Chromium Powder Research Articles

Preparation of Chromium Nitride Nanopowders Using High Pressure Gas-Solid Reaction Bed

Chromium nitride (CrN) nanopowders were successfully synthesized in high pressure gas-solid reaction bed with chromium hydroxide as a precursor. A special liquid-phase precipitation method was used to prepare the powders of chromium hydroxide.The aminolysis of chromium hydroxide was carried out for 2-4 hours at temperatures from 650°C to 850°C and at pressures up to 0.8MPa. The effect of the reaction pressure on the grain size and crystalline morphology of CrN powders were characterized by transmission electron microscopy and X-ray diffraction. The results indicate that high purity CrN powders with grain size of 40-60 nm can be obtained through the aminolysis at temperature of 650°C and pressure of 0.6MPa for 2 hours.

Tribological Behavior of Composite Electrodes for Spot Welding

The paper presents some aspects regarding tribological characteristics of composite tops of the electrodes for spot welding. The solution aims to improve the resistance to deformation and high intensity electric current conductibility without loses and excessive local heating. The composite material was obtained by mechanical alloying process, from a mixture of tungsten carbide, chromium, CuNiAl alloy and cooper powder which has been heated in furnace at 1100°C. The embedding process aims to avoid the toxic effects of metallic elements such as Be and Zr, which are usually introduced in cupper alloys for improving the mechanical characteristics. The tribological characteristics and microhardness of the melted zone were measured in order to estimate the strengthening effect obtained by the tungsten carbide presence for different mixture recipe.

The Contribution of the Bardin Central Research Institute of Ferrous Metallurgy to the Development of Powder Metallurgy in Russia

Scientific research that has been carried out since the middle of the last century at TsNIIchermet has contributed to the progress which has been made in the industrial production of powders of electrolytic iron, titanium, chromium, and stainless steels. The Sulinski Metallurgical Plant began the production of world-class reduced iron powders at the beginning of the 1980s. In the area of refractory materials, a technology was invented to make large powder semifinished products of Mo and W and use them to produce wrought heat-resistant and heavy alloys. In the 1970s, the Institute began work on the production of a new class of powder materials based on intermetallides of the system Ni–Al and alloys of the system Fe–Cr–Al. These materials are now being used in rocketry and aviation thanks to their exceptional resistance to creep, scaling, and heat.

On the Formation Mechanism of Chromium Nitrides: An in situ Study

AbstractThree different synthetic routes to chromium nitrides were investigated using in situ neutron diffraction (SPODI, FRM‐II) and thermal analysis in ammonia: (i) The direct conversion of chromium powder in ammonia, (ii) the ammonolysis of chromium(III) chloride CrCl3, and (iii) the ammonolysis of chromium(III) sulfide Cr2S3. During the direct conversion of chromium first γ‐CrN was formed above 385 °C, above 567 °C additionally β‐Cr2N was detected. Finally, a complete conversion of γ‐CrN to β‐Cr2N was observed at 1080 °C. The ammonolysis of chromium(III) compounds (CrCl3 and Cr2S3) revealed a so far in 3d metal nitride synthesis unprecedented redox sequence: The compounds are reduced to chromium(II) chloride CrCl2 and chromium(II, III) sulfide Cr3S4, respectively, prior to the reoxidation of those intermediates to γ‐CrN. In contrast to the ammonolysis of 3d metal(II) halides, for chromium(III) chloride no temporary formation of ammoniates was observed. Since formation of γ‐CrN was detected only after annealing chromium(III) sulfide in flowing ammonia for 4 h to 5 h at maximum temperature, the formation of sufficient amounts of chromium(II, III) sulfide as the rate‐determining step for the nitride formation appears likely.

Corrosion resistance of W–Cr–C coatings fabricated by spark plasma sintering method

To increase the corrosion resistance of tungsten, W–Cr–C coatings were fabricated on the tungsten matrix with spark plasma sintering (SPS) method from graphite and chromium powders. SEM and XRD analysis illustrated that W–Cr–C coatings with a thickness of 25μm were composed of tungsten carbide layer and chromium carbide (Cr7C3 and Cr3C2) particles distributed in the surface layer of the coatings. The incorporation of chromium carbide largely enhanced the compactness of coating surface and refined the WC grains. In the immersion experiments, W–Cr–C composite coating exhibited better corrosion resistance than pure WC coating after exposure to the de-ionized water for 38days at both room temperature and 60°C. Electrochemical measurement showed that the reinforcement of chromium carbides can largely improve the corrosion resistance of WC coating in a 3.5% NaCl solution.

