WROUGHT IRON Wrought iron, as compared to the total tonnage of steel, is an unimportant commodity. It amounts to less than 3% of the total production of steel. But it is important for its unique combination of physical properties, enabling it to survive since the time 75 years ago when it was the principal metal of engineer ing construction.
Puddling Process.—If about 600 lb. of pig iron and consiaer able iron ore are placed on the hearth of a small reverberatory furnace and melted, various reactions occur between the iron oxide from the ore and the impurities in the droplets of pig iron. The first reactions to occur may be simplified as follows: The iron thus reduced from the ore enters the pool of metal, and the silicon and manganese oxides join with the surplus ore to form a highly basic slag. This slag has the ability to absorb any phosphorus oxides which result from reactions between the phosphorus alloyed in the metal and the surplus iron oxide in the slag. The workmen stir the slag and iron together, an operation which is known as puddling, after the silicon and manganese have been substantially eliminated from the metal the carbon content begins to go down. The reaction may be written as follows: Carbon monoxide gas bubbling upward causes the bath to boil. Removal of all these alloying elements leaves practically pure iron, of a much higher melting point. Toward the end of the process, therefore, the bath is converted into a metallic sponge of white hot, pasty iron, filled with liquid slag. This is cut into zoo lb. balls by the furnaceman, surplus slag squeezed from the dripping mass, and the resulting balls rolled into flat muck-bars. The metal is hot enough to weld together satisfactorily, and the process is accompanied by a great spray of slag sparks, squeezed out by the rolls. Muck-bars are cut to short lengths, arranged and wired together in piles, heated to a welding heat and re-rolled into marketable shapes.
Nature of Ferrite.—From a knowledge of the manufacturing process it may be understood that commercial wrought iron is essentially pure iron containing a greater or lesser number of slag particles.
The chemical changes which occur in the process of refining are thus indicated : About 3% of the best wrought iron by weight is slag.
A view of a polished section through a microscope shows the structure of wrought iron (Plate III., fig. 9). The mirror-like metal appears white in the photograph; the stony lustre of the slag particles appears grey. The slag itself is not a simple glass, it is a high iron silicate containing more or less manganese and phosphorus and frequently develops well crystallized minerals.
Slight etching of the polished surface under examination develops characteristic markings; the metallic portion seems to consist of a number of closely fitted or interlocking grains. If one of these grains were picked out, it would have an irregular surface; nevertheless it is undoubtedly true that its inner structure is that of a true crystal—that is to say, the ultimate atoms are arranged in regular rows and ranks, in geometric order. Such tiny crystals of substantially pure iron are called ferrite.
Properties of Wrought Iron.—Wrought iron has superior properties for forge welding, because at high heats the slag particles melt and spread into a film protecting the metal from atmospheric oxygen. The purity of the iron is responsible for its toughness, resistance to shock and bending. It also resists atmospheric and aqueous corrosion, either by virtue of purity of the metal or by virtue of the slag inclusions, or for some other reason. These special properties cause it to be in demand for pipe, chain cable, boiler stays, sheet iron and blacksmith's bar iron. For some of these purposes a variety known as charcoal iron (q.v.) is preferred. Wrought iron is moderately strong in tension, developing from 45,00o to 55,000 lb. per sq.in. ultimate strength/. Furthermore it is a soft metal—Brinell hardness measuring 75 to 80. Likewise the production of big masses of it is tedious, and puddling requires a large amount of exhausting labour from skilled workmen. These facts all combined to hamper competition with the harder and stronger steel—the Bessemer's process showing how to make the latter in great quantity, rapidly and almost automatically. In 1883 7o% of all the pig iron made in England was made into puddled bar; in 1897 only 5%.
Aston's Process.—Mechanical puddling furnaces have been proposed and used to some extent, but the manufacture of metal in small batches is not in keeping with modern mass production. A more promising development consists of the process established in 1927 in America through James Aston. Knowing the essential characteristics of the ferrite and slag constituting wrought iron, he produced pure iron in a Bessemer converter, following the regular steel-making practice, and poured this iron into a pool of synthetic slag, which had been melted in a large cupola. Strong evolution of dissolved gas caused the metal to granulate into very fine particles; the resulting spongy mass of iron and slag was withdrawn, squeezed and rolled in the conventional manner.