Increasing Chromium Content Research Articles

Influence of Exposure Environment on the Corrosion Resistance of 2–9% Cr Steels

Corrosion tests on 2–9% Cr steels were conducted using steam and supercritical water under 0.1–25 MPa at 600 °C. The oxidation kinetics were similar in three different environments, and they showed near-parabolic behaviour. It appeared that the level of benefit arising from increased chromium content was not constant but was dependent on the exposure environment. Whilst it was evident that the increasing pressure increased the oxidation kinetics, it was also true that the presence of high levels of Cr in the alloy still imparted some enhanced protection.

On the Microstructure and Wear Behavior of Fe–xCr–4Mn–3C Hardfacing Alloys

The present paper describes an investigation aimed at evaluating the microstructural and dry sliding adhesive wear characteristics of Fe–xCr–4Mn–3C hardfacing alloys applied through shielded metal arc welding. The effect of chromium addition on the microstructure of hardfacings was carried out by using optical microscope, field emission scanning electron microscope and X-ray mapping. Dry sliding wear tests were performed on a pin-on-disc wear tribometer. From the experimental results, it was observed that the primary carbides were refined and increased with the increase of chromium content. The morphology of carbides revealed that the primary carbides were rod shaped. The increased chromium content was also found to be beneficial to enhance hardness and wear resistance of hardfacings. The correlation between hardness and wear resistance exhibited the reliability of hardness as an indicator of the wear performance of hardfacings.

ABOUT THE ISSUE OF CARBIDES FE3C AND FE7C3 FORMATION IN HIGH-CHROMIUM CAST IRONS

Purpose. This article analyzes the formation conditions, transformation and systematization of carbide phases formed in the system Fe – C – Cr.Methodology. Conversion of the elements’ content from mass % into atomic % and vice versa was carried out using standard methods. In order to identify the structural components and etching of carbides the Marble etchant was applied. Cast iron 300Х28Н2 in cast state without heat treatment and after isothermal holding at 1050 °С during 4.5 hours with further normalization was studied. Findings. Isothermal state diagrams of the Fe–C–Cr system don’t take into account the existence of Fe7C3 carbide alloyed with chromium. But there is evidence of the existence of chromic carbides containing 24…37.6 % of chromium, which exceeds its maximum solubility in cementite, but is not enough to form Cr7C3.Analysis of chromium and carbon content in carbide phases which are formed in high-chromium cast irons allowed to substantiate the formation of Fe7С3 carbide, stabilized with chromium. Assessment of the carbide phase by chemical composition in mass percent doesn’t allow determining the main carbide-forming element with sufficient accuracy. It is shown that with the increase of chromium concentration in carbides, mass content of carbon increases. Areas of existence of carbides of different types depending on carbon and chromium content in them were determined. Maximum content of chromium in the carbide (Fe, Cr)7C3 is 44.0 %. Allocation of alloyed cementite occurs on the present carbides Ме7C3 or grain boundaries, and with the increase of cooling rate – in the grain volume. This process is thermodynamically inevitable due to the decrease of carbon solubility in ferrite or austenite at temperatures when chromium diffusion is impeded, and only carbon diffusion is possible. At high chromium concentrations carbide Fe7C3 is formed, which transforms into Cr7C3 carbide as chromium diffusion takes place. Originality. The model of laminated structure of carbides, formed out of the liquid phase in the system Fe–C–Cr was built. The identification of the carbide phase based on the elements’ concentration in atomic percent was suggested. The model of the change of carbon content in carbides of different types depending on chromium concentration was built. Practical value. The suggested system of carbide classification and their structural model allows optimizing the cast irons’ composition and heat treatment modes for different exploitation conditions.

Structural and electrical properties of Cr doped nickel ferrite

The Chromium doped Nickel ferrite nanopowders prepared by sol–gel combustion route by nitrates of respective metal ions. Formation of structure, morphology has been confirmed by X-ray powder diffraction method, FTIR and HRSEM. Additionally, electrical properties of prepared chromium doped nickel ferrites were studied using four probe method. The conductivity of the prepared ferrites was found to be decrease with increase in concentration of chromium.

