The Stress Relieving Process:

1. Heating:

• The fastener or component is heated to a temperature below the material’s critical transformation point, typically ranging from 480°C to 650°C (900°F to 1200°F) for steel, depending on the alloy and application.

2. Soaking:

• The fastener is held at the target temperature for a sufficient time to allow the redistribution and relaxation of internal stresses. The duration depends on the component’s size and complexity, often ranging from 1 to 2 hours.

3. Cooling:

• After soaking, the fastener is cooled at a controlled rate, usually in still air, to prevent the reintroduction of stresses.

Effects of Stress Relieving:

Reduction of Residual Stresses: Internal stresses caused by prior manufacturing processes are reduced, improving dimensional stability and reducing the risk of cracking.

Improved Performance: Stress-relieved fasteners are less prone to failure under cyclic or dynamic loading, enhancing fatigue resistance.

Minimal Impact on Mechanical Properties: Stress relieving does not significantly change the hardness, strength, or microstructure of the material.

Enhanced Machinability: Reduced stresses can make machining operations smoother and more predictable.

Example of Stress Relieving in Fastener Manufacturing:

1. Material: Alloy steel bolt (e.g., 4140 steel).

2. Pre-Stress Relieving Process: The bolt undergoes cold heading and thread rolling, creating residual stresses.

3. Stress Relieving:

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

• Air cooling is used to avoid rapid temperature changes.

4. Outcome: Internal stresses are reduced, ensuring the bolt maintains dimensional accuracy and resists fatigue during use.

Benefits of Stress Relieving for Fasteners:

Dimensional Stability: Reduces the risk of warping or distortion during further processing or use.

Extended Service Life: Improved resistance to fatigue and stress-corrosion cracking enhances the durability of fasteners.

Ease of Assembly: Stress-relieved fasteners have better thread alignment and fit, simplifying assembly operations.

Enhanced Safety: Reduced internal stresses minimize the likelihood of sudden or catastrophic failure under load.

Applications in Engineered Fasteners:

Post-Machining: Fasteners that undergo significant material removal, such as bolts and studs, are stress-relieved to ensure dimensional accuracy.

Cold-Worked Components: Stress relieving mitigates the stresses induced during cold heading, forming, or rolling operations.

Welded Assemblies: Welded fastener assemblies, such as anchors or large bolts, benefit from stress relieving to improve their integrity and reliability.

Critical Fasteners: Fasteners used in high-stress environments, such as aerospace or nuclear industries, require stress relieving for enhanced performance and safety.

Challenges in Stress Relieving:

Overheating: Heating the material above its critical transformation temperature can alter its microstructure, defeating the purpose of stress relieving.

Oxidation: Without a controlled atmosphere, surface scaling or oxidation may occur, affecting the fastener’s appearance and performance.

Residual Stress Redistribution: Uneven heating or cooling can reintroduce residual stresses, so process control is critical.

Why Stress Relieving is Essential:

Stress relieving ensures engineered fasteners and components perform reliably in demanding applications by reducing internal stresses while maintaining their inherent material properties. This makes it a vital step in manufacturing high-quality fasteners for industries requiring precision, strength, and safety.