Iranian Journal of Materials Science and Engineering

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Carbon fibers are fabricated from different materials such as special polyacrylonitrile (PAN) fibers, cellulose fibers and pitch. But PAN fibers are recognized as the most widely used precursor for the present-day manufacture of carbon fibers. The process of fabrication carbon fibers from special PAN fibers is composed of two steps including oxidative stabilization at low temperature and carbonization at high temperatures in an inert atmosphere. Today carbon fibers are still expensive because of the high price of their raw material (special PAN fibers).This study focuses on making carbon fibers from commercial PAN fibers (low price PAN fibers used in textile industry). The results shows that in case of conducting complete stabilization process, it is possible to produce desirable carbon fibers from commercial PAN fibers. With some changes in conventional procedure of stabilization in terms of temperature and time of operation, the desirable conditions of complete stabilization are achieved.

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In this research pitting Corrosion of a sensitized 316 stainless steel was investigated employing potentiodynamic, potentiostatic techniques. Sensitization process was carried out on as-received alloy by submitting the specimen in electric furnace set at 650°Cfor five hours and then the specimen was quenched 25°C water. Potentiodynamic polarization of as received and sensitized specimens in 1M H2SO4 solution at room temperature and 70°C clearly revealed that the sensitization process has caused a magnificent change on electrochemical behavior of the specimen by changing critical current density for passivation, passivation potential and passive current density. Optical microscopy examination of the specimen surface after oxalic acid electrochemical etching also showed the deterioration of grain boundary of sensitized specimen due to chromium carbide precipitation in compared to as-received one. Several anodic potentiodynamic polarization on rode shaped working electrodes prepared from as-received and sensitized specimen in 3.5% NaCl test solution proved an average ~220 mV drop in pitting potential due to sensitization. Anodic potentiostatic polarization at 400 and 200 mV above corrosion potential also demonstrate the deterioration of pitting resistance of alloy as a result of sensitization. Scanning electron microscopy examination of anodically polarized of sensitized specimen at 700mVprior and after oxalic acid etching revealed large stable pits with lacy cover and also openpits with deep crevice for etched specimens.

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In situ refractories are defined as brick or unshaped products, which react internally or with furnace atmospheres and/or slag components so as to be enhanced in their performance. Examples of such products are discussed with emphasis on those that are currently employed and are being developed for the melting of iron and steel. Some strategies for the development of future in situ products are outlined.

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Iron and steel is an energy intensive industry and its contribution to the pollution of environment is considerable. Direct reduction iron (DRI) is a major element of an iron and steel production plant. Its share in natural gas and electricity consumption of total plant is estimated to be 70% and 15% respectively. Reduction gases are produced in natural gas reforming unit and its elements are CO and H2. A major consequence of using this technology is high level of CO2 emission, which pollutes the environment. An alternative to the existing technology is utilization of H2 as reducing agent. A comparison of various hydrogen production processes indicate that thermal decomposition of methane provides an attractive option from economical and technical point of view. Therefore, a system for producing hydrogen, based on thermal decomposition technique, has been designed in the framework of the present paper.

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Life assessment on the base of grain boundary creep cavitation of 1%Cr - 0.5%Mo low alloy steel has been discussed in this paper. Since microstructural degradation is one of the most important mechanisms that affects creep life, it is necessary to assess microstructural damage in order to estimate the life. Microstructural damage within the grain boundaries is a continuous phenomena starting from about the beginning of secondary stage of creep process. In this research, the amounts of damage accumulation in the form of grain boundary cavitations for various creep times up to the ends of secondary creep stage for each creep condition was found by using quantitative metallography technique, i.e. image analyser. Then from the data obtained for grain boundary area cavitated and number of cavities per unit area, which was about linear as a function of time for each of creep conditions, the amount of damage in the tertiary stage was estimated for various times. Then a creep damage parameter was proposed for the creep process. Finally, having this damage parameter (?) and using continuum damage mechanics (CDM), a new version of Rabotnov-Kachanov equation for tertiary creep rate was established.

