F. Ajersch

The effect of oxygen concentration on the oxidation of low-carbon steel in the temperature range 1000 to 1250°C

This paper describes the oxidation behavior of low-carbon steel samples in binary gas mixtures of oxygen and nitrogen, at oxygen concentrations ranging between 1% and 15% and temperatures ranging between 1000 and 1250°C. Sample weight gains versus time were analyzed, along with measurements and calculations of sample heating rates due to exothermic heat of reaction at the sample surface. It was found that initial rates of oxidation depended on oxygen content in the gasmixture and that these reaction rates were linear up to oxide thicknesses of 0.4 to 0.5 mm. Calculations of linear oxidation rate constants based on equations for mass transport of oxygen in the gas mixture to the sample surface showed good agreement with those measured experimentally, indicating that the initial period of oxidation is controlled by the mass transport of oxygen to the reaction interface. The linear rate constants showed little dependency on temperature, an activation energy of approximately 17kJ/mole being obtained. Measurements of sample surface temperatures have shown that within this linear-oxidation regime, interfacial temperatures of the samples increase with increasing oxygen contents in the gas mixture, owing to exothermic heats of oxidation. Subsequent oxidation kinetics were found to be parabolic. Measured parabolic rates constants were in good agreement with previous investigations, with activation energy values of approximately 127kJ/mole.

Semi-solid processing of hypereutectic A390 alloys using novel rheoforming process

The feasibility of semi solid processing of hypereutectic A390 alloys using a novel rheoforming process was investigated. A combination of the swirl enthalpy equilibration device (SEED) process, isothermal holding using insulation and addition of solid alloy during swirling was introduced as a novel method to improve the processability of semi solid slurry. The effects of isothermal holding and the addition of solid alloy on the temperature gradient between the centre and the wall and on the formation of α(Al) particles were examined. In additional tests, phosphorus and strontium were added to the molten metal to refine the primary and eutectic silicon structure to facilitate semi solid processing. The results show that the combination of the SEED process with two additional processing steps can produce semi-solid A390 alloys that can be rheoprocessed. The microstructure reveals an adequate amount of non-dendritic α(Al) globules surrounded by liquid, which greatly improves the processability of semi-solid slurry.

Experimental study of the effect of nitrogen on titanium-arc cathode erosion

Experimental data on the dependence of titanium erosion rate on cathode temperature, Ar, and N/sub 2/ pressure from vacuum to 1 torr and arc motion are presented. Erosion rate is found to decrease with conditions that promote cathode poisoning/contamination. Higher cathode temperatures result in enhanced nitriding (poisoning), leading to a reduced erosion rate. A critical nitrogen pressure (0.001 torr) exists where a sharp drop in erosion is measured. Steered arcs show lower erosion rate values of 38 and 15 /spl mu/g/C for argon and nitrogen, when compared to random arc values of 45 and 35 /spl mu/g/C. Erosion rate studies on TiN-coated cathodes show a low value of around 22 /spl mu/g/C.

Solid-Phase Synthesis of Calcium Carbide in a Plasma Reactor

A laboratory-scale spout-fluid bed reactor with a dc plasma torch was used to study the solid-phase synthesis of calcium carbide. Calcium oxide powder with a mean particle size of 170 μm was reacted with graphite powder (130 μm). Argon was used to initiate the plasma and hydrogen gas was then added to increase power and raise the plasma jet enthalpy. Experimental results showed that the reaction took place in the vicinity of the plasma jet and that conversion to calcium carbide increased linearly with reaction time. The rate of conversion increased exponentially with plasma jet temperature, indicating that chemical reaction was the controlling mechanism. Microscopic analysis of the solid product showed that calcium carbide was formed around both reactants, and that the reaction followed a shrinking core model. Although melting and agglomeration of partially reacted particles occurred at high temperature, resulting in instability of the bed and impeding the reaction progress, high conversions are expected in a continuous process with optimized reactor design.

Surface energy and chemical characteristics of interfaces of adhesively bonded aluminium joints

The deleterious effects of water on the physico-chemical aspects of the durability of adhesively bonded aluminium joints has been investigated. Emphasis was placed on analytical techniques that lead to the better understanding of changes induced at the metal/epoxy interface by the presence of water. Analytical techniques such as contact angle measurements, X-ray photoelectron spectroscopy and X-ray diffraction were practical in obtaining information on the chemical composition and structure of the samples. Chemical conversion-coated samples were found to be much more stable in wet conditions compared to samples without conversion coating. The surface structure on the adherend, after extensive immersion times, was found to consist of a compact layer of bayerite with traces of gibbsite. The calculations of interfacial energy, γsl, and work of adhesion in dry and wet conditions, WA and WAl were carried out to evaluate the maximum bond strength. It was found that the durability of the bond in the presence of water was quite high for the XD4300/6061-T6 joint systems.

THREE-DIMENSIONAL NUMERICAL SIMULATION OF TURBULENT FLOW AND HEAT TRANSFER IN A CONTINUOUS GALVANIZING BATH

This article presents the application of a three-dimensional finite-element solution algorithm for the prediction of velocity and temperature fields in an industrial continuous galvanizing bath. The effect of line speed, strip width, strip temperature, and inductor mixing are evaluated. Simulations were carried out using a parallel computational fluid dynamics (CFD) software developed at the Industrial Materials Institute, Natural Research Council of Canada. The incompressible Navier-Stokes equations are solved for turbulent flows using the k - k model. Both forced-convection and temperature-dependent density conditions are considered in order to assess the buoyancy effect. When considering the buoyancy, the flow induced by variations in density is especially apparent near the inductors and the melting makeup ingot, while little effect is observed in the sheet and rollers region. Thermal effects are also amplified when the inductor is at high capacity, during the ingot melting. Simulations allow visualization of regions of varying velocity fields and clearly illustrate the mixed and stagnant zones for different operating conditions.

Numerical simulation of flow, temperature and composition variations in a galvanizing bath

Abstract The modern hot dip galvanizing operation is a complex process subject to a number of configurational, physical, chemical and kinetic parameters. Small decreases in temperature can precipitate intermetallic dross particles, which can be entrained in the flow towards the strip leading to surface imperfections. The numerical simulations carried out in this study clearly define the spatial distribution of velocity, temperature and compositional variation in the bath. The modeling of the transient effects during ingot melting and non-melting periods have also identified the critical periods and zones within the bath where dross formation can occur within an operating cycle. L’opération moderne de galvanisation à chaud est un procédé complexe exposé à plusieurs paramètres configurationnels, physiques, chimiques et cinétiques. De petites diminutions de température peuvent précipiter des particules intermétalliques de scories, lesquelles peuvent être entraînées dans l’écoulement vers la bande, conduisant à des imperfections de surface. Les simulations numériques effectuées dans cette étude définissent clairement la distribution spatiale de la vitesse, de température et de la variation de composition dans le bain. La modélisation des effets transitoires lors des périodes de fonte et d’absence de fonte du lingot a également identifié les périodes critiques ainsi que les zones dans le bain où la formation de scorie peut se produire à l’intérieur d’un cycle d’opération.

Deposition of TiN films using a thermal plasma reactive forming technology

Abstract TiN films have been deposited on graphite substrates by a novel reactive plasma technique. In this process, NH3 and TiCl4 are reacted in the jet of a plasma torch and are deposited onto a substrate forming TiN films at a high growth rate (above 2 μm min−1). It was found that the process parameters strongly affect the microstructure, thickness, density, and growth rate of the TiN films. These parameters include N : Ti ratio, temperature of substrate, spray distance and radial position away from the impingement point. In general, it was found that thick and dense coatings were deposited when the N : Ti ratio was kept low and the substrate temperature was high. Very dense and thin TiN coatings were deposited 3–4 cm away from the impingement point. These coatings are believed to be the result of a gas phase (chemical vapour deposition type) reaction. The limitations and advantages of this technique are also discussed in this paper.

Oxidation of molten ferrous sulphide

Molten ferrous sulfide of sulfur content lower than stoichiometric FeS was oxidized at 1200 and 1230°C in a stream of oxygen and argon gas mixture under conditions where the overall reaction rate was controlled by the diffusion rate of the gaseous components. The weight change of the sample during the oxidation experiment was recorded continuously. Initially, the melt absorbed a certain amount of oxygen and reached a composition very close to the FeS-FeO pseudo-binary system. Following this, the FeS oxidized to produce FeO and the sample weight decreased. As the mole fraction of FeO increased, the activity of magnetite was found to increase rapidly and the formation of appreciable amounts of magnetite occurred.

Modelling of a plasma reactor for the synthesis of calcium carbide

Abstract Plasma synthesis of calcium carbide was investigated using calcium oxide and graphite powders. A semi-batch spout-fluid bed reactor with a DC plasma torch was used for the study. Argon was used to initiate the plasma and hydrogen gas was then added to increase power and raise the plasma jet enthalpy. Experimental results showed that the reactor consisted of two different zones : a high temperature reaction zone and a well mixed isothermal bed zone. The size of the reaction zone and the particle flow rate into the jet depended on the net plasma power. The reaction rate was correlated by a shrinking core, reaction control model and showed excellent fit for conditions where hydrogen was present in the plasma gas and heat transfer limitations were negligible. The apparent activation energy of the reaction was determined to be 377 kJ/mol (90 kcal/mol). Extrapolation of the reaction model and the experimental results indicated that a plasma fluid bed process could be a technically viable and a more efficient alternative for the production of calcium carbide.