Year:2007 | Volume:40 | Issue:7 (101)|Papers Count:11

Al2O3-SiC-C castables are one of the advanced refractories which are favored in many applications because of the high corrosion resistance, thermal shock resistance, thermal conductivity and mechanical strength. They are mainly used in troughs and slag/metal runners of blast furnaces. In Iran for lining of runners, ramming mixes which are the oldest refractories used for such application are still in use. In this study a typical Al2O3-SiC-C castable was selected and the influence of amount of silicon carbide on properties was investigated. Bulk density, apparent porosity, permanent linear change, cold crushing strength and cold modulus of rupture of samples was determined at 110, 1000, 1250 and 1500°C. Phase analysis was carried out on all specimens fired at different temperatures.Mean while crucible and finger corrosion techniques were performed to determine corrosion resistance of the samples. Micro structural investigations were also done on all fired and corroded specimens. The results showed that increasing SiC content caused a slight increase in water demand, as well as decrease in mechanical strength and caused porosity increase. However, bulk density was not affected significantly Corrosion resistance was also decreased.

Boron carbide, which has a high melting point, outstanding hardness, good mechanical properties, low specific weight, great resistance to chemical agents and high neutron absorption cross-section (10BX C, x〉4 x ) is currently used in high-technology industries for example, fast breeders, light weight armors, high-temperature thermoelectric conversion, wear resistance applications, e.g., as blast nozzles, wheel dressing tools and nuclear industry, e.g., shielding material and control rods etc.Boron carbide can be synthesized by a variety of high temperature methods, such as the direct reaction of carbon with boron; carbonthermal reduction of boron oxide (B2O3) over 1000ºC; reduction of  BCl3 by 4 CH at a temperature of 1500ºC with laser; thermal decomposition of a mixture of pure carbon and boron trilogies in an atmosphere of hydrogen; gaseous reaction between BCl3 and a methane-hydrogen mixture in r.f. argon plasma.The most commercially viable and industrial method is from reduction of boric acid with carbon black at a temperatures over 1750ºC .The product thus obtained is hard and consists of excess unreacted carbon. The hard mass is crushed and pulverized to the requisite mesh size and purified by chemical and thermal oxidation method because of their contamination from grinding media. In this work, boron carbide was produced from inexpensive raw materials such as boric acid (boron source) and citric acid (carbon source) by sol-gel method. Aqueous solution of boric acid in presence of citric acid forms a stable gel under controlled pH condition. The gel on subsequent pyrolysed under vacuum. The precursor from primary pyrolysis with stoichiometric composition was heated in a tube furnace under vacuum to 1250, 1350, 1450, and 1500ºC with a heating rate of 5ºC / min and held for 2.5 hr at temperature.XRD patterns were taken from the samples prepared at different temperatures. From the patterns, it was found that no reactions have occurred at 1250 ºC and the product contains B2O3 and C. By increasing temperature to 1350 ºC it looks that some reactions have taken place, but unreacted B2O3 and C are seen yet. t. Whereas, at 1450 and 1500 ºC no B2O3 is observed and the only impurity seen is C. This indicates that at 1450 ºC and higher temperatures the reaction is completed. From the SEM patterns it can be seen that the shape of the particles is somewhat between spherical and elliptical. The free carbon and particle size of the product were analyzed.

Microstructure and shape memory properties due to the formation of a rhombohedral phase (R-phase) formation in nickel rich binary Ni-Ti alloy containing 50.23 atom% Ni were investigated in this research. After annealing, the samples were aged for 1, 5 and 7 hours at 773K. Microscopic studies revealed that the rhombohedral phase heterogeneously nucleated and then grew within the regions near the grain boundaries of the alloy. It was also observed that the amount of this phase increased with the ageing time. Microstructural TEM observations indicated that a considerable number of dislocations were simultaneously produced during the R-phase formation as a result of the ageing process. It was, therefore, suggested that the formation of the R-phase regions was related to the presence of the stress fields produced around the dislocations. With increasing of these strain fields, possibility of direct austenite to martensite transformation apparently reduced; while the formation of the intermediate R-phase regions grew. A diverse new set of twins and self accommodated twin envelops were observed in martensitic microstructure of the samples together with the pseudo-stable R-phase regions.

