Investigations on nitride layer formation at the iron surface during gas nitriding

The morphology of the nitride layer forming at gas nitriding can decisively be influenced by the pretreatment of the surface. At a temperature of about 550 °C an activated surface very swiftly covers with iron nitrides. After passivating of the surface the nitride formation and the geometrical order of the nitride layer are determined by the heating rate. At high heating rates the formation of iron nitrides is localized to a few spots and the extension of the nitride layer proceeds by the growth of these spots. At low heating rates the surface is reactivated by the reducing gas before reaching the nitriding temperature. By further investigations is shown that nitriding of the surface starts with the formation of γ′-nitrides. ϵ-Nitrides occur in the further process of nitriding as a second layer on the γ′-phase.