The testings involved in this paper were part of a comprehensive research project conducted to find out what complex processes were taking place at metal mould contact surface. These investigations are of particular importance for metal castings from relatively high melting temperature, as that of steel (approx. 1600℃). Steel castings, in regard to their high manufactured temperature and other specifications in pouring, need a mould mixture of good quality. The used mould mixture, for casting steel class DIN 50Mn7, was based on the silica sand, and with water-glass as a binder and AlK(SO_4)_2 as an active component. This active component is, at pouring temperature of liquid steel, decomposed in the mould by thermal influence during pouring, crystallization and cooling the casting. In decomposition process of active component the gaseous products appear and the water-steam is generated from a chemically bonded water. These processes are influencing the molten metal oxidation. The penetrated molten metal into the mould surface is oxidized, too. At the high melting temperature and in the presence of gaseous products, the silicates were formed in the contact zone between metal and sand mould mixture. The objective of these tests was to find out to what extent the composition of the moulding mixture affects the formation process, shape, type and composition of the silicates.
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