Welding defects compromise product integrity and cause failure by reducing strength, creating stress concentrations, enabling crack propagation, causing leaks, and accelerating corrosion. Here's a breakdown of the key mechanisms:

• How Defects Cause It: Voids (porosity, shrinkage cavities), lack of fusion, lack of penetration, and excessive undercut effectively remove load-bearing material from the critical joint area. Inclusions act like internal voids or weak spots.
• Result: The welded joint cannot withstand the intended design loads (tension, compression, shear, bending). Failure occurs at loads significantly lower than the design strength of the base metal or a sound weld.

2. Stress Concentration:

   • How Defects Cause It: Sharp features like cracks, undercut, excessive reinforcement (weld crown), and surface irregularities act as stress risers. Under load, stress is magnified dramatically at these points.
   • Result: Even moderate loads can cause localized yielding or initiate cracking at these points, leading to premature failure. This is especially critical under cyclic loading (fatigue).

3. Crack Initiation and Propagation:

   • How Defects Cause It:
     • Existing Cracks: Weld cracks (hot cracks, cold cracks, reheat cracks) are inherent flaws that directly initiate failure under load or during service.
     • Creating Crack Paths: Porosity, inclusions, lack of fusion, and lack of penetration create discontinuities that act as preferential paths for crack growth under stress or fatigue loading.
     • Hydrogen Embrittlement: Certain defects (especially underbead cracks) can trap hydrogen, making the surrounding metal brittle and prone to cracking under stress.
   • Result: Cracks grow rapidly under service loads, leading to sudden, catastrophic fracture (brittle fracture) or progressive failure (fatigue crack growth).

4. Loss of Hermeticity (Leakage):

   • How Defects Cause It: Porosity, cracks, lack of fusion, lack of penetration, and excessive porosity create interconnected pathways through the weld or between the weld and base metal.
   • Result: Pressure vessels, pipelines, tanks, heat exchangers, and other containment structures leak fluids or gases, causing loss of product, environmental damage, safety hazards, and system failure.

5. Accelerated Corrosion:

   • How Defects Cause It:
     • Crevice Corrosion: Tight gaps under undercut, between weld beads, or around inclusions trap moisture and corrosive agents, creating localized corrosion cells.
     • Galvanic Corrosion: Dissimilar materials (e.g., slag inclusions, different phases in the weld metal) in contact with the base metal can create galvanic couples, accelerating corrosion at the defect site.
     • Stress Corrosion Cracking (SCC): Residual stresses combined with a corrosive environment can cause cracks to initiate and propagate rapidly at defect sites.
   • Result: Thinning of material, pitting, and crack formation weaken the structure over time, leading to premature failure.

6. Reduced Fatigue Life:

   • How Defects Cause It: Almost all welding defects (cracks, porosity, inclusions, surface irregularities, lack of fusion) act as stress concentrators and crack initiation sites under cyclic loading.
   • Result: The component fails after far fewer load cycles than a defect-free component, even if the peak stress is below the material's static yield strength. Fatigue cracks initiate at defects and propagate until catastrophic failure occurs.

7. Manufacturing Defects Leading to Service Issues:

   • Excessive Reinforcement: Creates a stress riser and can lead to undercut or lack of fusion on the opposite side.
   • Undercut: Creates a sharp notch, acting as a severe stress concentrator and potential crack initiation site.
   • Poor Weld Profile: Irregular shapes (e.g., concave welds) reduce throat thickness and strength; convex shapes create stress risers.
   • Spatter: Can interfere with fit-up or create points for crack initiation.
   • Result: These defects compromise the joint's functional performance and durability in service.

In Summary:

Welding defects act as flaws that disrupt the continuity and homogeneity of the material. They weaken the joint, concentrate stress, initiate cracks, allow leaks, and promote corrosion. Any one of these mechanisms can lead to premature failure. In critical applications (pressure vessels, bridges, aircraft, pipelines), even small, seemingly insignificant defects can have catastrophic consequences due to the high stresses, cyclic loading, or safety requirements involved. This is why strict welding procedures, qualified welders, and rigorous inspection (NDT) are essential to ensure weld quality and prevent product failure.