Computer modeling of diffusion-saturation kinetics during gas nitriding
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A mathematical model of the process of gas nitriding in a gaseous atmosphere based on nitrogen and ammonium diluted with its dissociation products, nitrogen, hydrogen, and inert gases, which makes it possible to investigate and predict the technological regimes of variation in the basic parameters of the process — temperature, pressure, and composition of gaseous mixture — is developed to achieve the prescribed layer structure and thickness.
The following are possible in the process of numerical experimentation on the model: to investigate the kinetics of diffusion saturation with respect to variation in the distribution of nitrogen concentration in the layer; to evaluate the kinetics of mass transfer (the flow of nitrogen through the gas-metal boundary), the amount of nitrogen in the layer, and the nitrogen concentration on the surface of the metal; to record the formation time of new phases on the surface and the moment of their resorption in the nitriding process; to assess the effect of deviations in temperature, pressure, and mixture composition from prescribed values on the formation kinetics of the layer; to investigate the effect of heating and cooling rates during nitriding on the structure and phase composition of the layer.
Experimental confirmation of the adequacy of the model indicated satisfactory convergence of computed and experimental data on the thickness of the diffusion layer with an accuracy to 10–15%. This error is caused by difficulty in determining the precise magnitude of the diffusion sublayer by the metallographic method.
Combined nitriding regimes make it possible to carry out computational design of multistage processes to produce diffusion layers with a prescribed composition and structure.
KeywordsMass Transfer Gaseous Mixture Diffusion Layer Gaseous Atmosphere Basic Parameter
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