Lamellar Graphite Research Articles

Исследование причин отказов цилиндропоршневой группы среднеоборотных двигателей

The article is focused upon the problems of reciprocating diesel engines. The main reason for the failure of cylinder-piston group (CPG) parts is insufficient strength, wear resistance, and resistance of the metal structure to vibration. There are studied the following periods of engine operation: running-in period; period of normal operation; period of intensive wear. In the burn-in period, the main cause of CPG failures is scuffing, when finger breaking or ring groove chipping take place. Sudden failures during normal operation occur due to the wear of the bushings to the limit size. In the third period of intensive wear the main cause of metal failures is the ultimate wear by cavitation erosion and breakage of the bushing collar. The block diagram of the CPG system with serial connection of elements is given. The rate of CPG failures is illustrated depending on the time of operation. It has been found that the failure rate during the running-in period due to scuffing is reduced because of stabilization of the structure, dimensions and mechanical properties of the piston at operating temperatures (300–350°C). The failure rate of the normal period of operation depends on the increase in the abrasive wear resistance of the metal of bushings, increased hardness, due to the replacement of gray cast iron with lamellar graphite by gray cast iron with vermicular graphite or by high-strength cast iron with spherical graphite inclusions. There are listed the positive and negative qualities of aluminum alloys in comparison with cast iron. Variants of damage to cylinder liners are considered in detail. The most dangerous defect in the bushing is the cracks in the upper liner collar, as a result of which about 40% of the cylinder bushings must be replaced. The formulas for calculating the actual wear rate of the CPG parts during the first (end of warranty periods), medium and major overhauls are given. The failure rate of the last period of operation (intensive wear) is reduced by reducing vibration in the CPG system, reducing the amplitude of forced vibrations and reducing the size of the thermal gap between the sleeve and the piston, increasing the rigidity of the sleeve. The rate of cavitation wear of modernized and conventional bushings is determined, the influence of the bushing stiffness on cracks under the shoulder and on cavitation wear up to the maximum permissible thickness is considered.

Influencing Factors on Premature Fracture of Cast Iron Support of Electric Motor in Aluminium Rolling Line

There is a limited increase in the temperature of the cast iron supports during rolling (up to 30…50°C), but there is a wide range of vibration level (2.6 - 4.4 mm/s) representing 58-98% (75-80% as medium values) from the maximum admitted level (4.5 mm/s, ISO 10816). The maximum level of vibration is identified in the area where a fan is installed to cool the electric motor. In all electric motor types it appears a difference between several measurements, which highlights the appearance of various conditions in the rolling process, or in the quality of the cast iron support. The fracture of the support takes place predominantly in the area of fixing it with a screw. The cracks connect firstly the areas with casting defects (inclusions, pinholes, shrinkage and micro-shrinkages), but also coarse graphite particles (Kish or Type C-ASTM, spiky graphite) or undercooled graphite morphologies (Type-D ASTM). Lamellar graphite cast irons (traditionally used in this domain) are characterized by a low level of fracture resistance but high vibration damping capacity due to which it is necessary to make parts without casting defects and with a controlled graphite phase (high amount of graphite, graphite Type-A ASTM, without undercooled graphite), usually obtained through high performance inoculation [1,2].

An experimental study on the wall collision of micro-sized graphite particles by high-speed photomicrography

The micro-sized graphite dust plays an important role in pebble-bed high temperature gas-cooled reactors, for the close relation with the fission products (FPs). However, an accurate estimation of the deposition rate of graphite dust is still challenging, due to the irregular shape of the particles. Particularly, a key parameter, the critical sticking velocity, remains unclear. Few experimental or theoretical investigations have been reported on the wall collision of graphite particles. As a result, the particle-wall model for the graphite particles in multiphase CFD is often quite crude, which is either models for spherical particles or even hit-and-stick assumptions. To address this issue, we present an experimental study on the wall collision of natural micro-sized graphite particles. The high-speed photography together with a microscope is used to dynamically film the wall collision process of micro-sized graphite particles. Various phenomena such as sticking, rebound and fragmentation are observed. The images show that the irregular lamellar graphite particles prefer the side-to-face orientation rather than the face-to-face orientation. Although the incident angle is usually small, the rebound angles are scattered and nicely follow a Gaussian distribution due to the irregular shape. With the increase of the particle size, the critical sticking velocity decreases, and correlated with the particle size in a power-law relation. As the incident velocity increases, the restitution coefficient increases first and then decreases, which becomes stable and follows the Gaussian distribution at high impact velocities. By incorporating the size effect, surface adhesion and local dissipation, we further present a semi-empirical correlation for the restitution coefficient over a wide range of particle size and incident velocity. Our work can provide some insights into the nature of sticking phenomena of graphite particles in HTGR.

