Reading a Weld: Undercut, Porosity, Slag and Overlap Each Name a Cause

**Undercut** — a groove melted into the parent plate along the toe of the weld, not filled by weld metal. It is a notch in the full section of the part, so it matters far more than it looks. Causes: current too high, arc too long, travel too fast, or a weave that did not pause at the edges. **Overlap** — weld metal spilled over the toe and lying on top of the unmelted plate, with a sharp crevice underneath. It looks like a generous weld and it is a built-in crack. Causes: current too low, travel too slow, wrong angle. **Excessive convexity** — a tall, rope-like bead that has piled up. Adds metal and weight, adds nothing to the throat, and puts a sharp angle at the toes. Causes: travel too slow, current too low. **Spatter** — beads of metal thrown clear and stuck to the plate. Cosmetic unless it is severe, but severe spatter means the arc is too long, the current too high, damp electrodes, or the wrong polarity. Undercut and overlap are opposite ends of the same axis, so a plate with both on the same bead usually means the hand speed is wandering, not the settings.

Porosity is round, smooth-walled holes in the weld metal — scattered, clustered, or strung in a line along the bead. Round and smooth is the signature, and it is what distinguishes gas from every other cavity. The gas comes from one of four places: **Contamination.** Oil, paint, rust or moisture on the joint, boiled to gas by the arc. The joint-preparation rung is the fix. **Damp electrodes.** The flux coating absorbs water from the air. Low-hydrogen rods in particular must be kept in a heated quiver; a damp low-hydrogen rod is worse than a dry general-purpose one. **Arc too long.** A long arc lets air reach the pool past the shielding, and nitrogen and oxygen come out of solution as it freezes. **Freezing too fast.** Cold plate, or too little heat, so the pool solidifies before the bubbles can rise out. Preheat helps. Porosity in a line along the middle of a bead usually means the electrode itself — damp, or the wrong type for the plate. Scattered porosity usually means the plate was dirty.

**Slag inclusion.** Flux that solidified inside the weld instead of floating out on top. Unlike porosity it is irregular and angular, and it is usually found along the toes of a previous run or in the deep corner of a joint. Causes: slag not chipped off between passes, too slow a travel so the slag runs ahead of the pool and gets buried, a joint angle too narrow for the slag to escape, or the wrong electrode angle (a stick weld must be dragged, not pushed). **Lack of fusion.** The filler froze against a face that never melted. It shows on a broken or sectioned coupon as a flat, smooth, often bright area with no torn grain. Causes: current too low, travel too fast, arc aimed at the filler pool instead of the joint face, or slag or scale on the face. Both of these are invisible from the top of the bead, and both are the reason that a weld carrying load is proved on a coupon or by a proper inspection method, not by looking at it. A fault you can see is rarely the one that fails.

**Hot cracks** form while the weld is still solidifying, along the centre of the bead or in the crater at the end of a run. The metal is a mush of crystals with the last liquid between them; pull it apart while it is in that state and it tears. Causes: a deep narrow bead whose shape concentrates the last liquid down the middle, too much restraint, sulphur or phosphorus in the steel, and — the commonest by far — an unfilled crater. **Cold cracks** appear hours or even a day after welding, usually in the heat affected zone rather than the weld itself. They need three things together: hydrogen from the arc atmosphere, a hard brittle microstructure from fast cooling, and tensile stress. Take away any one and they do not happen. That is why the answers are low-hydrogen electrodes kept dry, preheat to slow the cooling, and less restraint. A weld that passed inspection and cracked the next morning is the classic hydrogen crack, and it is why a critical job is inspected after a delay rather than straight off the bench.