Weld defects are not random bad luck — every one of the common five has a root cause in preparation, parameters, or technique, and every one can be prevented. Inspectors reject welds for the same handful of reasons year after year. This guide explains the causes and the fixes for the five defects that cost jobs.

POROSITY

Porosity is trapped gas in the weld. The puddle is contaminated, gas bubbles form, and they freeze into holes — either clustered at the surface or hidden inside the weld. It is the most common defect and almost always a cleanliness or shielding problem.

  • Dirty base metal — rust, mill scale, oil, paint, moisture. Clean to bright metal before welding.
  • Draft blowing shielding gas away from the puddle. Use wind screens outdoors.
  • Gas flow too low or too high, or the nozzle is too far from the work.
  • Damp electrodes or flux. Store rods in a rod oven at 120°C.

Fix order: clean the joint, check the gas, check the rod. Nine times out of ten that is the whole answer.

UNDERCUT

Undercut is a groove melted into the base metal along the toe of the weld that the weld metal does not fill. It looks like a burnt channel at the edge of the bead and it acts as a stress riser — inspectors reject it because it starts cracks.

  • Too much heat or current — the arc digs into the plate instead of melting the filler into it.
  • Travel speed too fast — the puddle does not have time to wet out into the sides.
  • Arc length too long or torch angle wrong, putting heat on the base instead of the puddle.

Drop the heat, slow down slightly, and hold the arc tight against the leading edge of the puddle.

Pro Tip

Watch the toes of the puddle. If the base metal edge melts away before the filler reaches it, you are undercutting — back off the heat and let the puddle catch up.

CRACKING

Cracks are the most serious defect because they are not always visible. There are two families: hot cracks that form as the weld solidifies, and cold cracks that form hours or days later.

Hot cracking

Happens in the solidifying puddle, usually down the center of the bead. Causes include high-sulfur base metal, too much heat, a deep narrow bead, and incorrect filler selection. Fix it with proper joint design, controlled heat input, and filler metal that matches the base metal.

Cold cracking (hydrogen cracking)

Hydrogen from moisture enters the weld, then diffuses into the heat-affected zone and embrittles the steel until it cracks — often hours after the weld cools. It is deadly in high-carbon and high-strength steels.

  • Use low-hydrogen electrodes (E7018 class) and keep them dry.
  • Preheat when the code or material requires it — preheat drives hydrogen out and slows cooling.
  • Grind out and repair cracks completely; do not weld over them.

LACK OF FUSION

Lack of fusion is base metal that never melted into the weld. It is often invisible from the outside — a layer between the weld and the plate, or between passes. It shows up as a discontinuity on a bend test or in radiography.

  • Heat too low or travel speed too fast — the puddle slides over cold base metal.
  • Poor joint prep: mill scale, varnish, or a tight root gap that the arc cannot reach.
  • Wrong torch angle — the arc pushes the puddle away from the joint wall.
  • Small electrodes on oversized joints that do not reach the root.

The fix is boring but reliable: fit it right, clean it bright, put the arc where the joint is, and let the puddle wet the sides before moving.

SLAG INCLUSION

Slag is the fused flux layer that protects the weld as it cools. When it gets trapped inside a multi-pass weld, it becomes a slag inclusion — a weak layer that fails under load.

  • Not cleaning between passes. Slag and scale from the previous pass must be chipped and wire-brushed out.
  • Narrow joint profiles that trap slag in the corners.
  • Weaving too wide or moving too fast so slag rolls into the puddle.
  • Poor tie-in at the start and stop of each pass.

Rule: every pass gets chipped, brushed, and inspected before the next one. There is no shortcut that survives an inspector's hammer.

THE PREVENTION CHECKLIST

  • Clean the joint to bright metal — no rust, oil, paint, or moisture.
  • Verify shielding gas flow and nozzle distance before the first arc.
  • Store electrodes and flux dry, and use rod ovens for low-hydrogen rods.
  • Set parameters from the welding procedure — never guess amperage.
  • Watch the puddle and toes; adjust heat and speed to the fit-up.
  • Chip, brush, and inspect between every pass.
  • Preheat and follow interpass limits when the material demands it.
Watch Out

If you see a defect on one weld, assume it is in the last twenty. Stop, find the root cause, and fix the process — not just the weld.

THE BOTTOM LINE

Defects are process failures, not bad luck. Porosity is a shielding or cleanliness problem, undercut is a heat problem, cracking is a material and moisture problem, lack of fusion is a heat and fit-up problem, and slag is a technique and cleaning problem. Fix the cause and the defect disappears.