One of the most important applications of the Fick's laws of diffusion in metallurgical engineering involves the carburization of carbon steel. Carburization is used in the production of bearing races, gears, rock-drilling bits, etc., wherever a high-carbon hard case and a low-carbon tough core are desirable.
Carburization is a surface-hardening technique in which carbon is added to the surface of low-carbon steel at temperatures generally between 850 °C and 950 °C. In this temperature range, austenite — which has a relatively high solubility for carbon — is the thermodynamically stable phase (see the Fe-C phase diagram). Hardening is achieved when the high-carbon surface layer is quenched to form a hard, wear-resistant martensite on a tough, low-carbon steel core.
During quenching, the transformation from austenite to martensite begins at the boundary between the case and the core and moves outward the surface. Because of the constraints imposed by the core, the volume expansion that accompanies the austenite-to-martensite transformation leaves the surface layers in compression.
In carbon steels, the maximum hardness of the case after quenching is achieved at a carbon level of about 0.8 wt. %. If the carbon content is higher than 0.8 wt. %, the hardness of the as-quenched case decreases due to the presence of retained austenite.
Case depth of carburized steel is a function of carburizing time and the available carbon potential at the surface. When prolonged carburizing times are used for deep case depths, a high carbon potential produces a high carbon content at the surface, which may result in excessive retained austenite and/or the precipitation of carbides. Both retained austenite and carbides have adverse effects on the distribution of residual stress in the case-hardened part. Consequently, a high carbon potential may be suitable for short carburizing times but not for prolonged carburizing.
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