The suitability of tool steel alloys, for making tools, comes from their hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). Tool steel is generally used in a heat-treated state. Such properties are obtained from alloying elements like Cr, Mo, Co, V and W.
Analysis of samples for melting control of tool steel requires the fastest possible turnaround of results. This minimizes errors due to melting losses and maximizes production. At the same time close specifications and tight control of expensive alloy additions demand the highest possible accuracy and precision. Speed, precision, stability, sensitivity and accuracy are important criteria applied by the tool steel manufacturer in using X-ray fluorescence spectrometry as 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, Si, P, S, Mn, Fe, Co, Ni, Cu, Sn Goniometer: V, Cr, Mo and W
Medium: Vacuum Shielding plate: 27 mm Rh plated
Sample holder: Stainless steel 27 mm aperture Flow counter window: 0.6 μm for carbon analysis
Flow counter window: 0.6 μm for carbon analysis
Sample preparation and measurement details
Sixteen standards (SS 481/1 – SS 487/1 and BS 31B, BS 32B, BS 33C, BS 34 – BS 39B) 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 for 14 elements were measured simultaneously for 30 s 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 determined through multiple regression by the software. No other corrections were applied.
The analysis speed is usually limited by the element with the lowest sensitivity (carbon in this case) and/or the lowest concentration (Sn in this case) 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 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 Co and Mo.
Table 1.Values for ‘root mean square’ (RMS), K factor, and lower limits of detection (LLD)
Figure 1. Calibration graph for cobalt
Figure 2. Calibration graph for molybdenum
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 the 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. No drift correction was applied.
Table 2. Analytical precision (measured on SS 481/1)
Figure 3. Short- and long-term stability measurements of Cr and W in SS 481/1 tool steel standard
The Malvern Panalytical Axios FAST is capable of analyzing tool steel precisely and accurately. A total analysis time of only 30 seconds is required to analyze 13 elements, including carbon.