M2 tool steel | 1.3343 | SKH51

4. Heat Treatment

M2 steel can only achieve the required performance through proper heat treatment. The main objective of heat treatment is to transform M2 steel from a softened annealed state (mainly composed of ferrite and alloy carbides) into a hardened and tempered martensitic structure, in which carbides are distributed in the optimal position to meet the necessary cutting performance requirements.

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

4.1 Pre-heating

Preheating of materials should be carried out in stages to minimize the risk of thermal shock, especially when there are significant differences in the cross-section of parts. If the furnace is first heated to 650°C (1200°F), we can place M2 parts on the top of the furnace to remove any chill, and then place it in the furnace. This helps to reduce thermal shock and lower the risk of cracking. It is then placed in the furnace and kept there for about 10-12 minutes.

4.2 Hardening（Austenitizing） 

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). The soaking time at the final hardening temperature for M2 high-speed steel is relatively short, only a few minutes, depending on the part’s size and furnace efficiency. 

4.3 Quenching

The quenching medium can be air, oil, or salt. Oil quenching works well for M2 steel parts with a cross-section of up to about 1 to 1.5 inches (25-38 mm). The martensitic transformation begins at about 316°C (600°F) and is completed at about 93°C (200°F). We recommend cooling M2 parts to approximately 65°C (150°F) after quenching, then proceeding to the next step of tempering.

4.4 Tempering

Tempering is performed after quenching M2 high-speed 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 2 to 4 tempering cycles at a temperature of no less than 540°C (1000°F), typically 3 cycles.

For example, after austenitizing at 1230°C (2250°F), the first tempering cycle is performed at 565°C (1050°F), followed by the second at 550°C (1025°F), and the third at 540°C (1000°F).

For each tempering cycle, the soaking time is 2 hours per inch (25 mm) of the thickest cross-section. After the first tempering, the M2 steel component must be completely cooled to room temperature before the next tempering can be performed.

Tempering M2 material serves to transform retained austenite into fresh martensite, causing the precipitation of complex carbides and significantly contributing to the steel’s secondary hardness. Multiple tempering cycles refine the microstructure, enhance wear resistance, and further relieve stresses.

It is important to note that M2 steel must not be undertempered.

Additionally, we recommend performing stress-relief tempering after subsequent processes such as grinding, welding, or electrical discharge machining (EDM), with the tempering temperature set 14–28°C (25–50°F) below the final tempering temperature.

[Reference: ASM International. (1989). ASM Handbook, Volume 16: Machining. ASM International.]