Nitrided compound layer

Nitrided parts have a hard, wear-resistant surface layer. But that's a problem when welding: the heat destroys it, and you won't get a clean weld there. Before welding, you have to completely grind off the nitride layer in the weld area, otherwise the weld metal won't stick properly and you'll get porosity. After welding, the layer is gone at that spot – if you need the hardness again, you'll have to re-nitride the part.