In this diploma work, we have been studied decarburization of 51 CrV4 plain steel with respect to the temperature and time of annealing. Experiments were done on ground samples, which were heat treated for 2 and 4 hours in an air atmosphere at temperatures: 760°C (Ac1<Tž<Ac3), 860°C (Ac3<Tž<TG) and 1092°C (Tž>TG). The distinctive temperatures have been chosen on the basis of the steel phase diagram. Using metallographic methods, we measured the thickness of the oxide layer and decarburization depth and analysed the influence of temperature on decarburization mechanisms. Considering the obtained results, we first programmed a simple one-dimensional mathematical model to determine the approximate values of the carbon diffusion coefficients at individual temperatures, and then a two-dimensional numerical model of decarburization (FDM), which takes into account the simultaneous oxidation of steel during decarburization and can predict the degree of decarburization at these temperatures and the annealing times also at the edges and grooves of the sample. With higher temperature and longer annealing time the depth of the decarburization increases, but because of the chemical inhomogeneity in the steel, it will not be constant even under a smooth ground surface. The calculated values of the carbon diffusion coefficients in the steel 51CrV4, therefore, represent the limit values. With a two-dimensional model that considers the simultaneous oxidation of steel, we get lower values of diffusion coefficients, and slightly larger differences are only at the highest temperature of the annealing. The calculated values of the diffusion coefficients are within the size class of the values given in the literature sources.
|
