Austenite is a face-centered cubic (FCC) phase present in steel at high temperature. Upon cooling, most of the steel is transformed into ferrite (a body-centered cubic (BCC) phase) or martensite (a body-centered tetragonal (BCT) phase). Depending on the cooling parameters and alloying content, some percentage of the steel (typically 0-40%) will remain as austenite. X-ray diffraction (XRD) can accurately measure retained austenite concentrations as low as 0.5%. To characterize the concentration of retained austenite in a sample, four x-ray diffraction peaks are collected by the instrument: two for the ferrite/martensite phase and two for the austenite phase. A comparison of the intensities of the four peaks yields the volume percent concentration of retained austenite in the sample.
The amount of retained austenite present can play a significant role in the performance, dimensional stability, and longevity of a steel component. Its presence can be either harmful or beneficial depending on the application. For example, if retained austenite is detected in bearings or gears made of high-carbon steel, it could have a positive effect by improving the fatigue life and fatigue strength in bending or other complex loading situations. The presence of retained austenite may also be detrimental, as it could decrease the load-carrying capacity of martensitic/austenitic structures, reduce their resistance to scuffing and indentation, and increase the likelihood that they will experience grinder burn and heat checking during grinding.
Proto’s AXRD Benchtop diffractometer and iXRD/LXRD residual stress measurement systems can be outfitted for retained austenite measurement.
We have a range of options:
Low cost: manual four-peak collection
Fully automated: four-peak collection
High speed: multiple-detector, simultaneous four-peak collection
All measurements are done in compliance with ASTM E975.