Surface hardening

Several methods are available to provide surface hardening of components. When steels having a carbon content of 0.45%C and above are involved, surface hardening can be achieved by the use of induction or flame hardening methods. Low carbon steels, having carbon contents around 0.15%C may be case hardened by carburizing and hardening, carbonitriding, nitrocarburising or nitriding.

When it is necessary to restrict surface hardening to a localised portion of a component’s surface it is possible to choose from several methods. If the end of a shaft or similar shaped component is the only area to be surface hardened, flame or induction methods can be employed with steels having 0.45% C and above. Case hardening steels can be treated in salt baths by immersion of the end only. Alternatively, the component can be carburised all over, annealed for machinability and then the surface to be kept soft can be re-machined to remove the carburised case, leaving the remaining carburised area to be hardened by re-austenitising and quenching. Another method involves carburising the component all over and induction or flame hardening the restricted area required to be hard. Another technique involves the use of electroplating (a fine grained copper deposit is necessary) to prevent carburising, or alternatively proprietary ‘stop-off’ paints containing copper salts may be used, which inhibit the diffusion of carbon into the steel, or those containing tin salts for similar use in nitriding.