Association between personal exposure to metals in fine particulate matter and urinary metals: baseline results from a panel study

To investigate the associations between 21 metals in fine particulate matter (PM2.5) and their corresponding concentrations in urine in the general population. Between April and May 2011, this panel study enrolled 120 residents using random sampling approach in Wuhan communities which contained 3 035 subjects. Participants were aged 18 to 80 years and had lived in the sampling buildings for at least 5 years. Data from basic questionnaires, physical examinations, and morning blood and urine samples under fasting conditions were collected. Participants with missing data were excluded. Finally, 83 particpants included. Participants were instructed to use personal air samplers to continuously monitor PM2.5 for 24 h. The following 21 metals were measured in PM2.5 and urine by inductively coupled plasma mass spectrometry: aluminum, titanium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, rubidium, strontium, molybdenum, cadmium, tin, antimony, barium, tungsten, thallium and lead. The associations between PM2.5 metals and urinary metals were investigated using generalized linear regression models. The age of the study population was (51.5±6.3)years. After adjusting for age, sex, smoking status, BMI, education and income, elevated urinary chromium was significantly associated with increased chromium concentrations in personal PM2.5. The least square means (standard deviation) of urinary chromium in participants classified as having low exposure (<12.491 ng/m(3)), intermediate exposure (12.491-32.388 ng/m(3)) and high exposure (>32.388 ng/m(3)) were (-1.334±0.756), (-1.114±0.813) and (-0.718±0.645) μg/mmol creatinine, respectively (P=0.009). However, the association between urinary and personal PM chromium was not observed after additionally adjusting for false discovery rate (P>0.05). Furthermore, the results demonstrated that other metals in PM2.5 were not related to their corresponding concentrations in urine of subjects. Urinary concentrations of metals did not reflect human exposure to metals in air, and may not be appropriate as an index to assess personal exposure to metals in particulate matter.

Influence of the chromium concentration on structure formation in liquid chromonickel alloys

The electrical resistivity of liquid nickel–chromium alloys is studied as a function of the temperature and concentration. Experimental data indicate that the dependence of the alloys’ electrical-resistance isotherms on the concentration is nonlinear. On the basis of the temperature and concentration dependence of the electrical resistivity of nickel and chromium alloys, the optimal conditions for the formation of microhomogeneous and equilibrium structure in the melt may be determined. The electrical-resistance isotherms of chromonickel alloys may be qualitatively explained on the basis of percolation theory and a quasi-chemical model of the microheterogeneous structure of molten metal alloys. The structure formation of chromonickel melts with increase in chromium concentration is characterized by the successive formation of clusters with different structure and dimensions.

Magnetic cluster expansion model for random and ordered magnetic face-centered cubic Fe-Ni-Cr alloys

A Magnetic Cluster Expansion model for ternary face-centered cubic Fe-Ni-Cr alloys has been developed, using DFT data spanning binary and ternary alloy configurations. Using this Magnetic Cluster Expansion model Hamiltonian, we perform Monte Carlo simulations and explore magnetic structures of alloys over the entire range of compositions, considering both random and ordered alloy structures. In random alloys, the removal of magnetic collinearity constraint reduces the total magnetic moment but does not affect the predicted range of compositions where the alloys adopt low-temperature ferromagnetic configurations. During alloying of ordered fcc Fe-Ni compounds with Cr, chromium atoms tend to replace nickel rather than iron atoms. Replacement of Ni by Cr in ordered alloys with high iron content increases the Curie temperature of the alloys. This can be explained by strong antiferromagnetic Fe-Cr coupling, similar to that found in bcc Fe-Cr solutions, where the Curie temperature increase, predicted by simulations as a function of Cr concentration, is confirmed by experimental observations. In random alloys, both magnetization and the Curie temperature decrease abruptly with increasing chromium content, in agreement with experiment.

Influence of material properties of stainless steel on the ignition probability of flammable gas mixtures due to mechanical impacts

Grazing impact experiments of various types of stainless steels were performed in explosive atmospheres of hydrogen, acetylene, ethylene or propane with air. Depending on the gas mixture, kinetic energy of the impact, and applied stainless steel, the dominant ignition sources are either separated particles or hot friction surfaces. An influence of chromium content on the ignition probability was not found, although an increase in chromium content results in a reduction of the oxidizability of separated particles. Additionally, the influence of the material properties thermal conductivity, specific heat, density and hardness on the ignition probability of the hydrogen–air mixture was investigated. With increasing thermal conductivity a decreasing rate of ignitions was observed. In contrast, an influence of the physical properties specific heat, density and hardness on the ignitability was not found.

