L6 tool steel | 1.2714 | SKT4

3. L6 Tool Steel Heat Treatment

Effective heat treatment is crucial for maximizing the performance and service life of L6 tool steel components. As a versatile chromium-nickel, low-alloy tool steel, L6 offers a unique balance of toughness and wear resistance when processed correctly. 

3.1 Key Stages in L6 Tool Steel Heat Treatment

3.1.1 Annealing

Following any hot working processes like forging, L6 tool steel must be properly annealed. This step is critical for preparing the material for subsequent machining and hardening.

• Process: L6 steel should undergo soft annealing or spheroidizing. Normalizing is not recommended. This treatment refines the steel’s microstructure, improving its machinability and ensuring a consistent response during the hardening phase.
• Supplier Responsibility: Typically, this annealing process is performed by the steel mill, ensuring the L6 tool steel is supplied in an optimal condition for your further processing.

3.1.2 Stress Relieving

To minimize distortion during the subsequent hardening process and to improve the overall stability of L6 tool steel parts, stress relieving is a highly recommended practice.

• After Rough Machining: This is particularly beneficial for L6 components with complex geometries, deep cavities, or after substantial material removal during rough machining. The recommended temperature range for this stress relief is 650°C to 730°C (1200°F to 1350°F).
• After Secondary Operations: A stress-relief temper is also strongly advised after operations such as grinding, welding, or Electrical Discharge Machining (EDM). For this, select a tempering temperature approximately 14°C to 28°C (25°F to 50°F) lower than the temperature used in the part’s last tempering cycle.

3.1.3 Preheating Before Hardening

Careful and controlled preheating is essential to prevent thermal shock and reduce the risk of cracking when the L6 tool steel is heated to the hardening temperature.

• General Guideline: A preheat temperature of approximately 650°C (1200°F) is typically recommended.
• For Larger or Intricate Sections: It is advisable to heat the L6 steel slowly and uniformly through the critical temperature range, which is roughly 650°C to 760°C (1200°F to 1400°F). This ensures temperature consistency throughout the component, minimizing internal stresses before austenitizing.

3.1.4 Hardening: Austenitizing and Quenching

The hardening process is what imparts the necessary strength and wear resistance to L6 tool steel. It consists of two primary phases: austenitizing (heating to the hardening temperature) and quenching (rapid cooling).

a. Austenitizing:

• Standard Temperature: The typical austenitizing temperature for L6 tool steel is around 830°C (1525°F).
• Temperature Range: Austenitizing temperatures can be increased up to approximately 955°C (1750°F) generally without causing severe austenite grain coarsening, which could otherwise compromise the steel’s toughness.

b. Quenching:

• Primary Medium: Oil quenching is the standard and preferred method for L6 tool steel due to its ability to achieve good hardness while minimizing distortion.
• Alternative Media: For L6 parts with simple designs or very heavy cross-sections, water or brine quenching may be considered. Notably, molybdenum-containing grades of L6 exhibit such high hardenability that they can sometimes be effectively air hardened.
• Objective: The purpose of quenching is to cool the steel rapidly from the austenitizing temperature. This rapid cooling facilitates the transformation of the austenite phase into martensite, which is the hard microstructure desired in tool steels. Proper selection of the quenching medium and ensuring adequate agitation are critical factors for achieving uniform and successful hardening. Agitation helps to maintain a consistent cooling rate across the part’s surface and prevents the formation of insulating vapor pockets, leading to more uniform hardness.

3.1.5 Tempering

Tempering is an indispensable step following the quenching of L6 tool steel. This heat treatment process involves reheating the hardened steel to an intermediate temperature to relieve internal stresses, reduce brittleness, and achieve the final desired balance of hardness and toughness.

a. Hardness Adjustment:

The final hardness of the L6 tool steel is inversely proportional to the tempering temperature; as the tempering temperature increases, hardness decreases, but toughness generally improves. Molybdenum-bearing L6 grades demonstrate better resistance to softening at higher tempering temperatures.

b. Application-Specific Hardness:

• For applications like woodworking saws, a tempered hardness of 45 to 50 HRC is often targeted.
• Other applications, such as certain dies and punches, might require higher hardness levels of 58 to 62 HRC, which are achieved by using lower tempering temperatures.

c. Ductility Improvement:

Tensile ductility of L6 tool steel tends to improve with tempering temperatures above 315°C (600°F).

d. Dimensional Considerations:

It’s important to recognize that dimensional changes (both contractions and expansions) occur during tempering. These are due to microstructural transformations and the relief of internal stresses. For applications demanding high-dimensional stability, multiple tempering cycles are common practice. These additional cycles can help ensure the complete transformation of any retained austenite and provide further relief of internal stresses, contributing to superior dimensional stability over time.

3.2 Summary of Key Heat Treatment Temperatures for L6 Tool Steel

3.3 Properties Achieved Through L6 Tool Steel Heat Treatment

Correctly heat-treated L6 tool steel exhibits several desirable properties:

• Good Toughness and Machinability: L6 is known for its excellent toughness, making it resilient under impact, alongside good machinability in its annealed condition.
• Adequate Wear Resistance: Due to its carbon content, L6 provides good wear resistance, especially suitable for applications not involving high temperatures.
• Low Distortion Risk: L6 tool steel has a relatively low susceptibility to distortion and cracking during the hardening process, particularly when oil quenched.
• Decarburization Resistance: It does not show a pronounced tendency to decarburize during the hardening cycle.
• Temperature Limitation: It is important to note that L6 tool steel has relatively low hot hardness and resistance to softening at elevated temperatures, making it generally unsuitable for high-temperature service applications.