Solid State Phenomena Vol. 216

Abstract: In this work we followed the chemical composition and structure of pellets produced by two recipes in the laboratory of Energy and Raw Materials Base of the Faculty of Engineering Hunedoara, followed by determining compressive strength of pellets after hardening process and chemical composition. Some of the experiments were performed in a stage carried out at Luleå University of Technology Sweden (SEM and XRD measurements). The experiments have watched also the evolution of hardening temperatures for these pellets, the data was interpreted in the form of charts in Excel.

Abstract: The aim of this study is to assess the quality of laser welded joints, by different methods such as: scanning electronic microscopy, metallography, microhardness testing, spectrographic and radiographic analyses. The alloys assessed are a titanium-based alloy used in manufacturing prefabricated pieces for implantology, a standard AuPd alloy for the metallo-ceramic technique and a Cr-Co-Mo alloy used for manufacturing the metallic components of partial dentures. The conclusion of the testing is that laser welded joints are generally mechanically satisfactory. Laser is best suitable to weld titanium and its alloys because they have higher rates of laser beam absorption and lower thermal conductivity than other dental casting alloys.

Abstract: Waste tires create big problem in the world as for their utilization. One domain in which this material is able to found promising application is civil engineering. The waste tires additive in the concrete manufacturing leads to change of mechanical properties such as: Young modulus, compressive strength, fracture toughness, energy absorption, brittleness, water absorption etc. It was partly reported in the literature only for the plain concrete. There is no investigations for a lean concrete - the material which has potential applications as a subgrade of roads. The aim of this paper is to investigate the importance of the rubber particles addition in the lean concrete production as for modification of the basic mechanical properties and further degradation of the material due to mechanical loading. The new technology of compaction of the concrete was elaborated to perform basic test: cyclic uniaxial compression and cyclic bending. In order to describe gradual degradation of the rubberized lean concrete a scalar damage parameter D related to loading history was proposed. The current value of the elastic modulus was expressed as E=E_o(1-D), where E_o denotes the initial value of the Young’s modulus. The paper includes also a numerical model of the beam made of the rubberized lean concrete, which is subjected to 3-point bending deformation. The stress distribution in this beam is highly non-homogeneous with visible stress concentrations around the rubber particles.

Abstract: The structural characteristics of low S (0.025%), low Al (<0.003%) and 4.0% carbon equivalent (CE) electric melted grey irons were studied at different solidification rates, for Ca and Ca,RE [Mischmeta as the inoculating elements. Despite a relatively high CE, this base iron is sensitive to undercooling during solidification. A conventional Ca-FeSi inoculant is not recommended for these critical conditions, especially to avoid undercooled graphite morphologies. It is only adequate to control free carbides, for castings with more than 15mm section. Rare earth (RE) bearing CaFeSi alloy appears to be a more effective solution, at least for castings with more than 5mm section to control carbides, even the end effect, and promote a homogeneous structure. Undercooled graphite presence was limited and avoided in more than 10mm section. Thin wall castings, less than 5mm section, requiring stronger control for carbides and undercooled graphite formation, should not be produced from this type of base iron, unless more efficient inoculation can be accomplished.

