Abstract: The experimental formation of aluminide coatings on 316 stainless steel grades and Ni by use of a chemical vapour deposition via the fluidised bed technique at relatively low temperatures (550, 560 and 570 degrees C for the steel and 550-650 degrees C for Ni) was Studied. A mathematical model that predicts the growth rate of the coating for both substrates is being Proposed. The experimental results were evaluated by means of optical microscopy. Uniform coatings even for reduced treatment times of a few minutes were produced. The measured coating thickness was compared to the predicted results from the model. The results indicate a linear correlation of coating thickness and treatment time for short treatment times and a parabolic correlation for longer ones. The low treatment temperature allows for energy saving and the mechanical properties of the treated parts remain unaffected. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: The effect of sieve boron waste (SBW) and borate-containing Evansite(R) on the thermal behaviour, microstructure and properties of a clay-based body was investigated. SBW and Evansite(R) were introduced in quantities that correspond to 0.6 wt.% B2O3 addition in the dry body for both cases. Cylindrical samples were extruded and fired at three different peak temperatures 900, 950 and 1000 degrees C. The reference body, R, and the body with SBW, RB, demonstrate a comparable dilatometric behaviour whereas the densification for the body with Evansite(R), RV, initiated 50 degrees C approximately lower and resulted in higher firing shrinkage. After firing at 900 degrees C, the physico-mechanical properties as well as the microstructure are comparable. Nonetheless, akermanite is formed in RB, whereas hercynite and mullite, the latter at 1000 degrees C, are formed in RV. For firing at 1000 degrees C, the role of borates is intensified. Water absorption is reduced by 16.1% and 18.0%, whereas bending strength increased by 27.6% and 40.8%, for RB and RV respectively, compared to the reference formulation. This is attributed predominantly to the enhanced vitrification that took place in the boron-containing bodies. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Abstract: The present research study investigates the synergy of industrial solid by-products from lignite combustion (fly ash and bottom ash) and aluminum production (ferroalumina) in the production of lightweight aggregates (LWA). The process consists of two stages, pelletization and sintering. Bottom ash (BA) is used as the principal raw material in mixtures while ferroalumina (FAL) is added in lower percentages (5-30wt%). BA carbon content is used as the fuel of sintering process in high temperatures, around 1250 degrees C, and gas generation is responsible for porous structure formation. Physical properties such as porosity, water absorption and bulk density, of sintering products are measured. Increase of FAL percentage in sintering mixtures results in decrease of porosity from 61% to 35% and of water absorption from 61% to 21% and in increase of bulk density from 1.02g/cm(3) to 1.80g/cm(3) of the produced aggregates. Aggregates produced by FAL addition up to 20wt% are characterized as LWA. Aggregates formed are used in the production of concrete specimens. Compressive strength of concrete increases by increasing FAL addition in aggregates from 5 wt% to 15 wt% (highest strength value), while decrease by increasing FAL addition from 20 wt% to 30wt%. FAL addition in lignite ashes sintering mixtures (up to 15 wt%) is considered as an important parameter for enhancing aggregates strength. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: The sessile drop technique has been used to measure the temperature dependence of the contact angle, theta, of the liquid metals Ag and Cu in contact with polycrystalline yttrium oxide (yttria, Y2O3) at the temperature range 1,333-1,773 K in Ar/4%H-2 atmosphere. Combination of the experimental results with literature data taken for nonwetted and nonreactive oxide/liquid metal systems permit the calculation of the surface energy of Y2O3 as gamma(sv) (J/m(2)) = 2.278-0.391 x 10(-3) T. For the same atmospheric conditions, thermal etching experiments on the grain boundaries intersecting the surface of the polycrystalline ceramic allow to determine the groove angles, psi, with respect to temperature and time as well as the grain-boundary energy of Y2O3 as gamma(ss) (J/m(2)) = 1.785-0.306 x 10(-3) T. Grain-boundary grooving studies on polished surfaces of Y2O3 annealed in Ar/4%H-2 atmosphere between 1,553 K and 1,873 K have shown that surface diffusion is the dominant mechanism for the mass transport. The surface diffusion coefficient can be expressed according to the equation D (s) (m(2)/s) = 1.22 x 10(-3) exp(-343554/RT).
