It is only proposed to deal with the subject of iron and steel from a builder's point of view, and not as would be required for an engineer or ironfounder, a brief resume being given of the various points of which the student is required to have some knowledge.
Wrought and cast iron and steel are all made from pig iron, extracted from the natural ore by the smelting process, which consists of heating the ore in a furnace, after it has been crushed into smaller particles, and then roasted or dried.
When in the furnace limestone is added to the prepared dry ore to extract its impurities, and a hot or cold blast passed over the aggregate, resulting in the molten metal leaving the lighter and more fusible impurities at the top, and sinking itself to the bottom, whence it is run into long channels with branches formed in sand called the "sow and her pigs,' which gives this iron its name.
This "pig" or unpurified cast-iron is divided into the foundry and forge qualities, distinguished by the combination and amount of the carbon each in the first instance contains, the resulting cast-iron having a proportion of from 2 to 6 per cent, while steel should only have from 0.15 to 2.00 per cent, and good wrought-iron should be practically free from it.
Carbon, although it plays such a very important part in iron and steel, is considered a foreign matter, the effect which it has on the different metals depending on the nature of its combination. When it is in chemical combination in cast-iron it produces a white, hard, and brittle metal, only fit for the commonest uses, such as sash-weights.
On the other hand, a mechanical mixture, in which particles of carbon appear as black specks, giving it a grey appearance, produces superior metals, that of a light grey colour being hard and brittle, and suitable for heavy castings, while the dark grey is weak, and only used for light ornamental work; but a medium quality in colour gives us a strong and durable metal, of weight-carrying properties, capable of resisting considerable pressure.
White cast-iron is made from forge and grey from foundry pigs, each of which is remelted and run into the moulds formed in sand according to requirement.
Good out-iron should be of uniform toughness, with an even surface and straight edges, to show even shrinkage, while a fracture of the best quality should be clean and bright, of a bluish-grey colour, and free from any air or sand-holes, the presence of which can easily be detected in the mass by the hollow sound given when the metal is hammered. .
All angles of castings should be gradual, to prevent uneven shrinkage, as a thin portion will naturally cool much faster than a thick one; and where these two are adjacent their junction must be gradual, so that the thin part shall not set first and act as a check on the other, and cause cracks. This point is of especial importance in castings for girders and cantilevers, on account of the great difference there is between the power of cast-iron to resist compression and tension, the former being 38 tons pet square inch, and the latter about 8 tons ultimate strength.
Fig. 414. Scale.
The "safe" load is about 8 and 1 1/2 tons per square inch respectively, or nearly in the proportion of 1 to 6; and when it is considered that both flanges of a girder are subject to the same amount of stress, the top one of compression and the bottom of tension, it is obvious that their respective sectional areas, to resist their strains, must be in the proportion of 1 to 6, otherwise there would be either a waste of metal or lack of strength in one or the other.
Therefore the section of a cast iron girder must be as fig. 414,.the bottom flange having six times the area of the top one; while the web, as the vertical portion is called, must make a gradual junction between the two at top and bottom.
"Girders," it will be understood, are bearers having supports at both ends, while a "cantilever," as fig. 415, has only one end fixed; and consequently the strains on the flanges are the reverse of those of girders (as will be explained in Chapter XXIV (Stresses. Definitions).), necessitating also the reversal of the section in cast-iron cantilevers, which should be, therefore, as shown in fig. 416.
"Cast malleable iron" results from the extraction of a proportion of the carbon out of cast-iron by means of embedding the casting in oxide of iron, and reheating it in an annealing or softening oven. This kind of iron combines the qualities of wrought and cast-iron, and is used for intricate work in wrought-iron specialities, being welded to the wrought-iron plainer portions, which cannot be done with cast-iron.