On a possibility of replacing grey cast iron for manufacturing cast elements of electrolyzer gas collecting bell

The main process leading to the destruction of the cast elements of gas-collection bell of electrolyzer, made of grey cast iron, is the oxidation of iron by oxygen, SO2 gas and sulfur vapors to form magnetite, hematite and pyrrhotin. The simultaneous formation of iron oxides and sulfides does not prevent further corrosion, since scale is formed with a loose structure that does not have protective properties. Reducing the length of the interfacial boundaries inside the material of the cast enables to reduce the rate of corrosion destruction, which can be achieved by modifying the cast iron to change the shape of graphite inclusions, i.e. obtaining high-strength cast iron with a spherical shape of graphite inclusions. However, the obtaining spherical graphite in cast iron using magnesium modification does not exclude the access of aggressive gases to the surface of the products and the possibility of their diffusion along the grain boundaries. It was shown that alloying can be an alternative, which leads not only to the exclusion of lamellar secretions of graphite in the structure of cast iron, but also to the formation of surface oxide layers based on the alloying element preventing the corrosion. Alloying with chromium gives cast iron high abrasive resistance due to the presence of a carbide component in the structure, as well as corrosion resistance due to the alloying of the metal base, heat resistance due to increasing the electrochemical potential of the metal base and creating a strong neutral oxide film on the surface of the castings, heat resistance, etc. An experimental comparative analysis of the corrosion resistance of cast iron used for manufacturing of gas collecting bell of electrolyzers showed that chromic cast iron ЧХ3 has a higher corrosion resistance than high-strength cast iron with spherical graphite ВЧ50 and much higher than grey cast iron with lamellar graphite. However, chromic cast iron ЧХ3 has low casting properties, is very sensitive to the cooling rate and has a large heterogeneity in structure, which makes it difficult to use it for the manufacture of gas collecting bell of electrolyzers.

DETERMINATION OF THE DIMENSIONAL AND TOPOLOGICAL PARAMETERS OF THE GRAPHITE PHASE IN CAST IRON BY FRACTAL ANALYSIS OF IMAGES OF ITS MICROSTRUCTURE

The basics of metallography and modern systems used for studying and analyzing the structures of materials are described. Special attention is paid to the techniques of quantitative microscopy, as a kind of ancestress of modern microstructure analysis systems. The analysis of modern computer programs used to analyze images of microstructures obtained from digital microscopes is presented. The basics of fractal analysis as a highly effective tool for calculating numerical values of parameters of geometrically complex objects are outlined. Using the example of the analysis of graphitized cast iron structure, the application of the fractal analysis method to determine such characteristics of the graphite phase as the shape of graphite inclusions and their distribution in the alloy volume is demonstrated. As part of the study, various classes of cast iron have been studied, such as graphitic pig iron with flaked graphite, cast iron with vermicular graphite, and high-grade cast iron with spheroidal graphite. To determine the shape of graphite inclusions, a fractal dimension has been proposed to be used, and the unevenness of the distribution has been estimated using such a function as lacunarity. The individual stages of determining these characteristics using a specialized FracLac module applied in the structure of the ImageJ program are presented. The obtained results showed high adequacy. Despite positive assessment, the shortcomings identified during the research on the use of fractal analysis methods for identifying geometrically complex dimensional and topological parameters inherent in the graphite phase in cast iron are noted. The ways for further improvement of these methods for solving a wide range of problems in metallography of alloys are proposed.

ON THE INFLUENCE OF THE SIZE OF FLAKE GRAPHITE INCLUSIONS AND ITS VOLUME CONTENT ON THE ACOUSTIC PARAMETERS OF CAST IRON AND THE CHARACTERISTICS OF ULTRASONIC LONGITUDINAL WAVE SIGNALS

Algorithms have been presented for calculating the velocity (in the approximation of a fine-layered model) and the attenuation coefficient of a longitudinal ultrasonic wave in cast iron, depending on the average size of graphite elements and its volume content, the calculation results for which are qualitatively confirmed experimentally. The calculation was performed using a fine-layered model of the structure, the graphite inclusions were described in the form of plane-parallel layers placed in an isotropic elastic medium (metal base). Computer simulation of acoustic paths for a mediumcast iron with flake graphite for standard direct converters is carried out in order to study the influence of such a medium on the characteristics of transmitted and received signals during ultrasonic testing. In the course of the research, a previously developed model was used to calculate the attenuation coefficient of longitudinal waves in cast iron with flake graphite due to their Rayleigh and phase scattering on graphite inclusions. Computer simulation of the acoustic characteristics of the signals of a direct linear probe with a phased array in cast iron with flake graphite was carried out, during which the shape of the acoustic pulses of the longitudinal wave was calculated, depending on the distance traveled by the wave and the value of the attenuation coefficient for various models of cast iron. The main modeled characteristics of the transducer include the directivity characteristic and the change in the signal amplitude along the acoustic axis. It is shown that for cast iron with flake graphite, there are cases when the directivity of the probe with a phased array transmitted into the cast iron is practically absent.

