Border Surface Changes on the Edge of the Castings with Nodular Graphite

The paper presents the study of the microstructure and ultrasound measurments by proeminent part of the nodular iron castings. The experiment has been made in the Havy Machine Factory in Clausenburg. The crossing speed of the ultasound increases near the edge of the castings. Analising the nodular iron microstructure by smoler castings, there occurs no graphite change. By bigger mouldings the shape of the graphite can change to a vermicular graphite near the edges. The thickness of the transformed stratum is of 0,1-0,3 mm. In the nodular graphite iron are acting the forces in the frontier-surface. After casting the evaporation of the magnezium increases near the edge of the castings. The changes of forces on frontier surface can be caused by the changes of the innere tension, and also by the interacting of moulding materials. Opinions regarding these two effects are contardictory in the technical literature. Our experiments have been made in the Heavy Machine Combinat (C.U.G) in Kolozsvar, on nodular iron containing different ferrit-pearlite proportions. The weight of castings varried between 0,5 kg and 8000 kg. The metalic melt necessary to nodulisation was produced inductive furnaces of 6,3 and 12 tons. Before treatment by magnezium alloy, the chemical composition of the iron melt varried between the following parameters, according to the different types of irons: 3,7-3.9% C, 0,2-1,4% Si, 0,1-1,0% Mn, max. 0,025% S, max. 0,05% P, 0,1-1,4% Cr,%, 0,05-0,3% Mo, 0,2-0,8% Cu, 0,2-2,0% Ni. The globularisation has been made in the Tundich-pot. The work-temperature was between 1450 and 1600 C. We used 1,8 – 2,2% FeSiMg and 0,4-1,6% FeSi for the nodulisation. Our target was the examination of the surface strata on the castings, using for mould mixtures with different chemical effect.