Band 62, Heft 4

Hard metals are in great demand in areas that require high hardness and wear resistance. Tungsten carbide excels in this area, especially with cobalt as a binder phase [1, 2]. For microstructural characterization, choosing an applicable preparation method is the most essential step. Publications concerning WC-Co materials were published by the National Physical Laboratory in Teddington, England, notably by Prof. B. Roebuck, though the influence of preparation methods was not entirely explored. Electron backscatter diffraction (EBSD) is one of the most common characterization techniques for metallurgical investigations. We use common preparation techniques and a few others we developed to analyze and compare the resulting surface quality and their applicability for EBSD [3, 4, 5, 6]. In addition to EBSD, the sample should be compatible with X-ray diffraction (XRD) and/or transmission electron microscopy (TEM) characterization. Our criteria consider EBSD grain indexing in orientation imaging microscopy analysis (OIM) software, changes in microstructure, cobalt leaching/cobalt losses, the size of the characterization area and ease of preparation procedure. We target the distribution of cobalt phases, hexagonal close packing (HCP) and face-centered cubic (FCC) with respect to their magnetic saturation. HCP is stable at room temperature, while FCC forms at temperatures higher than 417 °C [7].

A new metallographic preparation method for steel samples with large and brittle scale layer is presented. A combination of electrolytical nickel plating, mechanical grinding and polishing as well as conductive silver paste penetration is proposed to protect the samples from corrosion caused by residual water from the preparation as well as to reduce the height difference between base metal scale layer and regions of broken scale. Both measures drastically increase the conductivity of the sample, as local charging on corrosion products as well as on protruding scale parts is prevented. It is demonstrated that the new method enables high quality imaging (secondary electron, back scatter diffraction as well as energy dispersive spectroscopy imaging) without the need for focused ion beam sample preparation, which drastically reduced the necessary time for sample preparation.

Three steel grades, 1.2322, 1.2379 and 1.8550, were heated using a natural gas/oxygen and a hydrogen/oxygen mixture and scale formation on these steels was determined gravimetrically and examined by microprobe analysis. No differences in the structure and composition of the scale were observed when using a hydrogen-heated instead of natural gas-heated furnace. Even though a thicker scale layer must be expected when using natural gas instead of hydrogen, no differences in terms of decarburization were found between the process gases used.

An investment cast seal segment for an aeroderivative gas turbine engine was received at the authors’ laboratory in the grain-etched condition. The part exhibited a non-continuous surface crack in an area where there is a fin, with relatively sharp fillet radii on both sides. Both fillet radii were affected by the cracking. The cracking was caused by overload fracture from thermal stresses after solidification, in an area where casting microporosity coincided with geometric stress raisers.