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Metal Casting Technologies : December 2010
30 www.metals.rala.com.au a thermo-electric technique in which the %Si is obtained by measurement of the voltage developed when a heated copper probe is pressed into a sample of cast iron drillings contained in an unheated copper dish. The voltage developed depends on the Si content of the drillings and the method proved to be sufficiently accurate for process control purposes thus enabling %C content to be calculated in irons of known low %P content . Most engineering grades of grey iron have low %P levels. The later second method obtained the %C content directly from the cooling curve using a "carbon calculator", since composition can be related to the eutectic arrest temperature of the white iron reaction. This is not the case for the grey iron reaction arrest temperature which is sensitive to degree of nucleation as well as composition. This technique makes use of a tellurium coated sample cup, first developed for use with hyper-eutectic irons, to ensure that solidification follows the white iron reaction. In the original method the austenite liquidus arrest and the white iron eutectic arrest temperatures are entered into a rotating scale calculator to obtain an indication of carbon content of the iron. The method proved very useful for malleable irons and for base irons for ductile iron production as well as for flake irons providing that their compositions satisfied the requirement that: %C + %Si/9 + %P/3.5 < 4.3 (3) The use of a tellurium coated sample cup was a development of the use of plug in expendable cartridges first used in the mid sixties. Their use helped foundries to avoid the need to make their own test moulds and thermocouple arrangements. Each cartridge was produced as a shell moulded cup fitted with an expendable thermocouple inserted through the base, the inside of the cup and thermocouple being protected by a refractory coating. The cup is a push fit onto a special stand to enable the thermocouple to make contact with the leads from the stand to the recording instrument. The liquid iron sample is poured into the cup to solidify as a small ingot 50mm high by 30mm in diameter. By adding around 10% tellurium to the refractory coating it is ensured that the liquid iron sample solidifies as a white iron thus enabling CEL values for hyper-eutectic irons to be measured. In hyper-eutectic irons that solidify via the grey iron reaction (i.e. without the use of a tellurium coated cup mould) accurate measurement of the primary graphite liquidus temperature is very difficult due to the small amount of latent heat evolved when primary graphite forms, and due to the gravity segregation (floating) of this graphite in the remaining liquid. When tellurium is present the grey iron reaction is suppressed and the eutectic point is moved to higher CEL values (as indicated in the schematic double eutectic diagram in Figure 1). Solidification will then take place as formation of primary austenite (its liquidus is easily detected) followed by the white iron eutectic. Since Si content lowers the white iron eutectic temperature the higher the Si content of the iron then the higher is the CEL that can be detected so long as the sample solidifies white. In the original detailed study on flake graphite (FC) irons by Moore  it was found that, using both plain and Te coated cartridges, the austenite liquidus surface in unalloyed cast irons (i.e. Fe-C-Si-P alloys) could be described using: Austenite liquidus in oC = 1650 -- 121 (CEL) (4) Where CEL = (%C + 0.22%Si + 0.54%P) (5) For foundry use this relationship is expressed simply as: CEL=%C+%Si/4+%P/2 (6) Control of microstructure Following on from the development of CEL testing interest grew in the use of cooling curves to predict the effects of inoculation and Mg treatment in cast irons and the effects of grain refinement and modification in Al-Si based casting alloys. It was believed that the shape of cooling curves could provide information about nucleation and growth behaviour during solidification. Cooling curve features that can provide possible information about microstructure include undercooling, eutectic nucleation and growth temperatures, recalescence, undercooling time, eutectic growth time, etc. This lead to the use of computer aided differential thermal analysis to study ductile (FCD) and compacted graphite (CG) irons . It was found difficult to predict nodularity levels of Mg treated irons due to scatter in the data caused by factors such as pouring temperature, carbon equivalent and nodule count etc. and by inaccuracy of temperature measurement. Hence it was suggested  that predictions must be based on relative temperature measurements such as the rate of solidification after recalescence, the recalescence interval and solidification time. It had long been recognized that even in production of FC irons the austenite liquidus temperature (and hence measured CEL) could be influenced by charge materials and melting & holding conditions. To minimize melt variations many iron