Moscow Automobile and Road Construction State Technical University (MADI), Moscow, Russia:

L. G. Petrova, Dr. Eng., Prof., Head of the Dept. of Technology of Structural Materials
I. S. Belashova, Dr. Eng., Prof., Dept. of Technology of Structural Materials

Vyatka State University, Kirov, Russia:
O. B. Lisovskaya, Cand. Eng., Associate Prof., Head of the Dept. of Materials Science and Basics of Design
E. A. Marinin, Cand. Eng., Associate Prof., Dept. of Information Technologies in Mechanical Engineering, e-mail: e.marrini@gmail.com

The article considers thermo-cyclic processes of chemical-thermal treatment of armco-iron. Nitriding regimes involve alternation of low-temperature cycles of nitrogen saturation in flowing ammonia with high-temperature cycles of de-nitriding, during which the internal volume diffusion of nitrogen occurs due to the resorption of high-nitride phases. It is shown that by changing the duration of the cycles it is possible to regulate the phase composition and structure of the nitrided layer. After the thermal cycling, the thickness of the nitrided layer in armco-iron increases by 3 times compared to the thickness of the layer obtained after the traditional nitriding after the same period of time (6 h). It was found that the ε-nitride on the surface of the samples almost completely disappears at time of the high-temperature denitrogenation cycle >1.5 h. To recover it, a combined process is used including an additional nitriding after the thermocyclic process. It was found that during the final stage of saturation in ammonia for 0.5 h, a nitride area on the surface is formed again very quickly. Experiments showed that the nitride area thickness after thermocyclic nitriding and after the combined process is less than after usual nitriding. After short saturation cycles and internal volumetric diffusion, the nitriding process prevails over the deazotization process and the nitride area are not dissolved. After the long cycle, the process of internal volumetric diffusion is more intensiv than nitrogen saturation at the nitriding stage. It helps to obtain layers based on an α-solid solution. A combined process of thermocyclic nitriding with an additional completion in ammonia is ofered in the paper. This process makes it possible to restore a layer of a highly nitrogenous e-phase with hardness and corrosion resistance on the deazotized surface.
The material was prepared as part of scientific research under project No. FSFM-2020-0011 (2019-1342), experimental studies were carried out using the equipment of the MADI Research Equipment Sharing Center.

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