Comparison of solidification kinetics of compacted and lamellar cast irons

Thermal analysis is largely used in cast-iron foundry shops as a means to check melt preparation before casting. It has been suggested that the shape of the cooling curves could be related to graphite form, either spheroidal, lamellar or else compacted. The present work is part of a larger study intended to improve melt control for compacted graphite castings by using controlled additions of sulfur and magnesium. Experiments showing lamellar graphite and their counterpart with compacted graphite have been selected for analysis and simulation of the thermal analysis records. Simulation makes use of the fact that compacted graphite differs from lamellar graphite by the limitation of graphite branching in the former. Solidification of both types of irons may thus be simulated using the same basic growth law for irregular eutectics, though changing the parameters describing the branching capability of the faceted graphite phase. The increase of the undercooling of the eutectic plateau during solidification of compacted cast iron when compared to that of lamellar cast iron could be verified and reproduced by simulation. The limitations of the present approach which assumes the temperature in thermal cups is homogeneous at any time are also pointed out.

Graphite Nucleation in Compacted Graphite Cast Iron

During the last several decades, a multitude of theories have attempted to explain the process of graphite nucleation in lamellar (LG) and spheroidal (SG) graphite iron castings. Nevertheless, the complex 3D morphology of compacted graphite (CG) has hindered significant advances in similar theories for this type of graphite. To bring clarity to this issue, interrupted solidification experiments were conducted on compacted graphite irons at two different levels of titanium content in the melt (0.008% and 0.037%), with and without addition of commercial inoculants (Ce, MnZr). Nucleation sites were characterized through detectors, spectrums, mapping, and line scans utilizing FEG-SEM equipment. It was found that the nature of the inclusions acting as nucleation sites is directly related to the titanium content in the base metal. Nucleation in samples with low level of Ti occurs on Mg–Ca sulfides or Mg–Si–Al nitrides, which usually appear alone and seem to affect the growth of graphite. In the case of high percentage of Ti, double inclusions formed by Ti carbonitrides growing on Mg–Ca sulfides and restricting their growth seem to be the best combination for the nucleation of graphite. This is in line with our earlier findings for spheroidal graphite.

Modelling of compacted graphite cast iron solidification - Discussion of microstructure parameters

A melt maintained for hours in a press pour unit allowed the following changes over time from spheroidal graphite to compacted graphite iron by casting thermal cups at regular time intervals. This provided extensive experimental information for checking the possibility of simulating solidification of compacted graphite irons by means of a microstructure modelling approach. During solidification, compacted graphite develops very much as lamellar graphite but with much less branching. On this basis, a simulation of the thermal analysis records was developed which considers solidification proceeding in a pseudo binary Fe-C system. The simulated curves were compared with the experimental ones obtained from three representative alloys that cover the whole microstructure change during the holding of the melt. The most relevant result is that the parameter describing branching capability of graphite is the most important for reproducing the minimum eutectic temperature and the recalescence which are so characteristic of the solidification of compacted graphite cast irons.

INVESTIGATION OF THE POSSIBILITIES OF ULTRASONIC TESTING OF CAST IRON WITH LAMELLAR GRAPHITE USING STANDARD NORMAL PROBE

Computer simulation of acoustic paths for medium – cast iron with lamellar graphite for standard normal probe of several types was carried out in order to study the effect of such medium on the characteristics of transmitted and received signals during ultrasonic testing. In the course of studies to calculate the attenuation coefficient of longitudinal waves in cast iron with lamellar graphite, due to their Rayleigh and phase scattering by graphite inclusions, the previously developed model was used. Comparison of the results of calculations of spectra and pulses of the recorded signals for two samples of cast iron with lamellar graphite confirmed the possibility of using the developed calculation model for a description of real physical phenomena adequate for the practice of ultrasonic testing. The ratio of the recorded signal and structural noise levels for different models of transducers, reflectors and characteristics of the controlled cast iron is estimated. Shown that increased attenuation of the longitudinal wave in cast iron in comparison with steel as a result of scattering by inclusions leads to higher level of this noise and can greatly reduce the signal-to-noise ratio in the detection of the reflected from the defect signals.