The effect of bainitic microstructure obtained from austempering on mechanical properties of a series of microalloyed cast steels containing Ti-V-B and V-only in comparison with base composition is investigated. The use of microalloyed wrought steels is widespread throughout the construction industry. However, the use of microalloys to increase the strength of lowcarbon content cast steels, without significant reductions in toughness or weldability, is still in its infancy.For the wrought microalloyed steels, excellent toughness and weldability are achieved by lowering the carbon content. Strength is maintained by adding small amounts of microalloying elements (usually vanadium, titanium and niobium) that precipitation strengthen the material during thermomechanical processing. Similar demands for low-cost, higher-strength steel casting with good toughness and weldability indicate a wide range of potential applications where microalloyed cast steels may be appropriate.Although these overseas experiences with these new materials have been reported, detailed information on the alloying and processing variables affecting mechanical properties is not available. The physical metallurgy principles governing the production of cast and wrought microalloyed steels are similar; however, significant differences can be expected in the final compositions, microstructure and processing.In this paper, two different alloys of microalloyed cast steels and a base composition were prepared and effect of produced microstructures by austempering on mechanical properties was evaluated. The samples were austempered at 370, 410 and 450ºC for 5, 100, 300, 600, 900, 1200 and 1500 s.Results indicated that the hardness, strength decrease and toughness, elongation increase with increasing austempering temperature respectively. Also the hardness, strength decrease and toughness, elongation increase with increasing austempering time respectively. A good combination of strength and toughness is achieved by developing a microstructure of martensite and bainite.University College of Engineering, University of Tehran 3 Examination of the microstructure by optical microscope and SEM showed that the above microstructure is achieved in the composition containing Ti-V-B by austenitizing at 900 ºC followed by holding at 410ºC for 10 min. In this case an ultimate tensile strength of 850 MPa and an elongation of 15.5% were achieved.

Cold metal forming has received considerable attention in recent years. Considering that in most metal forming processes, there exists a compressive stress state, the study of deformation behavior of metals under this condition is of great importance. In this study, the cold upset of aluminum and brass solid cylindrical specimens with different aspect ratios (1.5, 1.0 and 0.5) under different frictional conditions has been investigated. The upsetting tests have been carried out at two different strain rates of 10-2 and 10-3 s-1 and under lubricated and unlubricated conditions. Using PTFE lubricant sheets decreases the friction and thus eliminates barreling phenomenon. In both of the materials under unlubricated condition, decreasing the aspect ratios decreases the stress required for deformation at a constant strain while in the lubricated condition, the behavior is reversed. Using the difference between the stress–strain curves in the lubricated and unlubricated conditions, the contribution of friction and redundant work to the deformation has been studied. For brass, the curves for the stress differences are closer to each other. This may be caused by the presence of undissolved lead in the brass specimens which act as an internal lubricant, causing a lower friction level and thus, a lesser difference between the curves. The calculated friction coefficients were found to have constant values of 0.15 and 0.08 for aluminum and brass, respectively. These values were independent of strain rate and aspect ratio. It has been also found that the value of this coefficient for brass specimens is lower than that of aluminum. It has been observed that change of strain rate does not affect the stress– strain curves for both aluminum and brass specimens.

Rhenium hepta-oxide (Re2O7) is the most prominent Re compound industrially produced as a by-product of molybdenum roasting process. This material is highly soluble in water resulting in formation of ReO4- anions dissolved in a weak acid medium. For ReO4- extraction, different ionexchange (IX) resins can be used to adsorb the metal containing anion. Ion exchange reactions are generally obscure with unknown stoichiometric coefficients simply assumed equal to unity by majority of researchers.These coefficients play a significant role in the thermodynamics and kinetics calculations carried out for design and optimization of the IX systems. Combining the experimental data obtained for the three different brands of resins used in this investigation with the equilibrium correlations, the stoichiometric coefficients of the ReO4- anion exchange reactions with Cl- anion were mathematically determined.Utilizing the stoichiometric coefficients obtained and measuring the effect of temperature on the rhenium distribution ratio between solution and the resins, the apparent standard Gibbs free energies of the ion exchange reactions for standard states were evaluated for followings: (a) ionic resin: resin leached in 1 normal hydrochloric acid aqueous solution, (b) metal containing solution: 1 normal potassium perrhenate and (c) salt produced by exchange reaction: 1 normal potassium chloride dissolved in water were determined. The said reactions were found to be as follows:ΔGºapp= 56.781-0.274T (j/mol) for Amberlite,ΔGºapp= 53.047-0.257T(j/mol) for VarionΔGºapp= 50.205-0.247T(j/mol) for PuroliteAnionic resins.

