Oxynitriding
A combined thermochemical treatment to produce a nitrocarburized surface layer followed by targeted oxide formation to optimize wear and corrosion properties.
Oxynitriding is a two-stage process. First, the part is nitrocarburized, meaning nitrogen and carbon are introduced into the surface layer to harden it. Then oxidation follows, forming a thin, wear-reducing oxide layer.
The result is a hard, wear-resistant surface. If you weld on it, you'll ruin the surface layer. Hardness is lost and you'll get cracks. Before welding, the layer must be removed, e.g., by grinding.
Make sure the bath composition and temperatures are correct. If you mess that up, hardness will be uneven and the part will wear on one side. Stick to the specified times, then it'll be fine.