Cast irons are a family of ferrous metals with a wide range of properties that are produced by being cast into shape instead of being formed. Cast irons contain 2 to 4 wt% carbon and 1 to 3 wt% silicon, but a wide range of elements are used to control specific properties. Cast irons have a wide range of mechanical properties which make them suitable for use in engineering components. The wide spread use of cast iron is a result of its low cost and versatile properties. Speed, precision, stability, sensitivity and accuracy are important criteria applied by the cast iron manufacturer in using X-ray fluorescence spectrometry as a method of analysis.
Axios FAST is a fully integrated simultaneous wavelength dispersive XRF spectrometer, complete with X-Y sample handler and state-of-the-art software. It is engineered for excellence in terms of both analytical and operational performance.
X-ray tube: Rhodium anode 4 kW SST-mAX tube
Tube power: 60 kV/ 66 mA
Fixed channels: C, Mg, Al, Si, P, S, Ti, V, Cr, Mn, Fe, Co,
Ni, Cu, Zn, As, Mo, Sn, W, Pb
Goniometer: Se, Zr, Nb, Sb, Te, Ce, Bi
Medium: Vacuum Shielding plate: 27 mm Rh plated
Sample holder: Stainless steel 27 mm aperture Flow counter window: 0.6 μm for carbon and magnesium,
6 μm for aluminium analysis
Sample preparation and measurement details
Eight standards (CKD 232 – CKD 239) were used to set up the calibration. The standards were freshly surfaced prior to measurement with a Herzog grinding machine using 150 grit Al2O3 sandpaper. The X-ray intensities of 27 elements were measured simultaneously for 60 seconds at the peak. The calibration was performed using the Fundamental Parameters (FP) regression model incorporated in the SuperQ software package. During calibration line overlap corrections were automatically performed by the software through multiple regression. No other corrections were applied concentration. As a general rule, K factors < 0.05 indicate accurate calibrations. Using the K factor it is possible to estimate the accuracy of a result, accounting for both instrumental error and errors associated with the certified concentrations of the reference materials defining the calibration. Figures 1 and 2 show the calibration graphs for Mn and Ni.
The analysis speed is usually limited by the element with the lowest sensitivity (carbon in this case) and/or the lowest concentration. With the Axios FAST excellent results are obtained for all elements, including carbon. Table 1 gives an overview of the elements with their concentration range, their respective values for the ‘root mean square’ (RMS) error, K factor and the lower limit of detection (LLD) based on a 100 seconds counting time. RMS (absolute error) and K (relative error) represent the accuracies of the calibration. The K factor is a weighted residual error and is a measure of the differences between the measured concentration and the supplied, certified chemical
Table 1. Values for ‘root mean square’ (RMS), K factor, and lower limits of detection (LLD)
Figure 1. Calibration graph for manganese
Figure 2. Calibration graph for nickel
Precision and instrument stability
The precision, repeatability and reproducibility of the Axios FAST is outstanding, not only for short-term measurements (20 consecutive measurements), but also for longer-term measurements (measurements carried out over a period of 10 days), see Table 2 and Figure 3. For comparison, the counting statistical error (CSE) expressed in concentration units is also shown in Table 2. The CSE is the theoretical minimum possible error and the data demonstrate the inherent stability of the Axios FAST instruments. The sample was freshly surfaced before each measurement. No drift correction was applied.
Figure 3. Short- and long-term stability measurements of C, Si and Mn in CKD 237 cast iron standard
Table 2. Analytical precision (measured on CKD 237)
The Malvern Panalytical Axios FAST is capable of analyzing cast iron precisely and accurately. A total analysis time of only 60 seconds is required to analyze 27 elements, including carbon.