Why does residual stress affect steel pipes?

Why does residual stress affect steel pipes?

Residual stresses will be generated during the processing of steel pipes (seamless pipes or welded pipes), and the higher ones are close to the yield limit. Most of the residual stress in steel pipes shows great harmful effects. 

For example, the strength of the steel pipe decreases, the fatigue limit of the workpiece decreases, causing stress corrosion and brittle fracture, and the components deform due to residual stress relaxation, affecting dimensional accuracy. Element. Therefore, it is very necessary to reduce and eliminate the residual stress of components.

The effects of residual stress on steel pipes include

1. Effect on the yield limit of steel pipe metal materials. If the material has tensile residual stress, it is equivalent to reducing the tensile yield limit of the material. If the material has compressive residual stress, the tensile yield limit increases and the compressive yield limit decreases.

2. The influence of residual stress in steel pipes on fatigue life. When a steel pipe is subjected to alternating stress, the fatigue strength of the steel pipe will increase when there is residual compressive stress, and when there is residual tensile stress, its fatigue strength will decrease. Due to differences in process current, plating solution type, temperature, etc., the distribution and value of residual stress in electroplating treatment vary greatly, so the impact of electroplating residual stress on fatigue strength also varies greatly. Residual tensile stress will be generated on the surface of most metals after electroplating, so the fatigue strength will be greatly reduced.

3. The influence of steel pipe residual stress on component deformation. Residual stress is an unstable stress state.

When a component is acted upon by an external force, the interaction between the applied stress and the residual stress causes some parts to undergo plastic deformation and the stress within the section is redistributed. When the external force is removed, the entire component deforms. Therefore, residual stress significantly affects the accuracy of machined parts.

4. The influence of steel pipe residual stress on metal brittle failure. Residual stress exists in components as initial stress, especially the superposition of residual tensile stress and external tensile stress will accelerate brittle failure.

5. The influence of residual stress in steel pipes on stress corrosion cracking. The damage caused by the chemical reaction between the metal and the surrounding medium is called corrosion. If stress is present when corrosion occurs, corrosion damage will be accelerated, known as stress corrosion cracking. 

That the characteristics are: First, tensile stress and corrosion coexist. Second, due to different compositions, structures and media of materials, the sensitivity to stress corrosion is also different. Sometimes in non-corrosive media, some metals can also undergo stress corrosion under the action of stress. Third, during the stress corrosion cracking process, pitting corrosion occurs first and then gradually expands into cracks. Cracks propagate mainly along the direction perpendicular to the maximum principal stress, microscopically along the material grain boundaries or through the grains.