Weld Defects

Porosity and Gas Entrapment

Porosity—spherical voids in weld metal—reduces mechanical properties proportionally to porosity volume. Small scattered porosity has minimal impact; extensive porosity can reduce strength 30% or more. Causes include inadequate shielding gas, contaminated wire, moisture, and incorrect parameter settings. Prevention focuses on eliminating gas sources and ensuring adequate shielding.

Radiographic inspection detects porosity readily as dark spots on radiographs. Ultrasonic inspection reveals cluster porosity and confirms internal voids. Acceptance criteria typically limit porosity size and number. Organizations implementing welding process control training reduce porosity incidence significantly.

Cracks and Stress Concentration

Cracks represent the most serious weld defects. Cold cracks initiate after cooling, typically from high-strength steel, high restraint, and hydrogen. Hot cracks occur during solidification, common in austenitic stainless steel and high-strength materials. Cracks concentrate stress, initiating fatigue failure at loads far below material yield strength. Any crack in critical-stress location mandates rejection.

Crack detection requires multiple methods. Ultrasonic testing excels at tight cracks. Magnetic particle testing detects surface cracks. Penetrant testing reveals fine surface cracks. Systematic multi-method inspection through comprehensive inspection programs ensures crack detection before service.

Lack of Fusion and Incomplete Penetration

Lack of fusion occurs when adjacent weld metal or base metal fails to melt and bond. Results are unbonded surfaces that act as stress concentrations, initiating cracks under loading. Incomplete penetration occurs when weld fails to penetrate through the joint thickness, leaving unbonded root areas. Both defects dramatically reduce effective section properties and stress-carrying capacity.

Ultrasonic testing readily detects lack of fusion and incomplete penetration through characteristic reflections. These defects appear obviously on radiographs. Prevention focuses on process parameter control—adequate heat input, proper root gap, and correct technique. Organizations implementing systematic process control prevent these defects.

Inclusions and Contamination

Inclusions—entrapped non-metallic material such as slag, oxides, and tungsten particles—reduce ductility and fatigue strength. Small inclusions have minimal effect; large or clustered inclusions significantly degrade properties. Sources include inadequate cleaning between passes, contaminated wire, electrode problems, and insufficient shielding.

Radiographic inspection readily detects inclusions as dark regions on radiographs. Acceptance criteria limit inclusion size and clustering. Prevention focuses on cleanliness—proper interpass cleaning, high-quality consumables, and appropriate procedures. Training through professional programs emphasizes defect prevention.

Surface Defects and Geometry Issues

Surface defects include undercut (groove melted into base metal adjacent to weld), overlap (weld metal folded over base metal), and spatter (adhering droplets). These defects create stress concentrations and reduce fatigue strength. Visual inspection detects these defects; magnetic particle and penetrant testing reveal fine surface cracks. Prevention focuses on parameter control and welder technique.

Excessive weld reinforcement (bead height) can be acceptable structurally but affects fatigue performance. Insufficient reinforcement suggests incomplete fusion. Geometric consistency indicates process control. Systematic visual assessment through quality programs prevents geometric nonconformances.