Many engineering applications require high surface hardness to resist wear, while complex service conditions demand adequate core strength and toughness to withstand impact and cyclic loading.
To obtain this combination of properties, two general approaches are commonly used:
1) Modifying the chemical composition of the surface layer before or after quenching and tempering. Common methods include carburizing, nitriding, cyaniding, and carbonitriding.
2) Hardening only the surface layer by localized heating followed by rapid quenching. The most widely used techniques are flame hardening and induction hardening.
Carburizing:
Carburizing is a thermochemical process in which carbon is diffused into the surface of a steel component to a controlled depth by heating it in a carbon-rich environment. The hardened surface layer, known as the case, provides high wear resistance while maintaining a tough, ductile core.
Case depth depends on the carbon potential of the medium, processing temperature, and time at temperature. Steels suitable for carburizing generally contain low carbon levels, typically below 0.30 percent, to ensure adequate core toughness after quenching. Carburizing temperatures usually range from 1550°F to 1750°F.
Three carburizing methods are commonly used:
1) Liquid carburizing, where steel is heated in molten cyanide salts, allowing both carbon and nitrogen absorption.
2) Gas carburizing, which uses a controlled gaseous atmosphere to precisely regulate carbon content in the case.
3) Pack carburizing, where steel parts are sealed with solid carbonaceous material in a gas-tight container and heated.
After carburizing, parts may be quenched directly or cooled and reheated to the austenitizing temperature prior to quenching. Selective carburizing can be achieved by masking areas that should remain unhardened using copper plating or commercial stop-off compounds.
Nitriding:
Nitriding is carried out by heating steel parts at temperatures between 900°F and 1150°F in an ammonia-based atmosphere. This process forms an extremely hard and wear-resistant surface layer through the formation of alloy nitrides.
The presence of strong nitride-forming elements such as chromium, molybdenum, or aluminum is essential. A key advantage of nitriding is minimal distortion, allowing parts to be fully quenched, tempered, and machined prior to treatment.
Cyaniding:
Cyaniding is a rapid surface-hardening process in which steel components are heated in a molten cyanide bath at temperatures just above the transformation range, followed by quenching. It produces a thin, hard case suitable for light-duty applications.
Carbonitriding:
Carbonitriding is similar to cyaniding but is performed in a controlled gaseous atmosphere containing hydrocarbons and ammonia. Both carbon and nitrogen diffuse into the surface, producing a hard case with improved wear resistance.
Processing temperatures typically range from 1425°F to 1625°F for parts that are quenched. Lower temperatures may be used where quenching is not required.
Flame Hardening:
Flame hardening involves rapid surface heating using a high-temperature gas flame, followed immediately by controlled quenching. Only the heated surface transforms to martensite, while the core remains tough and unaffected.
Steels used for flame hardening typically contain 0.30–0.60 percent carbon and sufficient hardenability for the required case depth. Immediate tempering is required to relieve residual stresses and reduce brittleness.
Induction Hardening:
Induction hardening uses a high-frequency alternating current to rapidly heat the surface of a steel component. The depth of hardening is controlled by current frequency, heating time, and material thermal properties.
Quenching is usually achieved by a water spray delivered through or near the induction coil. In some cases, oil quenching is used. Induction hardening offers excellent repeatability, precise control, and minimal distortion.
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