Stress Relief Annealing – Purpose and Procedure

Objectives of stress relief annealing

Mechanical processing (cold forming, milling, straightening, turning, etc.), uneven cooling during thermal processes (casting, welding, forging, etc.) or other machining processes can lead to components exhibiting high residual stress. Structural transformations or uneven cooling can also be reasons for residual stresses. The aim of stress relief annealing is to reduce these.

The process also offers the following advantages:

1. Reduction of distortion: By reducing the internal stresses, the risk of distortion during further processing or in use is significantly reduced, which enables low-distortion production.
2. Improvement of mechanical properties: The process can improve the mechanical properties of the material, such as ductility and fracture toughness, resulting in greater resistance to mechanical stresses.
3. Optimized further processing: After stress relief annealing, the material can be processed more easily, as a more stable and stress-relieved structure is available for subsequent processes such as welding or surface treatments.
4. Increased service life: The reduction of residual stresses reduces the risk of cracking and premature failure, which extends the service life of the component.

Process for stress-relief annealing

The heat treatment process for steel is divided into three basic steps: Preheating, tempering and cooling. Each of these steps is of central importance for the successful reduction of internal stresses in components:

Heating

• Careful temperature increase: To avoid thermal distortion, the components are heated carefully. This is done slowly to prevent abrupt temperature differences that could lead to new stresses.
• Reach the target temperature: The parts are gradually brought to a specific target temperature of between 450 and 650 °C, depending on the material, previous machining processes and wall thickness.

Tempering

• Permanent holding phase: After reaching the target temperature, the parts remain in this heat state for a defined period of time – often several hours.
• Reduction in tensile strength: During this holding phase, the tensile strength of the material is continuously reduced, which reduces the internal stresses without affecting the material structure.

Cooling

• Controlled temperature gradient: Cooling down must be just as slow and controlled as heating up in order to avoid the development of new stresses.
• Homogeneity: The core and surface of the component should cool evenly to ensure uniform stress reduction across the entire workpiece.
• Oven cooling: The part remains in the oven for cooling to ensure a controlled environment, which can result in a total process time of over 24 hours.

Through these three steps, the material is comprehensively treated to minimize stresses and improve the quality and processing properties of the components.

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