Abstract: Colemanite, ulexite and tincal are the main boron ores found in Turkey and account to 63% of the world estimated reserves. The production route of boron products results in significant amounts of different types of Boron Wastes, BW. Their open field disposal raises substantial environmental concerns in fear of leaching and groundwater pollution. The heavy clay ceramic industry can potentially absorb substantial quantities of BW. In the present study, BW produced from Kirka borax plants in Turkey, with 12.6 wt.% B2O3 was introduced in 0 wt.%, 5 wt.% and 15 wt.% in a heavy clay body mixture. Four peak temperatures, 800 degrees C, 850 degrees C, 900 degrees C and 950 degrees C, were examined for the dry pressed samples. The thermal behaviour analyzed by dilatometry and TGA shows that major loss in weight starts about 600 degrees C and continues to 700 degrees C approximately. For 5 wt.% BW addition and firing at 900-950 degrees C, the sintered bodies present comparable or improved physical and mechanical properties with respect to the reference formulation. The microstructure was analyzed by SEM whereas the main crystalline phases were identified by XRD. Samples fired at 900 degrees C with 5 wt.% BW present comparable properties with the reference ones. At 950 degrees C, the obtained properties were improved. Deformation occurs for samples with 15 wt.% BW when fired at >900 degrees C. (C) 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Abstract: Ceramics were developed using Bayer's process bauxite residue and a clay mixture in a weight ratio 1 : 1. Firing was performed in a propane firing kiln at 1000 degrees C for 1 or 4 h soaking. For oxidising atmosphere, water absorption varies between 18.9 and 19.5% and bending strength between 25.9 and 27.9 MPa. The crystalline phases identified are haematite Fe2O3, quartz SiO2, plagioclase [anorthite (Ca, Na)(Al, Si)(2)Si2O8], clinopyroxene [augite Ca(Mg, Fe+3, Al)(Si, Al)(2)O-6] and corundum Al2O3. The microstructure is comprised of Si rich vitrified areas that contain quartz and have closed, round pores and an assemblage of particles creating a mass with open and interlinked pores and no signs of glassy phase. For reducing atmosphere and 1 or 4 h soaking, water absorption varies between 18.2 and 18.7% and bending strength is similar to 29.5 MPa. The crystalline phases identified are qualitatively the same as for the oxidising atmosphere. The microstructure is comprised of two concentric zones due to the reduced gas permeability in the core of the body. The outer shell is highly vitrified with closed and rounded pores. The microstructure in the core of the body resembles that of samples fired in oxidising atmosphere; however, the average pore size is higher and the vitrified regions are more extensively formed. The build-up of compressive stresses in the outer shell may also account for the increase in the bending strength; however, cracks were developed in few occasions. In all cases, quartz participates in the development of liquid phase and dissolves in the melt. Increase in the soaking time results in a decrease in the quartz content in the body and an increase in the vitreous phase.
Abstract: Bauxite residue, the principal waste from the Bayer process, was dried, pressed and studied for its thermal and sintering behaviour under different atmospheres, up to 1100 degrees C. For sintering in air and N-2, shrinkage begins at 800 degrees C and ranges from 2.6% to 13.9%, after firing at 1000-1100 degrees C. Bulk density varies from 1.7 to 2.3 g/cm(3) whereas water absorption from 31.5% to 17.7%. The main crystalline phases identified on firing in air were hematite (Fe2O3), gehlenite (Ca2Al2SiO7) and perovskite (CaTiO3) whereas magnetite (Fe3O4) was also found on firing in N-2. Microstructures are characterised by irregularly shaped, <20 mu m Feret diameter, pores in a ceramic matrix with interconnected porosity. The average pore size is greater in samples fired in N-2. On sintering in 4%H-2/Ar, shrinkage begins at 710 degrees C. After firing at 1100 degrees C, shrinkage is 20.1% and water absorption 1%. The main crystalline phases are magnetite, wustite (FeO), gehlenite and perovskite. Mcrostructures are characterised by a compact heterogeneous matrix, with isolated <15 mu m. Feret diameter, closed pores. The grains have reacted with the adjacent phase and their shape is rounded with no sharp facets. Increased sintering temperature results in improved physical properties for all atmospheres tested and in higher average pore size when sintering takes place in air and N-2. The use of magnetite-reducing sintering conditions can potentially assist in the production of a variety of ceramic compositions containing bauxite residue. (C) 2008 Elsevier Ltd and Techna Group. S.r.l. All rights reserved.
