Carbonitriding

Carburizing introduces carbon into the surface layer and increases hardness there. When welding carbonitrided parts, you have to be careful: The heat breaks down the surface layer again, and you can get cracks if you melt the carburized zone along with it. Better weld before treatment or grind the layer completely away in the weld area. The treatment itself is done in a sealed furnace with a carbon-donating medium; the exact temperature and time depend on the material and the desired depth.