L6 tool steel | 1.2714 | SKT4

4. Heat Treatment

The heat treatment process for L6 tool steels generally involves four primary steps: preheating, austenitizing (hardening), quenching, and tempering.

4.1 Annealing and Stress Relieving

L6 steels should not be normalized due to their high hardenability, but instead, they should be annealed after forging and before hardening. The annealing temperature for L6 is 870°C (1598°F). Stress relieving is sometimes performed after rough machining and before final machining, usually by heating to 650-730°C (1200-1350°F) to minimize distortion during hardening.

4.2 Preheating

The preheating temperature for L6 tool steel is 1200°F (650°C), with a holding time of 10 to 15 minutes, or until the material is uniformly heated. Avoid prolonged soaking times, as this can disrupt molecular structure and cause adverse effects in subsequent processes. For large or complex L6 components, it is recommended to heat slowly within the range of 650–760°C (1200–1400°F). If placing the material directly into a furnace already heated to the preheating temperature, it is advisable to first position the material above the furnace to remove chill.

4.3 Hardening (Austenitizing)

The hardening temperature is 1525°F (830°C). Some sources list it as 790–870°C (1450–1600°F) or 790–845°C (1454–1553°F). This process is influenced by both time and temperature. The soaking time should be sufficiently long to ensure the workpiece is fully heated. During this process, the steel’s microstructure transitions from ferrite-pearlite to austenite, and the alloy carbides dissolve.

4.4 Quenching

The objective of quenching is to cool the austenite rapidly enough to transform it into a hardened martensitic structure. L6 tool steel is primarily an oil-hardened steel. It has good hardenability and achieves the required hardness through oil quenching. After quenching, parts should be cooled to a warm temperature of 125-150°F (52-65°C).

4.5 Tempering

When the part temperature drops to 125-150°F (52-65°C), tempering can be performed. Tempering reduces internal stress, improves the toughness of steel, and adjusts its hardness. L6 is often tempered once, but multiple tempering is recommended to reduce residual austenite and relieve stress. The holding time is 2 hours per inch (25mm) of cross-section.
The resulting hardness can be controlled by varying the tempering temperature: