Chromium Powder Research Articles

Reaction of Methyl Radicals with Metal Powders Immersed in Aqueous Solutions

AbstractMethyl radicals radiolytically produced in aqueous solutions react efficiently with Cr0, Mn0, Fe0, Ni0, Cu0 and Zn0 powders immersed in the solution. The Cr0, Mn0, Fe0, Ni0 and Zn0 powders reduce the radicals to form methane. On the other hand the Cu0 powder seems to oxidize the radicals. Surprisingly a part of the energy absorbed by the Cr0, Fe0, Ni0 and Zn0 powders is transferred to the aqueous solution, thus increasing the radical yield. CH4, C2H4, C2H6, C3H6 and C3H8 are formed when an aqueous deaerated buffer solution, pH 4−5, is added to powders of analytical iron, zinc, manganese and chromium. The source of these gases is carbon traces present, as atoms or atom clusters, in the “analytical” metal powders. These carbon atoms, when present on the surface of the metals, are reduced by the metal particles in aqueous solutions. This mechanism might be the source of light alkanes and alkenes in the prebiotic era. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Effect of oxygen content and stearate additives on sintering characteristics of Cu-25Cr composite

The sintering behaviour of Cu-25 wt-%Cr contact materials produced by powder metallurgy is investigated with respect to the processing variables, such as oxygen content, sintering temperature, time, stearate additives and microstructural features. The sintering temperature of the mixture is determined from the temperature of the liquid phase formation by differential thermal analysis (DTA). Properties of sintered products prepared with and without the reduction treatment of chromium powders are compared in terms of sintered density and electrical conductivity. The initial oxygen content of the chromium powders is decreased from 4500 ppm to 200 ppm by a new method of reduction treatment developed previously and it is observed that the sintered density is increased from 95% theoretical density to 98% theoretical density while the corresponding electrical conductivity increased from 28% IACS to 39% IACS (International Annealed Copper Standard). The sintered density further increased to 99% theoretical density with an electrical conductivity level of 45% IACS by the addition of 0·5 wt-% lithium stearate to the Cu-25 wt-%Cr (200 ppm O2) mixture. By cold working and age hardening, the electrical conductivity is increased to 55% IACS with accomplishing a hardness value of 154 HV5 in the 50% cold worked samples.

Composite reinforcement of the surface of cast iron by WC powder inserts

Inserts represent a convenient means of improving the properties of cast materials by the use of other materials, such as steel, iron, and ceramics. These reinforcement materials are generally used in the form of blocks or plates. The use of hard powders as reinforcement materials for inserts is also the topic of much research. The purpose of the present research is to investigate the reinforcement of gray cast iron locally by inserting tungsten carbide (WC) powders. Three WC powders were used for the inserts, having average grain diameters of 1 μm, 6 μm and 60 μm, respectively. The experiment on the inserts was carried out by firstly inserting WC powders only and then by mixing some alloy powders into the WC powders separately, such as electrolytic chromium, Fe-Mo alloy, and Fe-P alloy. The mold for the insert, which makes five cylindrical specimens, is made by the carbon dioxide process, and each of the specimens measures 30 mm diameter by 50 mm long. WC powders were mixed with a sodium silicate of 0.7 mass% as a binder and then filled into a bottom-mold about 5 mm thick. Gray cast iron (FC150) was used as a parent material for the insert. It was melted with a high frequency induction furnace and poured into the mold at 1623 K. The results confirmed that an approximately uniform inserted layer formed at the bottom of specimens regardless of the particle size of the WC. In the inserted layer, the dispersion form of the WC particles depended on particle size. Here, the matrix structure of the inserted layer remains the same as gray cast iron when only WC powders were inserted. In contrast, when adding chromium powders or Fe-Mo alloy powders to the WC powders, the matrix structure transformed to white cast iron. The hardness of the gray cast iron increases from about 200HV0.3 to about 400HV0.3 with the insert. When adding further alloying elements, the hardness increased further than when inserting WC powders only. Specifically, addition of chromium into WC powders under 6 μm in size increased the hardness to 700HV0.3 or above.