Characterization of nanostructured Cu[sbnd]Cr bulk composites prepared by high-energy mechanical alloying

In this paper, the structure and properties of metal matrix composite materials on the basis of copper reinforced with chromium (20–50 wt%) were investigated. Raw materials consisted of grinded copper chips 5000 μm in size and chromium particles with an initial size of 10 μm. The composites were produced by mechanical alloying in a high-energy planetary mill with a quasi-cylindrical grinding body followed by hot pressing at 650°С. The milling duration was 1–10 h. It was shown that microstructure refinement occurs in the composites with an increase in chromium content and milling time. The structure of Cu-50 wt% Cr bulk materials after mechanical alloying for 5 h followed by hot pressing consists of the Cu and Cr-rich phases with average crystallite sizes of 37 and 28 nm, respectively. During short time milling the hardness of this composite increases to 600 HV due to the high intensity of mechanical alloying. At the same time, its wear resistance increases almost by 30 times in comparison with 1 h milling. On the contrary, electrical conductivity decreases after mechanical alloying.

THE INFLUENCE OF CHEMICAL COMPOSITION OF HIGH-CHROMIUM CAST IRONS ON THE MACHINABILITY

Purpose. This research is aimed to obtain the regression dependence of the machinability on the chemical composition of pig iron (C, Cr, Mn and Ni) in cast state. Methodology. The method of active experiment planning was used to build a mathematical model. Cast irons of composition 1.09…3.91 % С; 11.43…25.57 % Cr; 0.6…5.4 % Mn; 0.19…3.01 % Ni were studied. Cutting tools with plates 10х10 mm out of ВК8 according to State Standard 19051-80 were used for turning. Cutting modes: cutting depth – 0.8 mm, longitudinal feed – 0.15 mm/rot., spindle’s rotation frequency during turning – 200…360 rot./min. Lubricating and cooling liquids were not applied. Evaluation of iron workability was produced by determining the linear tool flank wear per unit length of the cutting path. Findings. Mathematically probabilistic equation of the regression dependence of the cutting tool’s wear on the C, Cr, Mn and Ni content in the machined cast iron were obtained. It was established that with the increase of Cr content in the cast iron to 14.8 % the cutting tool’s wear decreased as a result of formation of carbide eutectic which destroyed the doped ledeburite continuous frame. Further increase of chromium content promoted appearing of chromic carbides with high microhardness which considerably increased the tool’s wear. The conducted research shown that the minimum cutting tool’s wear 0,18 mkm/m was observed during the machining of cast iron containing: 1.09 % C, 14.8 % Cr, 2.3 % Mn and 1.2 % Ni; and the maximum wear is 48,96 mkm/m – when the content was: 3.91 % C, 11.43 % Cr, 5.4 % Mn and 0.19 % Ni. The tool’s wear reached 47.61 mkm/m during the treatment of cast iron containing 3.91 % C, 25.57 % Cr, 5.4 % Mn and 0.19 % Ni. Originality. Mathematically probabilistic model of the dependence of the cutting tool’s wear on the C, Cr, Mn and Ni content in the machined cast iron has been elaborated by the author. Practical value. The model allows optimizing the compositions of wear-resistant cast irons for castings which require the significant mechanical machining. Cast irons compositions were recommended for different exploitation conditions.

Two-body abrasion resistance of cementite containing different chromium concentrations

Abstract

Real and Imaginary Parts of Magnetic Susceptibility of Fine Dispersed Nanoparticles Synthesized by Reverse Micelle: From Superparamagnetic Trend to Ferrimagnetic State

Ni0.6Zn0.4Fe2−xCrxO4 (x = 0–0.5) ferrite nanoparticles were prepared using reverse micelle process. X-ray diffraction patterns demonstrated that single phase of spinel ferrites are obtained. TEM micrographs demonstrate the formation of well dispersed nanoparticle with almost spherical morphologies. The EDS analysis proved that the chemical composition has almost uniform distribution in the whole series of samples. Magnetic dynamics of the samples was studied by the measurement of AC magnetic susceptibility versus temperature at different frequencies. The phenomenological Neel–Brown and Vogel–Fulcher models were employed to distinguish between the interacting or non-interacting system. Results exhibited that there is strong interaction between fine particles. The results obtained from vibrating sample magnetometer confirmed that with an increase in chromium content, the saturation magnetization increases. SQUID results at 2 K displayed the ferromagnetic state in nanoparticle and it was found that with an increase in substitution contents, the coercivity decreased.

