Case Hardening
Case hardening processes harden only the exterior of the steel part, creating a hard, wear resistant skin (the "case") but preserving a tough and ductile interior. Carbon steels are not very hardenable; therefore wide pieces cannot be through-hardened. Alloy steels have a better hardenability, so they can through-harden and do not require case hardening. This property of carbon steel can be beneficial, because it gives the surface good wear characteristics but leaves the core tough.
Read more about this topic: Carbon Steel
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