Peter the Great St. Petersburg Polytechnic University (St. Petersburg, Russia):

Kazakov A. A., Dr. Eng., Prof., E-mail: kazakov@thixomet.ru
Zhitenev A. I., Engineer
Salynova M. A., Engineer

RF State Research Centre JSC SPA “CNIITMASH” (Moscow, Russia):
Kolpishon E. Yu., Dr. Eng.

A method for an assessment of impurity rate of nonmetallic inclusions (NMI) for forged shells from a super large steel ingot weighing 420 tons was developed. The method is based on a separate assessment of small indigenous and large exo-indigenous NMIs according to the hypothesis that large ones are no more than 5% of the total number of all NMIs found. To assess fine indigenous NMIs ASTM E 1245 was used, and for large exo- indigenous ones the modified GOST 1778-70 with reference to the panorama of the entire surface of the sample (300 mm²) was used. This approach has made it possible to improve significantly the accuracy of measurements of NMIs impurity rate for forgings from a super large steel ingot with a mass of 420 tons of 15Cr2NiMoV high quality steel. By means of the developed procedure the pattern of distribution of fine and large NMIs, reflecting the features of the cooling rate and hydrodynamics of the melt during solidifi cation at various horizons of the super large steel ingot, the nature of the location of structural zones in it, including the transition from columnar to equiaxed crystals have been established. The distribution of small indigenous inclusions along the thickness of the padding and bottom rings submits to strict laws that are in good agreement with the features of solidifi cation of the ingot. Large exoindigenous NMIs are distributed irregularly along the cross-section of these rings, therefore, in order to describe them, it is necessary to improve experimental methods of research and methods of processing the data obtained. 

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