جستجوی مقالات مرتبط با کلیدواژه « کروم » در نشریات گروه « مواد و متالورژی »

U-500 is a nickel-based superalloy which has significant amounts of some precious elements such as cobalt and chromium. Due to the economic and strategic value of these metals, their recovery from the used superalloys is of great importance. Hydrometallurgy is an economical and effective method for recovering these valuable metals from the superalloy scraps. So in this investigation, the dissolution behavior of nickel, cobalt and chromium elements from the used U-500 superalloy in the electro leaching process as well as the effect of different process parameters (such as: sulfuric acid concentration, applied voltage, process temperature, stirring and cathode-anode distance) on the dissolution of these elements are studied in details. The results show clearly that by increasing the concentration of sulfuric acid in the electrolyte, the applied voltage, and temperature, as well as decreasing the cathode-anode distance, the anodic dissolution of the superalloy U-500 is improved. Electrolyte stirring has no significant effect on increasing the anodic dissolution of the superalloy. The optimum conditions for the anodic dissolution of superalloy U-500 in sulfuric acid electrolyte was determined as acid concentration of 125 g/L, applied voltage of 4 V, stirring speed of 300 rpm, and cathode-anode distance of 3 cm without temperature control (starting with ambient temperature as the initial temperature(. Under these conditions, about 75% nickel, 84% cobalt and 67% chromium were durnig 25 minutes.

In the present study, the effect of chromium carbide element on microstructure, hardness and abrasive properties of welded coating on the DIN-25CrMo4 substrate was investigated. For this purpose, three groups of roughing electrodes with different chromium content (0-13 wt%) were made and roughing operation was performed by hand electrode arc process (SMAW). Optical and scanning microstructural studies (SEM) along with XRD fuzzy analyzes showed that with increasing chromium content the microstructure of the coating is varied from a ferritic field with needle and morphology morphologies into a rich austenite field of carbide phases such as (FeCr) 7C3. The evaluation of the characteristics of roughened samples showed that the coating hardness increased by about 350% with increasing chromium content (the hardness of the chromium-plated coating and the hardness of the metal substrate were 875 ± 5HV and 200 ± 5HV respectively). Examining the wear behavior of the samples indicates a significant improvement in the wear resistance of the chromium-rich coating (an 80% reduction in the coefficient of friction and a 90% reduction in the amount of lost weight relative to the metal-tensile properties) The SEM study of washed surfaces implying a change in the wear mechanism from the two-body scrubbing wear to the cutting mechanism by increasing the volume fraction of carbide compounds in higher-chromium specimens.

In-situ synthesizing of Al/Cr was carried out to improve the strength of spot joining of Al plates. For this purpose, 0.03gr Cr powder with 10 μm particle size was inserted in the spot joining zone and the assembly was subjected to the Friction Stir Spot Welding Process (FSSW). Microstructure, formation of intermetallic compounds and mechanical properties of samples were investigated by optical microscopy (OM), scanning electron microscopy with spot and line EDS, tensile and microhardness measurements. Spot and line scan chemical analysis (Spot-EDS and Line-EDS) of joining area indicated that minimal tool rotation speed for formation of Al-Cr intermetallic compounds (Al13Cr2) is around 2500 rpm. Formation of Al-Cr rich intermetallic compounds in the joining area increased the hardness of joining zone more than two times of hardness of join without this components (140±5 HV and 60±5 HV). Furthermore, in-situ synthesis joining increase ultimate tensile strength of the joint from 40±5 Mpa to 130±10 Mpa

Chromium is one of the most important pollutants in aquatic environments that, at high concentrations, cause different ecological threats and health risks for human society. Thus, it is very necessary to remove it from aquatic Solutions. Therefore, the aim of this study was to investigate the efficiency of removal hexavalent chromium by biological absorbers (oak, pomegranate, and egg shell), and the study of isotherm and kinetics of their absorption. In order to achieve to the optimal removal of chromium, the variables affecting on adsorption process such as pH, coagulation speed, reaction time and initial concentration of adsorbent material were investigated. Also, for adsorption isotherms and kinetics study the first order and second order pseudo kinematic models and Langmuir and Freundlich isotherms were used. The results showed removal efficiency of Cr increased with increasing the dose of adsorbent, contact time, and coagulation speed. But by changing the pH, depending on the absorbent type, the absorption efficiency was changed and the highest absorption efficiency for egg, oak and pomegranate shell was obtained at pH 10, 4 and 2, respectively. The kinetics was second-order for all three adsorbents and absorption isotherms from the Freundlich model for egg and pomegranate and for oak shell was more consistent with Langmuir model. According to the obtained results can be concluded that egg, oak and Pomegranate shell respectively, have high more efficiency in removing hexavalent chromium from aqueous solutions.