Emission rate assessment of airborne brake particles by characterization of the pad and disc surfaces from a pin-on-disc tribometer

Particles from brake, road and tire wear contribute to about half of the emissions (PM10) of particulate traffic pollution. It is estimated that 50 to 70% of the brake debris material is transformed into an emission of polydisperse aerosols. In order to improve the understanding of the brake debris generation and its dependency on the brake material, the wear of a disc and a brake pad from a standard production car were studied. The disc was made of perlitic cast iron with lamellar graphite and subjected to standard braking cycles. Microscopic evaluation was performed on the disc track, as well as analyses by Energy Dispersive Spectroscopy (EDS). Finally, a metallographic section has been made in the longitudinal direction of friction to better understand the morphology. The study focuses on disc surface oxidation and morphology of a thin layer on both disc and pin surface. Particle concentrations increase with the friction power and the area of contact surface. The observations show that the generation of particles can be the result of the oxidation of the disc surface during friction by two- and three-body abrasion when braking.

Study of corrosion mechanism of cast iron in molten aluminum

The corrosion mechanism of three aluminum ingot mold materials used by Chinese aluminum electrolytic enterprises in molten aluminum was studied: The three materials are pearlite cast iron(PCI) contains vermicular graphite, lamellar graphite, and spherical graphite respectively. After 800 °C molten aluminum corrosion for 4 h, the intermetallic compound (IMC) layers of Fe2Al5 and FeAl3 is formed at the interface. It was found that the volume of spherical graphite in IMC was reduced to 20%, and lamellar graphite aggregated in the IMC layer. The average thickness of the Fe2Al5 layer in PCI1, PCI2 and PCI3 is about 180 μm, 100 μm and 101 μm, respectively. The weight loss rate of PCI1 is 2.57 (mg cm−2)min−1, PCI2 is 2.20 (mg cm−2)min−1, and PCI3 is 2.24(mg cm−2)min−1. There are dispersed Cr and Mo carbides in the IMC layer. Si-rich ribbon-shaped region was found at the interface between cast iron and IMC. The needle-like precipitates (Al4C3) were observed only at the only at the boundary between cast iron and IMC in PCI1.

Unconventional approach in the study of diffusion welding of marine grades grey cast iron to low carbon steel

Experimental research on the joining of gray lamellar graphite cast iron with low carbon steel by diffusion welding is carried out with the expected result of establishing a methodology for the optimization of welding conditions producing the ultimate welding capacity of cast iron to low carbon steel. Preliminary studies on diffusion welding experiments have been carried out on the last few material specimens using a standard hot press. While some experiments failed to produce reliable joints, further inquiries into the unique characteristics of the equipment resulted in the necessary adjustments to parameters and procedures that resulted in successful joints. However, due to a sudden indefinite breakdown of the available standard machine/equipment led to the development of an alternative and unconventional approach for subsequent diffusion welding few experiments. This paper describes the experimental development of the alternative approach and equipment usage that produce a few diffusion couples.

