Nondestructive Quantification of Retained Austenite by X-Ray Diffraction
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 a-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.
In terms of crystal chemistry, austenite is an interstitial solid solution of carbon atoms in the g-Fe lattice, i.e the iron atoms are located on the f.c.c. lattice points and the carbon atoms occupy interstitial positions. The incorporation of C in g-Fe will result into an isotropic expansion of its lattice with the cubic symmetry being conserved. Maximum solubility is ~2.1wt% C. When C is dissolved in the b.c.c. lattice of a-Fe, the solid solution is called ferrite. The solubility of C in a-Fe, however, is very limited (max. 0.025wt% C). A higher uptake of C into the a-Fe lattice will cause lattice distortion since there is less room for interstitial voids in the b.c.c. lattice. An anisotropic distortion forces this interstitial solution, called martensite, to lower its symmetry from b.c.c. to b.c.t. (body-centered tetragonal). It is these solid solutions whose concentrations have to be determined quantitatively by X-ray powder diffraction method for a 'retained austenite' measurement.
While several protocols, like ASTM E-975-84 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.
Analysis Services Provided by CAMET Research, Inc.
CAMET Research, Inc. is an independent materials characterization laboratory, which has been providing X-ray analysis services since 1984. We determine retained austenite by the Rietveld Phase Analysis Method, employing NIST Standard Reference Material 485. The retained austenite analysis is performed in adherence to ASTM E-975-84 and to Soc. Auto. Eng. SP-453.
CAMET Research, Inc. - 6409 Camino Vista #F, Goleta, California 93117 - Tel. (805) 685-1665 - Fax (805) 685-9082 - E-mail: camet@camet-lab.com