In this process, the surface of the steel that is treated has carbon added to it. The result is an outer layer of steel that is hard, with an inside core that remains softer and pliable.

Case hardening causes the top layer of steel to be more durable than the inside portion. This makes the steel better able to resist corrosion. The steel also has more protection against abrasions, which cause it to wear down.

Another name for case hardening is 'surface hardening'. The hardening process is complete when the now higher carbon outer layer is 'quenched'. 'Quenching' occurs when hot steel, submerged in cold water (or other liquid), cools. This quenching forms martensite. Martensite is a hard solid solution of iron and carbon. The final product is hardened steel. The top layer is wear and fatigue resistant martensite infused onto a low-carbon core.

The application of the case hardening process usually occurs after the steel assumes its final shape. Application of this process can occur before, though. If steel bars need their hardening element content increased, the application takes place earlier. The resultant hardened steel bar can then receive further modification through other processes.

Steel treated this way can have the carbon introduced to the top layer in different ways. This is done through carburizing. The different ways are gas, vacuum, plasma, salt bath, and pack carburizing. Through these methods, carbon transfers into the original steel, thus strengthening it at its outermost layer.

Gas, vacuum, and plasma carburizing involve the use of gas to infuse carbon into the steel.

Salt bath carburizing infuses the steel with carbon using liquid. Pack carburizing infuses the steel through solid compounds. Case hardening this way involves 'packing' a high-carbon compound around the steel.

A typical case hardening process for mild steel has these steps:

* The steel is heated. Only a portion of the steel may require case hardening so the application of heat may be to one area.

* After heating, submersion of the steel in a high carbon, hardening compound takes place. The steel then goes through a cooling stage.

* The steel receives a heat application once again. It becomes red-hot. The next step is submersion again; this time in cold water.

* The result of the process is an exterior layer, which is hard and abrasion resistant. The interior steel remains softer. This process can repeat to add additional hardness to the outer layer.

Some products that utilize case hardened steel are car camshafts, firearms, and screws and fasteners. Smaller items often receive case hardening through repeated applications of heat. After this heating, submersion in a high carbon environment occurs.

Steel that undergoes case hardening usually has carbon content in the 0.2% area. The hardened outer layer increases to around 0.8% to 1.0% in carbon content through the infusion of extra carbon. However, 0.9% is ideal as the martensite of the top layer becomes too brittle if higher than that.

Case hardening of steel is a process that results in steel that lasts longer. This helps in cost reduction, as steel that is case hardened does not need replacing as often. The case, for case hardened steel, in products that require durability, is a strong one.