Carbonitriding

Carbonitriding means deliberately introducing carbon and nitrogen into the workpiece from outside, usually via the weld seam or a heat treatment. This happens when the material is in the austenitic range so that the elements can properly dissolve in the lattice. Too much carbon makes the structure hard and brittle, so you have to watch out for cracking during cooling. Nitrogen provides strength, but if you get too much of it, the material becomes porous. Keep temperature and times under control, then you'll get a clean, tough result.