Relationship between mechanical behavior and microstructure evolution of sintered metallic brake pad under impact of thermomechanical stress

It is well known that on the brake pad material, the triptych microstructure-properties-solicitations is the key to better understand the phenomena caused by braking stress. The challenging issues are the evolution of this triptych, i.e., the impact of thermal stress and mechanical stress on the microstructure which undoubtedly induces changes in properties. In order to solve the issues without tackling them in all their complexity, this study proposes an experimental approach where physics is decoupled but inspired by the braking sequence in terms of applied temperature gradient and braking loads. Two experimental tests were carried out. The first one is the thermal solicitation test where a temperature gradient from 400°C to 540°C was applied to the material. The second one is the thermomechanical test where a compressive load at 20 MPa was applied under the same thermal gradient. The experiment time is fixed for two minutes, equivalent to the time of one braking stroke. The referred material is sintered metallic composite, which is widely used as brake pad material for high-energy railways. As result, it shows that coupled thermomechanical stress has a greater impact on the material properties than decoupled one. This impact is related to the microstructure where graphite inclusions play an important role.

Diffusion crushing of graphite in integrated cast iron processing

Purpose. It is necessary to investigate the possibility of crushing graphite inclusions in cast irons. The aim of the work was to study structural changes in graphite under explosive action followed by thermal cycling treatment of gray cast iron. Methods. Gray cast irons were subjected to explosive action (pressure is 90 GPa, the deformation rate is 100 s-1, the time is 10-6...10-7 s). Then the cast irons were subjected to thermal cycling: 950 °C, holding for 20 min, cooling with a furnace; 5 cycles. Metalographic ("Neophot-21"), micro-X-ray spectral ("Nanolab-7"), X-ray diffraction (DRON-2.0) research methods were used. Results. The features of diffusion crushing of graphite in cast irons with different types of metal matrix (ferritic, austenitic) and graphite shape (lamellar, spherical) have been studied. The features of structural changes in the cast iron matrix under explosive action, which determine the conditions for the transformation of graphite during subsequent thermal cycling, are discussed. It is shown that this process of diffusional crushing of graphite can be accompanied by local melting, which leads to the formation of regularly distributed dispersed particles of graphite. Scientific novelty. From the point of view of physical mesomechanics of plastic deformation, the processes of structural changes in the metal matrix of cast irons are considered. It is shown that nonequilibrium defect substructures of the cast iron matrix obtained during the explosion determine the features of diffusion crushing of graphite inclusions during subsequent thermal cycling. The conditions for diffusion crushing of graphite with reflow and in the solid-phase state have been established. Practical significance. It has been established that the complex processing of “explosive loading + thermal cycling” promotes crushing of coarse graphite inclusions, which is favorable for the mechanical and operational properties of cast irons. The use of the results obtained will make it possible to develop technologies for complex processing with regulated parameters of graphite inclusions, which will lead to an expansion of the field of application of gray cast irons. Keywords: cast iron; shock wave treatment; graphite; thermal cycling; stress relaxation

EBSD analysis of graphitized steel microstructure after compression deformation at room temperature

The graphitized steel has attracted considerable attention due to its excellent cutability and good properties at cold forming. Compression deformation at room temperature of graphitized steel (0.43 % C) with a ferrite-graphite microstructure was performed on a universal testing machine. Microstructures of deformed samples were studied using the analysis technique of Electron Back-Scattered Diffraction (EBSD). The evolution of microstructure morphology, texture, distribution of Kernel Average Misorintations (KAM) and the Taylor factor in the zone of large deformations of deformed samples with different degrees of deformation is discussed. The results show that the studied steel has a good ability to compression deformation. During compression deformation, with an increase in deformation degree the deformation morphology of the ferrite grain and graphite inclusions gradually stretch in the direction perpendicular to the compression axis and they are represented as fibrous forms. The orientation of ferrite grains in the matrix is gradually obvious, and the orientation of ferrite grains around graphite inclusions is not obvious, that is, the number of grains oriented to <100>, <111> in the matrix is much greater than around graphite inclusion. In addition, KAM and the Taylor factor in the large deformations region of compression samples show that the deformation degree of ferrite grains around graphite inclusions is less than that of ferrite grains in the matrix. The reason for this is that the soft graphite inclusions can reduce the degree of dislocation pile-up.

