What is Welding Stress?

Due to the uneven heat input during welding, which leads to rapid changes in the internal temperature field, stress field, and microstructure of components, uneven elastic-plastic deformation is prone to occur. Consequently, the impact of residual stress on workpieces processed by welding is more pronounced compared to other machining methods.

Types of Welding Stresses:

Stresses in welded structures can be categorized based on their causes and nature:

Thermal stress: Caused by uneven heating and cooling during welding, these are transient stresses that change during the welding process.

Phase transformation stress: Arises from uneven microstructural changes in the weld joint area, commonly seen in high carbon equivalent materials or with improper welding procedures.

Constraint stress: Generated due to the structure's own constraints or external constraints during welding.

Hydrogen-induced stress: Local stresses caused by the accumulation of diffused hydrogen at microdefects in the weld joint area after welding, more likely to occur with higher hydrogen content.

Hazards of Welding Stresses:

Causes welding cracks: Under the interaction of temperature, microstructure, and structural rigidity, welding stresses can lead to various types of cracks, affecting structural quality and potentially leading to repairs or scrapping.

Reduces structural load-bearing capacity: The superposition of residual and working stresses increases the stress level, effectively reducing the structure's carrying capacity or safety margin.

Leads to stress corrosion: Tensile residual stresses can cause stress corrosion cracking in corrosive environments, leading to stress corrosion and brittle fracture under low stress.

Affects structural dimensional stability: especially for structures requiring post-weld machining, which can disrupt the balance of internal stresses, causing deformation or instability in dimensions.

Factors Affecting Welding Stresses:
Structural form: The distribution of residual stresses varies with different joint types, such as plate butt joints and cylindrical shell circumferential welds.

Constraint effects: The level of constraint affects internal stresses, with narrower plates experiencing higher constraint stresses.

Plate thickness and groove form: Residual stress distribution changes with plate thickness, and the transverse stress component cannot be ignored.

Welding process parameters: Increased heat input leads to wider areas of tensile residual stress.

Welding direction: The direction of welding affects the distribution of transverse residual stresses.

Phase transformation effects: Welding high-strength steel with high carbon equivalent can lead to phase transformation stresses due to the change in specific volume during the transformation from austenite to martensite.

Methods to Prevent and Reduce Welding Stresses:

Use a reasonable welding sequence and direction: Prioritize welding the joints with the highest contraction.

Minimize welding heat input: Use small diameter electrodes, multiple passes, and controlled heat input techniques.

Preheat the entire structure: Reduces temperature differences between the weld joint and the structure, minimizing plastic deformation.

Hammering method: Post-weld hammering to induce plastic deformation in the weld metal reduces stress and deformation.

Reduce the influence of hydrogen: Use low-hydrogen electrodes and control the welding environment for high-strength alloy steels.

Stress relief treatment: Heat the welded structure to just below the phase transformation temperature to relax residual stresses through creep.

These methods aim to mitigate the effects of welding stresses and improve the quality and durability of welded structures.