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An investigation of the electrochemical noise generation during Stress Corrosion Cracking (SCC) of 70-30 Brass in Mattson's solution was conducted. The fluctuations of potential and current were monitored. The relationship between potential and current fluctuations has been evaluated in time domain and the obtained data has been analyzed in the frequency domain using Power Spectral Density (PSD). It is shown that 70-30 Brass has characteristic noise behavior during SCC that is step-by-step change in current and potential up to the final stage of fracture, and this may be used for SCC monitoring.

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Compositions of Al2O3+Si, SiO2+Al and Al+Si systems were prepared to study the effect of reaction bonding process on the mullite formation. The composition of each system was adopted according to mullite stoichiometery and sintered in 700-1600°C range. Results showed that the formation of reaction bonded mullite starting from Al2O3+Si mixtures, proceeded in two partially overlapping steps, the oxidation of Si to SiO2, and the reaction of SiO2 and Al2O3 to form mullite. In this system, up to 1400°C, conversion of Si to SiO2 was taken place and cristobalite formed, but mullite formation was not observed. Mullite phase started to form at 1450°C. Results indicated that complete reaction was not occurred up to 1600°C and 2 hours soaking time. XRD patterns of samples in Al+ SiO2 system showed that the reaction through sequences: (a) reduction of SiO2 by Al, (b) formation of a- Al2O3 and SiO2-rderived Si oxidation, and (c) mullite formation. X-ray diffraction patterns of heat-treated Al+Si system showed that reaction between Al and oxygen at 900°C was occurred with the reaction product being a- Al2O3 Oxidation of Si and formation of mullite were not detected in this system. SEM micrographs showed that both Al2O3+Si and SiO2+Al systems have similar microstructures, which consisted of a- Al2O3, mullite and free Si. The microstructures of the samples in Al+Si system consisted of a- Al2O3 free Al and Si with intermetallic Al-Si compound.

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In wire arc spraying, the atomizing air pressure and applied nozzle system are important factors influencing particles characteristics and coating quality. The aim of this paper is to study how the characteristics of particles such as size, velocity and temperature are influenced by the operating conditions in wire arc spray. For that, three types of wires are tested: solid wire of stainless steel 316L, cored wires 97MXC and 98MXC. Arc spray gun is an Arc Jet 9000 manufactured by TAFA. For each condition, the particles temperature and particles velocity are measured using an imaging CCD camera, Spray Watch (Oseir Ltd). Particles size distributions are determined with a laser grain meter. The morphology and composition of particles were evaluated with SEM, EDX and X-Ray diffraction. Results induce important modifications in the particles size distributions and particles velocity. A small diameter nozzle and high atomizing air pressure resulted in a reduction in particles size distributions and an increase in particles velocity.

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Ultra-fine particles of barium hexaferrite have been synthesized by co-precipitation method using barium nitride and iron chloride precursors with a Fe/Ba molar ratio of 11. Co-precipitation was carried out at 25 and 80°C using NaOH as a precipitant. Effect of coprecipitation and annealing temperatures on the phase composition and morphology of the products have been investigated using XRD and SEM, respectively. XRD results indicated existence of BaFeO3-x as a major phase in co-precipitated samples. Analysis of the XRD results also revealed that barium hexaferrite starts to form at a relatively low temperature of 700°C for sample synthesized at 80°C. SEM micrographs exhibit plate-like hexagonal particles of barium hexaferrite for calcined samples. The SEM results showed that the mean particle size of co-precipitated sample at 25°C is smaller than that of 80°C after calcining.

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Enhancing the properties of dental resin composites is of interest to researchers. The objective of the present investigation was to improve the strength and fracture toughness of dental composites via addition of silicon carbide whiskers and substitution of commonly used filler materials with stabilized zirconia ceramic powder. It was also intended to study the effect of powder- to- whisker ratio on mechanical properties of the resultant composites. The flexural strength and fracture toughness of composite samples with different whiskers loadings were measured. It was found that addition of whiskers to the composites enhances the mechanical properties of the composites. The strength and fracture toughness increased by increasing the amount of whiskers. The flexural strength of a composite having 60wt% whisker and 10wt% zirconia powder was about 210 MPa while that of the composite having only 60wt% ceramic powder was about 110 MPa. The microstructural examinations revealed that reinforcing mechanism was whiskers pull-out as well as crack deflection.