Model of assessment of the hardness of the iron rollers СПХН-43 and СШХНФ-47

Relevance of the article. The quality criteria of massive metal products is affected by many technological parameters. Changing some of these parameters can lead to a significant change in the quality of the target product. Therefore, various types of models are widely used in materials science to simulate individual processes or predict individual criteria. An analysis of literary sources indicates the prospects of using fractal geometry to model the structure and properties of various materials. It is proposed to apply multifractal analysis to assess the structure of cast-iron rolls. Materials and methods. The influence of the structure of cast irons on their hardness was investigated. The microstructure of cast iron was investigated using quantitative methods of metallography (ГОСТ 3443) and multifractal analysis. The calculation of the spectrum of statistical dimensions of the cast iron microstructure was carried out according to the Renyi formula. The hardness indices of the roll-forming rolls СПХН-43 (520 ´ 1 000 mm) and СШХНФ-47 (680 ´ 1 000 mm) were determined at three points evenly spaced along the length of the roll barrel. Results and its discussion. The influence of the spectrum of statistical dimensions D 0 , D 1 , D 2 , D -100 and D 100 of carbides, lamellar and spherical graphite on the hardness indices of cast iron rolls is studied. It was found that the pair correlation coefficients in the prediction of hardness according to the traditional characteristics of the structure (length, diameter, area) are R 2 = 0,73...0,87. The correlation coefficients in the prediction of hardness by multifractal characteristics for rolls СПХН-43 are 0,78...0,88. For rolls СШХНФ-47, the correlation coefficients are 0,81...0,93. The obtained results confirm the effectiveness of using multifractal analysis in assessing the quality criteria of cast-iron rolls. Scientific novelty. The influence of multifractal dimensions of graphite and carbides on the hardness of cast iron rolls СПХН-43 and СШХНФ-47 is established. The sensitivity of the СПХН-43 roll hardness indices to the information and correlation dimensions of carbides, as well as to the fractal and statistical D -100 dimensions of plate graphite, was revealed. The sensitivity of hardness indices to the fractal and statistical D 100 dimensions of carbides and to the fractal and correlation dimensions of spherical graphite was established for the СШХНФ-47 rolls. Conclusions. Based on the results obtained, an approach has been developed to assess the hardness indices of rolls СПХН-43 and СШХНФ-47, which includes: 1) Determination of the spectrum of statistical dimensions of structural elements of cast irons. 2) Determination of sensitivity coefficients of hardness indicators to the spectrum of dimensions of structural elements. 3) Construction of a mathematical model for forecasting roll hardness. The approach considered can be interpreted as an alternative method for assessing the quality criteria of cast irons based on an analysis of their structure.

Examination of the Size and Morphology of Austenite Grains in Lamellar Graphite Cast Iron

There is a current discussion concerning the austenite grains of lamellar graphite irons and its growth at different carbon equivalent contents. The term “grain” is often employed to refer not only to the primary austenite, but also to the colonies or cells formed during the eutectic solidification. This investigation applies advanced experimental methods to study the solidification of lamellar graphite iron. A better understanding of the solidification process should lead to the design of better casting procedures. Samples of lamellar graphite iron were cast and directly austempered after solidification in order to reveal their solidification macrostructure. A typical ingot structure was found, with a narrow columnar-to-equiaxed transition. Electron backscatter diffraction technique was applied to confirm these macroscopic observations. Color etching was employed to determine the size of eutectic colonies at the columnar and equiaxed regions. The EBSD results confirm previous proposals, revealing that the solidification grains result from the growth of austenite dendrites. These grains include several eutectic colonies or cells inside, all these colonies or cells having the same crystalline orientation as the primary austenite dendrite. An analysis of the critical gradient for columnar-to-equiaxed transition could be developed. The size of the eutectic colonies at the columnar zone was found to be smaller than the size of the eutectic colonies at the equiaxed zone. Besides, the size of equiaxed grains strongly decreased with the increase in cooling rate, while no evident differences were obtained when the size of the columnar grains was measured.

TRIBOLOGICAL PROPERTIES OF BRAKE DISCS COATED WITH Cr2O3–40% TiO2 BY PLASMA SPRAYING

In this study, the tribological properties of Cr2O3–40% TiO2 coating for brake disc materials were investigated in braking performances. Plasma spraying technique was used in order to deposit coating materials onto cast iron disc with ventilation channels substrate. The braking performances of discs were tested according to SAE J2430 test standard. Microstructures of discs were characterized by means of light microscope (LM), scanning electron microscope (SEM) and energy-dispersive spectrometry (EDS). The surface hardness and roughness were measured with testers. In general, it is noted from the LM and SEM images that there was an infinite gradation between coating layer, binding material and the lamellar graphite cast iron and that bonding resistance was excellent. The coated disc exhibited less wear, approximately the same coefficient of friction and longer life than the uncoated disc. As a result, the coated disc could be a much better alternative for new-generation brake discs in the motor vehicles than the uncoated disc.