Effect of Alloy Addition Method on Thermal Fatigue of Cr5-Deposited Metal

Given the same alloy composition in Cr5 deposited surfacing metal, the influence of two different methods of adding Cr and C in the flux-cored welding wire (1#-chromium carbide powder; 2#-graphite and chromium powder) was studied on the thermal fatigue (TF) performance of the deposited metal. Results show that (1) for either as-welded or tempered state, after the same number of thermal cycles, the crack length of specimen 1# was greater than specimen 2#; (2) it took about 200 more cycles for the initiation of cracking and failure cracking on specimen 2# than on specimen 1#; and (3) after the same number of cycles, the length of failure cracks of the as-welded state was greater than the tempered state. Studies of the microscopic structure, number of inclusions, and grain size of both types of specimens show that only a portion of chromium carbide added in 1# flux-cored wire was oxidized in high-temperature molten droplets or weld pool, plenty of incompletely decomposed chromium carbide grains and those generated in site lead to higher amount of inclusions in the deposited metal, compared with 2#. In addition, the boundary between inclusions and matrix was distinct, and the contents of S and P were higher in 1#; a larger proportion of residual austenite and twin structures, slower transformation of the residual austenite during tempering, and coarser primitive austenite and matrix grains were found in 1#. These findings suggest that optimization of the method of adding alloy elements might be an effective and cost-efficient way of improving the TF performance of deposited metal.

Cr3+ and Al3+ co-substituted zinc ferrite: Structural analysis, magnetic and electrical properties

Nanocrystalline powders of chromium and aluminum co-substituted zinc ferrites with general formula ZnFe2−2xCrxAlxO4 (0⩽x⩽1) have been synthesized for the first time. Using the sol-gel auto-combustion technique and the tartaric acid as combustion-complexion agent, materials with spinel mono-phase cubic spinel structure were successfully prepared. The materials were characterized by IR, XRD, SEM and 57Fe Mössbauer spectroscopy. The crystallite size estimated by Scherrer formula has been found in the range of 16–69nm. The experimental results combined with those obtained from a mathematical model suggested that all compounds have a mixed ionic distribution. Moreover, relationships between the structure features and the electric and magnetic properties have been established. The magnetic measurements showed that the hysteresis losses and the magnetization at 10 KOE linearly decreased when the Cr–Al content in Zn ferrite increased. The results obtained in dielectric study showed very low values of dielectric loss at frequencies over 1MHz.

Formation of chromium carbides in copper matrix during mechanical alloying in carbon-containing media

Structural and phase transformations that occur during mechanical alloying (MA) and subsequent annealing of nanocrystalline Cu-Cr-C alloys obtained from copper and chromium powders and graphite or xylene as the source of carbon have been studied. It is shown that, when using graphite, a supersaturated Cu(Cr) solid solution and an X-ray amorphous Cr-C phase are formed during MA. Heat treatment leads to their decomposition and the appearance of Cr3C2 in the nanocrystalline copper matrix. When xylene is used as the source of carbon, no strongly supersaturated Cu(Cr) solid solution and no X-ray amorphous Cr-C phase are formed, but the same volume fraction of chromium carbide, i.e., 20–24 vol %, appears. When graphite is used, the carbide is formed after shorter times of MA.

Parametric Optimization of PMEDM Process with Chromium Powder Suspended Dielectric for Triangular Electrodes

In this article, parametric optimization for material removal rate (MRR) and surface roughness (SR) study on the powder mixed electrical discharge machining (PMEDM) of EN-8 steel has been carried out. Response surface methodology (RSM) has been used to plan and analyze the experiments. Average current, duty cycle, diameter of electrode and concentration of micro-nickel powder added into dielectric fluid of EDM were chosen as process parameters to study the PMEDM performance in terms of MRR and SR. Experiments have been performed on newly designed experimental setup developed in laboratory. Most important parameters affecting selected performance measures have been identified and effects of their variations have been observed.

Influence of chromium ion substitution on the structure and properties of zinc ferrite synthesized by the sol–gel auto-combustion method

Nanocrystalline powders of chromium substituted zinc ferrites with the general formula ZnFe2−xCrxO4 (0⩽x⩽2) were obtained by the sol–gel auto-combustion method, using tartaric acid as a combustion-complexing agent. The solid phase chemical reactions were monitored using infrared spectroscopy, finally indicating the absence of organic phases. The XRD results confirmed the spinel mono-phase formation in the 24–36nm crystallite size range. Nanosized particle formation was confirmed through scanning electron microscopy. The cation distribution in the samples was estimated theoretically and the results showed that all the compounds had a mixed ionic distribution. The magnetic properties of the samples were studied using a vibrating sample magnetometer and showed that the increase in Cr3+ concentration in the Zn ferrite caused a reduction in the hysteresis losses and a non-linear reduction in the saturation magnetization. The electrical analysis showed very low values of dielectric loss at frequencies over 1MHz.