In this research bioleaching of Sarcheshmeh molybdenite concentrate was evaluated using a native strain of Acidithiobacillus ferrooxidans for extraction of rhenium. The research showed that dissolution of rhenium is mainly dependent to the complete dissolution of molybdenum. This may occur due to the substitutional replacement of molybdenum by rhenium in the molybdenite lattice, (because the atomic radius of rhenium is very close to that of molybdenum). The experiments showed that precipitation of molybdenum as molybdenum oxide in the solution was possible, but it was not possible for the rhenium. Hence, this was used to separate selectively rhenium and molybdenum in the solution. Experiments were expanded to determine optimum conditions for the dissolution of molybdenum from molybdenite and precipitation of molybdenum in the solution. The effect of applying other energy sources, different pulp densities and culture media in the experiments were investigated. Applying 1% pulp density, 9K culture medium and addition of ferrous sulfate bioleaching tests showed that maximum rhenium extraction was 7.3 % while molybdenum extraction was 0.54 %. The native strain of the Acidithiobacillus ferrooxidans was able to grow on the media that contain up to 250 mg/L molybdenum while pyrite was an additional source of energy. This caused approximately three times more molybdenum uptake than those reported in the literatures for Acidithiobacillus ferrooxidans.

Molten magnesium alloys oxidizes rapidly in contact with air and surface oxide layer forms over surface of melt immediately.Due to high rate of oxidation, the molten of magnesium alloys are sensitive to surface turbulence during casting. The influence of casting defects on the mechanical properties of aluminum castings has been studied by a number of researchers, with the common conclusion that oxide films play a major role in the formation of these defects. Campbell shows there is a critical velocity for any of molten metals that over it, surface turbulence is occurred. Experimental work consisted of the casting of plates, 10 mm thick, of AZ91 in resin – bonded sand moulds. The metal was introduced into the mould cavity at different mold entry velocities by changing the dimensions of a bottom-gated filling system.Video recording of the filling of glass-fronted molds was a valuable technique to see the filling behavior of the alloy and the mould entry velocity of flow through the gates were measured. The models were used to investigate the liquid metal flow behavior at different in gate velocities. Distribution of macro defects including gas porosity and shrinkage area indicated in x-ray radiography. Three point bending test carried out for determine of mechanical properties in horizontal direction in different velocities.Some of samples prepared for scanning electron microscopy (SEM) fractography. In addition, energy dispersive X-ray (EDX) was applied on fractured bend test specimens.Results indicate that In-gate velocity affected on distribution of properties and performance of castings. In 10-mm plates, smooth, layer- by – layer filling is seen at velocities up to about 300 mm/s, at 500 mm/s, the mushroom morphology of entry of the melt is well developed. Vertical jet geometry of filling is observed at velocities higher than 1000 mm/s.In lower velocities, surface turbulence is University College of Engineering, University of Tehran 5 decreased and the entrainment is lower, so superior properties are expected. Scanning electron fractograph showing dendrites in shrinkage pores in association with oxide films and EDX microanalysis of oxide film on fracture surface, revealing high oxygen and magnesium peaks. Due to be quiescent of melt in further from gate, mechanical properties will be improved.

Barium hexaferrite (BaFe12O19) as a eramic magnet with outstanding magnetic properties has various applications in telecommunication, data storage devices and electronics. Various non-conventional methods like sol-gel, co-precipitation and hydrothermal have been employed to synthesize such a magnets. In this work, barium hexaferrite particles were prepared by self-propagating high temperature synthesis method using iron, iron oxide and barium nitrate as starting materials. Fet/Ba molar ratio was adjusted to 9 with different ratios of Fe/Fe2O3. Effect of iron content on the phase constitution and the reactions occurred during SHS and annealing treatment were investigated. XRD, DTA/TGA, VSM and SEM techniques were applied to characterize the resultant powders. Heat released from oxidation of iron sustained the synthesis process in a propagating manner.The oxygen sources for oxidation of iron were 1. air ( external source) and 2. barium nitrate which acted as an internal oxidizing agent SHS process resulted in small content of barium hexaferrite, only in sample with Fe/Fe2O3=1.3. SHS process in sample with Fe/ Fe2O3=2 had a good speed and combustion front had a non-stop movement. XRD results showed this sample contained barium monoferrite, wustite, magnetite and hematite. DTA trace of sample with Fe/ Fe2O3=2 revealed an exothermic peak at 828ºC which may be corresponded to the formation of barium hexaferrite. Annealing of SHS products resulted in gradual formation of barium hexaferrite phase. The results indicated that the content of barium hexaferrite magnetic phase increased by increasing the annealing temperature and in 1150ºC hematite did not exist but small amount of barium monoferrite was still present. It can be concluded that for achieving single phase of barium hexaferrite, higher Fe/Ba molar ratio should be used.Scanning Electron Microscopy (SEM) showed hexagonal particles with particle size of 1 to 2 μm in sample annealed at 1150ºC for 2 h in air. Saturation magnetization and coercivity were measured 54.7 emu/g and 1.7 kOe, respectively for sample annealed at 1050ºC for 2 h in air.