Abstract: Red mud (RM), the digestion by-product of bauxite processing from "Aluminum Hellas", was dewatered by means of a filter press. The filtrand with water content from 28 to 32wt% was named ferroalumina (FA). In order to utilize it as a raw material in the production of Ordinary Portland Cement (OPC), mixtures were prepared with limestone, sandstone and 1, 3 and 5 wt% FA addition, respectively. The design of the mixtures was based on the cement's compositional indexes LSF, AM and SM. Burnability tests showed that less than 1 wt% free lime can be obtained in all cases for firing at 1450 degrees C, except for the mixture with 1 wt% FA addition, which requires 1550 degrees C. XRD analysis and optical microscopy examination showed that FA addition did not affect the mineralogical phases of the produced clinkers. The characterisation of the produced Portland cements indicated that differences on surface area, water requirement and setting time are negligible. Compressive strength results after 28 days of curing varies from 55 to 63 MPa, which ranks the produced cements in CEM I 42.5N category, More specifically, the cements with FA addition due to their relatively high 2-day strengths (>20 MPa) can be ranked in CEM I 52.5N category. Addition of FA increases the amount of water-soluble chromium proportionally to the amount of total chromium to the mixture; however, conversion of total Cr to hexavalent Cr remains practically constant, in the range of 32-35 wt%. The results indicate that FA can be used as raw material in the production of OPC up to 5 wt% according to the chemical composition of the other raw materials. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Dense sintered esseneite-wollastonite-plagioclase glass-ceramics have been successfully prepared from a vitrified mixture of important inorganic waste (Bayer process red mud, fly ash from lignite combustion and residues from the polishing of porcelain stoneware tiles). The enhanced nucleation activity of fine glass powders, favoured by particular oxidation conditions, caused a substantial crystallisation, even in the case of very rapid thermal treatments at 900 degrees C, which led to remarkable mechanical properties (bending strength and Vickers micro-hardness exceeding 130 MPa and 7 GPa, respectively) and a promising chemical durability. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: The thermal behaviour of clay body mixtures with soda-lime-silica waste glass for the production of traditional ceramics was assessed. The effect of calcite content in the body mixture, of the particle size distribution of glass and of the firing temperature was investigated. In the case of calcite-rich mixtures with glass, increased expansion may take place during firing, at temperatures slightly higher than 700 degrees C. This effect is attributed to the entrapment of released gases, mainly CO2. Sintering starts at lower temperatures for the samples with glass, whereas, a second shrinkage zone was observed for temperatures approaching 1000 degrees C. The content of calcite is one of the main factors determining the mineralogy of the sintered body and the extent of glass devitrification. Devitrite, cristobalite and wollastonite are the products of devitrification within the glass grains, whereas, sodium aluminum silicate, most probably nepheline, has been formed at the inter-granular rim, between glass and ceramic matrix. Increase in the firing temperature and/or decrease in the particle size distribution of glass promotes the densification of the body. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: "Bauxite Residue", BR, is the main by-product of the alumina-producing Bayer cycle. Aiming at its utilisation in the production of heavy-clay ceramics, the thermal behaviour of clay body mixtures with BR was investigated. The process parameters examined were the calcite content in the clay body mixture and the firing temperature, in relation to different BR additions in the clay body mixture. The firing process was studied by means of DTA-TG and dilatometry whereas the mineralogy was determined by XRD. The DTA-TG curves did not reveal cross-reactions between body mixture and BR. However, in the mixtures with BR, sintering initiated at a lower temperature and the firing shrinkage was increased. Moreover, a second shrinkage zone was observed for high BR content and firing temperature above 950 degrees C, suggesting the development of a low viscosity liquid phase. The main mineralogical phases present in the BR modified mixtures after firing were quartz, hematite, clinopyroxenes, gehlenite and plagioclase. The formation of clinopyroxenes and gehlenite seems to be dependant on all process parameters examined, i.e. calcite content in the clay body mixture, BR addition and firing temperature. