A Treatise On The Principles And Practice Of Dock Engineering

TRON AND STEEL. 133 IRON AND STEEL. Cast iron, wrought iron, and steel are essentially the same substance in ■combination with different proportions of otlier constituents. The prin- cipal ingredient in this connection is carbon, and the following percentages are generally recognised as forming the distinctive compositions of the three classes of metal, viz. :— From '0 to ■! per cent for wrought iron. ,, -3 ,, 1'8 ,, „ steel. „ 2-0 „ 5-0 „ „ cast iron, Unfortunately, this quantitative differentiation is not susceptible of too strict interpretation, because other ingredients, besides carbon, exercise a powerful modifying influence upon the compounds. Their properties also depend upon the form in which the carbon is present—whether as specks of graphite, or free carbon, mechanically mixed and easily detected, or in such intimate chemical combination as to be indistinguishable from the metal itself. A practical distinction is founded upon the behaviour of a bar of metal under certain treatment, as follows :— Steel attains great hardness when suddenly cooled, from a high tem- perature, by immersion in water or oil. This process has no effect upon wrought iron. Steel which has been hardened in this way may be softened again, or tempered, by heating it and allowing it to cool gradually. Cast iron may be hardened, but it cannot be tempered. One drawback to the efficacy of these tests is that some modern steels, containing elements other than carbon and iron, are made softer, and not harder, by sudden cooling. A third attempt at drawing a distinction relies upon the results obtained in the testing machine, but this method is too artificial to be of any practical value. Altogether, it must be confessed that, while the differences in the physical properties of iron and steel are sufficiently marked to preclude any misconception, it is no easy matter to lay down any definite line of démarcation between the metals themselves. Steels containing less than ■5 per cent, of carbon form an intermediate class insensibly shading into, and gradually acquiring the characteristics of, wrought iron. Such steels are commonly designated mild steels, and they furnish the bulk of the material used for structural purposes. Those compounds containing a higher percentage than 1'5 imperceptibly merge into the class of cast irons. The influence exerted by carbon in modifying the physical charac- teristics of iron, while largely dependent upon the manner in which it ■enters into combination with it, may be stated in general terms as follows: