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Experimental investigation Experimental details


A cubic block 30 cm in size was cast following a pro- cedure detailed previously.12


The composition of the ma-


terial as analyzed after casting was 3.68% C, 2.20% Si, 0.09% Mn, 0.035% P, 0.014% S, 0.040% Mg, 0.02% Cu, 0.005% Ti, 0.02% Cr, 0.011% V, 0.0123% Ce, 0.002% La (contents expressed in weight). Subsequent analyses made on an area with CHG graphite and another free of this de- fect did not show any chemical differences. According to a previous evaluation of the effect of various elements on the austenite and graphite liquidus,13


this alloy is slightly 25.0% in the central part.


hypoeutectic (carbon equivalent at 4.30%), the austenite liquidus temperature is 1167.6C (2133.7F) and the equi- librium eutectic temperature is 1158.3C (2116.9F). The micrographs in Figure 2 present the microstructure in the outer non-affected area (Figure 2-a) and in the inner zone (Figure 2-b). In this latter, several CHG cells are clearly visible. The central area affected with CHG (see Figure 1) amounts to 17.6% of the whole volume of the block, while the specific surface of CHG cells is as high as AA


=


Samples for DTA were machined in the outer area free of CHG (labelled no-CHG in the following) and from the centre (labelled with-CHG). The study specimens were cylinders 3.9 mm in diameter and 4 to 5 mm in length. The DTA experiments were designed to look for any dif- ferences in the cooling records that could be related to the origin of the samples, either with-CHG or no-CHG. Two series of trials were performed with one new sample for each run: •


heating to 1200C (2192F), holding 1800 sec. then cooling at either 2.5, 5, 10 or 20ºK/min;


In the first series of trials, the lengthy holding in the liquid state was designed so as to lead to full Mg fading and so- lidification with lamellar graphite. On the contrary, the tri- als with very rapid melting and short stay in the liquid state were expected to preserve alloy composition as much as possible. One additional trial consisted in applying a rap- id melting and solidification at 10ºK/min down to 1000C (1832F), then reheating at 1200C (2192F) and holding 1800 sec., finally solidifying at 10º K/min. This trial was defined for reproducibility checking.


Independent calibration of the DTA cell with pure alumin- ium and nickel showed that it underestimated temperatures by about 3C. All records presented in the following have been corrected accordingly.


results


The graphs in Figure 3 show the whole series of DTA records obtained upon cooling after sufficient time has been spent in the liquid state so that all the Mg introduced for spheroidi- zation has faded. In these graphs, the vertical dashed line indicates the calculated temperature for the stable eutectic. It is seen that the records show one main peak associated to the eutectic reaction, and a much smaller thermal arrest at higher temperatures. Comparison of the peaks associated to the eutectic reaction in Figures 3-a for with-CHG and 3-b for no-CHG materials shows that the DTA records on both materials are similar whatever the cooling rate, from 2.5 to 20ºK/min. As a matter of fact, the change in the cooling rate





heating to 1200C (2192F), holding 1 sec. then cooling at 5, 10 or 15ºK/min (in this latter case, the upper temperature of the heating stage was in- creased to 1225C (2237F) in order to ensure com- plete melting of the sample before cooling starts).


(a)


(b)


Figure 2. Optical micrographs of the cast material taken (a) in the non-affected outer area and (b) in the central part of the block.


36 International Journal of Metalcasting/Winter 2012


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