Hot cracking

Hot cracking happens while the joint is still hot, which is what separates it cleanly from hydrogen cracking: a hot crack opens as the pool finishes solidifying, while a hydrogen crack is a delayed mechanism that can take hours or days to appear in metal that has already cooled to ambient. The two are sometimes still called “hot” and “cold” cracking for exactly that timing distinction, and mistaking one for the other points an investigation at the wrong controls — chemistry and pool shape for one, hydrogen and preheat for the other.

The weld pool’s cross-sectional shape is the single biggest lever on susceptibility, and weld-pool solidification metallurgy gives it a widely used rule of thumb: keep the pool’s depth-to-width ratio at or below roughly 1. A wide, shallow pool solidifies with grains growing inward from both sides and meeting across a broad, shallow front, which spreads any low-melting segregate film over a wide plane rather than concentrating it. A narrow, deep — “finger” — pool solidifies with grains converging toward a single centreline, concentrating the same segregate film into one plane with almost nothing on either side to resist the shrinkage strain trying to pull it apart. The ratio is a proxy for that grain-growth geometry, not an arbitrary number, which is why it holds across processes and materials that otherwise have little in common.

Pool shape is necessary but not sufficient on its own: heat input, travel speed and current set the shape, but sulphur, phosphorus and other low-melting segregates in the filler or base metal supply the liquid film, and joint restraint supplies the shrinkage strain that opens it. Controlling only one of the three — flattening the pool while leaving a high-sulphur consumable in the procedure, for instance — reduces but does not remove the risk, which is why a hot-cracking investigation checks chemistry and restraint alongside the recorded bead profile rather than stopping at whichever cause is found first.