Cr(N,C) diffusion coating formation on pre-nitrocarburised H13 tool steel

The microstructural processes of Cr(N,C) coating formation by thermoreactive deposition and diffusion (TRD) on pre-nitrocarburised H13 tool steel were studied. Both nitrocarburising and TRD were performed in fluidized bed furnaces at 570 °C. During TRD, chromium was transferred from chromium powder in the fluidized bed, to the nitrocarburised substrates by gas-phase reactions initiated by reaction of HCl gas with the chromium. Addition of 30% H 2 to the input inert gas was found to increase the rate of coating formation, although hydrogen reduction resulted in rapid loss of nitrogen to the surface. The reason for the increased rate of coating formation could not be established without further investigation, although several possible explanations have been proposed. It was found that porosity and the formation of an iron nitride ‘cover layer’ during nitrocarburising were the biggest influences on the microstructure of the Cr(N,C) coating. Microstructural characterization of the coatings was performed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and glow discharge optical emission spectroscopy (GDOES).

High-nitrogen martensitic stainless steels produced with nitrided chromium powders: N.K. Filho et al. (Powder Metallurgy and Magnetic Materials Laboratory, Sa˜o Paulo, Brazil.)

High-nitrogen martensitic stainless steels produced with nitrided chromium powders: N.K. Filho et al. (Powder Metallurgy and Magnetic Materials Laboratory, Sa˜o Paulo, Brazil.)

Sinter-joining of different metal powder compacts using resistance heating

Feasibility of sinter-joining method making use of resistance heating with an adiabatic feature was examined. Modeled experiments were conducted using pure metal powders of chromium, iron, nickel, copper and aluminum with a wide variety of melting points or sintering temperatures. All pairs of green compacts were sinter-joined varying compacting pressure and electrifying voltage at fixed electrifying time of 1 s, and their sinter-joining performances were rated by visual inspection and optical microscope observation of vertical section across the joining interface. The results were as follows. Most of the pairs were successfully sintered and joined under properly arranged conditions of electrifying voltage and compacting pressures, but in the combination of chromium and aluminum with a large melting point difference, the aluminum compact melted and infiltrated into the openings of the chromium compact, and in the combinations of chromium and iron with copper with high thermal conductivity, the sintered compacts of chromium and iron had both a porous structure near the joining interface. Those undesirable phenomena were, however, suppressed considerably by employing more rapid heating measures. It was concluded that the sinter-joining method using resistance heating has not a little feasibility.

Anneling of as-mechanically alloyed ironchromiumaluminium powder : A.R. Jones et al. (University of Liverpool, Liverpool, U.K.)

Hardfacing with the martensitic electrode is the interesting way for prolonged life of severe wear in machine parts and very useful in industries because it is easier and lower cost than changing the new one. Powder addition in welding procedure has been a new technique to improve welding alloy properties. This work was carried out to evaluate the effect of chemical composition, macrostructure, microstructure, welding dilution and hardness of iron-based hardfacing with chromium powder addition on to low carbon steel. Using Submerged Arc Welding (SAW) process, the single hardfacing layer was deposited on low carbon steel plate. Commercial low carbon electrode and various amounts of chromium powder addition were used for the hardfacing layer. The hardfacing with powder addition was compared with the hardfaced layer using martensitic type electrode. Optical emission spectrometer (OES) and Optical microscope (OM) were used to analyze the chemical composition and macro/microstructure of hardfacing. Micro-hardness of the cross-sectional hardfacing was also studied. The results of these research revealed that microstructure of hardfacing layer without powder consist of ferrite and pearlite. The microstructure of hardfacing with powder addition showed martensite phase and ferrite. Increasing amount of powder mixture showed lower welding dilution and higher hardness values of the hardfacing layer, on the other hand, microstructure and hardness values of hardfacing welded with the martensitic type electrode are different to that of hardfacing welded with powder addition.