In this research a new type of magnetic Pb(II) and Cr(VI) ion-imprinted polymers were synthesized using Fe3O4@SiO2 magnetic particles as supporter, benzoyl phenyl hydroxylamine and dithizone as ligands for Pb and Cr ions respectively, 4vinyl pyridine as functional monomer, ethylene glycol dimethacrylate as crosslinker and 2,2′-azobis(isobutyronitrile) as the initiator. The ionic templates were achieved using HCl 2M as an eluent solution. The chemical structure, morphology, thermal behavior and magnetic properties of the sorbent polymers were evaluated using Fourier-transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric analysis and vibrating sample magnetometry methods. Optimization of affecting parameters of the adsorption of analytes (adsorption time, pH, kind and concentration of the eluent solution, elution time and temperature) were performed using flame atomic absorption spectroscopy method. The results showed that the maximum adsorption capacities for lead and chromium at 40 ◦C were 92.3 and 96.8 milligrams per gram at pH of 6 and 3 respectively. The synthesized sorbents had high selectivity and were successfully applied for these pollutants removing from waste waters in vdifferent stages of ammonium perchlorate production process.

In this study, nano yellow pigments were fabricated by doping of chromium and antimony elements into TiO2 using chemical precipitation and solid state method and after transforming them to ink, their printing performance on the ceramic surface were investigated. The precursor to doping antimony to white rutile structure of TiO2 was SbCl3, while Cr(NO3)3 and K2CrO4 were the precursors for chromium doping by solid state reactions. The fabricated nano pigments were characterization by x-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis which showed different morphology, structure and color. Particle sizes which were estimated by SEM image were between 50 to 170 nanometers which is appropriate for ink-jet printing on the ceramic. XRD results showed the pigment synthesized by Cr(NO3)3 (sample A) had a mono-phase compound while the sample synthesized by K2CrO4 (sample B) consisted of a multi-phase compound. The pigments were transformed to ink by milling method and using an ester media. After printing process, the ceramics were analyzed by Diffuse Reflectance Spectroscopy and colorimetric measurement (CIE L*a*b*). The h° value from colorimetric measurement of A nano pigment, was 76.59 while for B was 79.18. This shows better yellow color of B. After printing the prepared ink of each pigment, h° value changed to 69.59 for A and to 81.14 for B. Therefore, the ink of A nano pigment declined to golden yellow after printing while the ink of B nano pigment presented a pure and better yellow color.

In present work a tested enamel coat for Ni-Cr wire-wound resistor is studied for its electrical and chemical properties. In the present work it is tried to find appropriate coating by testing different compounds. Results showed that the optimum enamel is the compound with 1 wt% cobalt oxide، 3 wt% chromium oxide and firing temperature 840˚C. Variation of the electrical conduction of the enamel with temperature is measured by an instrument designed by Rasch-Hinrichsen et al. Results indicate that with increasing the temperature to 275˚C electrical conduction reaches to 10-7. 60 Ω-1. cm-1. Dielectric strength test shows 100 KV/cm for breakdown voltage. Weight loss in different liquids with various pH shows that the most chemical resistance is yielded in water and for neutral pH.

In the present study, the effect of addition of potassium permanganate on the cladding microstructure obtained from the simultaneous synthesis of chromium from chromium oxide and molybdenum from molybdenum trioxide, were studied. For this purpose, a paste of blended powders containing chromium oxide, molybdenum trioxide, nickel, urea and aluminum with various amounts of potassium permanganate was applied on the surface of AISI 304 stainless steel substrates and welded using GTAW process. Cross section of the cladded samples have been investigated using optical microscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The results revealed that the morphology of the claddings in the center of the weld is lathy ferritic in austenite matrix and in the interface of the substrate and cladding, the morphology was changed to skeletal ferritic. By increasing potassium permanganate in the cladding powders, the final microstructure of the cladding has been finer and the amount of ferrite in the weld metal has been increased. Furthermore, the results showed that the addition of potassium permanganate to the cladding powders would result in increasing the chromium content and subsequently decreasing the molybdenum content of the weld pool, without any significant changes in the manganese content of it.

In this study the feasibility of obtaining thin surface layer of chromium based intermetallic compounds on Al–6061 substrate was studied by using laser surface alloying (LSA). The initial layers of copper، nickel and chromium with a total thickness of 15 micrometers was electroplated on Al substrates and subsequently laser alloyed by using pulsed Nd: YAG at various output power and scanning rates with argon gas shielding. Analyses of samples using optical and scanning electron microscopy and X-ray diffraction (XRD) showed that adding nickel and chromium are suitable to reduce susceptibility of cracking and improving surface hardness of alloyed layer on aluminum due to the formation of intermetallic compounds such as Al11Cr2، Al2Cu، Al3Ni2. Microhardness test was performed on cross-sections of specimens and it showed a significant increase (6 times) in hardness of alloyed-layer compared to the Al substrate.