Behaviour of Cr Species in the Molten System NaF - AlF3 - (Al2O3)

Oxidation and reduction of Cr(III) species originating from dissolution of Cr2O3 in cryolite-based melts at 900 {degree sign}C was studied. The molar ratio n(NaF)/n(AlF3) was 1.4, which allowed us to perform measurements at the temperature of 900 {degree sign}C. It was found that Cr(III) can be oxidized to Cr(VI) both chemically by oxygen present in the air and electrochemically when using an inert anode. When a carbon anode was used, the formation of Cr(VI) species was not observed. The solubility of Cr(VI) species in the above-mentioned melt was found to be (1570 {plus minus} 16) ppm. Cr(II) species were prepared in situ by the reduction of Cr2O3 with chromium powder. It was found that saturation was achieved after several hours, the concentration of Cr(II) being (239 {plus minus} 14) ppm.

Solubility of Cr Species in Low Temperature Cryolite-Based Electrolyte at 900°C

Oxidation and reduction of Cr(III) species originating from dissolution of Cr2O3 in cryolite-based melts at 900°C was studied. The molar ratio n(NaF)/n(AlF3) was 1.4, which allowed us to perform electrochemical measurements at 900°C. Cr(II) species were prepared in situ by the reduction of Cr2O3 with chromium powder. Saturation was achieved after several hours, the concentration of Cr(II) being (237 ± 11) wt ppm. Cr(III) can be oxidized to Cr(VI) both chemically by oxygen present in the air and electrochemically when using an inert anode. When a carbon anode was used, the formation of Cr(VI) species was not observed. The solubility of Cr(VI) species in the above-mentioned melt was found to be (1395 ± 22) wt ppm at a total amount of Cr species in the melt of (1574 ± 16) wt ppm.

Surface Modification by EDM Using Co-Cr Sintered Powder Metallurgy Electrode

In this investigation, the powder metallurgy (PM) electrode developed from cobalt (Co) and chromium (Cr) powders was used in electrical discharge machining (EDM). This new Co-Cr sintered electrode was used to machine mild steel workpiece with kerosene dielectric fluid. The main objective is to modify its surface characteristics and secure increased surface hardness. The EDM variables of peak current, pulse duration and pulse interval were applied to investigate these surface properties. Scanning electron microscopy (SEM) / energy dispersive X-ray (EDX) spectroscopy analysis confirmed migration of electrode materials (Co and Cr) onto the EDMed surface. Improved hardness was obtained on the mild steel, the highest being 85.3 HRB. Thus, the Co-Cr electrode introduced into the EDM has the capability of modifying the mild steel surface.

High fracture toughness of powder chromium sintered in magnesium vapor

The fracture toughness of powder chromium sintered in magnesium vapor is higher by a factor of 53 than that of powder chromium sintered in hydrogen and by a factor of 5 than that of deformed low-alloy chromium VKh2K castings. This high fracture toughness is due to the skeleton formed of plastic interlayers of high-purity chromium. Chromium becomes highly ductile after fine purification in Cr–MgO alloys to remove interstitial impurities. The interlayers form on the surface of chromium powder particles under the refining action of magnesium vapor. Auger electron microscopy and data on fracture, chemical composition, and etching resistance lead to the conclusion that there are interlayers made of pure chromium. The high fracture toughness remains after annealing for 1 h at 1500°C.

Analyzing the quality of grinding and mixing of copper and chromium powders in a new-type mill

The potential use of a new-type mill developed at the Frantsevich Institute for Problems of Materials Science to produce copper and chromium powder mixtures intended for the manufacture of electrical-grade materials is studied. The mill ensures mechanical grinding of loose materials by mechanisms similar to those acting in vortex and jet mills. The greater powder refinement in this mill is due to the sliding interaction of particles with abrasive surfaces of the mill chamber. It is shown that common grinding of electrolytic copper and reduced or aluminothermic chromium for 10–30 min in argon promotes variation in the shape of particles, facilitates their mixing, and provides uniform distribution of components in the mixture.