Sintered High Nitrogen Stainless Steel Obtained using Pre-Mixed Powders

Nitrogen alloying in martensitic stainless steels has been considered as an alternative to improve mechanical properties and corrosion resistance. The use of conventional melting and solidification processes is not the most efficient way to produce high nitrogen steels (HNS) since nitrogen solubility in melt is quite low. Nowadays, HNS is being produced by Powder Metallurgy and, in this case, the nitrogen is alloyed in the solid state, especially considering reactions between high nitrogen sintering atmospheres and pre alloyed pressed bodies. This paper investigates another possibility regarding the introduction of nitrogen in stainless steel, i.e., the use of high nitrogen chromium powders (nitrided powders) mixed with ordinary atomized iron powder. This mixture was pressed and sintered at 1350 °C for 2 and 8 hours. As reference, two other set of nitrogen free samples were produced and characterized: sintered pre-alloyed 434L stainless steel (commercial powder); sintered pre mixed Fe + nitrogen free Cr. The discussion was based on shrinkage during sintering, microstructural aspects and corrosion resistance.

Chromium Steel Powders for Components

Chromium as an alloying element is of great interest due to its low cost, high hardenability and possibility to recycle. Use of pre-alloyed chromium powders has increased recently due to its accessibility at low cost. Low total cost is and has been the driving force to convert parts made from conventional steel to PM material. By use of pre-alloyed chromium material, the PM industry can further increase its market share, compared to conventional steel. Two pre-alloyed iron powders, one for high strength applications containing 3 % Cr and 0,5 % Mo and one for medium strength applications containing 1,5% Cr and 0,2% Mo are considered in this paper. The mechanical properties for mentioned materials in the carbon range from 0,2 to 0,8% are described.

Properties of Ni–Cr–Al Alloys Obtained by a Powder Metallurgy Method

Alloys of practically 100% density were formed by hot pressing mixtures of nickel, chromium and aluminum powders. The principal phases formed are solid solutions based on chromium, on nickel, and also the intermetallic NiAl. These correspond to phases in the phase equilibrium diagram of theNi ― Cr ― Al system. The strength and wear resistance of the alloys are determined by the ratio of phase components.

Mixing at micrometric and nanometric scale in mechanically alloyed Fe 60Cr 40

Mechanical alloying (MA) of iron and chromium powder mixtures was performed from 0 to 190 h. SEM, transmission electron microscopy with energy-dispersive X-ray analyses and Mössbauer spectrometry were used to study the mixing at micrometric and nanometric scale. At the stationary mixing state (≈20 h of milling), MA of Fe 60Cr 40 powder mixtures gives particles of several micrometers consisting of magnetic nanograin cores characteristic of crystalline Fe 60Cr 40 alloy surrounded by a disordered paramagnetic grain surface (at 300 K), which increases during milling. At 85 h, the nanograins are totally disordered. For a longer time of milling, a de-mixing occurs by partial recrystallization of this disordered structure.

EDM performance of Cr/Cu-based composite electrodes

Electrode materials for electrical discharge machining (EDM) are usually graphite, copper and copper alloys because these materials have high melting temperature, and excellent electrical and thermal conductivity. The electrodes made by using powder metallurgy technology from special powders have been used to modify EDM surfaces in recent years, to improve wear and corrosion resistance. However, electrodes are normally fabricated at high temperatures and pressures, such that fabrication is expensive. This paper proposes a new method of blending the copper powders contained resin with chromium powders to form tool electrodes. Such electrodes are made at low pressure (20 MPa) and temperature (200 °C) in a hot mounting machine. The results showed that using such electrodes facilitated the formation of a modified surface layer on the work piece after EDM, with remarkable corrosion resistant properties. The optimal mixing ratio, appropriate pressure, and proper machining parameters (such as polarity, peak current, and pulse duration) were used to investigate the effect of the material removal rate (MRR), electrode wear rate (EWR), surface roughness, and thickness of the recast layer on the usability of these electrodes. According to the experimental results, a mixing ratio of Cu–0wt%Cr and a sinter pressure of 20 MPa obtained an excellent MRR. Moreover, this work also reveals that the composite electrodes obtained a higher MRR than Cu metal electrodes; the recast layer was thinner and fewer cracks were present on the machined surface. Furthermore, the Cr elements in the composite electrode migrated to the work piece, resulting in good corrosion resistance of the machined surface after EDM.

