Comparison of solidification kinetics of compacted and lamellar cast irons

International audienceThermal analysis is largely used in cast-iron foundry shops as a means to check melt preparation before casting. It has been suggested that the shape of the cooling curves could be related to graphite form, either spheroidal, lamellar or else compacted. The present work is part of a larger study intended to improve melt control for compacted graphite castings by using controlled additions of sulphur and magnesium. Experiments showing lamellar graphite and their counterpart with compacted graphite have been selected for analysis and simulation of the thermal analysis records. Simulation makes use of the fact that compacted graphite differs from lamellar graphite by the limitation of graphite branching in the former. Solidification of both types of irons may thus be simulated using the same basic growth law for irregular eutectics, though changing the parameters describing the branching capability of the faceted graphite phase. The increase of the undercooling of the eutectic plateau during solidification of compacted cast iron when compared to that of lamellar cast iron could be verified and reproduced by simulation. The limitations of the present approach which assumes the temperature in thermal cups is homogeneous at any time are also pointed out

Is Thermal Analysis Able to Provide Carbon and Silicon Contents of Cast Irons?

International audienceThe determination of silicon and carbon contents by thermal analysis is based on the recording of a cooling curve of a melt whose eutectic solidification takes place in the metastable system. The temperatures of the austenite and eutectic arrests are evaluated and then related to the carbon and silicon contents by linear relationships that would be defined by the phase diagram if there were no undercoolings. However, published experimental values of austenite liquidus arrest and metastable eutectic arrest both show significant undercoolings when compared to the equilibrium metastable Fe–C–Si phase diagram. The undercooling of the austenite observed in hypo-eutectic alloys can be explained by tip undercooling during dendritic growth. On the other hand, it is pointed out that alloys with a composition close to the stable eutectic—either slightly hypo- or slightly hypereutectic—can show much higher undercoolings than expected if some stable eutectic precipitates. For such compositions, the thermal analysis could hardly be reproducible enough if the alloy does not solidify completely in the metastable system. Furthermore, the published experimental information on the temperature of the metastable eutectic is here supplemented by new results for silicon contents up to nearly 4 wt%. Both previous and new results show a similar undercooling of 5 to 15 °C compared to the calculated equilibrium metastable eutectic. Each foundry wishing to use thermal analysis for carbon and silicon evaluation must determine its own relationships for austenite liquidus and metastable eutectic to account for the specificity of its melting process and casting system. Care should be taken to avoid any graphite precipitation in the thermal cup, and testing should be performed to improve reproducibility as this defines the accuracy of the composition determination