text: Diffusion hardening can be done in many different ways to achieve different hardnesses and different surface finishes on metal parts.  Some of the different diffusion hardening operations include: Carburizing, Nitriding, Carbonitriding, Nitrocarburizing, Boriding, Titanium-carbon diffusion, and Toyota diffusion.  While diffusion hardening is performed mainly on steel parts and carbon is mainly the element used for diffusion, diffusion hardening can also be performed with other diffusion elements and with other metals.  In nitriding, nitrogen is diffused into the surface of steel, but can also be used with metals such as Aluminum, Chromium, Molybdenum, and Vanadium.  Besides metals and diffusion elements used, diffusion hardening processes differ in the temperature required for diffusion, the phase of the diffusion element, and additional treatments such as quenching and tempering.  These different factors greatly affect surface finish and dimensional accuracy of a part.  A quenched and tempered part does not have the same dimensional accuracy as a part that has not undergone such a process.  Also, they can affect the efficiency of the overall process.  In carburizing, the carbon can be in any of the solid, liquid, or gas phases.  Although using carbon in the solid phase is usually the safest and easiest of these to work with, the process is difficult to control and the heating is inefficient.  All these things must come into consideration when choosing a diffusion hardening process.