Observation of Oxidation and Mechanical Strain in Cr Nanoparticles Produced by Ball-Milling

Nanoparticles of Cr, produced by ball-milling a commercial chromium powder for up to 70 h in Ar-filled glove box, are characterized by room temperature X-ray diffraction (XRD) and magnetization (M) studies as a function of temperature from 5 to 300 K. Analysis of the XRD linewidths in terms of particle size (L) and mechanical strain (η) shows that with increase in milling-time, L decreases and η increases. However, for milling times >40 h, saturation in L (at 20 nm) and η is reached and presence of Cr 2 O 3 becomes evident in the XRD patterns. In the magnetic studies, M increases as L decreases, also reaching saturation values for milling time > 40 h. The variations of M versus T are indicative of nanoparticles effects such as blocking. However, it is evident that the effect of Cr 2 O 3 is present even for samples for which XRD patterns do not show the presence of Cr 2 O 3 (samples milled for less than 40 h). The question of the observed large magnitudes of M for Cr nanoparticles vis-a-vis bulk Cr in terms of undetected surface oxides and uncompensated surface moments is discussed.

One-dimensional assembling of diiodo[phthalocyaninato(1-)] chromate(III) molecules through neutral I(2) molecules. Alternating ferro- and antiferromagnetic interactions in the metal-radical system.

Crystals of diiodo[phthalocyaninato(1-)] chromate(III) diiodine, CrPcI(2).I(2), were grown directly in the reaction of chromium powder with 1,2-dicyanobenzene under a stream of iodine at about 250 degrees C. The CrPcI(2).I(2) complex crystallizes in the centrosymmetric space group of the triclinic system with one molecule per unit cell, with the cell dimensions a = 7.851(2) A, b = 8.402(2) A, c = 12.668(3) A, alpha = 80.32(3)(o), beta = 74.06(3)(o), gamma = 82.33(3)(o), and V = 788.7(3) A(3). The X-ray single-crystal analysis shows that each of the centrosymmetric CrPcI(2) molecules is bridged by a neutral I(2) molecule (detected also by Raman spectroscopy) and develops a polymeric one-dimensional structure. The magnetic measurements have been carried out in the temperature range 300-2 K. Temperature dependence of the effective magnetic moment, mu(eff), shows the ferro- and antiferromagnetic interactions in the system of the paramagnetic central Cr(3+) ion and surrounding pi-conjugated radical ligand Pc(1-). The conductivity measurement on a polycrystalline sample exhibits weak temperature dependence (dsigma/dT < 0). The UV-vis spectrum exhibits, besides the B- and Q-bands, one additional band assigned to the electronic transition from a deeper level to the half-occupied HOMO level in the one-electron oxidized phthalocyaninato(1-) radical ligand.

Structural and magnetic properties of grain boundaries in Fe60Cr40 alloysynthesized by mechanical alloying

Mechanical alloying of iron and chromium powder mixtures was performed from 0to 190 h milling times. X-ray diffraction, Mössbauer spectrometry and differentialscanning calorimetry were used to determine the structural and magneticproperties of the intergranular zone, which was associated with a very disorderedgrain boundary structure, paramagnetic at 300 K. This structure is assumed to bedue to the presence of oxygen and nitrogen atoms, forming a mixture ofamorphous FeCr + O + N alloy. Between 21 and 85 h, the hyperfine parameters ofthe magnetic contribution, and therefore the properties of the Fe60Cr40nanograin cores, are constant, but the paramagnetic fraction associated withthe intergranular zone increases. At 85 h, the paramagnetic contributionrepresents 100% of the Mössbauer spectrum.Moreover, after very long milling times, we have evidenced ade-mixing reaction which results in preferential oxidation andnitrogenation within the very disordered structure.

