austenite.
• Due to potential issues like oxidation, decarburization, and grain growth at these elevated temperatures, and the steel’s thermal conductivity, staged heating is commonly employed.
• Holding time at the final hardening temperature for M2 high-speed steel is relatively short, often only a few minutes, depending on the part’s size and furnace efficiency.

3.3 Quench

Following austenitizing, the M2 steel is quenched—cooled rapidly—to transform the austenite into the hard martensite structure.

• Suitable quenching media for M2 tool steel include air, oil, or salt.
• Oil quenching works well for M2 steel parts up to about 1 to 1.5 inches (25-38 mm) in cross-section.
• For M2 high-speed steel, the martensite transformation typically begins around 316°C (600°F) and finishes around 93°C (200°F).
• It is recommended to cool the M2 steel part down to approximately 65°C (150°F) after quenching.
• In the as-quenched state, M2 steel is hard but also highly stressed and quite brittle.

3.4 Tempering

Tempering is performed after quenching M2 steel to relieve internal stresses, improve toughness, and promote secondary hardness. This involves reheating the steel to an intermediate temperature below its critical transformation temperature.

• M2 high-speed tool steel requires multiple tempering treatments—usually 2 to 4 cycles.
• M2 specifically requires multiple tempers at a minimum of 540°C (1000°F). A common practice is to use a triple temper.
• For example, after austenitizing M2 steel at 1230°C (2250°F), the first temper might be at 565°C (1050°F), followed by a second at 550°C (1025°F), and a third at 540°C (1000°F).
• Each tempering cycle for M2 steel should include a soak time of 2 hours per inch (25mm) of the thickest cross-section.
• After the first temper, the M2 steel part must be allowed to cool completely to room temperature before the next temper.
• Tempering M2 steel serves to transform retained austenite into fresh martensite and causes the precipitation of complex carbides, contributing significantly to the steel’s secondary hardness. This multi-stage process refines the microstructure, enhances wear resistance, and further relieves stresses.
• Do not undertemper M2 steel.
• After subsequent operations like grinding, welding, or electrical discharge machining (EDM) on M2 steel, a stress-relief temper is strongly advised, typically at a temperature 14-28°C (25-50°F) below the final tempering temperature used.

3.5 Tool Steel Heat Treatment Parameters Summary