Nitriding

Ion nitriding is a case hardening process that creates a hard, thin and ductile surface layer on steel parts. This layer contains very hard nitrides of iron and certain alloying elements in the steel. It also creates a supporting layer below the hard surface by diffusing nitrogen into the metal matrix.

Parts are placed on an electrically isolated baseplate in a vacuum chamber. A mixture of hydrogen and nitrogen is introduced into the evacuated chamber. When an electric potential is applied to the gas in the chamber it ionizes the gas, creating positively charged gas ions and electrons that are attracted to the grounded chamber wall. Gas ions are accelerated to the parts and bombard their surface with high energy. Hydrogen ions break down the surface oxides and activate the surface to react chemically with the nitrogen ions to form nitrides. The transfer of the kinetic energy to the parts heats up the parts and promotes diffusion of nitrogen into the metal. Parts are heated to about 900 deg. F and held at that temperature for a number of hours to achieve the desired case depth.

Increase in Fatigue Strength

Parts such as crankshafts experience fluctuating high bending and torsion loads. These bending loads create tensile stresses at the surface. The nitrogen diffused in the metal during nitriding creates compressive stresses. These compressive stresses counteract the tensile load stresses and lower the resulting stress, thus increaseing the number of load cycle a part can withstand at a given load level. The thin and ductile ion nitride surface does not crack or spall under the dynamic load.

Tests on automotive crankshafts have shown an increase of 40% in the fatigue strength in steel crankshafts, a little less in cast iron crankshafts.

Wear resistance

The nitrides of iron, chrome and some alloying elements create a hard and wear resistance layer on steel parts. In steels this layer shows a hardness of about 62 HRC and on cast iron it is about 45 HRC.