Tempering Process:

1. Preparation After Quenching:

• After a fastener is quenched, it often has a hard but brittle martensitic structure. Tempering addresses this brittleness while retaining the strength gained during quenching.

2. Reheating:

• The fastener is reheated to a controlled temperature, typically between 150°C and 700°C (300°F to 1292°F), depending on the material and desired properties.

• For low-temperature tempering (150°C–250°C), hardness is maintained with slight toughness improvements.

• For high-temperature tempering (500°C–700°C), toughness and ductility are significantly increased, with some reduction in hardness.

3. Soaking Time:

• The fastener is held at the tempering temperature for a specific duration to allow changes in the microstructure. The time is determined by the material and the fastener’s size and application.

4. Cooling:

• After soaking, the fastener is air-cooled, water-cooled, or oil-cooled, depending on the process requirements and material.

Effects of Tempering on Fasteners:

Reduction of Residual Stresses: Quenching induces internal stresses due to rapid cooling. Tempering relieves these stresses, reducing the risk of cracking or distortion.

Transformation of Microstructure: The brittle martensite structure transforms into tempered martensite, which has enhanced toughness and stability.

Tailored Mechanical Properties: Tempering adjusts the balance of hardness and toughness, making fasteners suitable for specific applications.

Improved Wear Resistance: Tempering enhances the wear resistance of fasteners, ensuring durability in demanding environments.

Key Considerations in Tempering:

1. Temperature Selection:

• Lower temperatures maintain hardness but offer limited toughness improvements.

• Higher temperatures improve toughness and ductility
  but may reduce hardness.

2. Controlled Atmosphere:

• Tempering in a controlled environment (vacuum or inert gas) prevents oxidation or scaling on the fastener’s surface.

3. Material-Specific Parameters:

• Different materials require different tempering temperatures and durations to achieve optimal results.

Benefits of Tempering for Fasteners:

Enhanced Durability: Fasteners can resist mechanical and thermal stresses over extended periods.

Improved Safety: Reduced brittleness minimizes the risk of failure under load.

Customizable Properties: Tempering allows manufacturers to fine-tune properties for specific applications, such as aerospace or oil & gas.

Applications in Engineered Fasteners:

Tempering is essential for fasteners that must withstand mechanical loads, thermal stresses,
and harsh operating conditions. Common examples include:

Structural Fasteners: Bolts, screws, and nuts used in construction, automotive, and
aerospace applications.

High-Strength Fasteners: Fasteners for machinery, heavy equipment, and turbines.

Critical Components: Fasteners for nuclear power plants, oil rigs, and space exploration.

Challenges in Tempering:

Over-Tempering: Excessive temperature or time can overly reduce hardness, compromising strength.

Surface Oxidation: Without proper atmospheric control, oxidation can degrade the fastener’s surface quality.

Dimensional Stability: Large or complex-shaped fasteners may experience slight distortion due to
residual stress relief.

Tempering Example for a High-Strength Bolt:

1. Material: Medium-carbon alloy steel (e.g., 4140 steel).

2. Quenching: The bolt is quenched to form martensite.

3. Tempering:

• The bolt is reheated to 600°C (1112°F) for 2 hours.

• This reduces brittleness and increases toughness while maintaining sufficient hardness.

4. Resulting Properties:

• High tensile strength, improved toughness, and resistance to fatigue.