M35 TOOL STEEL | 1.3243 | SKH55

3. M35 Tool Steel Heat Treatment Guide

Proper heat treatment is critical for achieving the desired properties in M35 tool steel. The primary objective of this process is to transform the M35 steel’s annealed structure (primarily ferrite and alloy carbides) into a hardened and tempered martensitic structure containing the necessary carbides for optimal cutting tool performance. This typically involves four main stages: preheating, austenitizing, quenching, and tempering.

3.1 Preheating

Preheating is a necessary step for high-alloy tool steels like M35. It serves to minimize thermal shock and equalize the temperature throughout the part before the higher austenitizing temperatures are applied.

1. Recommended Preheat Temperature: For M35 tool steel, the recommended preheat temperature is 815°C (1500°F).
2. Considerations:

• For intricate or larger tools, a multi-stage preheating process is often beneficial.
• If placing the M35 part into a furnace already at the preheat temperature, it is good practice to first place the part on top of the furnace to remove any chill, further reducing the risk of thermal shock and cracking.

3.2 Austenitizing (Hardening)

During austenitizing, the M35 steel is heated to a high temperature to dissolve complex alloy carbides into the austenite phase, which is essential for developing its final properties. High-speed tool steels are typically heated to temperatures ranging from 1150°C to 1290°C (2100°F to 2350°F), depending on the specific grade.

• Recommended Austenitizing Temperature for M35: 1190°C (2175°F).
• Holding Time: The hold time at this high temperature is generally short, around 2 to 6 minutes, depending on the tool’s configuration and size. For example, a 6-inch thick cross-section might be held for about 5 to 6 minutes.
• Furnace Environment: Using a salt bath or a controlled atmosphere furnace is often preferred for austenitizing M35 to prevent surface degradation such as oxidation or decarburization.

3.3 Quenching

After austenitizing, the M35 steel is rapidly cooled (quenched) to transform the austenite into martensite.

• Recommended Quenching Media for M35: Oil quenching or salt bath quenching.
• Salt Bath Quenching: Typically, the salt bath is maintained between 540°C to 650°C (1000°F to 1200°F). After this, the part is air-cooled. Salt bath quenching helps minimize distortion and residual stress due to temperature uniformity.
• Oil Quenching: Direct oil quenching is not always practiced; sometimes, the steel is first cooled to an intermediate temperature (e.g., around 1000°C) before the oil quench to help avoid quench cracks.
• Cooling Rate: The cooling must be rapid enough through the critical temperature range to ensure the desired transformation to a martensitic structure.

3.4 Tempering

Tempering is a crucial final step. The as-quenched M35 structure (martensite and retained austenite) is highly stressed and brittle. Tempering increases the steel’s toughness, relieves internal stresses, and promotes secondary hardening through the precipitation of alloy carbides. For high-speed steels, tempering also transforms retained austenite to fresh martensite, which then also needs to be tempered.

M35, a cobalt-containing grade, generally has a higher working hardness range (65-67 HRC) and exhibits increased temper hardness and hot hardness compared to M2. Tempering for high-speed steels is typically done around 530°C to 570°C (980°F to 1060°F).

• Tempering Cycles for M35: Requires multiple tempering cycles, typically 2 to 4 times, to ensure proper transformation of retained austenite and tempering of newly formed martensite.
• Soaking Time: Each cycle involves heating to the desired temperature and soaking for usually 2-4 hours each, or 2 hours per inch of cross-section.
• Cooling After Tempering: Usually done in air.
• Timing: Temper the steel as soon as possible after it has cooled from quenching (ideally before it reaches room temperature, e.g., between 52°C and 65°C or 125°F and 150°F) to prevent cracking.
• Cooling from Tempering Temperature: Slow cooling is recommended to minimize residual stresses.

3.5 M35 Tool Steel Tempering Temperatures and Resulting Hardness:

3.6 Optional: Subzero Treatment for M35 Steel

A subzero treatment can be performed after quenching, particularly for high-carbon and high-alloy steels like M35, to further transform retained austenite into martensite. This can enhance hardness, wear resistance, and dimensional stability.

• Procedure: Involves cooling to -30°C to -120°C.
• Follow-Up: If a subzero treatment is used, it must be followed immediately by tempering to relieve stresses introduced by the transformation.