Análise numérica e experimental da cinética de nitretação a plasma de aços-ferramenta

Abstract

The plasma nitriding kinetics of tool steels AlSl H13 and AlSl D2 was studied. These steels were plasma nitrided at 520°C for times between 42 min and 11 h. Nitrided layer depths from 50 to 200 μm were obtained. A numerical model was developed to calculate the nitriding kinetics in the plasma-metal interface and in different depths from sample surfaces. Basically, the model consists in solving equations of mass transport, by means of the computational programs Thermocalc® and Dictra®. The model takes into account both the solute depletion of the matrix during the hardening and nitriding treatments and the delaying effect of the precipitates on the nitrogen diffusion in the matrix. The model predicts not only the maximum nitriding time - provided the thermal history of the steel, the temperature and the gas composition - to obtain samples without a compound layer, as well as the composition and the phases formed as a function of both the nitriding parameters and the distance from the surface. A good agreement was observed between the numerical simulation values and the experimental evidences. The N 2-%vol in gas mixture and the maximum time which allows obtaining samples without compound layer, t cr, can be expressed by the equation %N 2 = K(t cr) -0.5, where K is a constant depending upon the nitriding temperature and the hardening temperature.