Structure and phase composition of the nitride-oxide coating after nitrooxidation

With the aim of developing an industrial technology for nitrooxidation, we considered the production of a diffusion nitride-oxide layer on the surface of metal products by low-temperature gas nitriding at the first stage of saturation with nitrogen and sequential oxidation of the nitride layer in water vapor at the second stage of processing.For its further oxidation in water vapor, it is experimentally investigated to obtain a pore-free developed nitride layer depending on the nitrogen potential of the atmosphere upon saturation in an ammonia environment. The microstructures of the nitride - oxide layer were studied by nitriding at the first stage of the process in pure ammonia at below the eutectoid temperature for the “Fe-N” system and oxidation at the second stage in water vapor at a temperature below the eutectoid temperature for the “Fe-O” system.It was found that during oxidation, at the beginning of the process, denitrogenation occurs on the surface of the nitride layer with the formation of an oxide layer, which further prevents denitrogenation, acting as a barrier layer on the surface. Nitrogen from the high-nitrogen nitride phase continues deep into the surface with the formation of lower nitride phases with an increase in the thickness of the nitride phase, and at the same time diffusion occurs in the nitride zone of carbon from the steel matrix during decarburization.As a result of the oxidation of the nitride phase with oxygen from the saturating atmosphere and carbon from the steel matrix in the nitride layer, an oxycarbonitride phase is formed, which has the best physic mechanical and physicochemical properties. The change in the concentration of elements along the depth of the layer and X-ray structural analyzes of the nitride and oxide phases are also given.