Aluminum-silicon-magnesium (Al-Si- Mg) alloys, due to their excellent cast ability and high strength-to-weight ratio, are the most widely used aluminum foundry alloys. This alloy is extensively used in aerospace and automotive industries. The cast microstructure of these alloys consists of a primary phase, aluminum or silicon and a eutectic mixture of these two microcomponents.In addition, the mechanical properties of these alloys are mainly dictated by the characteristics of eutectic microcomponents in the microstructure. Furthermore, many investigations have been conducted on the semi-solid macrostructures to control the matrix (Al-alpha) and eutectic phase (silicon) morphologies.As the thermomechanical treatment (TMT) is one of the most effective materials processing techniques, the objective of this research was to study the influence of TMT parameters on Si morphology in aluminum 356 alloys with two semisolid and conventional casting macrostructures. Therefore, hot compression tests have been carried out in a range of temperature (420°C to 540°C) and strain rate (0.1, 0.01, and 0.001 sec-1). The results indicated that the morphology of Si phase was changed from acicular to spherical shape and:1. The observed dynamic flow softening was mainly related to silicon platelets fragmentation and spheroidization. 2. The faults driving force approach was adapted to explain the silicon platelets fragmentation and spheroidization under deformation at high temperatures. 3. Maximum UTS and El for alloy in 540°C, 0.1 sec-1 and 540°C, 0.001 sec-1 respectively.

The effects of microstructure on yield stress at different test temperatures of superalloy IN-738LC were investigated. IN- 738LC is the low carbon version of Nickelbase superalloy IN-738. It is designed to provide the gas turbine industry with an alloy which will have good creep strength combined with the ability to withstand long time exposure to the hot corrosive environments associated with the engines. The good mechanical properties of Ni-base superalloys are mainly due to the presence of the long range ordered ý precipitates with LI2 structure. For the purpose of studying size and distribution of precipitates in IN-738LC in Standard Heat Treatment (SHT) condition Scanning Electron Microscope (SEM) was used. The results showed that after SHT, the microstructure of superalloy IN-738LC consists of coarse precipitates with cuboidal morphology and fine precipitates with spherical morphology. Transmision Electron Microscopy (TEM) at SHT condition showed that the dislocation density in this condition is low. Tensile tests were performed on standard heat treated samples, in the temperature range of 25º C to 85 ºC. Results show that the yield stress decreases slightly between room temperature and 450ºC. At higher temperatures up to 700ºC it increases and at higher temperatures decreases sharply. In order to study the reasons of this behavior, TEM investigations were performed after 0.2% plastic strain. At the temperature of 450ºC distribution of dislocations was homogeneous and dislocation pairs and creation of anti phase boundaries in the precipitates were observed. At this temperature, there were few numbers of slip bands in the microstructure. Results also show that at 450ºC simultaneous cutting of matrix and precipitates occurs. This simultaneous cutting can be related to a same slip system in both ý precipitates and ý matrix at this temperature. With changing the slip system of ý precipitates at higher temperatures, the possibility of simultaneous cutting of precipitate and matrix diminishes. At 700ºC the distribution of dislocations was inhomogeneous and TEM investigations confirmed that the density of slip bands had increased significantly and the dislocations had been stored in the matrix between the primary ý precipitates. At this temperature simultaneous cutting of matrix and precipitates was not obseverd. The increment of yield stress at elevated temperatures can be due to the change of the slip system of ý precipitates at these temperatures, and for this reason the cutting of precipitates by matrix dislocations dosen't occur.