Microstructural Characterization of Carburized Steels

摘要

Carburizing has been a popular method for obtaining a unique balance of case/core properties in steels for many years. Steels with low carbon contents are characterized by good toughness and good ductility, but they have low strength and poor 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 or high carbon content steel with the same surface hardness. Naturally, the carburizing and heat treatment processes must be carefully controlled and the results must be verified relative to manufacturing specifications. This usually involves case-depth measurement, surface hardness, and microstructural characterization. Depending upon 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.