Malvern Panalytical offers a push-button solution according to the ASTM E975 norm, for the quantitative determination of retained austenite using X-ray diffraction. The method provides reliable results and is compliant with international normalization. The dedicated retained austenite software module is easy to use and can be run also by less experienced operators.
In the tool-steel industry, excessive retained austenite - produced as a consequence of the hardening process to which steel is subjected - is universally considered to be detrimental. Therefore, in order to improve the quality of steel it is crucial to carefully quantify the retained austenite phase.
In the tool-steel industry, excessive retained austenite - produced as a consequence of the hardening process to which steel is subjected - is universally considered to be detrimental. Therefore, in order to improve the quality of steel it is crucial to carefully quantify the retained austenite phase.
The retained austenite (RA) phase and the main martensite phase can both be measured and distinguished using X-ray diffraction (XRD). Non-overlapping peaks of both phases are obtainable in the diffraction pattern. According to the ASTM E975 norm, quantitative analysis is done using the ratio of the peak areas without the need for calibration with reference samples.
Malvern Panalytical offers a push-button solution according to the ASTM E975 norm, for the quantitative determination of retained austenite using X-ray diffraction. The method provides reliable results and is compliant with international normalization. Typical measurement times are in the range of 5 to 15 minutes. The dedicated retained austenite software module is easy to use and can be run also by less experienced operators.
Malvern Panalytical offers its expertise to provide a fully automatic analysis solution. The solution requires a full pattern or partially measured scan of a steel sample as input.
Malvern Panalytical provides a ready-to-use solution. It can be further customized based on the analytical requirements. The solution provides a control file for the analysis of retained austenite in steel samples according to the ASTM norm E975 using total pattern fitting method. It is setup for standard cubic phases. The measurements must be performed with Co radiation. Results are reported in a popup window and in an Excel readable table format. The solution gives texture and low austenite warnings. Instructions how to install the solution are delivered with the control files as well as example files.
The use of multiple peaks for each phase is preferred above using single peaks. This approach makes the analysis more reliable and also provides the possibility to assess the texture (non-randomness of the phases) present in the sample. The pattern of martensite in cubic form or martensite with low tetragonality combined with relatively broad peaks can be adequately described with the cubic ferrite phase (Table 1).
The use of Cu radiation is not recommended. There are two major disadvantages: X-ray fluorescence and limited penetration depth. The fluorescence cause a high background signal that will obscure the diffraction peaks. The ASTM norm indicates that a diffracted beam monochromator should be used. The penetration depth is restricted to a few microns only and about 5 times less compared to other radiations (Figure 1) and less reliable. There is no counter measure possible for this disadvantage.
Figure 1. Penetration depth (micron)
Customized versions of the solution are possible containing one or more of the following options:
Tetragonal martensite option
For martensite with visible tetragonality the tetragonal martensite phase can be used in the customized version.
Cementite option
A correction for the presence of iron carbide (cementite, Fe3C) will improve the determination of low amounts of retained austenite (Figure 2).
Alternative wavelength option
The standard version supports measurements with Co radiation only. Customization is possible to support another wavelength of your choice: Mo, (Cu), Fe, Mn, or Cr.
Reporting option
Reports can be customized to your preferences. Do you want to see more parameters or maybe less? Do you want to add a pass/fail criterion? All is possible in the customized version.
Table 1. Crystallography and XRD patterns of phases in steel *Exact numerical values depend on presence of alloying elements (e.g. interstitial C and/or substitutional Cr, Mn).
Figure 2. XRD scan with Co radiation showing ferrite and austenite phase identification. The insert shows the cementite phase overlap on retained austenite.
Figure 3. The software provides automatic electronic reporting of multiple measurements.
An automatic setup comprises the on-screen display of the actual measurement and automatic electronic reporting of multiple measurements (Figure 3). Warnings for exceeding the validity range of the ASTM norm are included as well (Figure 4). This concerns the amount of texture (non- randomness) of martensite and/or austenite and the detection limit for the quantitative retained austenite. In all cases the numerical result will be calculated and reported. If iron carbide is present in the sample (for example >1%) an optional correction for iron carbide is available.
Figure 4. Example report screens showing the outcome of an individual analysis, without (left) or with (right) cementite correction included. Additionally, the software provides warnings for exceeding the validity range of the ASTM norm.
The international norm ASTM E975 ’Standard Practice’ for X-ray Determination of Retained Austenite in Steel with Near Random Crystallographic Orientation summarized:
Malvern Panalytical’s solution for the analysis of retained austenite can be tailored for any of the Malvern Panalytical XRD systems (Empyrean, X’Pert3 Powder, X’Pert3 MRD, CubiX3). For this analysis Malvern Panalytical RoboRiet software is required.
The retained austenite determination is successfully tested on ball bearing steel (DIN 100Cr6, AISI 52100) with the optional correction for cementite (Fe3C). Malvern Panalytical recommends to measure a full pattern scan with Co radiation. Each solution is customized for one steel alloy type
Retained austenite is an unwanted remainder of the hardening process of steel. To harden steel a temperature sequence of heating, quenching and tempering is applied. At room temperature the stable steel phase is ferrite. At high temperature the stable phase is austenite. By rapid quenching into water or oil the hard and brittle martensite phase is formed. In the final tempering step the desired hardness and ductility is obtained.
The remaining phase portion not transformed into martensite is called retained austenite. The amount of retained austenite must be as low as possible (for example < 2%) as it involves a risk for failure during the service life of high precision steel components (e.g. ball bearings). Retained austenite is an instable phase and may transform into stable ferrite. This transformation goes along with an increase in volume, which does compromise the tight tolerances in a component.
