Some product applications require surface properties of hardness and strength to resist penetration under high pressure and to provide maximum wear qualities. Where through- hardness and the maximum strength associated with it are not necessary, it may be ore economical to gain the need surface qualities by a "case-hardening " process. Case hardening involves a change of surface properties to produce a hard, wear resistant shell about a tough, fracture-resistant core. This is usually accomplished by a change of surface material chemistry. With some materials, a similar condition can be produced by a phase change of material already present.Case hardening may be more satisfactory than through-hardening in those cases where a low cost, low carbon steel with a hard shell may be used instead of a higher cost, high carbon or alloy steel need for through-hardening. The process is much less likely to cause wraping or cracking, and the product, because of its soft, ductile core, is less subject to brittle failure than a through-hardened product. Case hardening is often suitable for heavy sections that would require very special high alloy steels for through-hardening to be effective.
Ese depth measurement is sometimes checked by destructive methods: cutting the object, etching the cut surface and checking the cut depth with a measuring microscope. A faster and more usable method when knowledge is needed directly for service arts is by use of electromagnetic test.
Carburazing
Carbon is caused to diffuse into the steel by heating the material above its critical temperature and holding in the presence of excess carbon. The rate of penetration slows down as the depth increases, so that for large depths, relatively long periods of time are necessary. Carbon may be supplied from a Gas, Liquid or Solid environment, parts to be carburized may be packed in carbon or other carbonaceous material in boxes that are sealed to exclude air and then heated in a furnace for the required length of time, in a process sometimes referred to as "pack hardening".
Grain-size control necessary for best properties. Quenching from above the critical temperature will produce a hard martensitic structure in the high carbon surface material but will have little or no effect on the low carbon core. As in the case of most through-hardened steels, tempering is usually required to toughen the outside shell.
Flame hardening.
Surface must be heated above transformation temperature. Another case-hardening process that does not require a change of composition in the surface material is "flame hardening". This method can be used only on steels that contain sufficient carbon to be hardneable by standard heat-treating process.As in the case of carburizing, the surface may be then reheated to temper if toughness improvement. The depth of hardening is controlled by the temperature to which the metal is raised, by the rate of heating and by the time that passes before quenching.