Automated Inclusion Microanalysis in Steel by SEM 1089
hemispheres, and sunken spheres. For Al Kα X-rays from micro-inclusions, the net effect is that the yield ratio (20–10 kV) is slightly lower than 1 for inclusions with dia- meter smaller than 1.5 μm. The same effect is expected for the two other major light elements often present in oxide inclusions, Si and Mg, as they have similar Kα energies. For Ca Kα X-rays from shallow inclusions, the yield ratio is also slightly lower than 1 (because a smaller fraction of the beam electrons interacts with inclusions at 20 kV), but the ratio is much larger for deeper inclusions. The net effect is that theX-ray count rate from micron-sized
inclusions is expected to be similar or slightly higher at 10kV than at 20kV for Al (at the same beam current), and somewhat lower for Ca. The implication is that the required EDX analysis time at 10kV need not be much longer than at 20kV. Measurement on inclusions in steel confirmed this trend
(Fig. 13b): The Al Kα count rate (relative to beam current) was similar at 10 and 20kV (higher at 10kV for inclusions shallower than ~1.5 µm), whereas the Ca Kα count rate was higher at 20kV (and more so for deeper inclusions). For these measurements, the depths of the inclusions were estimated from the relative height of the Fe Kα peak (at 20kV) and the radius of the inclusion section at the sample surface, using the procedure described previously (Pistorius et al., 2013).
Analysis Speed
The total analysis time for a steel sample is mainly the sum of three steps: stage movement, BSE image acquisition, and EDX spectrum acquisition (sometimes together with recording of a higher-magnification BSE image of each feature) (Izraeli et al., 2014). The time for each step can be estimated from the instrument settings (BSE pixel dwell time, EDX analysis time, total area analyzed, magnification, and image size) and the expected (or actual) number of features per unit sample surface area analyzed. The general calculation procedure, relevant constants, and values for a specific example are shown in Table 3. Figure 14 shows a good match between the calculated time and the measured time for several automated inclusion microanalyses performed on the XL30 SEM. These analyses used the same SEM settings, but were conducted on different samples with different inclusion concentrations and analyzed areas. As expected, BSE imaging is often the slowest step (with one exception, which was an exceptionally “dirty” sample with ~700 inclusions/mm2, marked in Fig. 14).
SUMMARY
Automated inclusion microanalysis by computer-based SEM is an indispensable tool for steel cleanliness
Table 3. Calculation of the Total Analysis Time (With the “Dirty Sample” Marked in Figure 14 as Example). (a) Instrument settings
BSE imaging Symbol
Value EDX
Symbol Value
Mechanical Symbol
Value
Magnification M
880 Image size (pixels)
Width (nw) Height (nh) 1,024
896
Live time per feature (s) τX
2
Average time to move between fields (s) τM
1.45 (b) Sample
Symbol Value
Parameter
Area of one field Number of fields BSE image time
Stage movement time EDX analysis time
Area analyzed (mm2) AA
5.7 Formula
Af =c(W/M)2 nf =AA/Af
nf×nw×nh×τB nf×τM
AA ×(nI/A)× τX /(1−f ) BSE, backscattered electron (BSE); EDX, energy-dispersive X-ray.
Feature density (mm−2) nI/A
700 (c) Calculations Value
0.00723mm2 785
5.78 ×109 µs=96 min 1,143 s=19 min 9,172 s=153 min
Image aspect ratio Image width (mm) Dwell time per pixel (µs) c=nh/nw
0.875
Fraction dead time f
0.13
W 80
τB 8
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