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: Laser-Induced Breakdown Spectroscopy (LIBS) is used for the analysis of steelmaking slags (EAF, Ladle Furnace (LF) and Vacuum Ladle Degasser (VLD)). In view of the potential industrial application of LIBS for rapid slag analysis, relatively simple criteria were employed in order to obtain calibration curves for the elements of interest. The LIBS experiments were performed on samples prepared after crushing and milling to a size less than 0.2 mm and on as delivered solidified slag samples. In general, the analysis of the solidified as delivered slag samples exhibited better results than the corresponding analysis of the pressed slag, mainly for the elements distributed in the matrix. Thus, for LIBS application, the time consuming sample preparation of the slag can be avoided. According to the results obtained, the potential of LIBS technique for in-situ, multi-element analysis of slag was examined at the 60t- EAF of Helliniki Halyvourgia SA (HH-Greece). The most important parameters that influence slag analysis are the laser pulse energy, the number of accumulated laser shots, the laser beam focusing conditions and the gating conditions of the detector. LIBS accuracy was found to depend on both the distribution of elements in the slag and on their environment (i.e. if they are embedded in the matrix or in a specific crystallized phase) as well. LIBS limitations for quantitative slag analysis are also discussed. The accuracy of FeOn LIBS analysis for as delivered solidified EAF and VLD slags of the steel plant of Georgsmarienhutte was satisfactory showing a Regression Coefficient (RC) of 0.93 and 0.95 respectively. The accuracy of SiO2 and MgO analysis of VLD slags with RCs of 0.94 and 0.86 respectively was also satisfactory. In contrast, for the EAF slags the RC for MgO and SiO2, due to the observed non-homogeneous crystallized phases have shown insufficient values, with 0.55 and 0.43 respectively. Similar results were gained for the FeOn, MgO and SiO2 analysis of the HH slags. Due to the higher intensity of scattering, the analysis of CaO, Al2O3 and Cr2O3 was of lower accuracy than for FeOn, MgO and SiO2.
Abstract: Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Alternatives ways of recycling soda-lime-silica scrap glass (SLS) glass, apart from remelting can offer certain advantages and may be implemented when glass production is not suitable. For the ceramics industry, the incorporation of glass into the ceramic body may improve the quality of fired products and save energy Mixtures of SLS glass and ceramic raw materials used for the production of porcelain stoneware tiles and heavy-clay products were developed and examined. In the case of porcelain stoneware tiles, several modified mixes were prepared by replacing different amounts of the fluxing component, sodium,feldspathic sand For roofing tiles, SLS glass was added at a fixed percentage in the body mixes. The fired samples were characterised in terms of linear shrinkage, water absorption, flexural strength and reliability Microstructure and the main phase transformations in the fired samples were examined by x-ray diffraction and by scanning electron microscopy along with EDS microanalysis. The results, for the porcelain stoneware body mix, shou, that a substitution of the fluxing component in the range of 5-10 wt% is feasible The new formulations had lower water absorptions, adequate flexural strengths and a consistent increase in reliability due to improved homogeneity. For substitutions exceeding 10 plagioclase was crystallised, which inhibited the formation of mullite and did not favour densification. The results regarding the modified body mix for heavy clay products have shown that a glass additions up to 30 wt% are feasible. New crystalline phases were developed within and at the interface of the glass grains. The particle size distribution of the glass and the firing temperature both influenced the physical and mechanical characteristics whereas they had a minor effect on the mineralogy. The fired products demonstrated improved characteristics in terms Of water absorption and bending strength.