Study of the Dielectric Behaviour of Cr-Doped Zinc Nano Ferrites Synthesized by Sol-Gel Method

The series of Cr-Zn nano ferrites having the general composition CrxZnFe2-xO4 (0 ≤ x ≤ 0.5) have been synthesized successfully in the nanocrystalline form using the sol-gel method. The samples were sintered at 900°C for 3 hours. The effect of chromium substitution on dielectric properties of Zn-ferrites is reported in this paper. The analysis of XRD patterns revealed the formation of single phase cubic spinel structure for all the Cr-Zn ferrite samples. The FTIR spectra show two strong absorption bands in the range of 400 - 600 cm-1, which corroborate the spinel structure of the samples. The average grain size was found to be in the nanometer range and of the order of 43 - 63 nm obtained using TEM images. The lattice parameter and crystallite size decrease with increase in Cr concentration (x). The investigation on dielectric constant (e'), dissipation factor (D) and ac conductivity (σac) was carried out at a fixed frequency 1 kHz and in the frequency range of 100 Hz to 1 MHz at room temperature using LCR meter. The plots of dielectric constant (e') versus frequency show the normal dielectric behavior of spinel ferrites. The value of ac conductivity (σac) increases with increase in frequency for all the compositions. The appearance of the peak for each composition in the dissipation factor versus frequency curve suggests the presence of relaxing dipoles in the Cr-Zn nano ferrite samples. It is also found that the shifting of the relaxation peak towards lower frequency side with an increase in chromium content (x) is due to the strengthening of dipole-dipole interactions. The composition and frequency dependence of the dielectric constant, dielectric loss and ac-conductivity are explained based on the Koop’s two-layer model, Maxwell-Wagner polarization process, and Debye relaxation theory.

EFFECT OF CHROMIUM CONCENTRATION ON THE STRUCTURE FORMATION PROCESS OF LIQUID CHROME-NICKEL ALLOYS

The temperature and concentration dependences of the electrical resistivity of the nickel-chromium alloys in the liquid state have been studied. Experimental data suggest a nonlinear concentration dependence of the electrical resistivity isotherms of the studied alloys. The results of studies of the temperature and concentration dependencies of the electrical resistivity of nickel-chromium alloys allow determining the optimal conditions for the formation of microhomogeneous and the equilibrium structure of the melt. Using of percolation theory and quasichemical model of microheterogeneous structure of liquid metal alloys gives the possibility to explain the nature of the isotherms of the electrical resistivity of nickel-chromium melts. The process of structure formation of nickel-chromium melts with increasing chromium concentration has been presented in the form of schemes, reflecting the gradual formation of different clusters, featuring by their structure and size.

Biosorption of Chromium from Aqueous Solution by Gracilaria corticata (Red Algae) and Its Statistical Analysis Using Response Surface Methodology

The present work deals with, the potential of biosorption technique for the removal of chromium from aqueous solutions using marine based material viz. Gracilaria corticata powder. The experiments are carried out in a batch operation to understand the equilibrium studies, kinetics and thermodynamics of the biosorption. The maximum removal efficiency of 93.68% was obtained at a temperature of 323 K, pH 4, equilibrium time of 240 min and at an optimum biosorption dosage of 20 g/L of 44 μm particle size. The percentage biosorption increases with an increase in initial concentration of chromium. The statistical analysis of biosorption using Response Surface Methodology (RSM) was studied. The theoretical and experimental values for % biosorption of chromium were observed as 85.39% and 85.49% respectively. It is evident that experimental values of % biosorption are in close agreement with that values predicted by Central Composite Design. The experimental data are well described by Freundlich, Langmuir and Redlich isotherm models. The biosorption data follows pseudo second order kinetics. Thermodynamic results showed that biosorption is endothermic, irreversible and feasible.

Enhanced Catechol Oxidase and Peroxidase Activities as Possible Markers for Measuring Chromium Tolerance in Sorghum (Sorghum bicolor L.) Genotypes

The aim of the present study was to estimate the effect of chromium on catechol oxidase and peroxidase activities in ten sorghum genotypes grown in soil amended by different concentrations of chromium. Ten sorghum genotypes, (Tabat, Wad Ahmed, L4, L7, L12, L14, L16, L25, L32 and L34) were obtained from the Department of Agronomy, Faculty of Agriculture University of Khartoum., Seeds were sown in polyethylene bags filled with 2 kg soil (clay and sand, 2:1). Two weeks after sowing, the seedlings were irrigated with eight levels of chromium (Cr-VI) concentrations (0, 2.5, 5, 10, 20, 30, 40, and 50 mg/l.). Samples for assaying enzyme (catechol oxidase and peroxidase) activities were taken three times (on the 8, 12, and 15 day) following chromium application. The results revealed that the level of enzyme activity of most genotypes was increased significantly with increasing chromium concentration. The magnitude of increase in enzymatic activity was also dependent on the genotype and duration of time after chromium treatment. These findings suggest that, the activities of both enzymes might be used as indicator for selecting chromium tolerant sorghum genotypes.

