Volume 58, Issue 5

Since marginal sharpness is often an issue in metallographic samples preparation, various mounting compounds are commercially available which address these issues. Nevertheless, the conductivity of such products is not sufficient for electron backscatter diffraction measurements (EBSD) and products which are electrically conductive often result in a lack of marginal sharpness. Therefore, this study is focussed on the modification of commercially available embedding compounds (epoxy based and phenolic based hot embedding compounds) with graphite to increase their electrical conductivity. To verify the applicability of the modified embedding compounds, the austenitic steel grade 1.4301 was chosen for investigation via scanning electron microscopy and EBSD. Results indicate a good performance during SEM-investigations of the epoxy-based resins, even at 0 wt.-% graphite contents, whereas the phenolic resin-based embedding compound is only applicable for SEM-investigation > wt.-5 % of graphite. Best performance at EBSD measurements were achieved with phenolic resin based hot mounting compounds with glass fibres and an addition > 10 wt.-% graphite.

This work aimed at developing a methodology for examining the naturally grown passive layer of a thickness of just a few nanometers of an austenitic CrMnN steel by means of atom probe tomography and gaining knowledge on the structure of this alloy’s passive layer. The sample surface was ground, polished, cleaned, degreased, electrolytically polished, and oxidized in air to produce a reproducible passive layer. The oxide layer was subsequently coated with a silver layer of a thickness of 3 μm. The silver layer protects the oxide layer during the preparation of the atom probe tips in the focused ion beam microscope and the alignment of the tip in the atom probe. The samples were measured in the atom probe’s pulsed-voltage mode. The findings show that an enrichment of oxygen, molybdenum, nitrogen, and chromium and a depletion of manganese, nickel, and iron occur in the area of the passive layer.

A six-cylinder crankshaft in a 12L diesel engine was locked after testing for about 840 hours in the bench test. The fractography investigation indicates that fatigue is the dominant failure mechanism of the crankshaft. It is found that the fatigue crack mainly initiated at the fillet of the crankweb between the 6 th main journal and 6 th crankpin. The fatigue crack initiation area lies outside the induction surface hardened zone. From detailed metallographic inspection, abnormal microstructure containing Widmannstatten was found in the fatigue crack initiation area and the 5 th main journal, while that was not found at the severely deformed crankweb. Since the region containing Widmannstatten has lower hardness, the root cause of the failure may be that the abnormal microstructure lowered the fatigue strength at the stress concentrated fillet. The crankshaft prematurely fractured under the complex stress condition in the bench test.