Casehardening (eBook, PDF)

Casehardening is the process of increasing the carbon content of the surface of steel comparatively low in carbon, so that it can be hardened by the usual method of being heated to the hardening temperature and quenched in a cooling medium. The term casehardening, by it self, implies the hardening of the surface or skin of an article, and in order to fully understand the process and its object, it is necessary to briefly consider the facts and laws upon which it is founded. Car bon has a very great affinity for iron and combines with it at all temperatures above a faint red heat. Advantage was taken of this fact in the production of steel by cementation, an old process which consisted of rolling wrought iron into thin strips and then placing these in boxes with some material containing a fair proportion of carbon. These boxes were then heated to a very high temperature and the carbon was gradually absorbed by the iron. The process of casehardening is, in fact, only an improvement on this old cementation process usednin times past for making steel from wrought iron. The steel is heated in packing boxes in the presence of a carbonaceous material and when the surface of the steel has absorbed enough carbon so that it will harden the same as high carbon steel, it can be quenched in oil or water, according to the requirements. For many purposes, in machine work, articles are required which must have a perfectly hard surface and yet be of such internal structure that there is no chance of breaking them when in use. In many instances, this result can be obtained better by using casehardened mild steel than by using high-class crucible steel. For example, in making axles, cups, cones and many similar parts for bicycles, it is extremely difficult to obtain perfect hardness combined with great resistance to torsional, shearing or bending stresses. For such purposes, nothing meets these requirements so well as do articles which have been casehardened. A great improvement has been made in casehardening processes during the last few years. The advance was begun with the develop ment of the bicycle industry; and the necessity for casehardened parts of the highest quality in automobile manufacture has caused a still further improvement in this field.