Abstract
	The article presents the calculation of diffusion layer and penetration of substitutive elements in welded joint of two different steels, based on results of measurement of concentration profile. Steels have identical structure of ferritic type; from the viewpoint of chemical composition they represent a polycomponent system. Fundamental calculation method is developed on the solution of unsteady diffusion along the straight line, both in positive and negative direction. The starting point is the second Fick’s law. The input data have been obtained by experimental measurements using the method of energy dispersive (ED) X-ray microanalysis. The evaluation of concentration profiles on both sides of welded joint has been performed by means of numerical method. Non-linear regression algorithm optimising model parameters had to be used. The Levenberg – Marquardt’s method has been adapted and developed for this purpose. The parameters of the model, i.e. coefficient of diffusion, the effective concentrations of  substitutive element in great distance from the welded join as well as the inaccuracy in location of the contact of both metals have been estimated. The output comprises also statistical results expressing the errors of the above mentioned quantities. The procedure is demonstrated numerically and graphically on diffusion of chromium. Obtained results can be potentially used for practical information about chromium heterogeneity in the given or similar type of welded joint. It is necessary to emphasize that the conceptions of the thickness of diffusion layer and the penetration depend on the appropriate definition, what is important for comparison of different substitutive elements. This paper is close related to the former publication dealing with the same problem of silicon. [1].