106 Prakt. Met. Sonderband 52 (2018)
FEN
4. METALLOGRAPHIC ASSESSMENT x
When examining the prepared samples, great care has to be taken when trying to assess |
the quality of the preparation, which is mostly determined by oxide breakout. Most of the
oxide scales grown during the reheating step of hot rolling or simulations thereof exhibited
some degree of inherent porosity. Cavities which have been infiltrated with mounting agent
can safely be assumed to have been present before grinding and polishing. However, infil-
tration will usually not be complete, for example due to gas bubbles in the resin or closed
porosity. An example of the latter can be seen in Fig. 5a. This porosity was most likely
caused by solidification of the surrounding fayalitic oxide, which was liquid at oxidation
temperature. Damage caused by grinding and polishing is often recognizable by its anisot-
ropy, for example in the form of long scratches, as seen in Fig. 5b. Many of the defects in
this sample also originate from the phase boundaries between different oxides where sep-
aration occurs more readily. Often the only way to verify whether porosity is inherent to the
sample or is caused by oxide breakout is to incrementally polish the sample for a few
minutes at a time and compare the resulting images for changes in the amount of pores
visible, as was done for the sample in Fig. 3 and 4.
(a) Fayalitic oxide with shrinkage porosity (b) Oxide breakout caused by preparation
Fig. 5: SEM-SE images of oxides with different types of defects
5. SUMMARY
Damage to the oxide scale on steel samples during sectioning, grinding and coarse polish-
ing is virtually inevitable. Samples with mostly dense scales can be hot mounted, but high-
ly porous samples should be cold mounted using vacuum infiltration. Mitigation of defects
introduced during preparation involves extensive polishing with 3 pm diamond suspension
(about 15 min) and avoiding prolonged use of napped clothes for polishing. When evaluat-
ing the preparation quality of samples, porosity inherent to the scale has to be distin-
guished from apparent porosity caused by oxide breakout during grinding and polishing.
