Case hardening is a method of hardening an iron alloy that generally has too little carbon in it to undergo sufficient hardening throughout the material. This is usually about 0.5% carbon at the lower limit. Additional carbon-bearing material is packed around the part, then heated until the low-carbon steel becomes a solid solution, allowing the surrounding carbon outgassing from the heated carbon-bearing material to dissolve into the steel to a degree, not unlike dissolving sugar into hot water. Being a solid, and a metal at that, carbon can only dissolve into a low-carbon steel to certain depth, typically no more than 100–200 micrometers. This gives a case-hardened part good surface hardness, but the interior of the part will not harden as well, or at all, depending on the material used.
Induction hardening isn’t really a chemical process like case-hardening, at least not in the same way. Induction hardening uses alternating frequency passing through a coil (the primary) to induce an electric current into a metal part (the secondary) usually placed next to or inside the coil. The concept is identical to how electrical transformers or a wireless charging system operates.
Induction hardening is usually a through-hardening process that doesn’t involve chemical enrichment. Case hardening is a surface-hardening process that explicitly relies on chemical enrichment by the introduction of carbon from an external source.
Case hardening was traditionally performed by packing a low-carbon steel part into a container surrounded by organic materials- horse hooves, urine, leather, charcoal- materials containing high concentrations of carbon, and other surface-hardening elements like nitrogen and phosphorous, that also increase surface hardness by creating hard nitrites and phosphates on the surface of the part.
Induction hardening is a modern industrial process that is usually done in a reducing or normal atmosphere.
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