Band 56, Heft 5

This study analyses the causes leading to the failure of a high-voltage cable in regions close to it “mooring” at the end near to one of the high-voltage towers. The horizontal span of the cable between moorings was over 1040 m, with a difference in elevation between high-voltage towers of approximately 300 m. The conductor was made up of 30 aluminium strands with a 7 strands eutectoid steel core. The aluminium filaments were distributed in 2 layers, an outer layer with 18 filaments and an inner layer with 12. The steel filaments were distributed in a layer composed of 6 filaments surrounding a single central filament. All the filaments had a diameter of 2.6 mm. Cold drawn and twisted aluminium and the possible presence of recovery substructures in regions close to the fracture were analysed by means of optical microscopy. The steel was similarly studied, verifying its pearlitic nature and assessing the existence of softening phenomena due to recovery or recrystallization. The thicknesses of the zinc coating on the steel filaments were determined. The fracture faces of the aluminium and steel filaments were analysed by scanning electron microscopy and the fused remains resulting from possible arcs between the aluminium filaments were examined. In order to assess the deterioration of the filaments, tensile tests were performed on all the individual filament in regions adjacent to and at a certain distance from the fracture. The failure of the conductor had its origin in the outer aluminium filaments due to causes imputable to fatigue phenomena. The reduction in the cross-section of the conductor led to an increase in temperature, leading to the weakening of the remaining aluminium filaments and their subsequent failure, the electrical current being subsequently transferred to the steel core. The increase in electrical resistivity characteristic of steel led to a significant increase in temperature that drastically decreased the mechanical strength of the steel cables.

The suitability of different preparation methods for imaging nano-carbides by scanning electron microscopy (SEM) was evaluated taking the example of cold work steel X153CrMoV12 manufactured by powder metallurgy. The ion beam slope cutting method, carbide etching using sulfurous acid, and electrolytic polishing using the Struers electrolyte A2 were considered in this study. The preparation aim was to develop an appropriate method for the qualitative detection of nano-carbides using the SEM to characterize relatively large sample areas and to substitute time-consuming transmission electron microscope examinations. The carbide detection was performed on heat-treated samples using sulfurous acid, applying the ion beam slope cutting method, and by electrolytic polishing using the electrolyte A2. In the SEM, nano-carbides could be observed on the sample surfaces prepared by ion beam slope cutting and the electrolytically polished ones. The examinations were performed on conventionally quenched and tempered material samples of cold work steel X153CrMoV12 (reference condition) as well as on samples subjected to deep cryogenic treatment (DCT). Deep cryogenic treated samples showed a significantly increased number of nano-carbides.

An important aspect in describing the microstructure of non-heat treatable Al wrought alloys is their constitution in the form of dissolved alloy elements or precipitated second-phase particles. These characteristic structural constituents, which practically can be subsumed as microchemistry, have an impact on the physical and, in particular, on the mechanical properties of Al materials. The present work examines the formation of constituent phases during solidification as well as the microchemical changes during a subsequent homogenization annealing of the alloy AA 3103 (AlMn1). The forming phases were statistically analyzed in a scanning electron microscope regarding their size, shape, area density and chemical composition. The results were matched with complementary characterization methods to explain the microchemical changes during solidification and the subsequent homogenization.

Cracking and leakage, not repairable by welding, was found in premix piping of heavy-duty gas turbine engine for power plant applications. The subject laboratory specimen, a pipe section cut out on site and received at the investigating laboratory, failed by transgranular stress corrosion cracking (SCC). The stress corrosion cracking has probably been induced by chlorides (halide-induced stress corrosion cracking). Chlorine peaks were found in EDS spectra acquired on the inner pipe wall surfaces. However, it cannot completely be ruled out that those were artifacts caused by handling, cleaning, and cutting. The cracking is located adjacent to welds.