Abstract: The feasibility of Al deposition on Mg and Mg-alloys by means of Fluidized Bed CVD (FBCVD) or Pack Bed CVD (PBCVD) processes is evaluated by means of thermochemical computations and experimentation. For the thermodynamic analysis, a closed system consisting of Al2O3 as filler, Al as donor, halide or halide compound as process activator and Ar as inert gas was considered. Different activators were evaluated in the temperature range from 280 to 420 degreesC. Among them NH4Cl, and iodine are the best candidate compounds due to the relatively high partial pressures of the formed aluminium halides. The analysis indicates that the deposition of aluminium on magnesium takes place through a displacement reaction mechanism on the surface followed by limited inward diffusion of Al. Pack bed experiments have shown that by use of NH4Cl activator, aluminium is deposited locally. Iodine activator results in more even but porous coatings. The process is simple and promising for the formation of corrosion and wear resistant coatings on magnesium. However, in order to provide a long-term protection, the coating should be deposited under conditions that eliminate through-thickness pores. Further developments are required in order to achieve dense, pore-free coatings. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: The formation of aluminide coatings on Ni, Inconel 738, NiCr23Fe, SS304 and ARMCO Fe by a Fluidised Bed Chemical Vapour Deposition (FBCVD) process at low temperatures (550-650degreesC) was studied. Aluminium powder and a HCl activator were used to form precursor halide vapours of Al. This led to the aluminide coating formation on the substrate's surface. The examination of the treated samples by means of optical microscopy and SEM/EDX indicated the deposition of dense and homogeneous coatings over the entire surface even for small treatment times. Coating thickness growth and coating morphology were evaluated for various treatment times and temperatures. The low temperature process has advantages compared to the high temperature process since the mechanical properties of the treated parts are not affected, although the coating thicknesses achieved are smaller.
Abstract: The influence of laser pulse duration on laser-induced breakdown spectroscopy (LIBS) calibration curves is investigated in the present work. Two Nd:YAG lasers providing pulses of 35 ps and 5 its, respectively, both operating at 1064 nm, have been used to create plasmas on aluminium, manganese, iron, and silicon targets and on prepared stoichiometric samples of these metals in a matrix. The time-resolved, space-averaged plasma temperatures have been deduced using Boltzmann plots, while the electron number density has been determined from the broadening of spectral lines. The effect of laser pulse duration on the plasma characteristics is discussed, and comparisons are made with previously reported data measured under similar experimental conditions. The optimum experimental conditions (i.e., time delay, gate width, laser energy) have been determined for reliable use of LIBS for quantitative analysis for both pulse durations. For each of the metals of interest, calibration curves have been constructed for concentrations ranging up to 2%. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: In this paper the utilization of ferronickel electroreduction furnace (FeNi-ERF) slag for the production of high-alumina cement (HAC) is investigated through laboratory and pilot-scale tests. The process followed consisted of smelting reduction of slag mixtures with low-grade diasporic bauxite and limestone. In the laboratory-scale trials the main process parameters were defined, concerning raw material proportions, kinetics of the reductions and cooling rate of the product. The presence of a carbon-containing iron bath enhances FeOx reduction from the slag. Products from laboratory tests developed satisfactory compressive strengths relative to those of commercial HAC. According to the results of the laboratory tests, pilot-scale heat treatments were carried out in a 5-t electric arc furnace (EAF) and about 4 t of final mixture were produced. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Aluminide coatings have been deposited on ARMCO iron and an iron-chromium alloy (15 at.% Cr by a fluidised-bed chemical vapour deposition (FBCVD) process at 1000 degreesC with the use of FeAl donor powder and an NH4Cl activator. Examination of the coated samples by means of optical microscopy, SEM and microanalysis has shown the formation of dense and homogeneous coatings consisting of an FeAl outer phase and an alpha-solid solution inner phase. A simplified process model based on the diffusion equations of aluminium in pure iron, considering the intermetallic compounds according to the FeAl phase diagram, has been derived. The model yields the aluminium composition profiles as a function of time for particular process conditions and can be used for evaluation of the coating growth rate and morphology during aluminisation. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The potential of VFB(very fine bubbling)-technology in steelmaking, developed for the production of super clean steels, was investigated. Recent R&D work has proven that with very fine argon bubbling through a developed special porous plug (SPP) at low flow rates, the total oxygen content of low carbon steel grades can be lowered to a level of 6 ppm under industrial vacuum conditions [1; 2] and to a level of 10 ppm under argon protective atmosphere [2]. The perspective of industrial application of the VFB technology to a 56-t ladle furnace of Helliniki Halyvourgia S.A., Greece, in order to improve steel cleanliness, requires additional R&D efforts. It is important to define the limits of VFB technology in respect of alloys dissolution, mixing time and homogenisation of steel and slag/metal reactions. In this work, a gas driven bubble aqueous reactor model simulating the bottom gas stirred ladle by means of gas injection through a SPP and a conventional porous plug was studied. Various operating conditions as well as different positions for the porous plug with and without a top oil layer were simulated. Tests concerning mixing time, solid-liquid mass transfer and critical gas flow rate, liquid/liquid mass transfer, using the SPP and a conventional porous plug have been performed. The evaluation of experimental results delivered important information for the design and operation of steel ladles, applying VFB-technology. Experimental results with SPP bubbles' agitated steel (1600 degreesC) in laboratory and technical scale experiments in IF and VIF are presented and discussed.
Abstract: Fine bubblers to be used as special porous plugs (SPP), have been devised for fine bubbling within steel melts for improving the cleanliness efficiently by floating the micro-inclusions and removing the dissolved gases. The development principles of the magnesia-spinel porous plugs are presented. Laboratory water simulator tests have shown that small (< 1 mm), non-coagulating bubbles, with uniform distribution are formed. The characterisation and testing of the SPP has shown that under steelmaking conditions clogging from metal infiltration is limited. Erosion, cracking or changes in microstructure were not detected in high temperature long-term endurance tests. With combined treatment in a vacuum induction furnace under a 3 kPa pressure and Ar purging of 18 min through the new SPP, the total oxygen content of the steel melt was decreased from 65 to 7.7 ppm. An application has been filed for a patent for the method for the production of fine bubblers to be used as porous plugs.
Abstract: The present work concerns the formation and modelling of aluminide diffusion coatings onto nickel and nickel alloys by means of a fluidised bed chemical vapour deposition (FBCVD) process. In this process, the treated object is suspended within the FB reactor, which contains the treating agent. The latter is a mixture of powders of the donor of the coating-forming element (FxAly), and of a filler material (Al2O3). As an activator of the process, a halide compound is used (NH4Cl). The bed is fluidised by inert gas (Ar). The precursor vapours of the element to be deposited are formed in situ, by the reaction of the donor with the activator. The produced coatings are characterised by optical microscopy, SEM and EPMA. The modelling of the nickel-aluminide coating growth on nickel, has been based on the diffusion equations of aluminium in pure nickel, considering the intermetallic compounds predicted by the Ni-Al phase diagram. The model predicts the aluminium composition profiles as functions of the time and the concentration of Al in the donor, for the aluminisation conditions under consideration. The model is in satisfactory agreement with experimental results. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: The present work concerns the formation of chromium and/or aluminide diffusion coatings on nickel, by means of a pack and fluidised bed chemical vapour deposition (PBCVD and FBCVD) process. The object to be treated is suspended within the reactor, which contains the treating agent The latter is a mixture of powders of the donor of the coating-forming element (Al, Cr, Al-Cr) and of a filler material (Al2O3). As an activator of the process, a halide compound is used (NaCl, NH4Cl, NaCl + NH4Cl, HCl) in an inert atmosphere (Ar). The precursor vapours of the element to be deposited are formed in situ, by the reaction of the donor with the activator and react with the substrate surface for the formation of the coatings. The produced coatings are characterised by optical microscopy and SEM techniques (EDS), in order to obtain their surface composition, the coating thickness and quality and the aluminium and chromium distribution along the coating-substrate interface, The formation of Al-Cr diffusion coatings was achieved in a short time PB and FB process, accompanied by a clearly visible diffusion zone.
