Short Answer:

Residual stresses are caused by non-uniform deformation, heating, cooling, or phase changes within a material during manufacturing or service. These stresses develop when some parts of a material expand, contract, or deform differently from others, and the body is restricted from fully relaxing.

In simple words, residual stresses are formed because of uneven internal strain in materials due to processes like welding, casting, machining, rolling, or heat treatment. When temperature or deformation varies across the component, some regions try to expand or contract more than others, creating locked-in stresses that remain even after external forces are removed.

Detailed Explanation:

Causes of Residual Stresses

Residual stresses are internal stresses that remain in a material after all external loads or forces are removed. They arise because of non-uniform plastic deformation, thermal gradients, or phase transformations that occur during manufacturing or processing. These stresses can exist without any visible external load and are often “locked” within the material.

Residual stresses can be both beneficial or harmful, depending on their type and location. While compressive residual stresses can increase fatigue strength and resistance to cracking, tensile residual stresses can weaken the component, causing distortion, cracks, or even premature failure. Understanding the causes of these stresses helps engineers design better manufacturing and heat treatment processes to control them effectively.

The major causes of residual stresses are explained below:

1. Non-Uniform Plastic Deformation

Residual stresses often develop when a material undergoes uneven plastic deformation — meaning some parts of the material deform more than others.

• When a component is subjected to mechanical working processes such as rolling, bending, drawing, or machining, different regions experience different strains.
• The outer layers may deform plastically, while the inner layers remain elastic.
• Once the external load is removed, the elastic regions try to recover their original shape but are restrained by the plastically deformed zones.

This internal imbalance of strain leaves behind locked-in residual stresses.

Example:
During cold rolling or bending, the surface layers are compressed while the inner layers are stretched. When the load is removed, these opposite effects cause internal residual stresses.

2. Non-Uniform Heating and Cooling

One of the most common causes of residual stresses is uneven temperature distribution during heating or cooling of materials.

When a body is heated or cooled, different parts may reach different temperatures because heat transfer is not uniform. The outer surface may cool faster than the inner core, or vice versa. This difference causes unequal expansion or contraction, resulting in thermal stresses that remain even after the temperature becomes uniform again.

Example:

• In welding, the area near the weld is heated to a very high temperature, while surrounding regions remain cooler.
• During cooling, the weld metal contracts more than the base metal, leading to tensile stresses near the weld and compressive stresses in adjacent regions.

Similarly, in casting, the outer layers solidify and cool earlier than the inner portions, generating internal stresses that remain locked inside the material.

3. Phase Transformations

During heat treatment, certain materials (especially steels) undergo phase changes, such as transformation from austenite to martensite. This transformation involves changes in volume and crystal structure, which can cause residual stresses if it happens unevenly across the material.

• When cooling is rapid (as in quenching), the surface cools and transforms before the inner core.
• The resulting difference in volume change creates tensile and compressive residual stresses.
• If these stresses are too high, the component may crack during or after heat treatment.

Example:
In the hardening of steel, the surface transforms into hard martensite while the core remains softer and expands later, creating residual stresses.

4. Mechanical Processes (Machining, Grinding, Forming)

Manufacturing processes that involve material removal or deformation can also introduce residual stresses.

• Machining and grinding remove surface material and generate heat, causing local expansion and contraction.
• The uneven distribution of temperature and strain between surface and subsurface layers leads to internal stresses.
• In some cases, these processes also introduce compressive residual stresses at the surface, which can improve fatigue resistance.

Example:
During grinding, excessive heat buildup at the surface layer causes thermal expansion. When the surface cools, it contracts more than the inner region, creating tensile residual stresses that can lead to cracks (called grinding cracks).

5. Differential Cooling in Casting and Welding

In casting, molten metal poured into a mold cools and solidifies at different rates in different regions. The outer surfaces, in contact with the mold, cool and shrink first, while the inner material is still hot and expanded. This differential cooling leads to permanent internal stresses.

Similarly, in welding, the localized high temperature at the weld zone causes expansion, while surrounding areas remain cooler. During cooling, the uneven contraction between the weld and the base metal produces residual stresses that remain in the structure.

Example:

• Large castings often develop cracks due to residual stresses from uneven cooling.
• Welded joints can distort or warp due to thermal contraction stresses.

6. Cold Working and Forming Operations

Processes like forging, drawing, pressing, or rolling involve plastic deformation, which introduces strain differences across the material.

• The outer regions may experience compression, while the inner parts experience tension.
• After removal of the applied force, the mismatch between the elastic and plastic regions leaves residual stresses.

Example:
In cold-drawn wires, the surface experiences compressive residual stresses while the core remains under tension. This improves surface fatigue strength but may cause internal cracking if not controlled properly.

7. Surface Treatments and Coatings

Surface treatments like shot peening, carburizing, or nitriding intentionally induce residual stresses to improve fatigue and wear resistance.

• Shot peening introduces beneficial compressive stresses at the surface by bombarding it with small steel balls.
• Carburizing and nitriding cause expansion in the surface layers due to chemical reactions, creating residual compressive stresses that increase hardness and durability.

While these processes can be beneficial, improper control can lead to uneven stress distribution and failure.

8. Environmental Effects

Residual stresses can also develop due to corrosion, oxidation, or differential swelling of materials exposed to environmental changes.

For instance, corrosion causes local expansion due to oxide formation, producing internal stresses that can lead to cracking.

Conclusion

Residual stresses are primarily caused by non-uniform deformation, temperature gradients, phase transformations, and mechanical or thermal processing. These stresses remain within materials even after external loads are removed. While compressive residual stresses can enhance strength and fatigue resistance, tensile residual stresses can cause cracking and failure. Understanding their causes helps engineers control or minimize them through proper design, heat treatment, and stress-relief methods. Managing residual stresses is crucial for ensuring the durability, accuracy, and safety of mechanical components and structures.