INFLUENCE OF GEOMETRY OF FUSION LINE ON DECARBONIZATION IN DISSIMILAR WELD JOINTS AFTER HEAT TREATMENT

Abstract

Welded joints of dissimilar steels are widely used in various components of the steam-water circuit at thermal and nuclear power plants. In such welded joints after tempering and during high-temperature operations, carbon migrates through the fusion surface from less alloyed steel to more alloyed steel due to the difference in the chemical potential of carbon in these steels. Decarburization in the weld-adjacent area of ​​the heat affected zone of less alloyed steel, which occurs due to carbon migration, can lead to the formation of defects and subsequent failures in service. It was noticed that the thickness of the decarburized layer varies depending on the geometry of the fusion line: after heat treatment in places of convexity of the more alloyed weld in the base metal the thickness of the decarburized layer in the heat affected zone is less than in places of concavity of the weld. To numerically estimate the influence of the shape of the fusion line on the intensity of decarburization in the weld-adjacent zone, it is proposed to use a geometric factor. The aim of the work was to find such a function for use as the geometric factor, which would allow to estimate locally the variable concavity of a complex curve (in our case - the fusion line) from a point outside the curve and express it through a scalar parameter. An integral function ФL(t) is proposed, which "scans" the fusion line L from the point t in the heat affected zone; the obtained numerical value of this function for each point t can be interpreted as the order of decarburization of this point during tempering or high-temperature operation at a given geometry of the fusion line L, and can be used to construct a scalar field of the decarburization order in the heat affected zone of less alloyed steel by implementation of ФL(t) using computer vision software.