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Weld Defects Chart: Types, Causes, Detection and Remedies

Rejected welds cost rework hours, schedule and sometimes certificates. This table pairs each common weld imperfection with likely causes, the inspection method that finds it and the usual fix.

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Imperfection	What it is	Common causes	Typically found by	Remedy
Porosity (scattered)	Gas pockets trapped in weld metal	Shielding gas loss, wet electrode, contamination, oil or paint	Visual (surface), RT	Check gas flow and leaks, dry consumables, clean joint
Wormholes (elongated pores)	Elongated gas cavities	Excess moisture or primer, rust on joint	RT, sometimes UT	Remove coating, dry consumables, adjust technique
Slag inclusion	Non-metallic slag trapped between passes	Poor interpass cleaning, wrong travel angle, too low current	RT, UT	Clean each pass, adjust angle, raise current
Lack of fusion	Weld metal not fused to base metal or previous bead	Low heat input, fast travel, wrong angle, cold lapping	UT, RT, visual at toes	Raise heat input, slow travel, correct angle and bead placement
Incomplete penetration	Weld does not reach the root of the joint	Root opening too small, low current, wrong electrode size	RT, UT, visual from root side	Widen gap or root face, raise current, use a smaller electrode for the root
Undercut	Groove melted into base metal beside the toe	Excess current or voltage, long arc, wrong angle, fast travel	Visual, with gauge	Lower current, shorten arc, pause at the edges
Overlap	Weld metal flows over base metal without fusing	Low current, slow travel, wrong angle	Visual, MT or PT on toes	Raise current, speed up travel, correct angle
Crater crack	Star-shaped crack where the arc stops	Abrupt arc termination, shrinkage in the crater	Visual, PT, MT	Fill crater, use crater fill control, back-step
Solidification (hot) crack	Crack along weld centerline as metal solidifies	High sulfur or phosphorus, deep narrow bead, high restraint	PT, MT, UT	Reduce restraint, widen bead profile, use low-impurity filler
Hydrogen (cold) crack	Delayed crack in the heat affected zone or weld	Hydrogen, hard microstructure and stress together	MT, UT, often after hours or days	Use low hydrogen consumables, preheat, control interpass, hold hydrogen bake out
Lamellar tearing	Stepped cracking in rolled plate under through-thickness stress	Weak through-thickness plate, high restraint at tee or corner joints	UT, visual	Use plate with through-thickness properties, change joint design, buttering
Spatter	Droplets thrown out beside the weld	High CO2 shielding, wrong voltage, damp consumables	Visual	Adjust voltage and wire speed, change gas mix, use anti-spatter
Excess reinforcement	Weld face higher than the code allows	Slow travel, excess filler, low voltage	Visual, gauge	Increase travel speed, lighten deposit, grind if permitted
Underfill	Weld face below the surface of adjacent base metal	Insufficient filler, too fast travel, missing cap pass	Visual, gauge	Add a cap pass, slow travel
Burn-through	Hole melted through the base metal	Excess heat on thin material, large root gap	Visual	Lower current, use pulsed process, add backing
Tungsten inclusion	Tungsten fragments embedded in weld metal	Electrode touching the pool, overcurrent, wrong tip	RT	Use proper electrode size, avoid contact, regrind
Arc strike	Local burn on base metal outside the weld	Striking the arc off the joint	Visual, MT or PT	Strike arc only in the joint or on run-on tabs
Misalignment (high-low)	Offset between members at the joint	Poor fit-up, no clamps, tack weld movement	Visual, gauge	Use fit-up fixtures, check before welding
Distortion	Shrinkage-driven warping of the part	Heat input, weld sequence, unbalanced welds	Dimensional inspection	Balance weld sequence, use fixtures, lower heat input
Root concavity	Root surface below the base metal surface	Excess purge pressure, high heat, wrong root pass technique	Visual, RT	Lower purge pressure, adjust root parameters

Acceptance limits come from the governing code and drawing, not from this table. NDT abbreviations: VT visual, PT penetrant, MT magnetic particle, RT radiography, UT ultrasonic.

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How to use this table

Search by what the inspector wrote, such as undercut or lack of fusion, and read across to the likely causes and the remedy. Welding engineers can use it as a starting point for a corrective action, and welders can use it as a quick coaching aid. Export it to Excel to attach to nonconformance procedures or training material.

How imperfections are classified

ISO 6520-1 classifies imperfections into groups such as cracks, cavities, solid inclusions, lack of fusion and penetration, imperfect shape and miscellaneous. Surface imperfections can be found by visual and penetrant or magnetic particle testing, while internal ones need radiography or ultrasonic testing. Whether an imperfection is a defect depends on the acceptance level in the governing code, such as AWS D1.1, ASME B31.3 or ISO 5817.

Common mistakes in troubleshooting

Fixing the symptom without finding the cause just repeats the nonconformance.

Grinding out and rewelding without changing the parameters that caused the fault.
Confusing lack of fusion with incomplete penetration: one is bonding, the other is root reach.
Inspecting hydrogen cracking too early: delayed cracks may appear after hours or days.
Blaming the welder when the WPS parameters are out of range.
Skipping a record of the repair procedure and the number of repair attempts.

Frequently Asked Questions

What is the difference between lack of fusion and incomplete penetration?

Lack of fusion means weld metal did not bond to base metal or to a previous bead. Incomplete penetration means the weld did not extend to the root of the joint. Both reduce strength, but they have different causes and are often found in different locations.

What causes porosity in welds?

Gas trapped during solidification: shielding gas loss from drafts or leaks, damp electrodes, oil, paint, rust or moisture on the joint, and wrong parameters. Check gas flow, clean the joint, dry consumables and shield the arc from wind.

How do I prevent hydrogen cracking?

Control the three drivers: use low hydrogen consumables stored dry, apply preheat and interpass temperature suited to the steel and thickness, and limit restraint. Post-weld hydrogen bake out can help on sensitive steels. Follow your WPS and code.

Is every imperfection a rejectable defect?

No. Acceptance depends on the applicable code and quality level. Small amounts of porosity or minor undercut may be acceptable in one code and rejected in another. Cracks and lack of fusion are generally not accepted in most codes.

If you want weld repairs and nonconformances tracked against jobs, heats and welders in your ERP quality module, Netray can describe common patterns.

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