Microstructural Characterization of Carburized Steels

*Member of ASM International arburizing has been a popular method for obtaining a unique balance of properties in steels for many years. Steels with low carbon contents are characterized by good toughness and good ductility, but low strength and low wear resistance. By locally increasing the carbon content at the surface, followed by heat treatment to produce a very high hardness in this layer while maintaining a lower hardness in the core, the “best of both worlds” can be obtained—a hard, wear-resistant surface with good fatigue strength and a tough, ductile core to inhibit the growth of any cracks that might form at the surface. Such a steel will be tougher than a through-hardened medium carbon content steel with the same surface hardness. Naturally, the process must be controlled and the results verified according to specifications, which usually involves case-depth measurement, surface hardness, and microstructural characterization. Depending on the application, retained austenite may be either desired to some amount or may be considered undesirable. Grain boundary carbide networks are universally undesirable. Surface decarburization is also undesirable. This paper gives examples of the use of metallography to assess the quality of carburized specimens.