Abstract
A new application of a centerless x-ray diffractometer was proposed that goes beyond residual stress measurements during the regular operation of such equipment. During depth residual stress measurement, it is possible to fit the measured interference function of the ferrite (211) with a different number of curves as a function of depth. The different curves corresponded with the appearance of ferrite fractions with different grain sizes and concentrations in ferrite, perlite, bainite, or martensite. Accordingly, different microstructures were non-destructively detected through the thickness of the carburized layer, and a good correlation was found with results from metallography and hardness testing. This novel method was validated on solid and gas carburized samples, but more work is needed to apply it to other microstructures.
Highlights
Carburizing is a case-hardening process in which carbon is diffused into a thin steel surface layer at elevated temperatures, while the steel is typically austenitic
The decreasing nature of the macrohardness data is consistent with the variation in the amount of martensite determined by x-ray diffractometers (XRD) or image analysis
There is no direct correlation between hardness and martensite volume fraction because one but several types of microstructural features are formed besides the martensite, and their quantity
Summary
Carburizing is a case-hardening process in which carbon is diffused into a thin steel surface layer at elevated temperatures, while the steel is typically austenitic. Carburizing is usually followed by a quenching and tempering process to produce tempered martensite at the surface (case depth), under which there is a more ductile bainitic core. The carbon gradient from carburizing creates a gradient in the steel surface hardness, strength, and wear resistance of the component. Hosford (Ref 1) and Reti (Ref 2) have shown that the main advantage of carburizing is that it produces compressive residual stress on the surface of the component. Especially wear and fatigue resistance, can be significantly improved by the well-controlled residual surface stress state of various components. As the magnitude of the tensile residual stress increases, the rolling contact fatigue life decreases linearly
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