Effect of Salt Bath Composition on the Chromium Diffusion on Plain Carbon Steels by TRD Process

The thermo-reactive diffusion (TRD) process is used for diffusing an element to the metallic steel substrate. TRD is carried out by using either salt bath or fluidized bed methods. In this research, the molten salt bath method is used. Ferro chromium was dissolved in the molten borax as the source of chromium in the salt. Samples of cylindrical shape of plain carbon steel with 10 mm diameter and 20 mm height were treated at 1000°C for 14 hours in different baths including either low carbon ferro chromium (LCFC) or high carbon ferro chromium (HCFC) powder. The purpose of this research is to investigate the effect of the salt bath composition on the diffusion of chromium and formation of chromium compound layer on plain carbon steel by using the salt bath method. The coating thickness layers were measured by SEM. The different phases formed on the samples, due to different amounts of carbon in treating salt bath, were determined by X-ray diffraction. It was found that in molten borax salt with high carbon content (high carbon ferro chromium) very few amount of chromium diffused into the plain carbon steel. The thickness of the diffused chromium layer in low and high carbon content ferro chromium in molten borax, is around 32±8 µm and 6.8±1.2 µm, respectively. A number of tests were conducted to address this effect.

<i>In Situ</i> Synthesis of Cr<sub>2</sub>Nb Reinforced Copper Alloy by Liquid Metallurgy Route

Cu-8Cr-4Nb alloy with high strength and high thermal conductivity have received a lot of attention over the last decades due to promising role in space application of rocket combustion chamber of reusable launch vehicle. In the present work, the copper alloy is synthesized in induction furnace by liquid metallurgy route wherein elemental powders of chromium and niobium was used added into premelted copper melt under argon atmosphere. The cast product is characterised for hardness, optical, SEM, EDS and XRD. It is possible to produce this alloy with reproducible average hardness of 135 VHN. The precipitate of Cr2Nb showed moderate distribution in and around the grain boundaries of copper with precipitate size varying from 1.33 μm to7.0 μm.

Effect of chromium powder suspended dielectric on surface roughness in PMEDM process

In this paper, a surface roughness (SR) study on the powder mixed electrical discharge machining of EN8 steel has been carried out. Response surface methodology has been used to plan and analyse the experiments. Average current, pulse on time, diameter of electrode and concentration of chromium powder added into the dielectric fluid of electrical discharge machining were chosen as process parameters to study the powder mixed electrical discharge machining performance in terms of SR. Experiments have been performed on a newly designed experimental set-up developed in the laboratory. An empirical model has been developed for SR. In addition, the recommended model has been verified by conducting confirmation experiments.

Parametric Optimization of PMEDM Process with Chromium Powder Suspended Dielectric for Minimum Surface Roughness and Maximum MRR

In this article, parametric optimization for material removal rate (MRR) and surface roughness (SR) study on the powder mixed electrical discharge machining (PMEDM) of EN-8 steel has been carried out. Response surface methodology (RSM) has been used to plan and analyze the experiments. Average current, duty cycle, diameter of electrode and concentration of micro-nickel powder added into dielectric fluid of EDM were chosen as process parameters to study the PMEDM performance in terms of MRR and SR. Experiments have been performed on newly designed experimental setup developed in laboratory. Most important parameters affecting selected performance measures have been identified and effects of their variations have been observed.

Effect of High-Temperature Heat Treatment on Mechanical Property of Cr-Geopolymer Composite

Chromium powder reinforced geopolymer composite (Cr/geopolymer) was prepared in order to enhance its thermal conductivity and mechanical properties. The phase composition, microstructure and mechanical properties of Cr/geopolymer before and after heat treatment at 900, 1000, 1100 and 1200°C were investigated by the X–ray diffraction (XRD), scanning electron microscopy (SEM) and three–point bending test. With increasing heat treatment temperature from 900 to 1100°C, mechanical property of Cr/geopolymer increased gradually and at 1100°C flexural strength got the peak value, which was 325% higher than that of specimens without heat treatment. Cr/geopolymer treated at 900°C did not completely transform into crystalline phase, and many micro cracks around the chromium particles were observed. Cr/geopolymer treated at 1000–1100°C completely crystallized into leucite phase, and the metal chromium remained its original state. Meanwhile, most of the micro cracks were closed. When the temperature further increased to 1200°C, many visible defects were observed in Cr/geopolymer, and chromium oxide appeared in the interface of Cr/geopolymer, which had detrimental effect on the heat conduction and mechanical property of the composite.