In some cases the metal is run into the moulds direct from the blast furnace, but in superior work it is generally specified that the cast iron is to be of the "second melting;" that is, from pigs remelted in a cupola.
The cupola is somewhat similar to a small blast furnace, and acts in the same way. A little limestone is added as a flux, which combines with some of the impurities left in the pigs, and removes them in the form of slag.
There are several varieties of cast iron - made from the different qualities of pig-iron - and they are classified by engineers in a somewhat similar manner.
No. 1 is of a dark-grey colour, caused by the profusion of specks of graphitic carbon throughout its mass; it melts into a very fluid state, which adapts it for very fine sharp delicate castings not requiring much strength.
It is, however, not so strong as the other varieties of cast iron, and is very soft, yielding readily to a chisel.
When broken it gives out a somewhat dull leaden sound, and shows a large, dark, bright grain.
No. 2 contains less free carbon than No. 1, is therefore lighter in colour, closer in the grain, and more difficult to melt; but being harder when cold is better for machinery, girders, castings to carry weight, or in any position where strength and durability are required.
No. 3 is of a lighter grey, with less lustre, and contains still less carbon than No. 2. It is therefore harder and more brittle, and is employed in heavy castings.
White Cast Iron is made from forge pigs; it contains very little free carbon; is of a silvery hue, extremely hard and brittle, and is unfit for castings, except those of the very commonest kind, such as sash weights.
Mottled Cast Iron contains both the grey and white varieties, which can easily be distinguished. The fractured surface is either chiefly white with grey specks, or grey with white spots and patches.
White and mottled cast iron are less subject to be destroyed by rusting than the grey kind.
They are less soluble in acids, are hard, brittle, and not so elastic as the softer lands.
This effect may be produced by placing in the mould over those parts where a hard skin is required, pieces of cold iron of suitable shapes, thinly coated with loam. Where these are touched, by the molten metal its surface is suddenly chilled and converted into white iron.
Thus the running surface of the wheel of a railway carriage is chilled, and covered with a hard skin of white iron, while the remainder of the wheel is of tough grey iron.
Malleable Cast Iron is made by extracting a portion of the carbon from ordinary cast iron in order to assimilate it to the composition of wrought iron, and thus increase its toughness. This is generally done, in the case of very small castings, by embedding them in powdered haematite ore, or in scales of oxide of iron, and raising to a bright red heat in an annealing oven.
Malleable castings "may be easily wrought cold, but become very brittle when heated, breaking to pieces under the hammer at an incipient white heat; at a higher temperature the kernel of unaltered cast iron melts, so that articles that have been subjected to the process cannot be united by welding, but may be brazed without difficulty."
Mr. Kinnear Clark states that the tensile strength of annealed malleable cast iron is "guaranteed by manufacturers to 25 tons per square inch," and that it "is capable of supporting a tensile stress of 10 tons per square inch without distortion." 1
Castings treated by this process, though they have not the peculiar fibrous structure characteristic of wrought iron, become to a certain extent malleable, and can be hammered or bent when cold without fracture.
They are specially suitable for intricate forms which could not be forged in wrought iron without much difficulty and expense.
The depth to which the casting is effected by this process depends upon the time during which it is exposed. Pieces about half an inch thick are rendered malleable throughout; thicker pieces have merely a skin of wrought iron, the interior remaining unaltered.
This process is applied to the manufacture of buckles, gun-locks, snuffers, pokers, tongs, etc.; and on a larger scale it has been used for toothed wheels of machinery, screw propellers, and other purposes where a certain amount of toughness is required combined with intricate forms.
Mr. Matheson recommends that malleable iron castings should be used for the shoes and connecting pieces in roof structures.2
Toughened Cast Iron is produced by adding to the cast iron and melting amongst it from 1/4 to 1/7 of its weight of wrought iron scrap.
Great experience is required in order to know exactly what descriptions of pig-iron to choose in order to make castings for any particular purpose.
Mixtures of pigs classed under different numbers, and even selections from different localities and makers, are recommended for large and important castings.
1 Clark's Tables.
Sir William Fairbairn recommends1 the following mixture as being of "great value in castings, such as girders for bridges, beams for buildings, etc., where rigidity and strength are required: -
Low Moor, Yorkshire,
No. 3, 30 per cent.
Blaina, or Yorkshire,
No. 2, 25 „
Shropshire or Derbyshire,
No. 3, 25 „
And good old scrap,
Many other recommendations as to different mixtures were made before the Royal Commissioners who reported on the employment of iron in railways.
It is now, however, generally considered better by engineers to stipulate that the iron shall stand certain tests, leaving the mixture to be used to the judgment of the ironfounder.