FRACTOGRAPHIC ANALYSIS OF HIGHLY OXIDIZED SPHEROIDAL CAST-IRON SURFACE

摘要

The process of cast-iron components failure analysis can be significantly hampered by the marked tendency of this material to oxidize; in fact, the morphology of the fractures - once covered by oxidation - can no longer be interpreted by analysis carried out with instruments and, in particular, the SEM. Sometimes the experience of the operator makes up for this deficiency but, at best, the lack of objective data invalidates the results of the analysis that, for example, cannot be used as evidence in legal proceedings. In the automotive field this problem has far-reaching implications, since the time that elapses between the failure of the component and the visual identification is very long and the fractured surface usually undergoes notable damage by oxidation. Even ultrasonic etching with ammonium citrate (selective removal of the oxide) - a technique often used by the fractographic operator - can be misleading since the resulting material -far from showing the original appearance -presents ambiguous and easily deceptive morphologies. If not conducted correctly, the interpretational process can lead to completely incorrect conclusions.