The mechanism of generating residual stress of metallic?

Generating residual stress in metals is a complex process that can occur due to various mechanisms related to thermal, mechanical, and metallurgical phenomena. Here are the primary mechanisms through which residual stresses are generated in metallic materials:

1. Thermal Mechanisms:

a. Mismatch in Thermal Expansion:

When a metal undergoes uneven heating or cooling, different parts of the material expand or contract at different rates. If one region expands or contracts more than another, internal stresses are created due to the restraint imposed by the adjacent material. For example, during welding, the heat-affected zone experiences rapid heating and cooling, leading to residual stresses.

b. Phase Transformations:

Some metals undergo phase changes that involve a change in volume. If these transformations occur while the material is constrained, residual stresses can develop. For instance, the transition from austenite to martensite in steel involves a volumetric expansion, which can generate significant residual stresses if not allowed to occur freely.

2. Mechanical Mechanisms:

a. Plastic Deformation:

During processes like rolling, forging, extrusion, or stamping, metals experience plastic deformation. The rearrangement of atoms and dislocations resulting from this deformation leads to residual stresses. Typically, the outer layers of the metal experience more deformation, leading to compressive residual stresses near the surface and tensile stresses in the core.

b. Quenching:

Rapid cooling (quenching) after heating can create residual stresses due to the differential rates of

 cooling across the material's thickness. Surface layers cool faster and contract, while the interior cools slower and tries to restrain the surface contraction, resulting in residual stresses.

3. Metallurgical Mechanisms:

a. Grain Structure:

The orientation and size of grains in a metal can affect the distribution of residual stresses. Grains with different orientations may behave differently under external forces, leading to internal stresses. Also, grain boundaries can act as barriers to dislocation movement, contributing to residual stresses.

b. Work Hardening:

As metals are subjected to plastic deformation, they work harden, meaning their strength increases. This hardening is often non-uniform across the material, leading to residual stresses. The material's surface typically hardens more than the interior, creating a gradient of stresses.

4. Welding Mechanisms:

a. Localized Heating and Cooling:

Welding involves localized melting and solidification of the metal. The weld and surrounding areas experience rapid heating and cooling cycles, which can lead to significant residual stresses due to the mismatch in thermal expansion and contraction.

b. Solidification Shrinkage:

As molten metal solidifies during welding, it shrinks. This shrinkage can generate residual stresses, especially if the metal is restrained from shrinking freely.

Understanding these mechanisms is essential for predicting and controlling residual stresses. Techniques such as stress relief heat treatments, mechanical straightening, shot peening, and vibration stress relief can be employed to reduce or redistribute residual stresses and improve the performance and reliability of metallic components.

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