Cation distribution and magnetic study of Cr-substituted lithium ferrites

Nanocrystalline Li0.5Fe2.5−xCrxO4 (2.5 ≥ x ≥ 0) systems were prepared by the sol–gel method. Formation of single phase cubic spinel structure for all the compositions was confirmed from their X-ray diffraction patterns. The lattice parameter shows a decreasing trend with the increase in Cr content. Infrared spectra showed two main absorption bands in the range 400–800 cm−1 arising due to tetrahedral and octahedral stretching vibrations. A room temperature magnetization result shows a ferromagnetic behavior decreases with increase in chromium content. The detailed results of structural and magnetic have been discussed so as to bring out the role of Cr substitution in lithium ferrite.

Direct Determination Of γ′ / γ′+γ / γ Phase Boundaries In Ni-Al-Cr System Based On Enthalpy Of Formation Results Obtained By Calorimetric Solution Method

AbstractThe work is a continuation of the research carried out on a high-temperature calorimeter solution type on alloys from Ni-Al-Cr system. Thanks to the construction innovation introduced by authors the device allows the determination of the formation enthalpy of alloys at ambient and elevated temperatures. Experiments described in this article were carried out at three temperatures: 873K, 996K and 1150K on the alloys of the chemical compositions from the Ni75Al25÷ Ni87Cr13section of the Ni-Al-Cr system. On the basis of changes in the enthalpy of formation with increasing chromium content of the alloys, points corresponding to places of phase boundaries γ′ / γ′+γ / γ in Ni-Al-Cr system were determined. A similar relationship was observed in previous studies of alloys from Ni75Al25÷Ni75Cr25section. For precise determination of these characteristic points a statistical model was applied

Effect of chromium concentration on the structural, magnetic and electrical properties of praseodymium-calcium manganite

The influence of Cr doping on magnetic, magnetocaloric and electrical properties in a polycrystalline sample of Pr0.7Ca0.3MnO3 is investigated. Structural studies show that our samples are single phase. The magnetization shows that the Pr0.7Ca0.3Mn1−xCrxO3 ceramics exhibit a paramagnetic–ferromagnetic transition with a large magnetic entropy change. The relative cooling power (RCP) values are comparable to those of other manganite. DC conductance GDC measurements show that all samples are characterized by a semiconductor behavior. It is found that GDC decreases by two decades when increasing chromium concentrations. For the parent compound, dc-conductance is characterized by the appearance of a saturation region at a specific temperature (Tsat = 200 K). For the doped compound, Tsat go beyond room temperature. Conduction mechanism is found to be dominated by the small polaron hopping (SPH) process at high temperature and by variable range hopping one (VRH) at low temperature. AC conductance study confirms that the conductivity is governed by hopping process and obeys to the Jonscher universal power law. The exponent ‘n’ variation with temperature is in good agreement with Mott theory. Its variation as a function of chromium content indicates that the material turns from metallic to semi-insulating behavior when chromium composition increases. Impedance analysis proves the presence of electrical relaxation phenomenon in the material and confirms that grain boundaries played a main role in the conduction process.

Influence of Chromium on Corrosion Behavior of Low-alloy Steel in Cargo Oil Tank O2-CO2-SO2-H2S Wet Gas Environment

As international maritime organization (IMO) draft 289 was adopted to develop a low-alloy anti-corrosion steel for the deck of cargo oil tank and to understand corrosion mechanism, corrosion behavior of a low-alloy steel with chromium contents was studied in O2-CO2-SO2-H2S wet gas environment. Corrosion rate was measured, and the microstructure and morphology of corrosion product film were characterized by scanning electron microscopy (SEM). The phase and chemical composition of the corrosion product film were investigated by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), respectively. The effect of misorientation distribution on corrosion resistance of steel was evaluated by electron backscattered diffraction (EBSD). The results showed that corrosion rate decreased with increasing chromium content in the low-alloy steel, and the corrosion type was general corrosion. The phenomenon of chromium enrichment was found in corrosion product film consisting of α-FeOOH, γ-FeOOH, sulphur, FeS2 and Fe1–x S. The increase of chromium content decreases the amount of high-angle grain boundaries, thus resulting in the improvement of corrosion resistance.