Often manufacturers of steel components are looking for differing properties in a single component. A good example of this is can be found in case hardening grades, where a softer metal is required for forming and bending, but harder steel is required for resistance to wear and cutting. This can be achieved with case hardening grades. These typically have low to medium carbon content, and are supplied as plain carbon manganese or alloy grades. Case hardened steel is usually formed by diffusing carbon and/or nitrogen into the outer layer of the steel at high temperature. The carbon combines with the steel to make it nearly glass-like in its hardness. The core of the metal stays soft. This gives you a piece of metal that you cannot cut with a saw, but also will not shatter.
Case hardening produces a hard, wear-resistant surface or case over a strong, tough core. The principal forms of casehardening are carburizing, cyaniding, and nitriding. Only ferrous metals are case-hardened. Case hardening is ideal for parts that require a wear-resistant surface and must be tough enough internally to withstand heavy loading. The steels best suited for case hardening are the low-carbon and low-alloy series. When high-carbon steels are case-hardened, the hardness penetrates the core and causes brittleness. In case hardening, you change the surface of the metal chemically by introducing a high carbide or nitride content. The core remains chemically unaffected. When heat-treated, the high-carbon surface responds to hardening, and the core toughens.
More recent developments with Boron as an alloying element have replaced some of the older grades, which are no longer as readily available but would include:-
Case Hardening SteelsEuronorm France Germany UK USA EN 10132 - 2 NFA 37-503 DIN 17210 BS 1449 ASTMC10E XC10 CK10/C10 CS12 SAE 1010C15 XC18 CS20/CS17CS3016MnCr5 16MC5 16MnCr5 - -