Gradient structural-phase states and properties of surface layer after nitro-oxidation

Abstract

The phase equilibrium is considered, the formation of nitride phases during gas nitriding due to the presence of carbon in the iron steel matrix in all formed structural-phase states of the nitride layer, carbides are released, the dispersion and dispersion of which in the nitride layer depends on the nitrogen potential of the saturating atmosphere. During subsequent oxidation in water vapor, simultaneously with the formation of a surface oxide film during the exposure period, especially in water vapor with additives of complexion agents, during reaction-diffusion, gradient structural-phase states of low-nitrogen carbonitride, oxycarbonitride, and nitride phases are formed in the nitride layer. At the same time, the presence of oxygen in the furnace atmosphere positively affects the formation of the nitride layer and the internal nitriding zone. The corrosion resistance of gradient nitride-oxide diffusion coatings depends on the presence of a surface oxide layer consisting of magnetite and the ratio of modified nitrides and γ'-phase composition. At the same time, in the composition of the nitride layer, the composition of oxycarbonitrides with a larger proportion of the γ'-phase has the highest corrosion resistance with a lower rate of development of corrosion damage on the surface. The presence of a surface oxide layer in the structural-phase states of the nitride-oxide layer, providing a positive property gradient, can provide conditions for external friction during sliding friction.