Graphite Nodules Research Articles

Graphite and Solid Fraction Evolutions during Solidification of Nodular Cast Irons

Ductile iron casting production is strongly affected by austenite and graphite distribution obtained after the solidification process. At the same time it is accepted that solidification behavior can be considered as hypo-, hyper- or eutectic depending on the chemical composition; there is still some misconception about the growth evolution of graphite nodules and about solid fraction progression. Quenching experiments were performed on two different carbon equivalent compositions using inoculated and non-inoculated thermal analysis standard samples with the aim of freezing the existing phases at different solid fractions for each alloy. As a result of these experiments, it was possible to study the structural features found at different locations of each sample and at different stages of solidification. Additionally nodule evolution during the liquid-solid transformation was also analyzed and discussed regarding the chemical and processing characteristics of the prepared alloys.

Effect of Tempering Temperature and Time on Strength and Hardness of Ductile Cast Iron

The effects of tempering temperature and time on the mechanical properties of ductile cast iron is investigated in the current work. The specimens were austenitized at 900°C for 120 minutes and then quenched in mineral oil at room temperature. Immediately after quenching the specimens were tempered at 400°C and 200°C for 60min, 90min, and 120min. In the tempering temperature range of 200°C-400°C, there is sudden increase in impact strength, ductility and toughness of the materials, as the temperature and time increase. The UTS drops initially, and hardness of materials will depends on amount of phase of martensitic and retained austenitic and graphite nodules. In this work alloying elements also effected the microstructure of the specimen. And due to increase tempering time the amount of martensitic phase will decrease and retained austenitic phase will increase, retained austenitic phase is softer then martensitic so hardness will decrease.

Effect of Processing Parameters on the Mechanical Properties of Heavy Section Ductile Iron

The main objective of the current work is to investigate the influence of different inoculation conditions on the microstructure and mechanical properties of heavy section ductile iron (DI) castings. Inoculation treatment was done via one step and double step treatments with different amounts of inoculants. The mechanical properties of the fabricated samples were evaluated. The best inoculation procedure in terms of graphite nodules characteristics and mechanical properties was double inoculation with 0.8% inoculants added at first and 0.2% in the late inoculation step. The presence of Sb in one of the cast alloys controlled the growth of graphite nodules in these heavy section ductile iron castings; however low impact toughness was recorded. The matrix structure of ductile cast iron showed a significant influence not only on the strength and impact properties but also on the fracture mode during testing.

Determination of effective elastic properties of ferritic ductile cast iron by computational homogenization, micrographs and microindentation tests

A comprehensive procedure for the prediction of the elastic behavior of ferritic ductile iron by means of multiscale analysis is introduced in this work. The procedure combines computational procedures for the homogenization analysis, micrographic analysis to retrieve the geometries for the finite element analysis, and microindentation tests to assess the elastic behaviors of the different phases of the microstructure. The size of the representative volume element (RVE) is assessed in terms of geometrical descriptors of the microstructure (graphite fraction and nodule count and size) and the invariance and isotropy of the homogenized elastic responses. The RVE is sized to contain at least 50 nodules, and it results from 100× micrographs. The results for the homogenized values for the Young’s modulus and the Poisson’s ratio are found in excellent agreement to the data retrieved from tensile tests, values reported in the bibliography and analytical formulas available in the literature. The proposed procedure can be easily extended to the characterization of cast irons with more complex microstructures.

Pearlitic Ductile Cast Irons: Fatigue Initiation Micromechanisms

Damaging micromechanisms in Ductile Cast Irons (DCIs) are strongly affected both by the matrix microstructure and by the graphite nodules morphology peculiarities (e.g., nodularity level, dimension, distribution etc.). The role played by the graphite nodules depends on the matrix microstructure and by the loading conditions (e.g., static, quasi-static or cyclic loadings): although,in the past, matrix – graphite nodules debonding was considered as the unique damaging micromechanism and graphite nodules were considered analogously to mere voids embedded in metal matrix, recent experimental results showed the presence of an internal mechanical properties gradient inside the graphite nodules and other damaging micromechanisms have been identified (e.g., “onion-like mechanism). In this work, the fatigue damaging micromechanisms in a pearlitic DCI have been investigated by means of scanning electron microscope and digital microscope observationsof the lateral surfaces of notched specimens during a fatigue tests.

