Tula State University (Tula, Russia):

Malenko P. I., Cand. Eng., Associate Prof., Chair of Welding, Casting and Technology of Construction Materials, e-mail: malenko@tsu.tula.ru
Relmasira K. J., Post-Graduate, Chair of Metal Physics and Metal Science
Leonov A. Yu., Post-Graduate, Chair of Welding, Casting and Technology of Construction Materials

Volgograd State Technical University (Volgograd, Russia)
Krokhalev A. V., Dr. Eng., Associate Prof., Dean of the Faculty of Contruction Materials Technologies

Peculiarities of saturation with carbon and nitrogen during cementation with the addition of ammonia and their impact on the formation of structural zones of the layer are observed in the article. It was established as a result of conducted investigations that addition of ammonia in the hardening furnace leads to increase of nitrogen concentration in the surface layer to 0.2%, while the concentration profile of nitrogen is characterized by a sharp drop. As for the profile of carbon concentration on the surface, its reduction to 0.78% takes place, which leads to decrease of microhardness in the surface layer. Troostite on the tooth working surface lies not in the form of a solid strip, but as separate sections along the grain boundaries. Addition of ammonia in the hardening furnace not only at the last stage, but also at the stage of cementation leads to increase of nitrogen concentration in more deep layers and to smooth varying of thickness of the hardened layer. The total process time is shortened by 4 hours, while the eff ective layer thickness is practically not reduced. Further increase of ammonia consumption in a carburizing furnace and a secondary heating furnace leads to increase of nitrogen capacity in the areas of carburizing and hardening furnaces, as well as to increased nitrogen content in the foil, passing through all zones of the unit. Nitrogen content in this case rises both on the surface and in more deep layers of the components. Consumption of the main part of nitrogen for formation of nitrides of alloying elements leads to nitrogen depletion of solid solution around nitrides and forming of troostite selections. Thus, it is shown that increase of ammonia consumption aimed on rise of nitrogen content in steel at the stage of heating for hardening is restricted due to possible nitrogen supersaturation in the surface layer at a small depth (up to 0.2 mm) with forming of nitrides and troostite at the grain boundaries.

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