Austenite has FCC crystal structure which allows dislocation motion more comparing with ferrite, pearlite, bainite and martensite and increases the toughness due to this movement in the crystals. However, in accordance with the needs of customer with respect to their processes and products, it should be measured and retained austenite ratio in the part should be determined.
Surface-centered cubic austenite and volume-centered cubic ferrite are the crystallographic phases of iron. In normal hardened and tempered low-alloy steel, both phases coexist as α-Fe (or ferrite) and γ-Fe (or austenite). During the heat treatment of steel, austenite forms at elevated temperatures, a process known as austenitizing. After austenitizing, steels are usually quenched to transform austenite into the much harder phase martensite.
Crystal structures of α-Fe and γ-Fe.
The retained austenite content is the volume fraction of austenite at room temperature in the sample that has not undergone the transition to martensite. The amount of retained austenite depends on the chemical composition of the steel alloy as well as the conditions of the heat treatment.
There are different methods to determine the volume fraction of retained austenite in steel-based materials. These can be listed as XRD, microscopy and magnetic methods.
Using the four peak method, the integrated intensities of two ferrite and two austenite diffraction peaks are used to determine the retained austenite content according to ASTM E975.
Residual austenite measurement by X-ray diffraction (XRD) is standardized and quantitative. It is non-destructive and measurements can be made at any point in the production process.
The portable X-ray diffractometer offers a wide range of options, making it possible to measure components of various sizes, from small bearing balls to meter-long steel bars.
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