Volume 58, Issue 9

Color etching provides additional information that standard etchants do not or only insufficiently reveal – in different steels but also in virtually all non-ferrous metals. This works aims at presenting the possibilities this technology provides by using the examples of aluminum, copper, nickel, titanium materials, and cast iron, while also pointing out its limits.

Fragments of the “Campo del Cielo” meteorite were first reported by conquistadores in Argentina in as early as 1576. This meteorite belongs to the group of iron meteorites and is classified as a group IA coarse octahedrite. The use of meteoric iron to make objects of daily use or hunting weapons in prehistoric times is only confirmed by very few finds. Metallographic examinations were performed to assess if it was realistic and in fact feasible to process meteoric iron. The bulk of the examined “Campo del Cielo” meteorite fragment found in Argentina is kamacite (α-Fe) with approx. 5.3 wt.% of dissolved Ni. Graphite was also detected in addition to various non-metallic phases. Due to high temperatures during processing, graphite can dissolve and a steel microstructure would form as a result.

The aluminum rim of a passenger car was claimed by the customer because a crack had appeared on the inner rim flange, which ran into the rim well. The cause of this damage was to be determined. Accumulations of oxide skins in areas close to the surface were proven to be the cause of failure by metallographic and scanning electron microscopic examinations. The cause of the damage was a manufacturing defect.

Just as 10 years ago, an examination of drills used in human medicine revealed cracks in the microstructure which can be attributed to fractured carbides. The drills are made of corrosion-resistant chromium steels with special carbides which are of the composition (Cr,Fe) 23 C 6 and (Cr,Fe) 7 C 3 (e. g. material no. 1.4125 or 1.4112). The cracks occur in the coarser carbides during the forming of the drills. It was considered problematic that the cracks are filled with contaminants which cannot be fully removed in the cleaning process. One of the drills examined for this study also contained MnS inclusions which is consistent with a “rust and acidresistant steel / free-machining steel” (e. g. material no. 1.4005).