Abstract: Cr-Al diffusion coatings have been modified by Hf addition using a one step halide activated pack cementation process. The substrate was a low alloy steel 2.25Cr-1Mo. Cyclic Oxidation properties of the coatings were evaluated, and a Hf modified coating was the most resistant coating. Codeposition pack thermodynamics were computed and analyzed using available thermodynamic tools in order to compare experimentally obtained results with theoretical predictions.
Abstract: The present work is concerned with modelling titanium carbide coating growth on carbon containing steels by CVD without the presence of hydrocarbons in the gas phase. The model is based on a simultaneous solution of the mass balance and diffusion equations of the system, allowing solutions for variable carbon concentration in steel. The obtained numerical results have been compared to experimental coating growth measurements of TiC coatings produced on steels from previous studies using conventional CVD, as well as the fluidized bed CVD (FBCVD) process. The model results are ill a good agreement with the experimental results in both cases. (C) 1999 Elsevier Science S.A. All rights reserved.
Abstract: A Fluidised Bed Chemical Vapour Deposition (FBCVD) process was applied for the formation of aluminide coatings on iron, nickel and NiCr23Fe nickel alloy. This process is promising since it can lead to a flexible surface treatment combining adaptability with relatively low capital and operational costs. The method involves a treating agent consisting of a powder mixture of Al2O3 with FeAl which is fluidised at elevated temperature with Ar+10% H-2. As an activator of the process NH4Cl is used. The results of optical microscopy, electron probe microanalysis (EPMA), X-ray diffraction, microhardness, surface roughness and hot corrosion tests indicate that with the FBCVD process the production of high quality nickel/iron aluminide coatings is feasible.
Abstract: A thermodynamic approach of a fluidized bed chemical vapour deposition process for the formation of Ti-bearing coatings on steels is attempted. For the thermochemical analyses, a system consisting of Al2O3 or SiO2, Fe2Ti, Fe and C as well as NH4Cl or NH4I as activators of the process was considered. The partial pressures of the equilibrium gaseous species and the condensed phases of the above closed system have been calculated for amounts of the above constituents analogous to those used in the experimental practice and for different amounts of NH4Cl activator. The analyses indicate, that in all cases, the Ti-halide vapours, which are necessary for the coating formation, exist in the examined temperature range 773-1473 K. Different modes for the coating deposition are discussed. With NH4Cl as activator higher deposition rates of the coatings can be expected, in comparison to the NH4I. SiO2 is not a good candidate as filler oxide as it reacts with the fluidizing medium. Although the process is feasible without hydrogen, addition of 10% of the latter is beneficial for the process as it increases the partial pressures of the Ti-chlorides.
Abstract: Nickel losses in the slag generated at Larco electric reduction furnaces (ERF) during smelting of Greek laterites account for about 20% of the total nickel of the feed. The recovery of the nickel contained in the slag as well as in its magnetic concentrate was studied under different methods. The experimental results have shown that a nickel recovery in the range of 75-90% is possible by allowing the slag to settle. Washing by low carbon steel can be performed during the final stages of the settling process as it improves the finally attained nickel recovery up to 94.4%. The simultaneous carbon reduction and settling process do not favour the Ni- recovery as the produced carbon monoxide and dioxide cause boiling of the bath which promotes the flotation of the metallic grains. In all cases a reoxidation of nickel was observed after 20min approximately. Settling of the ferronickel grains was described by a mathematical model. The model's results are in good agreement to the experimental ones for the initial 20min of the process. A modified model was elaborated combining the settling of the metallic grains with the nickel reoxidation reaction. This model describes with adequate accuracy the whole Ni- recovery process. The developed model can be proven helpful in the design of a slag cleaning step for metal recovery utilizing a slag settling furnace operating on line to the ERF.
