Solidification de la fonte

Pages: 11 (2510 mots) Publié le: 24 février 2011
Hot Topics in Thermal Analysis
Information on Thermal Analysis for Foundries

Dual Cup Analysis for Iron – October 2010
The dual cup analysis system was pioneered by MeltLab in the early 1990’s as a way of getting chemistry, chill, and inoculation information in a single step. The method was to simply pour both a tellurium cup and a plain cup from the same spoon of iron in quick succession.Besides the extensive analysis of both kinds of cup (chemistry and microstructure) we were also able to compare the “distance” separating the grey and white eutectics (stable and metastable). The greater the distance between the two in terms of temperature, the softer the iron was. We published a paper with AFS on comparing chill wedges to the separation between curves back in 1996. There is a lotof human variation that goes into chill wedge readings that can be removed by DualCup. The concept of chill measurement by thermal analysis is documented in my paper “Chill measurement by Thermal Analysis”, AFS Transactions, 1996, page 969. The separation between the graphitic eutectic and the white/metastable eutectic is both a measure of the grade of iron, and the chill potential of the iron. AG25 iron will have a large separation, while a G45 iron will have a much smaller separation. But rather than just specifying the grade, we can draw a finer distinction between the different heats to the point of actually predicting the chill by using the metastable eutectic and the eutectic undercooling temperature. Since the degree of undercooling is directly affected by the inoculation, thisfactors in both the overall chemistry and the inoculation into the chill calculation. Calculated Chill = (k1 * C.E.) + (k2 * (TEU – TCE) + k3 Calculated Chill = (k4 * Liquidus) + (k5 * (TEU – TCE) + k6

or

C.E. = Carbon Equilivant derived from the Liquidus temperature TEU = Temperature of Eutectic under cooling TCE = Temperature of Carbidic Eutectic (metastable or white eutectic)

The exampleto the left was from an Italian foundry with 25˚ C separation between curves. The screen shot is from the previous version of MeltLab. The upper graphitic curve shows a beautiful cap or ceiling eutectic arrest showing that the 1140.1˚ C is the true eutectic temperature and was not affected by the rate of cooling. The eutectic undercooling of 3.4˚ C indicates the degree of inoculation.

Dual CupAnalysis for Iron – October 2010
Though I am no expert in all the ins and outs of chill wedges, here are a few things I have learned:     Chill wedges come in different sizes. The correct size for you depends on the wall thickness you are pouring. I have seen wedges from 1 inch high to 8 inches high. The wedge mold must be free of dirt and loose sand that would blunt the edge of the wedge.The heat of the iron will affect the chill, so try to pour the wedge at a constant temperature. If also pouring thermal analysis cups, pour the wedge first, not last, as it is more affected than the cups by temperature. The best technique I ever saw was in an AFS inoculation course. The technician took a second chill mold and struck off the top of the poured mold to wipe off any excess iron and themiscues. Then, promptly at 60 seconds, he tossed the entire mold into a bucket of water. (You might have to change that time with 8 inch wedges.) The wedge needs to be broken at the same distance from the pour basin each time. Closer to the pour basin will show less chill, further away will show more. Most molds have a small break line embedded into the core. And finally, there is somedisagreement on how to measure the chill. Some measure from the point, some measure the width of the wedge at the first point of no chill. I hold with the width measurement because it is less affected by dirt in the point of the mold.





Images from the paper Chill measurement by Thermal Analysis, AFS Transactions, 1996, page 969 by David Sparkman and Charles Bhaskaran available for a fee at...
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