Where is the eutectic point?
The traditional iron-carbon phase diagram shows that the eutectic point is around 4.3%. However, that is during equilibrium conditions with a slow cooling rate. Therefore, if the chemical composition of an alloy is higher than 4.3 e.g. 4.5 but the cooling rate is high, the solidification will probably NOT be hypereutectic as expected but eutectic! An alloy that is hypereutectic as calculated with the formula C+Si/4+P/2, will solidify with primary graphite in a casting only if the cooling rate does not exceed a certain level. Especially for ductile iron it is important to use an alloy that does not solidify with primary graphite in order to avoid defects. Therefore the optimal active carbon equivalent (ACEL) value must be chosen depending on the cooling rate of the castings. In order to be able to keep a certain target for ACEL, thermal analysis using grey solidification as in ATAS should be used for maximum accuracy. The diagram above illustrates the approximate location of the eutecti
