Heat-affected zone crack
A crack forming in the thermally affected base material outside the actual fusion zone, typically caused by structural inhomogeneities and local embrittlement processes.
Heat expansion cracks occur due to too rapid cooling after welding. The material then contracts unevenly and internal stresses build up, which eventually crack. This is especially critical for thick or clamped components, as they cannot contract freely. To avoid this, you have to slow down the cooling. Let the part cool slowly, for example with a cover layer or by not artificially blowing away the heat. Cooling water or forced cooling is the wrong way here, it only makes it worse. If you cover the weld after welding and give the cooling time, the stresses remain low and the risk of cracking is gone.