Volume 56, Issue 6

Recently, the problem of hydrogen flaking resurfaced, when internal defects were detected in the reactor pressure vessels of two Belgian nuclear power plants. These defects turned out to be hydrogen flakes formed during the fabrication of these pressure vessels. The goal of this publication is to provide important insights into the phenomenon of hydrogen flaking, the different parameters that play a role in the mechanism, as well as the typical morphology and location of these flakes. Therefore an extensive literature study was combined with a detailed metallurgical characterization of a significant number of flakes. Hydrogen flaking is a fabrication problem, which is strongly linked with segregation phenomena. A combination of a sufficient amount of hydrogen, stresses and a sensitive microstructure causes hydrogen flaking. For these reasons hydrogen flakes inside large reactor pressure vessels are found in the so-called ghost lines, which originate from segregation processes during casting.

Kurzfassung Die Bewertung erdverlegter Gashochdruckleitungen nimmt immer mehr an Bedeutung zu, da Dauer und Kosten von Instandhaltungsmaßnahmen sowie die Sicherheit der Gashochdruckleitungen wesentliche Größen der Versorgungssicherheit mit Erdgas darstellen. Fortschrittliche Bewertungsverfahren und Kenntnis der realen Beanspruchungszustände verbessern die Situation. Die Messung realer Spannungsverhältnisse trägt wesentlich zu einer besseren Bewertung der Standsicherheit erdverlegter Gashochdruckleitungen bei. Prinzipiell sind für derartige Aufgaben Messungen mit Dehnungsmessstreifen, die röntgenografische Spannungsanalyse sowie Methoden der Ultraschallspannungsmessung geeignet. Diese unterschiedlichen Varianten der Erfassung von Spannungszuständen müssen auch unter Praxisbedingungen prüfpraktisch umsetzbar gestaltet werden. Dies setzt eine zweckentsprechende Qualifizierung in Anwendung und Auswertung voraus. Die Messmethoden werden erläutert und zugehörige Ergebnisse präsentiert.

Damages related to hydrogen absorption in steels are well known and described since decades. As hydrogen sources often moist electrode covering as well as etching and galvanic processes and corrosion are mentioned. In contrast, the hydrogen absorption during heat treatment processes in hydrogenous atmospheres was rarely referred. In the context of this review this hydrogen source will be described and specific cases of damage will be reported. The case of damage always occurred in combination with a missing or unfavorable heat treatment and/or previous material conspicuity. In contrast, the heat treatment processes which were applied in an appropriate way for million times, offer high process reliability without damages caused by hydrogen.

Hollow silver particles with three different shapes, i. e., spherical, cylindrical, and octahedral, were used as raw material to produce open-cell silver foams. The shape difference of the particles resulted from the reduction of different shaped silver sulfates. Fabrication of silver foams was carried out using SDP process with disaccharide particles as space holder. The resultant silver foams show high porosity with distinctive pore microstructures. The examination of pore structure by SEM shows that there are three levels of porosities which are unique for each type of foam. The foams, using spherical and cylindrical silver particles, have a larger surface area and a higher density than the foam with the silver particle of octahedral shape. A control of pore architecture can be performed by tailoring material and process parameters. The difference in pore architecture resulted in a different compressive behaviour of the foams. As expected, the foam, using octahedral silver particles, has lower compressive strength than the others, mostly due to lower foam density.

AlTiC master alloys were improved greatly and used as a new type in grain refinement of aluminum and aluminum alloys. In the present work, Al-Ti-C grain refiners with different Ti/C ratios were in-situ produced by the addition of K 2 TiF 6 and elemental carbon powder mixture to the commercial pure aluminum. The effects of casting temperature and Ti/C ratio on the grain refinement process were investigated by the so-called Alcoa Cold Finger test and respective optimum conditions were determined. The results indicate that the produced Al-Ti-C master alloys contain TiC and TiAl 3 particles within the aluminum matrix. The proper conditions for evaluating the efficiency of the produced Al-Ti-C master alloy to obtain a minimum grain size are: Temperature of 1300 °C, holding time of 20 minutes, and a Ti/C 0.60 wt.-%

In this study, the effects of three control parameters for the resistance spot welding of hot-stamped parts were studied. These control parameters are the welding force, welding time, and welding current. Al-Si coated 22MnB5 (USIBOR 1500P) sheets with thicknesses of 1.7 mm and 2 mm were used for the experiments. The Taguchi L 9 orthogonal array was used in the experiments. Evaluation of the experiments was based on the shear strengths of the welded specimens. The higher is better equation was selected to analyze the results. The relative importance of the spot welding parameters and their optimal combinations were determined using ANOVA. Additionally, an optimization of the welding parameters to maximize the weld tensile shear was conducted using the differential evolution algorithm, as well as the optimal parameters were determined.

