A Treatise On The Principles And Practice Of Dock Engineering

136 DOCK ENGINEERING. Classification of Iron.—A description of the varions processes employed in the manufacture of iron and steel is quite beyond the scope of the present work. A brief classification of mercantile products, with their most note- worthy features, is all that can be attempted. Pig iron is the name given to the coarse bars of unpurified metal run off from the blast furnace. These are roughly divisible into two kinds— those having a dark grey fracture, due to a large proportion of uncombined carbon, and those having a silvery fracture, with very little uncombined carbon. The first are distinguished as foundry pigs, being particularly suitable for castings, and the second as forge pigs, being only adapted for conversion into wrought iron. Special varieties of pig are generally assigned to the manufacture of steel. For what is known as the acid process (see below), the metal must be comparatively free from phosphorus and sulphur, such, for instance, as the pig produced from hæmatite ores. By the basic process much impurer ores, containing a large proportion of phosphorus, can be utilised, but the product is scarcely so satisfactory. Gast iron is obtained by remelting pig iron to eliminate its impurities. The process may be repeated with beneficial results as many as a dozen times. After that point has been reached the metal begins to deteriorate. According to Sir William Fairbairn, the transverse strength and elasticity decrease after the twelfth remelting, and the compressive strength after the fourteenth. Cast iron comprises three classes—grey, mottled, and white cast iron, following the structural nature of the pigs from which they are cast. The first contains a profusion of carbon in graphitic specks, the last is free from uncombined carbon, and the second represents an intermediate condition. Chilled iron is a product of casting in which the surface of the metal is allowed to corne into contact with a cold substance, with the result that it becomes hard and brittle while the interior remains tough. Wrought iron is iron from which all carbon has been eliminated as far as practicable. It is developed in a pasty mass which is much improved by cutting, piling, and rolling. Hence there are three qualities, each an amelioration on the preceding by a repetition of the process—viz., puddled bars, merchant bars, and best bars. Steel is capable of production on two systems (1) by eliminating the carbon from pig iron until the requisite proportion is left, and (2) by adding a definite amount of carbon to wrought iron. The cementation process based on the second system produces, first, blister steel of very unequal quality, and secondly, shear steel, in which the metal is rendered more homogeneous by piling and rolling. Cast steel is obtained by melting, in crucibles, wrought iron which has been previously bedded in charcoal powder in a furnace. The Bessemer process yields a steel of that name, which is due to the combustion of the carbon contained in suitable pig iron, by means of a volume of air forced at high pressure through the molten mass, leaving