In this research, effect of adding iron oxide nanoparticles on efficiency of algal biomass produced for chromium elimination. Initially to this end dried and crushed algal biomass, in continuation mixture of algal biomass with iron oxide nanoparticles absorbents used for chromiumelimination from water. The water that used in these experiments had four different concentrations of chromium, and results showed that the best absorber for water refinement is the mixture of algal biomass with iron oxide nanoparticles absorbents. Also 10 ppm concentration is best concentration on other hand in all experiments that used of iron oxide nanoparticles Cr6 was conversion to Cr3.

In this survey, the method of chromium extraction from potassium chromate and sedimentation on copper cathode by the method of hydrometallurgy has been considered. This method includes two stages: solvate and electrolysis. In the solvate stage, the effect of the concentration ofchromiumand the sulfuric acid to potassium chromate ratio was studied and in the electrolysis stage, the effect of density of current, pH, the distance between anode and cathode, the concentration of electrolyte and the temperature has been studied. The optimum conditions of extraction of chromium from electrolyte in the electrolysis stage have been specified so: density of current of 8-10 A/dm2, pH 1.8-2, the distance between anode and cathode 1.5-3.5 cm, concentration of chromium of 5 g/l, the concentration of nanographene powder of 1 g/l in the temperature of 40-50 oC has been specified. In this study, characterization of nanocomposites has been done to study their resistive features against any type of scratch or rubbing. In these surveys increase in resistance for chromium-graphene nanocomposite was shown. Also it was concluded that although a copper plate with chromium layer has enough surface solidity, maximum tensional resistance, rendition resistance and elongation percentage, but resistance against stroke for this plane is low and the copper plate with chromium-graphene plate exhibits better pulse resistance characteristics.

As a high temperature structural ceramic، cermets exhibit less brittle than the ceramic-ceramic composites or the monolithic ceramics in presence of the ductile metal second phase. On the other hand، they have higher refractory، higher creep and thermo-mechanical resistance and higher abrasive resistance in comparable of metals. In the present study، Alumina-Chromium cermets with different proportions of Al2O3، Cr2O3 and Al، different ball-milling time and different heat treatment temperatures have been fabricated. In order to evaluate the optimum manufacturing conditions، specimens with different composition of ball-milled starting materials and for different times were subjected to DTA and TGA analysis. Obtained materials were characterized by microstrutural observations (SEM)، phase analyses (XRD)، mechanical properties measurements (toughness and strength- three point bending test) and by hardness testing (Vickers). The results of DTA show that for ball-milling more than two hours، there is a decreasing of intensity and temperature of exothermic peak relating to thermit reaction. Higher proportion of x in starting materials، 3xAl، xCr2O3، (1-x) Al2O3، cause increasing the density and strength until x=0. 5 and increasing toughness of sintered samples until x=0. 7. The hardness however، constantly decreases on increasing of the x value. In addition، the mechanical properties of sample with x=0. 5 is improved by increasing of the sintering temperature to 1550 °C. he difference between thermal expansion coefficient of the metallic and ceramic phase، oxidation of the chromium phase، low wetting of alumina by chromium phase and a coarser structure are the principle responsible for decreasing of properties in higher value of x and high sintering temperature.

Cu-Cr alloys with high electrical and thermal conductivity, high strength and good resistance to fatigue and corrosion, as well as good ductility have many applications in industry. These alloys are capable of being age-hardened. As Cr- content in solution before aging increases, their strength increases since more fine particles can be produced in the matrix during ageing stage of hardening. Therefore intensive works have been done to increase Cr solubility in Cu especially by rapid solidification of molten copper containing chromium. In the present work attempts have been done to introduce a method for production of Cu-Cr alloy with acceptable Cr-content with reasonable cost. Results show that maximum solubility of Cr in Cu matrix about 1.12 wt.% can be achieved.

Effect of chromium content on the microstructure and inclusions formation in low alloy steel multipass welds was investigated. Manual metal arc welding was performed in flat position according to AWS A5.5-96 standard. The ferro-chromium content in the coating was varied such that the final weld metal consists of 0.05, 0.53 and 0.91 wt.% chromium. Microstructure was studied by optical and scanning electron microscope in both columnar and reheated regions of the weld metal. The results showed increase in acicular ferrite formed at the expense of primary ferrite and ferrite with second phase with steady refinement of microstructure. Amount of microphases also increase as the chromium content of the weld metal increases. It is believed that chromium does not affect apparent characteristics (such as volume fraction and average size) and chemistry of inclusions.