foundries to lead to casting technological properties differ- ent from those known from conventional materials. There- fore several tests were conducted to examine the material’s susceptibility to dross formation, its mould filling behaviour and shrinkage tendency.
In a dross test, the surface of the metal bath was exposed to oxygen (air) for 1 to 1.5 minutes in a basin. The complete melt including the dross layer was then poured through a filter, as illustrated in Figure 9. The pouring capacity was logged by means of a digital scale. Reproducible results of these tests were that cast iron with an Si content of 2.5% led to blocking of the filter, while with 4% Si in the cast iron the filter remained open (Figure 10). Hence, the result is contrary to what was originally expected, namely that a higher Si content would promote dross formation.
Further tests were conducted with a test casting specially developed for these materials (a casting harp with plates and webs of different thicknesses between 0.12 and 0.24 in.). The filling behaviour of the alloys in the “casting harp” was independent of the Si content. The differences if any resulted primarily from variations in the casting temperature.
Additionally, different plates, cubes and cylinders were cast using alloys with different Si contents (CE value was 4.3 in all cases). From sections taken from the feeders and feeder necks it could be told that the feeding requirement and extraction behaviour of Si-alloyed ferritic grades are virtually the same as with conventional cast iron grades. The comparison of the exothermal top feeders of, for ex- ample, different cast cylinders (height and diameter each 4.7 in.) with 2.2 and 4.0% Si showed no significant differ- ences in the feeder weight and the appearance of the sec- tions and the cut surfaces of the feeders. The evaluation of the feeder neck samples (cubes with edge length of 1.58 in. and at the side, arranged different dimension feeder necks)5 showed on the other hand that it might be necessary to de- sign the feeder neck somewhat larger when feeding alloys with higher Si contents under pressure.
Dynamic and Cyclic Mechanical Properties
Charpy impact tests were performed at temperatures be- tween -94°F and 248°F with different conventional and so- lution strengthened ductile cast iron grades. As expected, the impact resistance of the solution strengthened grades was lower than that of the conventional grades (e.g. GJS-500-7: 7 J and GJS-500-14: 3 J at 77°F). Even at a test temperature of 248°F, the specimens of GJS-500-14 and 600-10 did not exhibit a clearly identifiable upper shelf.
In terms of the cyclic properties, solution strengthened ductile cast iron performs better. Figures 11 and 12 show the fatigue strength values of conventional ferritic/ pearlitic GJS-500-7 and of ferritic solution strengthened
International Journal of Metalcasting/Volume 8, Issue 2, 2014
Figure 10. This graph illustrates weight versus test duration for basin filling with varied Si and Mg contents.
39
GJS-500-14 in stress-cycle diagrams. At 40,624 psi, the fatigue strength of the solution strengthened ductile cast iron is higher than that of the ferritic/pearlitic material (37,235 psi).
Summary
Due to the favourable properties of Si-alloyed ductile cast iron grades (high strength at fairly good elongation, uniform hardness distribution and better machinability), a strong demand for these grades can be expected for the future. In Germany, this new group of material grades is already being used in various applications, e.g. continuously cast products, rollers and pinion cages.
Figure 9. Testing device for the measurement of the dross generating behaviour of cast iron melts.
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