Abstract: Attempts have been made to clarify the feasibility of a fluidized bed CVD (FBCVD) process as another method for the formation of cementing and diffusion coatings in order to improve wear, oxidation and corrosion resistance of metals. Treating agent powders consist of a donator, usually a powder of the metal, an alloy or a metal carbide or nitride of the coating forming metal, an activator, usually a halide or alkali metal halides, and a filler oxide, usually Al2O3, SiO2 etc. The precursors of the coating forming metal are formed ''in situ'' the fluidized bed reactor by the reaction of the donator with the activator. The bed can be fluidized by inert or reactive gas or gas mixtures like Ar, N-2, Ar + H-2 etc. As the parts can be charged and withdrawn while the furnace is at treating temperature, this process can be integrated into the heat treatment cycles. The FBCVD process was applied for the formation of Ti-, V-, Cr-, Al-, and Si-bearing coatings. The produced coatings were dense and uniform over the whole specimen surface. According to the results, the use of the FBCVD process is promising since it can lead to a clean, flexible surface modification technology, combining adaptability with relatively low capital and operational costs.
Abstract: The use of a fluidized bed process for the production of hard wear- and corrosion-resistant coatings of carbides, nitrides and carbonitrides of cementing metals is promising since it can lead to a flexible surface treatment technology combining adaptability with relatively low capital and operational costs. In this paper the feasibility of a fluidized bed process for the formation of titanium carbide coatings using chemical-vapour-deposition-based reactions is discussed. The method involves a treating agent consisting of a powder mixture of Al2O3 with FeTi which is fluidized at elevated temperature with Ar. As an activator of the process NH4Cl was used. The use of explosive hydrogen is avoided by the formation of the vapours of the materials to be coated in situ in the reactor. With this method, adherent and dense coatings of TiC were applied on various steel grades.
Abstract: The sessile drop technique is used to measure the contact angles of molten Si, Sn, Cu and Ni in contact with mono- and polycrystalline alpha-SiC as well as CVD beta-SiC in purified argon atmosphere and at various temperatures. The contact angle of silicon, near its melting point, is about 38-degrees on a mono- as well as polycrystalline alpha-SiC substrate and about 41.5-degrees on beta-SiC. Tin does not wet the SiC. Using data from the available literature, the work of adhesion and the interfacial energy between SiC and Si or Sn were calculated. In the alpha-SiC-Sn system, both quantities are linearly dependent on temperature in the investigated temperature range 523-1073 K. The metals copper and nickel react with silicon carbide. The silicon content of the copper drop depends on the annealing temperature. The nickel drop after cooling forms the compound Ni3Si2. The interferometric measured groove angle of SiC (thermal etching) in vacuum at 2020 K gives a mean value of 157.6 +/- 5.8-degrees.
Abstract: A mathematical model of the electro-reduction furnace process (ERF process), for the production of ferronickel from laterite ores, has been tested at Larco's metallurgical plant at Larymna Greece. The theoretical predictions for the progress of the nickel reduction have been compared with the operational results. The tests were carried out in the ERF no. 2 of Larco's plant. The influence of the main process variables on the nickel recovery and nickel losses in the ERF slags have been investigated in conjunction with the variation of the analysis of the feed and the nickel content of the metal bath. The further application of the model to the metallurgical practice is discussed. According to the predictions of the model the further development of the process in the direction of the production of low-nickel ferronickel (Ni < 15%) is analyzed. The advantages of the production of low-nickel ferronickel are presented and the possibility of its straight conversion to nickel-bearing steel-grades is discussed.
