M42 tool steel | 1.3247 | SKH59

3. M42 Tool Steel Heat Treatment

A perfectly treated M42 tool undergoes several distinct phases: preparation and preheating, austenitizing (hardening), quenching, and tempering. Each stage is indispensable for developing the desired microstructure and mechanical properties.

3.1 Preparation and Preheat

Meticulous preparation is crucial before subjecting M42 tool steel to high hardening temperatures. This often includes initial processes like annealing, which softens the steel, refines its microstructure, and prepares it for effective hardening. Tools may also be spheroidized to enhance machinability.

M42 tool steel heat treatment typically incorporates a staged preheating phase, vital for minimizing thermal shock and potential cracking, especially in tools with varying cross-sections.

• A common preheat temperature for M42 is 815°C (1500°F).
• The goal is to uniformly bring the steel to temperature, reducing stress before the critical austenitizing stage.

3.2 Austenitizing (Hardening)

Austenitizing is the fundamental transformation stage. Here, annealed M42 tool steel (mainly ferrite and alloy carbides) is heated to a specific high temperature. This converts the structure to austenite and dissolves complex alloy carbides, essential for final hardness and wear characteristics.

Key parameters for austenitizing M42 tool steel include:

• Temperature: Typically between 1180°C to 1190°C (2155°F to 2175°F). One study identified 1180°C (1453 K) as an optimal austenitizing temperature.
• Holding Time: Relatively short for high-speed steels, generally 2 to 6 minutes once the steel reaches the target temperature. For instance, a 4-minute soak at 1190°C (2175°F) is a common recommendation. Holding M42 at 1180°C for 15 to 30 minutes has shown consistent microstructure and hardness.
• Caution: Exceeding the recommended temperature or holding time can be detrimental. It can lead to excessive carbide dissolution and undesirable grain growth, negatively impacting the steel’s properties, including a potential decrease in hardness due to phenomena like Ostwald ripening if held for too long (e.g., 45 minutes at 1180°C).

3.3 Quenching

After austenitizing, M42 tool steel is rapidly cooled (quenched) to transform austenite into martensite. Martensite is the hard crystal structure providing cutting and wear-resistant properties.

• Quenching Media: This can be achieved using air, oil, or a salt bath. M42 tool steel has deep hardenability, achieving good hardness throughout its cross-section.
• Cooling Rate: A high cooling rate is important. It influences the martensite start (Ms) and finish (Mf) temperatures and can affect the amount of retained austenite.
• As-Quenched Hardness: M42 tool steel typically achieves an as-quenched hardness of around 65-68 HRC.
  Untempered martensite, while very hard, is also highly stressed, brittle, and unstable. Tempering is therefore essential.

3.4 Tempering

Tempering, a critical step after quenching, involves reheating M42 tool steel to an intermediate temperature below its critical transformation point. This significantly enhances usability and performance through several mechanisms:

• Stress Relief: Relieves internal stresses from quenching, reducing susceptibility to cracking.
• Toughness Improvement: Significantly increases toughness, making the steel less brittle.
• Retained Austenite Transformation: Transforms retained austenite (which can reduce hardness and dimensional stability) into fresh martensite, tempered in subsequent cycles.
• Secondary Hardening: M42 exhibits secondary hardening. Fine, complex alloy carbides precipitate during tempering, further increasing hardness. M42 is known for excellent hot hardness, reaching up to 69 HRC after tempering (typically around 538°C / 1000°F). Optimal results often involve tempering on the high side of the secondary hardness peak.

Multiple Tempering Cycles:

M42 tool steel requires multiple tempering cycles (usually two to four) to ensure full transformation of retained austenite and to temper new martensite.

• Temperature: Common tempering temperatures for M42 are between 510°C (950°F) and 538°C (1000°F).
• Soaking Time: Each tempering cycle typically involves soaking for 2 to 4 hours at the chosen temperature.

Stress-Relief Temper (Post-Machining):

After significant grinding, welding, or Electrical Discharge Machining (EDM) on heat-treated M42, a final stress-relief temper is strongly recommended at a temperature 14-28°C (25-50°F) lower than the last tempering temperature.

3.5 Summary of M42 Tool Steel Heat Treatment Parameters

To provide a quick overview, the typical parameters for M42 tool steel heat treatment are summarized below: