Why are welded joints considered the weakest link in a welded structure?

Welded joints are considered weak points in welded structures primarily due to the following key factors:

1. Inhomogeneity of Microstructure and Properties: During welding, the weld and its surrounding metal undergo rapid and uneven heating and cooling. This results in significant differences in the microstructure and properties of the weld joint area (including the weld, fusion zone, and heat-affected zone) compared to the base metal. For example, the brittle transition temperature of the heat-affected zone is often higher than that of the base metal, making this region more susceptible to brittle fracture at low temperatures. Furthermore, the chemical composition and microstructure of the weld metal may differ from the base metal, further exacerbating the inhomogeneity of properties.

2. Presence of Welding Defects: Various defects can occur during welding, such as porosity, slag inclusions, cracks, and incomplete penetration. These defects significantly reduce the strength and toughness of the weld joint, becoming potential sources of structural failure. For example, porosity reduces the effective load-bearing area of ​​the weld, while cracks can propagate rapidly under stress, leading to structural damage.

3. High Tensile Residual Stress: Due to the uneven local heating and cooling during welding, welded joints generate high residual stress. These residual stresses, when the structure is in operation, superimpose with external loads, increasing stress concentration at the joint and reducing its load-bearing capacity. Especially in the heat-affected zone and fusion zone, the distribution of residual stress is more complex, easily leading to fatigue cracks or brittle fracture.

4. Generation of Stress Concentration Variations in the geometry (such as bevel and weld shape) and microstructure of welded joints can cause stress concentration. For example, the inhomogeneity of the microstructure in the fusion zone and heat-affected zone causes stress to concentrate in these areas, while welding defects (such as undercut and incomplete penetration) further exacerbate stress concentration. Stress concentration significantly reduces the fatigue strength and static load strength of the joint.