Carburization
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Carburization (often also called carburizing) is the process by which carbon is introduced into a metal.
One does it to make the surface harder and more abrasion resistant. But because too great a concentration of carbon makes metal brittle and unworkable, metals are also often decarburized.
In earlier times one used a direct application of charcoal packed onto the metal for carburization, but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide or methane).