Plastic zone
The small region immediately ahead of a crack tip where the material's yield strength is exceeded. Its size significantly influences local stress state and toughness.
If a crack in a workpiece is under load, stress builds up ahead of its tip. If the material is tough enough, it deforms plastically there, the crack tip blunts, and the crack needs more energy to propagate. In high-strength steels, this deformation zone is small because the material hardly yields – the crack stays sharp and spreads quickly in a brittle manner.
When welding, you often make it worse: the heat-affected zone becomes hard and brittle, and shrinkage creates residual stresses that additionally 'pull' on the crack. Add hydrogen, which embrittles the structure. The result is a small, hard zone that cannot stop a crack.
In practice, this means: use a material with good toughness, preheat the part to prevent hard microstructures from forming, and don't let the weld cool too quickly after welding. Sharp edges and notches must be ground smooth beforehand – every notch is a starting point for a crack. If you check hardness and the weld is too hard, only one thing helps: heat treatment or change welding parameters.