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plasma-sprayed silicon layers have been used to produce silicon nitride layers with fibrous microstructure which optimizes fracture toughness and strength. SEM examination of the specimens shows that the surface is covered by fine needles and whiskers of Si3N4.In order to study the oxygen contamination effect as well as other contaminants introduced during spraying and nitridation processes, surface sensitive analysis techniques like AES and XPS have been used to determine concentration of these contaminants.

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In this research, rotating bending fatigue test at minimum to maximum stress ratio of R=-1 was used for investigating the fatigue behavior of Ti-6Al-4V alloy. Both smooth and notched specimens, with elastic concentration factor, kt, of approximately 3.6 and 4.1 were used for this purpose.In addition, the effect of variation in ultimate tensile strength, UTS, on the fatigue behavior of this alloy was studied. S-N curves were drawn and the value of notch sensitivity was obtained or each case.The results showed that the presence of notch in Ti-6Al-4V alloy has a different amount of sensitivity when the notched specimens were subjected to high cycle fatigue (HCF) and low cycle fatigue (LCF) tests. However, the notch sensitivity of this alloy was shown generally to be much lower than steel alloys with similar UTS values. Thus, considering the high compatibility of this alloy with the body environment and its low sensitivity to notch, one can strongly recommend this alloy for use in biomedical application.

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Partially stabilized zirconia (PSZ) has been proven to be an excellent candidate as a thermal barrier coating (TBe) for hot sections in, for instance, heat or internal combustion engines and gas turbine parts. The main functions of these coatings are reducing heat losses, reducing fuel consumption, increasing efficiency, and extending durability and life. One of the main problems involved is wear behavior in the development of such coatings for these applications. Using the air plasma spraying (APS) technique, conventional and nanostructured 8 wt % yttria partially stabilized zirconia (Y-PSZ) coatings were deposited on austenitic stainless steel (AISI304) disc-shaped substrates. The coated substrates were subjected to pin-on-disc wear tests, using 10 mm silicon nitride and zirconia balls as the pin. The coefficient of friction was recorded in real time. The weight loss of coated substrates was measured. Coatings were characterized before and after being subjected to wear testing by various techniques including optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). Effects of various experimental parameters such as wear distance, test temperature, and counter face material were also investigated. Results obtained revealed that, regardless of experimental conditions applied, the nanostructured zirconia coating shows better wear and tribological properties than that of the conventional one.

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Wear of various basic refractory materials to substitute to currently used magnesia chrome bricks has been studied, measuring matte and slag penetration and dissolution, through different cup and rotary slag tests. High magnesia with and without impregnation, magnesia graphite, magnesia-alumina spinel with and without impregnation, olivine-magnesia and olivine magnesia- graphite bricks, as well as magnesia-graphite and olivine magnesia castables, have been tested. It has been shown that carbon impregnation and graphite introduction into basic refractories are feasible ways to enhance their corrosion-dissolution and penetration resistance against fayalite as well as calcium-ferrite slags. Olivine-based refractories (castables or bricks) may be considered as viable candidates to use in copper-making furnaces. At this point, evaluation of the thermo-mechanical properties of this new class of materials is still missing.

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In this research the influence of Ti contents on the amounts of inclusions formation and mechanical properties of a high alloy high strength steel, C300, has been investigated. For this purpose several bars were casted under the same solidification conditions, but different amounts of Ti element. All the seven casted bars were homogenized at 1200°C for a period of 2 hours. Then, they were immediately hot rolled after homogenization so that the out rolling temperature was kept in the range of 1000-1200° C. The specimens were then solution annealed at 820°C for hour and finally they were aged for a period of 3 hours at 500°C. The samples were subjected to tensile, impact and hardness tests in order to relate the variation in volume percent of inclusions due to different amount of Ti, to mechanical properties. The results showed that by increasing the amount of Ti a serious decline in toughness properties of the alloy due to increase in inclusion population occurred. So this research provides a very useful information about the relation between volume fraction of inclusions and mechanical properties of a C300 high strength steel.

