Formation Mechanism of Precipitated Graphite Shapes in Hyper Eutectic Steel

In the solid phase of the Fe-C alloy system, C atoms precipitate as graphite into defects such as voids in alloys. Therefore, the morphorogy and distribution of graphite may depend on those of the defects. In order to verify this assumption that had been previously presented by us and other investigators hyper-eutectoid Fe-C-Si alloys were heated to the austenite temperature range and held isothermally. Thereafter, the alloys were quenched in water or cooled to the graphitization temperature in a rate 0.17 K/s prior to graphitization.Results obtained are as follows:(1) In specimens containing Mg, spherical cavities less than 5 μm in size were observed and their number was decreased by the sweeping effect mainly due to migration of grain boundaries during the isothermal treatment. Micro-cracks were also observed in all quenched specimens.(2) After the graphitization annealing, graphites in these specimens had a shape and distribution similar to the above-mentioned defects.(3) It was suggested that the formation mechanism of graphites, which might be precipitated into spherical cavities and micro-cracks, would be substantially the same in these two cases, because characteristics of crystals resembled each the other in spite of much difference in their shapes in the observation of micro-structure of graphites.(4) These experimental facts would directly prove the above-mentioned assumption on the precipitation mechanism of graphite.