メカニカルアロイングを利用したＣｒＳｉ２熱電材料の作製

Elemental powders of chromium and silicon have been mixed to give the desired composition of Cr-66.66 at%Si and milled in a planetary ball mill under an argon atmosphere. CrSi2 has been synthesized in situ during ball milling. The obtained powder milled for 360ks consisting nano-sized Cr, Si and CrSi2 enables us to prepare dense compacts at the consolidating temperature 1290K which is about 170K lower than that of the other milled powders for various times. This is due to a decrease of powder particles and grains. Power factor of Cr-66.66 at%Si compact prepared by mechanical alloying and pulse current sintering is higher than that of other silicides below 650K, and shows the maximum value of 1.4×10-3 (W⋅K-2⋅m-1) at 550K.

Nonthermal Nature of Unsteady Combustion of Chromium in Nitrogen

When specimens pressed from a chromium powder react with nitrogen at pressures of 1–8 MPa and relative densities of 0.47–0.55, unsteady combustion of a nonthermal nature, which was not known previously, is observed. It is shown that the unsteadiness is due to cracks formed ahead of the combustion front. The specimens fail as a result of an increase in the condensed-phase volume upon formation of nitrides.

Theoretical Foundations and Implementation of the High-Temperature Electrosynthesis of Chromium, Molybdenum, and Tungsten Silicides and Borides in Ionic Melts

Melts containing a Group VIA metal and silicon or boron were studied by thermodynamic methods and voltammetry, and conditions were found for the high-temperature electrosynthesis of powder chromium, molybdenum, and tungsten silicides and borides.

The interfacial stability of the coated-SiC/Fe couple

SiC piece was successfully coated by interaction with chromium powder. When the SiC piece was coated at 1373 K for 1 h, the obtained coating layer of 6 μm is dense and well distributed on the SiC piece. The coating layer is composed of Cr 3Si, Cr 7C 3 and Cr 23C 6, and in a sequence of Cr 3Si/Cr 3Si+Cr 7C 3/Cr 7C 3/Cr 23C 6 from the SiC terminal to the growing surface. The coating layer on the SiC piece has high thermal stability, which may improve the thermal stability of the coated-SiC/Fe composite. The interfacial stability between the coated-SiC and Fe was examined by annealing the coated-SiC/Fe couples in the temperature range from 1273 to 1423 K for times from 1 to 22.5 h. The results showed that the coating layer improved the interfacial stability between the coated-SiC and Fe by inhibiting the reaction between SiC and Fe. The interdiffusion of the Fe and the coating layer’s component atoms resulted in the dissolution of coating layer into Fe piece at 1373 K. The dissolution of the coating layer increased with the increasing annealing temperature and time. After annealed at 1423 K for 5 h, the coating layer of the coated-SiC/Fe couple was found to be completely dissolved, and the reaction between SiC and Fe was detected.

The effect of chromium on the microstructure/toughness relationship of C–Mn weld metal deposits

Submerged-arc single-pass welds were prepared with varying chromium content in the range of 0–3.8%. Two carbon contents were obtained by dilution using different welding procedures. The variation in the chromium content was obtained by the addition of different amounts of chromium powder to the weld groove. The relationship between microstructure and toughness of weld deposits was studied by means of hardness, Charpy-V notch and metallographic tests in specimens cut transversely to the weld beads. Qualitative and quantitative analyses of microstructural constituents and fine phases were made by light optical and scanning electron microscopy, respectively. In this investigation, a selective etching method was employed in order to distinguish the martensite–austenite (M/A) “microphase” constituent from carbides. The results show that chromium impairs impact toughness, although it promotes an increase in the percentage of acicular ferrite (AF). In addition, it was observed that an increase in carbon content promoted a further decrease in impact toughness due to the higher volume fraction of the M/A constituent.