Heat treatment is a technology that has been developed in metallurgy from ancient times. Here, Surface Heat Treatment which is focused on surfaces among the heat treatments.

Some fixtures and tools do not reach the hardness required by the normal hardening and annealing processes, thus they will have short product lives. In order to solve this problem, surface heat treatments are applied.

Summary of surface heat treatment

Surface heat treatment is defined by JIS as "Heat treatment processes to give required properties to metal product surfaces", and can be classified into Surface Hardening, and Thermal Diffusion Treatment as shown in [Table 1]. Both use heating to apply surface treatments, and inevitably, atomic diffusion phenomena are occurring on the surface of the treated material.

Classification of surface heat treatments:

Surface hardening (High energy hardening) Flame hardening Induction hardening Electron beam hardening Laser hardening Thermal diffusion treatment Non-metal diffusion Carburizing (Carbon) Carbonitriding (Carbon + Nitrogen) Nitriding (Nitrogen) Soft nitrocarburizing (Nitrogen + Carbon) Sulfurizing (Sulfur) Sulfonitriding (Nitrogen + Sulfur) Boronizing (Boron) Stem treating/Homo-treatment (Oxygen) Metal diffusion Chromizing (Chrome) Aluminizing (Aluminum) Carbide coating (Vanadium, Chrome)

In general, metals are solid solutions, and ceramics are compounds. A solid solution is where multiple elements are blended into a single phase. A compound is where multiple elements do not blend but has changed into a heterogeneous material.
There are two types solid solutions, the Interstitial solid solution where atoms intrude into the other atomic space, and Substitutional solid solution where the atoms of each other have been replaced by the other.

The interstitial atoms that constitute the interstitial solid solutions must be small enough, and are non-metal such as nitrogen and carbon. Both elements of the substitutional solid solutions are metal.

The compounds are formed by non-metals diffusing on metals. For instance, oxygen (O) diffuses on titanium (Ti) forming Titanium oxide (TiO2), Titanium carbide (TiC) if its carbon (C).