Regulation of the phase composition of the nitrided layer in iron during chemical thermal treatment under thermo-cycling conditions

Metallurgy and Material Science


DOI: 10.34759/vst-2022-2-237-245

Аuthors

Belashova I. S.*, Petrova L. G.**

Moscow Automobile and Road Construction State Technical University (MADI), MADI, 64, Leningradsky Prospect, Moscow, 125319, Russia

*e-mail: irina455@inbox.ru.
**e-mail: petrova_madi@mail.ru

Abstract

The article considers the thermo-gas-cyclic nitration method, consisting in alternating the process stages with high and low nitrogen potential, being conducted at temperatures, respectively, below and above the temperature of the eutectoid transformation in the Fe-N system. At the half-cycle with high saturation capacity of the atmosphere at the low dissociation degree of ammonia, a high-nitrogen nitride zone is being formed on the surface. It transforms into an extended γ’-zone and an internal nitrating zone due to internal diffusion in a cycle with a low saturating capability of the atmosphere, or at a high degree of ammonia dissociation. The processes with alternate changing of the nitrogen potential contribute at certain stages to accelerated growth of the nitrided layer. Besides, this allows controlling the process and obtaining the required combination of phases, determining these or that product properties, necessary for various operation conditions, namely:

— The presence of a high-nitrogen nitride zone on the surface contributes to the running-in of friction units and, some cases, increases the corrosion resistance;

— Under wear-out condition at the increased specific pressures, the multi-layer structure from the surface nitride zone bearing on the internal nitriding zone, appears to be the most steadfast one;

— The extended zone of internal nitriding with minimal surface nitride layer should be formed for the parts operating in the dynamic wear-our mode and shock loading.

In some cases, such as corrosion-resistant steels nitriding, a diffusion layer based on an internal nitriding zone (solid solution) without a nitride zone is advantageous.

The control principle is based on maintaining the nitrogen potential at the level of values corresponding to the solubility of nitrogen in a given phase of the Fe-N system. Chemical-thermal treatment with alternate supply of ammonia and air (gas-cyclic process) allows fourfold duration reduction of the diffusion layer forming process of a specified thickness in alloyed steels. The phase composition of the surface layer after various nitriding process modes and kinetics of its individual sections growth were studied.

Possibilities of intensifying nitriding and controlling the phase composition of the layer by a rational choice of process parameters, namely the number of half-cycles and their duration are shown.

Keywords:

nitration, thermo-cycling, gas cycling, microstructure, phase composition, internal diffusion

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