Metal Casting Technologies : MCT-MARCH-2014
28 www.metals.rala.com.au METAL casting Technologies March 2014 29 TEcHNicAL fEATURE TEcHNicAL fEATURE Since from literature it was found that the feeding distance for plate casting is 4.5T, the simulation was started with a length of 4T without chill. Subsequently the work was extended with increasing length, i.e. 5T and so on. Chills were placed at the end of the section and at the top and bottom of the plate near the edge where shrinkage was found in the casting. The size ofallchillswas25mm×25mm×15mmandthechillswere placed in a manner shown in Fig.1. The size of the riser used was 75 mm dia. and 75 mm height. The casting alloy used was 0.24 % C steel and the mould material was silica sand. Temperature dependent heat transfer co-efficient (HTC) was assumed at cast alloy – sand mold, cast alloy – sleeve and at sand mold – sleeve interfaces, whereas constant HTC was assumed at cast alloy – chill and at sand mold – chill interfaces. The results of the simulation in the form of solidification time and Niyama criterion with varying lengths of the plate casting are shown in Fig.2 to Fig.7. In case of length of casting of 4T, the plate is totally sound, as evident from Fig. 2(b). When the length of the plate is increased to 5T, lower Niyama value region is observed at the middle of the length of the casting (Fig.3b). It indicates that shrinkage porosity is likely to exist in this region. It is well established that when the length of plate casting exceeds certain limit (e.g. 4 .5T), shrinkage porosity occurs in between the riser zone and end zone. This fact is confirmed from the low Niyama value region in Fig.3(b), since the length of the plate has exceeded 4.5T. Use of end chill in this plate casting of 5T length eliminates the shrinkage, as shown in Fig.4(b). No region with low Niyama value is observed here. This observation corroborates that the use of end chill increases feeding distance. The simulation results of the plate casting with 6T length without and with different types of chill are shown in Fig.5 to Fig.7. The plate casting without chill contains substantial low Niyama value region as shown in Fig.5(b). Use of end chill reduces this shrinkage zone existing at the middle of the length of the plate casting (Fig.6). When both end chill and top and bottom chill near the edge of the plate casting are placed, the shrinkage almost disappears, as shown in Fig.7(b). Obviously, the end zone of the feeding distance has improved by the use of top and bottom chill in the plate casting. This finding is Figure 2a. Solidification time of plate casting of 4T length without chill. Figure 4a. Solidification time of plate casting of 5T length with end chill. Figure 3a. Solidification time of plate casting of 5T length without chill. Figure 5a. Solidification time of plate casting of 6T length without chill. Figure 2b. Niyama criterion of plate casting of 4T length without chill. Figure 4b. Niyama criterion of plate casting of 5T length with end chill. Figure 3b. Niyama criterion of plate casting of 5T length without chill. Figure 5b. Niyama criterion of plate casting of 6T length without chill.
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