Understanding Weld Defects and Their Impact
Weld defects compromise the integrity, strength, and service life of welded structures. Even minor defects can initiate failure under stress, fatigue, or corrosive conditions. Understanding the causes of common defects and how to prevent them is essential for producing quality welds that meet specification requirements.
Defects in stick welding generally fall into several categories: porosity, cracks, lack of fusion, slag inclusions, and dimensional problems. Each defect type has specific causes and prevention strategies. Recognizing the early signs of potential defects allows welders to make corrections before the defect becomes established.
The cost of defects extends beyond the immediate rework. Inspection time, repair materials, schedule delays, and potential liability all add to the true cost. Prevention through proper technique and procedure adherence is always more economical than correction.
Porosity
Causes of Porosity
Porosity—gas pockets in the weld metal—results from gas that doesn't escape before the metal solidifies:
Moisture: The most common cause of porosity. Moisture in electrode coatings, on base metal, or in shielding gas dissociates in the arc, releasing hydrogen that forms pores.
Contamination: Oil, grease, paint, rust, and other contaminants on base metal or electrodes release gas when heated.
Excessive Arc Length: Long arcs allow atmospheric nitrogen to enter the weld pool, causing nitrogen porosity.
Inadequate Shielding: Insufficient gas coverage from the electrode coating allows air contact with the molten metal.
Dirty Electrodes: Electrodes with damaged or contaminated coatings don't provide proper shielding.
Preventing Porosity
- Store electrodes properly; bake low-hydrogen electrodes before use
- Clean base metal thoroughly before welding
- Maintain proper arc length (approximately equal to electrode diameter)
- Use appropriate amperage for the electrode
- Keep electrodes dry and clean
Repairing Porosity
Minor surface porosity can be ground out and rewelded. Internal porosity detected by NDT requires removal of the defective section and rewelding. Extensive porosity may require complete weld removal and replacement.
Cracks
Types of Weld Cracks
Hot Cracks: Form during solidification when low-melting constituents segregate to grain boundaries. Also called solidification cracking.
Cold Cracks: Form after solidification, often hours or days after welding. Also called hydrogen-induced cracking or delayed cracking.
Crater Cracks: Form in the weld crater at termination points due to rapid cooling and shrinkage.
Toe Cracks: Form at the weld toe where stress concentrates and the heat-affected zone may be hard and brittle.
Causes of Cracking
Hydrogen: The primary cause of cold cracking. Hydrogen from moisture or contaminants diffuses into the HA
Lack of Fusion
Causes of Lack of Fusion
Lack of fusion occurs when weld metal doesn't properly bond with base metal or previous passes:
Insufficient Heat Input: Low amperage or fast travel doesn't provide enough heat for fusion.
Poor Technique: Incorrect electrode angle, excessive weave, or inadequate pause at edges prevents proper fusion.
Inadequate Joint Preparation: Insufficient bevel angle, excessive land thickness, or small root opening blocks access.
Slag or Contamination: Slag from previous passes or surface contamination prevents fusion.
Excessive Arc Length: Long arcs spread heat over a larger area, reducing penetration.
Preventing Lack of Fusion
- Use appropriate amperage for material thickness
- Maintain proper electrode angle
- Ensure adequate joint preparation
- Clean thoroughly between passes
- Maintain proper arc length
- Pause at edges to ensure sidewall fusion
Repairing Lack of Fusion
Lack of fusion defects require removal and rewelding. Grinding or gouging removes the defective area, which is then rewelded with corrected parameters or technique.
Slag Inclusions
Causes of Slag Inclusions
Slag inclusions occur when slag becomes trapped in the weld metal:
Inadequate Cleaning: Slag from previous passes not removed before subsequent welding.
Improper Technique: Weave patterns that create pockets where slag collects.
Cold Welding: Insufficient heat prevents slag from floating to the surface.
Narrow Groove: Limited access makes slag removal difficult.
Poor Electrode Manipulation: Allowing slag to run ahead of the arc.
Preventing Slag Inclusions
- Clean thoroughly between passes
- Use proper weave patterns
- Maintain adequate heat input
- Ensure adequate groove access
- Keep slag behind the arc, not ahead
Repairing Slag Inclusions
Slag inclusions require removal by grinding or gouging, followed by rewelding. Radiographic testing locates internal inclusions that aren't visible.
Undercut
Causes of Undercut
Undercut is a groove melted into the base metal at the weld toe that isn't filled by weld metal:
Excessive Amperage: High current melts base metal that isn't replaced by filler.
Excessive Travel Speed: Moving too fast doesn't allow filler metal to fill the melted area.
Incorrect Electrode Angle: Angles that direct arc force away from the edge.
Excessive Weave: Wide weaves that overheat edges without adequate fill.
Preventing Undercut
- Use appropriate amperage
- Control travel speed
- Maintain proper electrode angle
- Pause at edges to ensure fill
- Adjust weave width
Repairing Undercut
Minor undercut may be acceptable per code if not excessive. Significant undercut requires grinding and rewelding, or building up with additional weld metal.
Excessive Reinforcement and Poor Profile
Causes of Profile Problems
Excessive Reinforcement: Too much weld metal built up above the surface.
Insufficient Reinforcement: Not enough weld metal.
Convexity: Excessively rounded bead surface.
Concavity: Sunken or hollow bead surface.
These problems result from incorrect parameters, technique, or joint preparation.
Prevention and Repair
Follow WPS parameters and use proper technique. Profile problems may require grinding to acceptable contour or adding weld metal where insufficient.
