The absorption of nitrogen atoms by a metal. The metal may remain dissolved or it may form metal nitrides. A case hardening process that depends on the absorption of nitrogen into the steel. All machining, stress relieving, as well as hardening and tempering are normally carried out before nitriding. The parts are heated in a special container through which ammonia gas is allowed to pass. The ammonia splits into hydrogen and nitrogen and the nitrogen reacts with the steel, penetrating the surface to form nitrides. Nitriding steels offer many advantages: a much higher surface hardness is obtainable when compared with case-hardening steels. They are extremely resistant to abrasion and have a high fatigue strength.