Introduction and Perspectives

Carbon case hardening, through natural evolution, commercialism, and economics, has become a process for which the possible number of variables is so large that it is hardly likely that any two companies will process exactly the same. There will always be some difference in choice of materials, equipment, or technique, and there will often be differences in the quality of the product. There may even be conflict of opinion regarding what is good practice and what is bad, and what is a valid test and what is meaningless. For each component treated, there is an optimum material and process combination, but who knows what this is for any given component? Most conflicts stem from there being too great a choice of materials or process variables and from the wide range of components that are required to be case hardened. Despite all this, what the carburizing processes have in common is that they produce at the surface of the component a layer of carbon-rich material that after quenching, by whichever technique, should provide a surface that is hard. Regrettably, this is no indication that the casehardening process has been successful. Additional microstructural features may exist along with, or instead of, the aimed-for martensite, and these indeed can significantly influence the properties of the component, thereby affecting its service life. The microstructural features referred to are internal oxidation, decarburization, free carbides, retained austenite, and microcracks in the martensite. Further modifications to the martensite in particular can be effected by tempering, and the proportions of austenite and martensite can be altered by subzero treatment after quenching. Cold working by either peening or rolling can modify the surface microstructures and have significant bearing on the life of the component, as too can surface grinding. One must not overlook the value of the microstructure and properties of the core or of the influence of inherent features such as microsegregation, cleanliness, and grain size. The aforementioned structural variants are the subject of this review, and where possible, examples of their effect in terms of properties are given. Those properties mainly referred to are bending-fatigue strength, contact-fatigue resistance, hardness, and wear resistance. These properties were chosen because it is to promote one or more of these properties that the carburizing treatment is employed. A gear tooth is a good example in which each of these must be considered. Some significance has been placed on the residual stresses developed during carburizing because these are additive to the applied stresses.