Structure and microhardness of binding for diamond tools based on tungsten carbide obtained by impregnation of iron-carbon melt

In this work, an experimental modeling of the technology for producing a matrix by sintering a diamond-containing briquette with a filler of tungsten monocarbide powder impregnated with a Fe-C eutectic melt in a vacuum is carried out. The microstructure, elemental and phase compositions of the products formed in the process of sintering a diamond-containing matrix with impregnation with a Fe-C eutectic melt in vacuum have been studied by scanning electron microscopy, X-ray spectral and X-ray phase analyzes, and Raman spectroscopy. It was found that the matrix consists of 61.0% tungsten carbide phases, 17.0% of iron carbide, 16.5% of α-Fe, and 5.5% of graphite. The eutectic Fe-C alloy, which serves as a matrix binder, consists of a ferrite-pearlite metal base with graphite inclusions. It is shown that at the diamond - matrix interface, graphite inclusions are formed not as a continuous layer, but as discontinuous areas along the perimeter of diamond grains. The microhardness of the WC-based matrix impregnated with the Fe-C melt is ~ 11 GPa, which is more than 3 times higher than the microhardness of the WC-Co-Cu hard alloy matrix obtained by sintering with copper impregnation.The research results can be used in the development of technology for the manufacture of wear-resistant matrices of diamond tools of a wide class used in the processing of materials with a high level of hardness.

INFLUENCE OF THE NUMBER AND SIZE OF GRAPHITE INCLUSIONS ON THE STRENGTH PROPERTIES OF CAST IRON

Рассмотрено влияние размера и количества графитовых включений на прочностные свойства высокопрочного чугуна. Установлено, что размер и количество включений шаровидного графита в высокопрочном чугуне влияет на временное сопротивление при растяжении. Показано, что при одинаковом объеме свободного графита временное сопротивление при растяжении при крупнодисперсной фазе в 1,4 раза больше, чем при мелкодисперсной фазе.

Method of the Analysis of Materials’ Microstructure Based on the Fractal Analysis of Images

The basics of metallography and modern systems used to study and analyse the structures of materials are presented. Special attention is paid to the methods of quantitative microscopy. The review of modern computer programs for analysis of image microstructures obtained from digital microscopes is given. The fundamentals of fractal analysis as a highly effective tool for calculating numerical values of parameters of geometrically complex objects are described. The analysis of the graphitized cast iron structure is provided as an example; the application of the fractal analysis method for determining such characteristics of the graphite phase as the shape of graphite inclusions and their distribution in the amount of the alloy is demonstrated. In the course of the research, different classes of cast iron have been studied. To determine the shape of graphite inclusions it was suggested to use fractal dimension. The nonuniformity of the distribution was estimated by such function as lacunarity. The separate stages of determining these characteristics with a specialized FracLac plugin within the ImageJ program are presented. The results obtained have shown high adequacy. In spite of positive assessments, there are shortcomings revealed in the course of the research on the application of fractal analysis methods for identifying geometrically complex dimensional and topological parameters of the graphite phase in cast iron. The ways to further improve these methods in order to solve a wide range of problems in metallography of alloys are suggested.

Using Computer Microscopy Methods to Control the Microstructure of Malleable Cast Iron Product with Spherical Graphite

The method of identification of objects on images of the microstructure of cast iron with spherical graphite of the correct shape with uniform distribution is presented. Morphological analysis techniques were used to identify shrinkage pores and graphite inclusions in microstructure images. Geometric features of the shape of graphite inclusions were used as methods for identifying graphite, in particular, particle size analysis, which is widely used to identify various objects in computer microscopy. The computer analysis of the image was performed with the program ImageJ. To determine the pores against the background of graphite inclusions, two characteristics were used - the shape and size of the objects themselves. The pores, presented on the image, differ from graphite inclusions by a complex, fractal border and comparatively large areas. For the visualization of the research results, the combination of the graphite part with the calculation and analytical part was used. Such presentation of the results is the most significant and allows to perform the most correct evaluation of the graphitized cast iron microstructure in accordance with GOST 3443-87.