There are many varieties of pig-iron, which themselves also differ pretty much according to the proportion of carbon contained by them.
Before proceeding to consider the different varieties, it is necessary to understand that there are two distinct forms in which carbon occurs in cast iron.
1 Percy's Metallurgy, p. 102.
1. In the state of Mechanical Mixture. - In this state the carbon is visible in the shape of little black specks interspersed throughout the mass, which give the iron containing them a dark-grey colour.
These little black specks are particles of free carbon, otherwise known as graphite or plumbago.
2. In the state of Chemical Combination. - The carbon in this state is not visible, and can be detected only by analysis.
The properties of cast iron depend not upon the absolute amount of carbon it contains, but upon the condition in which that carbon exists.
The varieties containing a large proportion of free carbon are of a dark-grey colour, are soft, and run freely into moulds.
When the carbon is all, or nearly all, in chemical combination with the iron, there are no black specks; the metal is white, very hard, brittle, and forms, when fused, a somewhat pasty mass, which will not freely fill a mould.
The former of these classes merges gradually into the latter, and between them there are several gradations. Some varieties contain both free and combined carbon.
White cast iron sometimes contains as much carbon as the grey varieties (about 4 per cent), but of this very nearly all is in a state of chemical combination, whereas in the grey iron a very large proportion of it is free, in the shape of distinct specks of plumbago, only about 1 per cent being in chemical combination with the iron.
The impurities mentioned below are originally derived either from the ore or fuel, and unless eliminated in subsequent processes, they will injure the respective metals produced in the manner stated.
Silicon is, next to carbon, the most common constituent of pig-iron. It is derived from the ore and from the fuel. A good deal of it is got rid of in the slag produced by smelting, and also during the refining and puddling processes.
In many respects silicon resembles carbon, and it affects cast iron in nearly the same way.
1/2000 part makes it cool and solidify without bubbling and agitation; more makes it brittle. 5/10 per cent makes it unforgeable.
Phosphorus is very readily taken up by the iron during the smelting process, and is one of the worst impurities it can contain.
Cast Iron is hardened by it, but is made more readily fusible. Its tenacity is reduced.
Wrought Iron is injured by it in proportion to the quantity present.
1/10 per cent does not reduce the strength of wrought iron; and improves its capacity for welding.
3/10 per cent makes it harder, but not weaker.
5/10fa per cent makes it "cold short " (see p. 275).
8/10 decidedly cold short.
1 per cent makes it very brittle, and unfit for any but special purposes.
Steel is injured by a very minute proportion.
1/20 per cent makes it unfit for the best cutlery.
1/10 per cent makes it cold short, and useless for tool-making of any kind.
Manganese nearly always exists in cast iron. It tends to produce the white variety, in which a large proportion is generally to be found.
In Wrought Iron and Steel it counteracts red shortness, probably by encouraging the departure of the sulphur and silicon (see p. 306).
Sulphur is derived from the pyrites in the ore and coal.
In Cast Iron it tends to produce the mottled and white varieties.
In Wrought Iron fa to fa per cent produces red shortness.
Copper has the following effects : -
In Cast Iron 2/10 per cent does no harm.
In Wrought Iron 3/100 per cent reduces tenacity. 5/10 per cent makes it red short.
In Steel 5/10 per cent makes it red short. 2 per cent makes it brittle.
Arsenic is not a very frequent impurity in iron.
In Cast Iron a smaller proportion is said to be good for chilled castings.
In Wrought Iron it causes red shortness.
Among the impurities met with more rarely, or in smaller quantities, are Tin, which makes wrought iron cold short.
Antimony, which makes wrought iron both hot and cold short.
Titanium, which tends to produce mottled cast iron. The so-called " titanic steel" contains no traces of titanium. The good qualities attributed to it must arise from some indirect action.1