Abstract: The high temperature oxidation behaviour of Ni-Cr-B-Si coatings with a higher Si-content was investigated in order to evaluate the suitability of such materials especially for novel applications concerning highly aggressive environments like metal dusting. Metal dusting is a corrosion phenomenon that occurs in reducing-, carbon-supersaturated (a_c>1) gaseous atmosphere, containing CO, H₂, CO₂ and H₂O, at elevated temperatures between 400 and 800°C. Metal dusting reactions can be classified into two types. The first one concerns Fe-alloys, where Fe₃C is growing on the surface. The second one is related to the reaction of Ni, Co and their alloys, where the destruction takes place through inward growth or direct ingrowth of graphite, without forming the metastable Fe₃C. Regarding to the literature, metal dusting is typically encountered in industrial furnaces, but mainly in the chemical or petrochemical industry. The way to suppress metal dusting is to stop the dissociation of the carbon source or to stop the carbon ingress in the material. One possibility in order to avoid the carburization of Fe, Ni, Co and their alloys is to preoxidize the samples. Based on the reducing atmosphere, where metal dusting occurs, the isothermal outsourcing for the formation of a protective Al-, Cr- or Si-oxide layer on the samples in air is mostly necessary. The role of a stable Al₂O₃ and Cr₂O₃-layer on the sample as a diffusion barrier against the carbon ingress, based on their low solubility for carbon, has already been investigated and proved by many scientists. The formation of a protective and thermodynamically very stable SiO₂ scale was also investigated. Within the scope of this work, the influence of a higher Si-content (4,5 wt%) in NiCrBSi-alloys, depending on the temperature, was analyzed. For this purpose the samples were oxidized in air at 600, 700 and 800°C respectively. The surface morphology and the phase composition of the grown oxide scales were characterized by means of scanning electron microscopy combined with energy dispersive X-ray (SEM/EDX) and by X-ray diffraction (XRD) technique. The experimental results demonstrate the importance of silicon content on the coatings properties, respectively on the stability of the formed oxide scale (free of micro cracks, no spallation). This element is able to form beside chromium, a dense oxide layer on the sample surface, protecting it against further degradation induced by the atmosphere in different high temperature applications. Moreover, the increased chromium content of the feedstock powder (from 10 wt% in previous work to 12,5 wt%) demonstrated that the Ni-Cr-B-Si coatings exposed at 600°C, 700°C as well as at 800°C were not susceptible to internal oxidation.

Abstract: Push rod method for determining linear thermal expansion using vertical differential dilatometer was used in the study of the thermal compatibility of metal-ceramic systems for dental applications. The purpose of this study consisted in evaluating the effectiveness of dental coating by determining the ceramic metal bonding strength of metal-ceramic couples (Ni-Cr and Co-Cr alloy coated with dental ceramic) and correlation with the difference of linear thermal expansion coefficients of metals and ceramics.

Abstract: Components soldered with Sn-based alloys are susceptible to the growth of whiskers. Tin whiskers have been proven to be responsible for equipment failures in a wide range of industries. In order to reduce defects in electronic components a new solder alloy is proposed based on the Sn-Cu alloys. The Sn-Cu-Ga alloy utilised in this study was fabricated as a ribbons using melt-spinning method. These ribbons were then soldered onto electroplated tin layers. Preliminary characterization of the solder alloy is presented in this paper, including scanning electron microscopy, EDX mapping and X-ray diffraction. Key words: tin whiskers, PCB, printed circuit boards, melt spinning, solder, ribbons

Abstract: In steelmaking and casting, there are used, stored and handled chemicals which are risk factors either in the normal operation of the technical plants, or in predictable and unpredictable failure conditions. In the steelmaking and casting plants, the substances and liquids considered risk factors are: methane, oxygen, liquid steel and water used for the cooling system. This paper deals with the risk assessment within the premises of electric steelworks, in predictable failure situations, based on the sheets of assessment and reduction of the risk associated with explosive atmospheres of methane and oxygen, in the presence of high temperatures existing at the electric steelworks.

Abstract: The mechanical properties of the welded structures are directly related to the weldability of the steels, thus the estimation of the microstructural constituents in the weld and maximum hardness in the HAZ according to the welding process parameters represent a problem of great interest. The microstructural changes in the HAZ are estimated using a five-parameter logistic function (5PL), which is very accurate in the fitting highly asymmetric data. Also, the same 5PL function can be used in order to predict hardness and toughness in the HAZ based on the heat flow, cooling rates between 800-500 oC and chemical composition of the material. A discussion about the parameters of the 5PL function and fitting experimental data is presented and a studied case for welding S355J2 steel plates is also analyzed.

Abstract: In welded structures subjected to dynamic cyclic loads may appear and propagate fatigue cracks due to local structural damage. These cracks may initiate due to technological damages induced by welding process, or by environmental operating conditions. In the present work, it is determined, by the means of Finite Element Method (FEM), the natural frequencies and mode shapes of more steel specimens that are welded together. The analysis is carried out in undamaged condition as well as in damaged one, after the damage has been artificially induced. The experimental measurement of the vibration response is carried out by using a condenser semi-professional microphone Samson C01U, which is suitable for high-fidelity acoustic measurements in the frequency range of 40 18.000 Hz. A first post-processing of the vibration responses of the welded specimens, in free-free conditions, is carried out using an algorithm based on Prony series. A second post-processing is carried out by means of signal processing using DWT together with Shannon entropy. The results are compared to modal parameters estimated using FE Analysis.