Production, Structurization, and Properties of Wear-Resistant ‘Iron–Carbon Alloy–Granular Hardmetal Reinforcement’ Materials

The production, structurization, and resultant properties of metal matrix ‘iron–carbon alloy–granular hardmetal reinforcement’ materials are studied. The vacuum suction method ensures almost complete filling of interparticle voids with molten metals and results in composites containing hardmetal reinforcement with 0.17–0.7 mm particles. The composites show the microstructure that has no pronounced transition zone at the hardmetal–metal interface and has a ‘granular’ austenitic cast iron matrix and a significant amount of lamellar graphite. The mechanical properties and abrasive impact wear resistance of the white cast iron and carbon steel materials are examined. The optimum combination of properties is exhibited by the composite consisting of steel 45 and 60 vol.% KKhNF15 hardmetal.

Simultaneous thermal and contraction/expansion analyses of cast iron solidification process

Simultaneous thermal and contraction/expansion analysis of hypoeutectic grey (lamellar graphite) and ductile (nodular graphite) cast irons solidification is recorded, using an equipment with a special designed ceramic cup, incorporating a thermocouple and contraction/expansion measuring device. Both cooling and contraction/expansion curves and their specific parameter values are displayed in a real time. The two compared cast irons occupy different and opposite positions during solidification process evolution: lower level of temperatures at the beginning and especially at the end of solidification, but higher level during the eutectic reaction for ductile cast irons. In the tested conditions, solidification of hypoeutectic cast irons with lamellar graphite is characterized by a greater undercooling during the eutectic reaction (lower ΔT1 and ΔT2, referring to the metastable eutectic temperature), with higher value for eutectic recalescence (ΔTr) and the maximum recalescence rate. Iron castings, including nodular graphite (ductile irons), show an end of solidification at higher undercooling (ΔT3), with less negative pick level of the first derivative of cooling curve. More graphitic initial expansion [edi(gr)], which favours the shrinkage formation, characterized nodular graphite iron castings. There is a good relationship between some parameters on the cooling curves and corresponding events on the contraction/expansion curves, such as solidification undercooling degrees, comparing to metastable eutectic temperature, and graphitic expansion [edi(gr)] for both tested cast irons: higher the level of ΔTr, and of ΔT1, ΔT2, ΔT3 (less negative) and GRF1 (graphitic factor), higher the [edi(gr)] level is.

Wear and corrosion resistance of Cr2O3%-40%TiO2 coating on gray cast-iron by plasma spray technique

In this study, lamellar graphite cast iron was coated with atmospheric plasma spraying (APS) technique. Cr2O3-40%TiO2 ceramic composite powder was used as a coating powder. The microstructure and mechanical properties of cast iron and coating were assessed. The new coating surfaces were analyzed by optical microscope (LM), scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS). Micro-hardness tests were carried out with 200 g load (Wolpert micro hardness device). Wear tests were performed with a reciprocating wear test device under normal atmospheric conditions (room temperature and 30% humidity) while corrosion tests were performed in a glass cell according to ASTM standards (G1-03, G5-94) in three electrode techniques at room temperature in 3.5% NaCl solution. The results show that the coating microstructure is porous, micro-cracked and contains some non-homogenous structures such as oxide. Based on the EDS analysis, the presence of the existing phases revealed that the coating process was successful. In addition, the presence of hard particles in the coated surface layer increased the hardness of the composite coating compared to that of cast iron. Accordingly, the wear resistance was also high. The Cr2O3-40%TiO2 coating showed an improved corrosion resistance compared to the uncoated cast iron sample.

