Volume 60, Issue 4

The properties of the compound layer have a significant influence on the lifetime of nitrided surface layers. In addition to a sufficient thickness, the compound layer characteristics include the phase composition and the porosity. Complex methods such as GD-OES or XRD are already used to qualitatively and quantitatively determine the phase composition of the compound layer. Particularly with regard to the characterization of the pore seam, no method has yet been identified that can be clearly recommended. In this work, the extent to which compound layers can already be characterized with the aid of microscopic examination methods is shown. For this purpose compound layers with varying thickness, phase composition and porosity were formed in the surface area of the material EN31CrMoV9 and EN42CrMo4 by different nitriding processes. It is demonstrated how the phase composition of compound layers can be studied qualitatively by means of special etchants on metallographic cross sections and subsequently quantitatively by image analysis. A more extensive characterization of the pore seam beyond the state of the art could be performed by scanning electron microscopy studies as well as 3D X-ray microscopy.

The deposition of metal in hoards appear throughout the European Bronze Age and there is on-going dispute about motivations behind the practice of hoarding metals at that period. Therefore, nine fragments of bronze swords from a late Bronze Age hoard have been investigated by metallography and X-ray fluorescence analysis (EDXRF) in order to evaluate their conditions. The objects do not show compositional deviations from contemporaneous material, but they are of poor quality showing considerable casting defects and pronounced dendritic segregation. They may never have been finished. Nonetheless, the reasons for their destruction and burial do not become evident.

A metallographic section from the crack area of an exhaust gas turbocharger was supplied by the customer. The shaft made of quenched and tempered low-alloy steel and the charger wheel made of a nickel-based superalloy were joined by laser welding. For design reasons, the root side of the weld contained a geometric notch, at which a high cycle fatigue crack initiated during the test facility operation of the turbocharger. At the time of examination, it went through almost the entire weld root. As a corrective action, it was recommended that the welded joint be replaced by a shrink fit.