From trials conducted by Ledebur, it appears that cast iron is rendered
suitable for foundry purposes--i.e., to fill the moulds well and to yield
sharp and definite forms free of flaws, to be cut with a chisel, and
turned on a lathe--through a certain percentage of graphite, whose
presence depends on that of carbon and silicium. Cast iron free of
silicium yields on cooling the entire amount of carbon in the amorphous
state, while presence of the former metal gives rise to the formation of
graphite, and, consequently, causes a partial separation of carbon. Iron
suffers on casting loss of graphite, assumes a finely-grained texture,
becomes hard and brittle, and is changed from gray to white. In view of
the fact that samples of cast iron with equal percentage of silicium and
carbon yield on casting a different product, it has become necessary to
institute experiments as to the cause of this behavior. Samples of cast
iron were therefore repeatedly melted, and thin sections of each melt
examined; these sections exhibited a gray color, though less apparent
than in the unmelted sample, and possessed sufficient softness to admit
boring and filing. During these processes of fusing, the amount of
silicium, carbon, and manganese had been gradually decreased, and
amounted to 12.7, 17.6, and 24.4 per centum for silicium in the three
samples examined. It also was observed that the more manganese the iron
contains the less readily the percentage of silicium is diminished; and
since manganese is more subject to oxidation than silicium, it is capable
to reduce silicic acid of the slag or lining to metal, and thus to
augment the amount of silicium in cast iron. The percentage of carbon
also suffers diminution by oxidation, which latter process is impeded by
presence of manganese, a fact of some importance in melting of cast iron
in the cupola furnace. An excess of manganese renders cast iron hard and
brittle, and imparts to it the properties to absorb gases, while an
amount of 1.5 per centum, as found in Scotch iron, undoubtedly has the
effect to produce those properties for which this iron is held in high
repute. The amount of copper is not visibly altered by fusion, but that
of phosphorus and sulphur slowly increased.
Experiments in regard to the relation between chemical composition and
strength of the material have established that a large amount of
silicium, graphite, manganese, and combined carbon reduce the elasticity,
strength, and tenacity of cast iron, and that a limited percentage of
silicium counteracts the injurious influence produced by an excess of
combined carbon. On remelting of cast iron, increase in tensile strength
was observed, which attained its maximum in iron with a small percentage
of silicium after the third, and in such with a large amount after the
fourth melting. The increase in tensile strength was accompanied by a
loss of silicium, graphite, and manganese coupled with a simultaneous
augmentation of combined carbon. A fifth melting of the cast iron renders
it hard, brittle, and white, through oxidation of silicium and subsequent
lowering of the amount of carbon. On lessening the percentage of combined
carbon with formation of graphite the injurious influence of the
accessorial constituents of cast iron is diminished, especially that
produced by the presence of phosphorus.--_Eisenhuettentechnik._
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