Optimization of Microstructural Evolution during Laser Cladding of Ni Based Powder on GCI Glass Molds

In glass industry, laser cladding is an innovative surfacing technique allowing to deposit a layer of nickel to protect glass mold against corrosion, abrasion and thermal fatigue. This method (powder fusion by projection), well known in additive manufacturing represents a real technological leap for the glass industry. But during laser cladding of Ni-based powder on gray cast iron, cracks can be observed for some process conditions. These cracks are often due to the Heat Affected Zone that creates structural stresses linked to the development of a martensitic structure in the ferritic matrix of the lamellar graphite cast iron. The aim of this work is to observe the impact of laser cladding (without substrate pre-heating usually employed to limit cracking) on the coating behavior but also on the flake-graphite cast iron substrates. The microstructure and the mechanical properties were studied (SEM and microanalysis, microhardness) around the interface cladding/substrate. Also, the impact of the processing parameters (power P (1500-2300 W), scanning speed v (2.5-10 mm/s) and powder feeding rate PFR (24.5-32.5 g/min) was studied by using the ANOVA (ANalysis Of VAriance) technique. It has been observed that laser cladding on graphite cast iron is possible without cracks by limiting the linear energy induced by the process. Also, an optimization of the processing parameters (P, v, PFR) in order to obtain the industrial expected geometry of the coating has been proposed.

EBSD Analysis of the Primary Austenite Grains in Lamellar Graphite Cast Iron

This work investigates the size and morphology of the primary austenite grains present in lamellar graphite cast iron. Samples of lamellar graphite iron were cast and austempered during the post-solidification cooling stage in order to preserve part of the original austenite at room temperature. Electron backscatter diffraction technique was applied to reveal the regions of different crystalline orientation of the austenite. The eutectic colonies and the last-to-freeze areas were also examined by color etching of the same regions previously analyzed by electron backscatter diffraction. The results reveal that the solidification units of lamellar graphite iron are formed by austenite grains that include several eutectic colonies inside, all of which have the same crystalline orientation as the primary austenite grain. For all the samples studied, it can be confirmed that there are no eutectic colonies with a different crystalline orientation than that of the primary austenite dendrites.

Tribological performance assessment of lamellar and compacted graphite irons in lubricated ring-on-cylinder test

A constant challenge in the automotive industry is to find ways to reduce the friction. Reductions in the weight of components can play this role, but for using smaller thickness in the engine blocks more resistant materials should be employed. The present investigation discusses the tribological performance of two cast irons used for manufacturing engine blocks, grey cast iron (GCI) grade FC250 and compacted graphite iron (CGI) grade GJV450, and the effect of two different thicknesses – 3.9 ( ± 0.6) and 7.4 ( ± 0.4) mm – on their friction and wear behaviors assessed in a ring-on-cylinder test under mixed lubrication. The tests were 8-h long with a normal force of 75 N, a stroke of 10 mm and a frequency of reciprocating movement of 5 Hz. A new parameter was introduced for evaluating the tribological performance, called Tribological Merit Index (TMI), which considers the coefficient of friction (COF) and the changes in the surface topography, determined using a 3D optical interferometry. The thickness of the samples did not affect significantly the COF values. Nevertheless, the best result in terms of the friction behavior was obtained for the CGI-thin sample (0.111). For the contact area estimated by the topographical parameters, the thickness was not relevant just for the samples of lamellar cast iron; and the best result,was obtained for the CGI-thin. Considering the TMI parameter, a ranking could be established, from the best to the worst tribological performance: i) CGI-thin; ii) CGI-thick; iii) GCI-thick; and iv) GCI-thin.

The Effect of the Substitution of Silicon by Aluminum on the Properties of Lamellar Graphite Iron

In cast iron, silicon and aluminum are elements that promote graphite precipitation and strengthen the alloy by solid solution. In the present work, Si has been substituted by Al leading to values that easily surpass standard properties of Fe–C–Si gray cast irons, reaching a strength of 466 MPa at moderate hardness (250 HB) for an iron with 3.08% C, 3.15% Al and 0.16% Si. Sequences of heats increasing Al content were carried out. It was found that the UTS increases with the Al content, reaching a maximum at about 3% Al, after which it decreases. Graphite area measurements show a minimum at the maximum UTS, with graphite exhibiting random orientation. Carbides are avoided above 1.5% Al for very low Si content (about 0.2%) in Y2 keel blocks (25 mm thickness). Plates as thin as 2.5 mm were cast free of carbides from irons with more than 3% Al, which makes this material very desirable for thin wall castings. Additions of other elements were necessary, including 0.35% chromium, 0.55% manganese and 0.075% tin to obtain a pearlitic structure.