Properties Of Steel

Properties Of Steel

When steel first came into practical use, its distinguishing characteristic was its ability to harden if heated to a red heat and cooled suddenly, as in water or oil. Present methods of steel making have, however, brought out a product of iron containing too little carbon to harden when cooled suddenly, yet its composition differs from the old form of steel only in containing less carbon.
Primarily, the differences between wrought iron, the several grades of steel, and cast iron are due to the per cent of carbon in each class of metal, and for this reason steel is said to occupy a place between wrought iron and cast iron. However, the processes of manufacture give steel a composition and a molecular structure which affect its properties aside from this simple relation. The properties of steel depend primarily upon the carbon it contains, influenced by the kind and quantity of the other ingredients (or impurities, they may be called), and further influenced by the cooling of the steel from its molten state. This last-named influence determines the size and composition of the crystals which steel assumes upon cooling.
Despite the somewhat complex conditions determining the properties of steel, the grades of steel are classed according to their hardness due to their contained carbon. The higher the per cent of carbon, the greater the strength and brittleness, and the less the elongation before breaking. The grades of steel merge so gradually one into another that only two classes are distinguished, viz., mild steel which will not harden when suddenly cooled, and high-carbon steel which will harden when suddenly cooled from a red heat. This property of hardening begins to show when the steel contains .25% of carbon though is not of much practical use in hardening tools until the carbon has reached about .75% in the steel.
A quick means of showing whether a piece of iron is wrought iron or steel is to place it in a somewhat dilute mixture of sulphuric and hydrochloric acids, after it has been cleaned to show a metallic surface. Steel shows a granular and wrought iron shows a fibrous structure after a few minutes action of the acid.
The conditions determining the properties of iron and steel can only be touched upon lightly here, and the pursuit of this subject is in itself a special study.
III. The Manufacture of Wrought Iron.