Abstract
	The processes, during which interstitial elements diffuse often against their own gradient of concentration, are labelled as reversed diffusion, or up-hill diffusion. Due to the fact that namely at low temperatures (500 to 800 °C), when majority of welded joints of steels in power engineering equipment works on a long-term basis, diffusion of substitutive elements in comparison with interstitial elements is practically negligible, it is appropriate to use at description of diffusion of interstitial elements in welded joints of steel the term of redistribution and to describe their diffusion in the given system as quasi-stationary diffusion. It is understood by this term, which was introduced already in 1966 ?1?, that substitutive elements diffuse in solid solutions of iron regardless of interstitial elements, while interstitial elements must perfectly accommodate to a distribution of substitutive elements, e.g. by their concentration at the welded joint boundary.
	The paper deals with a model of quasi-stationary diffusion of carbon in welded joints of steels. The model is presented in the form of engineering interpretation; the reason for such approach is its practical application. The possibility of relatively prompt prediction of carbon concentration characteristics in the neighbourhood of the boundary of welded joints of two steels (steels (1) and (2)) is presented in general manner. Steel (1) represents carbon or low-alloy steel and steel (2) represents medium- up to high-alloy steel, alloyed with element such as e.g. chromium which significantly minimises carbon thermodynamical activity.
	The presented phenomenological solution of carbon redistribution in welded joints of the type Fe-C-j, where the element j is a substitutive element, enables prediction of carbon distribution in joints formed by the steels (1) and (2), which represent solid solutions of the same type, e.g. of the type (1)-austenite/(2)-austenite. Solution is continuous for x in the interval x??????? and for t ? 0.