For centuries, humans have used heat to transform and enhance the hardness of the metal. At sufficiently high temperatures, solid metal alloys liquefy. Since typically melting occurs closest to the application of a heat source, exterior surfaces usually become molten before the interior core. This fact enables manufacturers to use heat treatments to modify the grain structure and the chemical composition of ferrous metals.

For instance, a manufacturer may harden only the surface of a metal part or may uniformly “through harden” the entire workpiece. The disruption of the crystalline structure of solid metal through melting facilitates thermochemical manufacturing processes. Some common heat treatment techniques include quenching (cooling suddenly by immersion) and annealing (heating and then cooling).

Ancient people relied on heat to help shape metal. Blacksmiths tempered swords and horseshoes to harden metal surfaces, for instance.

The advent of the industrial age witnessed significant advances in surface hardening technology and fabrication processes. By the late 1800s, metallurgists widely appreciated the importance of thermochemical changes to metal.

Metallurgists rely on case hardening (also called “surface hardening”) through heat treatment to create hardened exterior ferrous metal surfaces while retaining soft, ductile cores capable of absorbing stress.

During induction hardening, a manufacturer heats a ferrous metal alloy for an extended period of time to treat the surface and then quenches the part. A variety of methods exist to accomplish this goal. For instance, some manufacturers electrify a water-cooled copper coil containing the ferrous metal workpiece for specified periods of time and then quench the part in an oil bath.

Despite an initial investment in manufacturing technology, this process may occur cost-effectively.

Manufacturers produce the threaded pipes utilized during oil field drilling using induction hardening.

Ferrous metal alloys and steel undergo carburizing by remaining for several hours in a carbon-rich environment at high temperatures. Carbon gradually diffuses across the molten surface of the metal during carburizing, hardening it. Today manufacturers control parameters such as the length of exposure and the temperature to accurately influence carbon penetration.

A blast furnace and access to sources of carbon enable the production of carburized metal components in high volumes.

Carburizing contributes to the production of ball bearings, gears and roller bearings.

The nitriding process resembles carburizing, except nitrogen diffuses over the surface of the metal alloy, not carbon. Nitrogen combines with iron or other alloys on the exterior to form nitrides. Typically manufacturers achieve this type of case hardening by heating steel at high temperatures (950 degrees Fahrenheit to 1050 degrees Fahrenheit) in the presence of ammonia gas for extended periods of time e.g. up to 100 hours.

The widespread availability of nitrogen helps “low tech” production facilities utilize nitriding.

Nitriding plays an important role in the generation of cam shafts, and valve stems. It also assists in the production of fuel injection pumps.

Manufacturers place metal work pieces in baths containing cyanide salt to prepare the surface to absorb carbon and nitrogen. They later quench the metal in a liquid, such as oil, and mechanically temper the surface to achieve a desired hardness level.

This production method proves comparatively inexpensive. It was widely popular around 1900. Although now limited in many locations due to its negative environmental impacts, cyaniding still occurs in “low tech” production settings.

Manufacturers sometimes produce bolts and nuts with the assistance of cyaniding.

This process resembles carburizing, except it occurs at lower temperatures and manufacturers infuse small quantities of nitrogen into the environment, as well. The carbon atoms will not penetrate as deeply into the metal, so carbonitriding offers more superficial case hardening.

Despite the reduced carbon penetration, the exterior often displays enhanced wear resistance since nitrides enhance the surface carburizing process. Carbonitriding occurs very economically in some metal parts fabrication environments.

Manufacturers frequently use carbonitriding to case harden small stamped metal components in the absence of a blast furnace.

This process closely resembles nitriding, except it produces shallower case hardening and requires considerably lower temperature ranges. Manufacturers typically place metal components in a liquid salt bath of alkali cyanate or within a proprietary gas mixture or inside an intense electrical field in order to thermochemically diffuse nitrogen, carbon and a small quantity of oxygen across the surface of a metal work piece.

Since work pieces undergo nitrocarburizing at lower temperatures, they may demonstrate reduced dimensional distortion during surface hardening in some manufacturing environments.

This process helps manufacturers fabricate surface hardened spindles, dies, cams, hydraulic piston rods and gears.

Through hardening will harden certain metal alloys uniformly, as opposed to case hardening (which hardens a metal part’s surface only). Both processes hold advantages and disadvantages. The decision to use one or the other largely depends upon desired engineering specifications.

Today case hardening creates metal parts displaying hard surfaces and soft, ductile interiors. This combination may allow a component to better withstand fatigue and high stress in a mechanical environment. Some common applications for case hardened metals include anti-friction ball bearings and engine gears.