Why Does Residual Stress Affect Steel Pipes?

Residual stresses in steel pipes can arise during the manufacturing process, particularly through processes like rolling, forging, welding, or cold drawing. These stresses are not intentionally introduced for structural purposes but are instead a byproduct of the manufacturing techniques. Here's how residual stresses can affect steel pipes:

1. **Dimensional Accuracy**: Residual stresses can cause steel pipes to expand or contract, leading to dimensional changes that may not meet the required specifications. This can affect the fit and functionality of the pipe in a system.

2. **Strength and Toughness**: Excessive residual stresses can reduce the overall strength and toughness of the steel pipe. This makes the pipe more susceptible to failure under applied loads, especially if the stresses are concentrated at certain points, such as near welds or joints.

3. **Corrosion**: Residual stresses can create microcracks or stress concentrations that can promote the initiation and propagation of corrosion. These cracks can serve as pathways for corrosive agents, leading to premature failure of the pipe.

4. **Leakage**: In the case of seamless steel pipes, residual stresses can lead to internal or external leaks. If the stresses cause the pipe to expand, they may exceed the sealant's capability to maintain a tight seal, resulting in leakage.

5. **Ease of Deformation**: Pipes with high residual stresses are more prone to deformation under pressure or temperature changes. This can lead to bending, twisting, or other forms of distortion that affect the pipe's performance.

6. **Welding Issues**: Residual stresses can be particularly problematic in welded steel pipes. The heat-affected zone of a weld can have different properties due to the uneven heating cooling during the welding process, leading to residual stresses. These stresses can affect the integrity of the weld and potentially lead to weld failure.

To mitigate the negative effects of residual stresses, steel pipes are often heat-treated or stress-relieved after manufacturing. Heat treatment involves heating the pipe to a specific temperature and then slowly cooling it down to reduce residual stresses. Stress relieving is a process where the pipe is heated to a lower temperature and then cooled slowly to achieve a more uniform stress distribution throughout the material. These processes help to ensure that the pipe will perform as expected in service with reduced risk of failure.