These treatments, numbering more than a dozen, impart a hard, wearresistant
surface to parts, while maintaining a softer, tough interior, which
gives resistance to breakage due to impacts. Hardness is obtained through
quenching, which provides rapid cooling above a steel’s transformation
temperature. Parts in this condition can crack if dropped. Ductility is obtained
via tempering. The hardened surface of the part is referred to as
the case, and its softer interior is known as the core. Four of the more
popular surface hardening treatments are carburizing, carbonitriding, nitriding,
and ferritic nitrocarburizing.