How to relieve residual stress in metals?

Relieving residual stress in metals is a critical aspect of metallurgical processing and fabrication to ensure the stability, longevity, and performance of metal parts and structures. There are several methods to reduce or eliminate residual stresses, each with its own advantages, limitations, and applications. Here are some common techniques for relieving residual stress in metals:

1.Annealing: Annealing is a heat treatment process that involves heating the metal to a specified temperature above its recrystallization point, holding it there for a certain period to allow the stress relaxation to occur, and then cooling it slowly. This process softens the metal, improves ductility, and reduces residual stresses by allowing the atoms to rearrange more freely.

2.Heat Treatment Stress Relief : Similar to annealing, stress relief heat treatment involves heating the metal to a lower temperature below its critical point. The metal is held at this temperature for a period and then cooled slowly. This process is less severe than annealing and is typically used for normalizing the residual stress levels in weldments and castings.

3.Normalizing: Normalizing is another heat treatment process that involves heating the metal to a slightly above its upper critical temperature, followed by air cooling. This process refines the grain structure and can help in reducing residual stresses, particularly after hot working processes like forging or rolling.

4.Quenching and Tempering: Quenching involves rapid cooling of the metal from a high temperature to harden it, while tempering is a subsequent heat treatment that reduces hardness and brittleness, enhancing toughness. Together, quenching and tempering can also help in relieving some residual stresses by modifying the microstructure.

5.Vibration Stress Relief (VSR): As previously discussed, VSR uses mechanical vibrations to induce stress relief by exciting the natural frequencies of the metal part. This method is less energy-intensive than heat treatments and can be effective for certain applications.

6.Shot Peening: Shot peening is a mechanical process where the surface of the metal is bombarded with small spherical media, creating compressive residual stresses on the surface. This can counteract tensile stresses and improve fatigue strength.

7.Hydraulic Stress Relief: This method involves applying pressure to the metal component, which can help in relieving stresses. It is particularly useful for large components and structures where other methods may not be practical.

8.Mechanical Straightening: For components that have deformed due to residual stresses, mechanical straightening techniques such as stretching or pressing can be used to correct the shape and relieve some of the stresses.

9.Thermal Cycling: Repeated heating and cooling cycles can help in redistributing and relieving residual stresses, although this method is more commonly used in conjunction with other processes.

Each of these methods has its own set of considerations regarding cost, time, equipment requirements, and the specific material and part geometry. The choice of method depends on the nature of the residual stresses, the material properties, the size and shape of the component, and the desired final properties of the metal. Often, a combination of methods is used to achieve the best results. It's important to consult with materials engineers or metallurgists to determine the most appropriate stress relief strategy for a given application.