It is well-known that there is a clear correspondence between the qualitative characteristics of the metal under study and its internal structure. This made it possible to evaluate the mechanical properties of a metal through the analysis of its internal structure. In many cases the size of the grain is used as the main parameter of the metal in the analysis of its internal structure, because it has a significant effect on the properties of the metal. In this approach to research interesting from the point of view of material science is to study the kinetics of grain growth of the material under the influence of some factors of external influence, such as temperature. Different approaches are used to determine grain size. Among them are the method of determining the grain size on the basis of comparison with the reference scales, the method of counting grains and the method of calculating the intersections of grain boundaries. The above methods have a number of significant drawbacks, among which are the following. First, in the above methods and in other approaches of this type, they operate on average statistical values, and the object of analysis is not individual grains, but some, sometimes quite large, group of grains. Since on the thin section may be grains of different sizes, this leads to a methodological error in calculating the geometrical parameters of the grain. Second, the methods considered are suitable for the analysis of grains of convex shape only. When the shape of the grains differs from the convex one, it will also give rise to additional methodological error. Third, the methods described in the paper are not automated, which requires additional time to calculate the number of grains, the number of points of intersection of lines and boundaries of grains, etc. The paper analyzes one of the most commonly used approaches to calculating the average diameter of steel grains based on metallographic images. It is a method of crossing grain boundaries, based on the requirements of regulatory documents. It is established that the average diameter of the grains of metal in the plane of thin section calculated according to this method does not correspond to the real average diameter of the grains in three-dimensional space. An error analysis is performed, which is that the same cross sections in the plane of the thin section do not always correspond to real grains of the same size in three-dimensional space. To overcome this drawback, a method using a correction coefficient is proposed, the values of which depend on the number of grains used in the calculation of the average diameter. In general, the proposed method will improve the accuracy of the calculations of the average diameters of the metal grains.
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