Abstract
The graphite nodule count, size distribution and homogenization of the nodules distribution are the factor which are of more significance for the properties of the material. By just increasing the inoculants or Mg will not help to get rid of problems like the un-even size distribution and or increasing the nodule count. The cooling conditions of the melt prior to solidification and during solidification can control these two parameters to a large extent. In this research, it is more emphasized on the nucleation sequence of MgO particles and on the cooling rates. The nucleation of MgO at different temperatures and at different cooling rates was found to have a great influence on the nodule size distribution and the homogenization of the microstructure. A mathematical model was derived to relate the Mg concentration in the liquid to the cooling rate, prior to solidification. The MgO particles count was calculated as a function of cooling rate. It was found that at higher cooling rates, MgO can be nucleated in multi steps during cooling process, which can increase the nodule count tremendously.
Highlights
Cast iron is a composite type of alloy in which the graphite shape, size, count and distribution, all are affecting the mechanical and thermal properties of the alloy
Where XXMeeMeMeM is the equilibrium amount of Mg in the liquid with MgO, XXMLLMMM is the Mg content in the melt which varies during cooling, XXMMMMMMMMMM is the Mg content in the MgO particle which is assumed to 0.5 mole fraction
VVmMmMMMMM is the molar volume of MgO particle (1.14 × 10−5 mm3/mmmmmm), VVmMmMMM is the molar volume of Mg in the liquid in equilibrium
Summary
Cast iron is a composite type of alloy in which the graphite shape, size, count and distribution, all are affecting the mechanical and thermal properties of the alloy. At a longer solidification intervals, such as in wind turbine, the micro segregation plays a dominant role on the graphite morphology, resulting in non-homogeneity in the microstructure from surface towards the centre of the casting. Improper nucleation sequence of graphite nodule and micro segregation of different elements, can cause the non-homogeneous distribution of nodules and colonies of various sizes are formed. In the areas where many nodules are formed at the same time as a colony, as in the last solidifying regions, the inter-nodular spacing will be lower and the fracture toughness will decrease. The larger is the size of the colony with many nodules, the lower will be the fracture toughness, behaving like a local defect in the material
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