Retained Austenite

Quantification of Retained Austenite

Austenite is formed in the hardening process of low carbon and alloyed steels. During rapid cooling from the stable austenitic region, the diffusion of carbon is sufficiently suppressed for hard martensite to form instead of the softer α-Fe + carbide aggregate. The rate of cooling, cooling temperature and  alloy composition will determine how much austenite will be 'retained' in the microstructure at room temperature. The presence or absence of austenite has significant consequences in crucial metallurgical applications such as roller bearings, tool steels, high strength steels etc.  To monitor austenite content, X-ray diffraction method has been found to be most effective and accurate over other methods, in particular for concentrations of 15% or less.

While several protocols, like ASTM E-975-03 and  Soc. Auto. Eng. SP-453 are in place to guide through the analysis of retained austenite using X-ray diffraction, these documents fail to address a rigorous analysis approach when high concentrations of carbides are present. Carbide diffraction lines do interfere with austenite lines and common carbide phases like Me23C6 (Me: alloy metal) in 440C steel will pose a challenge when present in concentrations >10vol%. XRD pattern analysis after Rietveld can overcome this difficulty since it is capable of deconvoluting peak profiles.

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Quantitative XRD of high carbide steel
XRD pattern of 440C alloy steel
XRD pattern of NIST SRM 485
CAMET Research, Inc.
An X-Ray Diffraction (XRD) Laboratory
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