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 ~2.1 wt % 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. 0.025 wt % 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.


Portable X-Ray analyzer.
Image by courtesy of Stresstech Group.