This explanation of hardening, which is known as the “allotropic theory” is not universally accepted, although it is difficult to find a better or more complete explanation of the remarkable phenomena involved. However, the fact remains that the great accomplishments which have been made by the men of science and understanding have caused remarkable results to have taken place in the manufacturing world of today and the fine and obscure lines which these patient and careful laborers are continually drawing upon the map of knowledge are doing much to make the world a better and safer and more wonderful place in which to live. CHAPTER V. THE GENERAL EFFECT OF THE MORE IMPORTANT ELEMENTS IN TOOL STEELS. We know that all metals of engineering nature are crystalline in character, that is, the crystals form when the metal solidifies. If these crystals were free it would be easy to determine definitely just what properties the metal would have. However, the crystals are not free, but exist in the steel in combination with many other types of crystals. This results in many complicated and complex possibilities in the finished product, and will bring us presently to the subject of “Alloy Steels”. CARBON STEELS. Carbon Steels are those which do =not= contain enough of any element =other= than carbon to materially affect the physical properties which the steel will have when hard. Carbon is one element used above all others by manufacturers in getting required physical properties. An increase of one hundredth of one per cent (.01%) gives a tensile strength of about one thousand pounds per square inch, but even this amount of carbon also regularly decreases the ductility of the finished product. When steel is heated red hot and plunged into water, the carbon in the metal unites with the iron in some peculiar way so that it produces a compound of extreme hardness. If the steel contains nine-tenths of one per cent (.90%) of carbon, a sharp point so quenched will almost scratch glass. With one per cent (1.00%) of carbon it reaches nearly its limit of hardness. Now carbon steels with this percentage carbon can be used for some of the harder tools, which do not require much ductility or toughness, but with higher carbon contents than this percentage, the brittleness increases so fast that the usefulness of the metal is decidedly limited. Therefore, when the steel must meet requirements other than just that of hardness, such as, strength, ductility, toughness, resistance to repeated shock, “red hardness”, etc., then it is necessary to resort to other means and combinations for obtaining the required needs. It is to be remembered that such methods and combinations will materially increase the cost of the final product. ALLOY STEELS.