Fusion zone

The bond zone is the transition between the molten weld metal and the unmelted base material. Here, the grains grow directly into each other during cooling, forming a solid metallic bond. This is the most critical spot for cracks, especially when the material cools quickly and becomes hard and brittle. If you work with high-carbon or alloyed steel, you need to slow down the cooling, for example, by preheating. Also, make sure to use a dry, low-hydrogen filler material, otherwise you risk cold cracks.