In this study, glass fiber-reinforced composite materials were produced by hand lay-up with different number of layers of glass fiber, and tensile, flexural and impact fatigue strength of composite materials were investigated. The samples were also metallographically characterized. Depending on the number of fiber layers of samples mechanical properties and impact fatigue strength were examined. Due to the increase in the number of fiber layers, the mechanical properties of the samples were found to be improved.

In this study, Al 2 O 3 -CNT nanocomposites were sintered at 800–1000 °C under 5 GPa for 10 minutes using two different powders as starting material, γ-Al 2 O 3 and AlOOH, respectively. The effect of transition alumina powders on the properties of the high pressure sintered Al 2 O 3 -CNT nanocomposites were studied. Al 2 O 3 ceramics have been widely used for structural, electrical, and optical applications. Carbon nanotubes (CNTs) have recently been considered as reinforcing elements in ceramic matrix composites due to their unique mechanical properties. Reactive pressure sintering at low temperatures took place during the fabrication of CNT reinforced ceramic composites to avoid the agglomeration of carbon nanotubes. The hardness and densities were investigated and correlated with the microstructure of the dense materials. SEM analysis was used to determine the CNT distribution in the matrix, the morphology, and size of the particles.

In this paper, some properties of borided ferritic GGG40.3 ductile iron samples were investigated. Box-boronizing was carried out at 850 °C and 950 °C for 1 h, 2 h, and 4 h with Ekabor I powder. After the experiments, the formed FeB and Fe 2 B phases in the ductile irons were confirmed by X-ray diffraction analysis. The tooth-shaped morphology of the boride layers was examined by optical microscopy. Fracture toughness and the hardness of boride layer were measured by Vickers method. Abrasive tests were done using ball-on-disc test apparatus. An aluminum oxide (Al 2 O 3 ) ball was used in the abrasion experiments and the samples were subjected to abrasion under a load of 30 N at a constant sliding speed. It was determined that the thickness of boride layers obtained by using Ekabor I powder ranged from 38 μm to 198 μm, and the hardness of boride layers ranged between 1173 HV and 1820 HV. The fracture toughness values ranged from 2.79-3.78 MPa m 1/2 .

The results of an experimental investigation of structural, mechanical and electrical characteristics of different Cu-Al-Zn alloys are presented in this paper. The investigations were conducted using light optic microscopy, SEM-EDX analysis, hardness and electroconductivity measurements.

In this work, the particle image velocimetry (PIV) technique was applied to study the compressible supersonic flow regimes in the multi-Mach number high-speed (MMH) wind tunnel. To generate sufficient light scattering and flow across shock waves, tracer particles and the seeding technique were used, as well as scattering and tracking of particles were investigated using experimental observations and theoretical analysis. It was determined that the actual particle diameter should be around 200 nm to guarantee an adequate aerodynamic response to velocity gradients in the flow. It was further shown by the PIV measurements over a sharp cone that the uniformity of seeding particles dispersion was very important for the operations of the seeding facility. The present study indicated that the PIV technique could be an efficient method for monitoring the flow field of shock wave and subsonic flow regimes.

Depreciation expenses are tax free and thus they affect project profitability. There are different methods for depreciation expenses calculation. Some countries impose a well-defined depreciation method in its taxation system while others do not. Therefore, this paper aims at investigating the effect of the applied depreciation method on the profitability of a Saudi gold processing plant where a strict depreciation schedule of the taxation system is not defined. The study was applied to a Saudi gold mine in the southern part of the kingdom. The plant profitability – expressed by NPV, IRR and PBP was estimated applying different methods of depreciation estimation (straight line method, double declining method, sum of year digits method, and the fixed declining method). The obtained results showed that the plant profitability is highly affected by the method of depreciation calculation. An average change in the plant profitability indices may reach up to 30% due to the method applied for estimation of depreciation expenses. The method that returned the highest plant profitability was the fixed declining method for calculating depreciation (NPV = 95.7 Mw, IRR of 15.4% and PBP of 4.8 year). While using straight line method returned the lowest project profitability (NPV = 76.2 M$, IRR of 11.8% and PBP of 6.4 year).