Retained austenite measurement
The amount of retained austenite can influence the material properties and performance of a manufactured steel component e.g. toughness, ductility, weldability, thermal expansion, stress corrosion cracking and magnetic properties.
The desired level of retained austenite depends on the application. Many critical products including bearings, gears and shafts may contain unwanted retained austenite caused by inadequate heat treatment. Retained austenite can slowly transit to martensite at room temperature. This can result in dimensional changes and cracks. The amount of retained austenite can be easily measured utilizing X-ray diffraction (XRD).
Why retained austenite should be measured?
Retained austenite values can be measured from raw material or a finished part. If values are measured from the raw material, rework and expensive scrap can be avoided.
When measuring raw material is not possible, retained austenite measurement can be used as quality control to separate good quality components from unstable ones. This will decrease warranty cases and help to avoid brand value loss.
What is retained austenite?
Face-centered cubic austenite and body-centered cubic ferrite are crystallographic phases of iron. In usual 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 is formed at high temperatures, a process known as austenitizing. After austenitizing steels are often quenched to transform the austenite into the much harder phase martensite. Retained austenite content is the volume fraction of austenite at room temperature inside the sample, which did not undergo 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.
How retained austenite can be measured?
Different methods exist to determine the volume fraction of retained austenite in steel-based materials.
XRD
Using the four-peak method, the integrated intensities of two ferrite and two austenite diffracted peaks are used to determine the retained austenite content in accordance with ASTM E975 standard.
- Two diffraction peaks from both phases are measured.
- Areas of the peaks are determined.
- Retained austenite content is analyzed from the peak area ratios.
Microscopy
Ferritic/martensitic and austenitic phases are inspected optically by means of a microscope. The volume of retained austenite can be estimated based on the visualized microstructure of the sample. Special methods like scanning electron microscopy (SEM) and transmission electron microscopy (TEM) make use of the absorption and scattering of electrons for imaging of retained austenite.
Magnetic methods
The different properties of ferromagnetic martensite and paramagnetic austenite are used to determine retained austenite by means of magnetic methods. For that purpose, the attenuation of the sample’s magnetic field in the presence of an external alternating magnetic field is measured inductively.
What are advantages of retained austenite measurement by XRD?
Retained austenite measurement by X-ray diffraction method is standardized and quantitative. It is non-destructive and measurements can be performed at any point in the manufacturing process.
Stresstech provides X-ray diffractometers and a wide range of options which makes it possible to measure various sized components from tiny bearing balls to meters long steel bars. With intuitive software the measurement can be performed by any operator. Prior metallurgical or diffraction expertise are not required.
With the latest X-ray diffraction technology (Xstress DR45) measurement time is less than five minutes for a material with 2% retained austenite content.
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