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Abstract: Static recrystallization (SRX) behavior of a composite based 7075 Aluminum alloy reinforced with SiC particles was studied during annealing the deformed samples at high temperatures. The results showed an absence of SRX in the samples annealed after hot working at the same deformation temperature, however, a rise in annealing temperature of 100-1500 􀁱C above that the deformation temperature led to full recrystallization. This can be ascribed to the relatively moderate dynamic recovery and the presence of dispersions which stabilize the substructure. Particle stimulated nucleation (PSN) had a significant effect on the grain size in deformed samples at low temperature, but no PSN was observed in samples strained at high temperatures. The possible cause might be that at high temperature the dislocations can be annihilated by climb process around the particles together with the absence of deformation zone for nucleating the recrystallization.

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Abstract: The performance of an Imidazoline based commercial corrosion inhibitor in CO2 corrosion of a gas-well tubing steel was studied by employing Electrochemical Impedance Spectroscopy (EIS) technique. Inhibitor performance was investigated by means of its efficiency at various concentration and also its behavior at various exposure time. EIS results showed that inhibitor interaction to the electrode surface obeys Lungmuir adsorption isotherm. Interpretation of some parameters such as Rct, Rpf, Cdl, and Cpf associated to the equivalent circuit fitted to the experimental rsults showed that not only inhibitor efficiency and surface coverage improve by increasing in inhibitor concentration in the solution but also at constant inhibitor concentration both surface coverage and efficiency improve with exposure time and reach to their highest value after 4 hours.

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Abstract: The effect of phase development on peel strength of alumina-copper metalized joint has been investigated. The alumina-copper joint was prepared in three stages. The alumina substrate was, first, metalized at 1500°C in H2-furnace by a new formulation. In the second step, a nickel layer was electroplated on the metalized layer with approximately 10µm thickness. Finally, copper strips were bonded to metalized alumina with Ag-Cu (72-28) filler metal. The peel strength of the joint was 9.5±0.5 Kg/cm which shows approximately 30% increase in comparison to previous works. By study of fracture surface and crack propagation path, it has been concluded that this increase is due to the formation of more spinel phase.

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Abstract: In this paper, kinetics of reductive leaching of manganese dioxide ore by ferrous ion in sulfuric acid media has been examined. Experimental results show that increasing temperature from 20 to 60 °C and decreasing ore particle size from −16+20 to −60+100 mesh considerably enhance both the dissolution rate and efficiency. Molar ratios of Fe2+/MnO2 and H2SO4/MnO2 in excess to the stoichiometric amounts were needed for successful manganese dissolution. Under the optimum condition (ore particle size of −60+100 mesh, Fe2+/MnO2 molar ratio of 3.0, H2SO4/MnO2 molar ratio of 2.0) manganese could be extracted with over 95% efficiency by 20 minutes leaching at room temperature. A kinetic analysis based on dimensionless time method showed that shrinking core – ash diffusion control model fits the experimental results reasonably well. Value of activation energy was found to be 28.1 kJ/mole for the proposed mechanism.

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Abstract: Dolomite based refractories are widely used in Iranian steelmaking plants. In the present research, wear and corrosion of refractories used in steel-making converter lining in Esfahan Steel Company was studied. Post-mortem analysis of refractories clarified that the wear started with oxidation of carbon followed by chemical corrosion. Iron oxide from slag reacted with calcia, resulting in formation of low melting phase, and subsequent washout process, caused the refractory corrosion onset from the hot face. In addition, the effect of aluminum as an anti-oxidant and graphite on the corrosion resistance of refractory was investigated. Tar-dolomite samples containing different amount of graphite (0, 4, 7, and 10 wt. %) were prepared in order to study their physical properties, before and after coking. SEM micrographs employed to analyze the microstructures to determine the effect of graphite and antioxidant on corrosion behavior of the refractory. Results showed that oxidation process of carbon in the system was hindered and improved corrosion resistance by introducing graphite and antioxidant into the refractory composition.