Gibbs' phase rule

The phase rule is simple at its core: it tells you how many degrees of freedom you have when welding before the material's state changes. Look at the phase diagram, then you'll see where melt and solid phase exist side by side. Exactly in this range, the solidification interval, lies the problem: the wider it is, the greater the risk of hot cracks, because the remaining melt can no longer flow in. With aluminum alloys, use fillers with lots of silicon; they solidify almost without an interval and are thus uncritical. With steel, the point where you cool slowly in the two-phase region (ferrite + austenite) is tricky – that's where those hard, brittle islands form that ruin toughness. So: don't put in too much heat, and when annealing, always stay well below the transformation temperature, otherwise you'll ruin the structure again.