by J.M.Drew St. Paul Publishing Company 1918
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Steel is simply iron to which has been added a very small amount of carbon. Carbon is one of the most common substances. We are most familiar with it in the form of charcoal. The diamond is almost pure carbon. There are other elements to be found in steel in very small amounts, but for all practical purposes if we take pure iron and add to it a small percentage of carbon we will have steel. Steel such as is used for making cold chisels contains less than 1 per cent of carbon.
In the usual process of making fine tool steel, rods of pure wrought iron are packed in charcoal in long iron troughs, or boxes, which are sealed with fire clay and placed in a furnace, where they are subjected to a high heat for several days. The heat is so regulated that the rods do not melt, but are kept near the melting point. During this process the iron absorbs some of the carbon from the charcoal, and is thus changed to steel. The proper amount of carbon is determined by drawing out one of the rods occasionally and testing it. The steel made by this process is called “blister steel,” for the reason that the surface of the rods is covered with small blisters. What is known as “shear steel” is made by heating these rods of blister steel and welding them together under a steam hammer, or by running them between rolls. “Cast steel” is made by melting blister steel in earthen pots called “crucibles” and pouring into molds forming ingots, which are afterwards heated and rolled or hammered out into bars.
It will easily be seen that cast steel is much better for all tools than blister steel or shear steel, because the melting insures a thorough mixture of the carbon so that all parts are sure to contain the same amount. In blister steel the outside portions of the rods contain a much greater percentage of carbon than the centers; and in the shear steel the welding process does not cause such a complete mixture as in the process of melting which cast steel undergoes. What is known as “mild steel,” or “machine steel," is a steel which contains so little carbon that it is practically of the same nature a good wrought iron, excepting that it is somewhat stiffer and more durable when subjected to wear. It is produced by several different processes, the most important of which are the Bessemer and the open hearth processes.
In the Bessemer process the pig iron is melted in a large crucible or converter, and air is forced through the molten metal from the bottom. This causes the carbon to burn out and leave a mass of nearly pure iron. Then a quantity of iron having a known percentage of carbon is added, thus giving any desired percentage of carbon to the steel. The open hearth process consists in melting pig iron in large furnaces built of fire brick and keeping it at a very high temperature, until the impurities are practically all worked or burned out, leaving a liquid mass of nearly pure iron, which is then poured into molds forming ingots. These ingots are afterwards heated and rolled the same as wrought iron.
These processes have been so cheapened recently that soft steel is largely taking the place of wrought iron for many use. It is now cheaper than the better grades of iron, and is taking the place of iron in almost all cases excepting where extreme softness, (for example, the making of rivets), is required. For this purpose nothing is better than Norway iron.
Iron is the most useful of all the metals: First, because of its great strength; secondly, because it is so easily forged or changed in shape when hot, but becomes rigid, and at the same time tough, on being allowed to cool. Wrought iron and all the milder forms of steel may be readily welded below a burning temperature. Steels containing a high percentage of carbon (in general, all tool steels) may be welded if protected from the air by borax or other flux which will withstand a high heat.