Fatigue Crack Propagation in a Ferritic-pearlitic DCI: Overload Effects on Damaging Mechanisms

Ductile iron discovery in 1948 renewed the interest on the cast iron family. These cast irons (DCIs) combine the good castability of gray irons and high toughness values of steels; they are also characterized by an interesting fatigue crack propagation resistance. Considering their interesting mechanical properties, DCIs are widely used in the critical automotive parts (e.g., crankshafts, truck axles, etc.), and in many other application, like pumps, pipes or turbine components. DCIs can be considered as special-type composites, with graphite nodules embedded in a metal matrix. DCIs performances are strongly affected by the graphite elements morphological peculiarities (e.g., graphite elements nodularity, volume fraction, density, distribution, dimension). Different combinations of the mechanical properties can be obtained depending on the matrix microstructure: focusing on ferritic-pearlitic DCIs, different combination of ductility or tensile strength values can be obtained depending on the phases volume fractions and distribution.In this work, a ferritic-pearlitic DCI was considered. Compact Type (CT) specimens were metallographically prepared, chemical etched and, then, fatigue precracked and overloaded. According to a step by step procedure, lateral surfaces were observed by means of a Scanning Electron Microscope (SEM) and by means of a Digital Microscope (DM), in order to investigate the influence of the matrix microstructure and of the graphite nodules on damaging micromechanisms.

Graphite Nodules Influence on DCIs Mechanical Properties: experimental and Numerical Investigation

Ductile cast irons (DCIs) are characterized by very interesting properties, with a good castability (typical of cast irons) and a very interesting mechanical behaviour, that is comparable to low alloyed steels. From the point of view of the microstructure, these alloys can be considered as natural composites, with graphite nodules embedded in a metal matrix. Their mechanical properties are influenced both by the matrix microstructure (e.g., ferrite, pearlite, ferrite-pearlite, austenite, etc.) and by the morphological peculiarities of the graphite elements (nodularity, element distribution, dimensions etc.). In this work, an experimental and numerical procedure has been optimized in order to obtain a 3D representation of a ferritic DCI considering the effective distribution of the graphite elements. Starting from this 3D representation, the behavior of the investigated DCI has been simulated by means of finite elements approach.

Austenitization of FerriticDuctile Iron

Abstract Austenitization is the first step of heat treatment preceding the isothermal quenching of ductile iron in austempered ductile iron (ADI) manufacturing. Usually, the starting material for the ADI production is ductile iron with more convenient pearlitic matrix. In this paper we present the results of research concerning the austenitizing of ductile iron with ferritic matrix, where all carbon dissolved in austenite must come from graphite nodules. The scope of research includedcarrying out the process of austenitization at 900° Cusing a variable times ranging from 5 to 240minutes,and then observations of the microstructure of the samples after different austenitizing times. These were supplemented with micro-hardness testing. The research showed that the process of saturating austenite with carbon is limited by the rate of dissolution of carbon from nodular graphite precipitates

Study on the (Fe,Cr)<sub>7</sub>C<sub>3</sub> Particles Reinforced Iron-Based Composite Coating

An iron based composite coating reinforced by (Fe,Cr)7C3particles, with a thickness of 4.0 mm, was fabricated on the ductile cast iron substrate by means of a centrifugal casting and in situ techniques. The microstructure and properties of the composite coatings were investigated by optical microscope, X-ray diffraction, scanning electron micrograph, energy dispersive spectrum, microhardness tester and pin-on-disc wear tester. Results show that the composite coatings consists of primary (Fe,Cr)7C3carbides as the reinforcing phase and interdendritic (Fe,Cr)7C3/γ eutectics accompanied by negligible graphite nodules as the matrix. The reinforcements are metallurgical bonded to the ductile cast iron substrate. Compared with unreinforced ductile cast iron, the composite coatings offer virtually a unique wear resistant behaviour under the condition of dry sliding with loads.

Modeling of microstructural evolution during divorced eutectic solidification of spheroidal graphite irons

A two-dimensional multiphase cellular automaton model is proposed for the simulation of microstructural evolution during divorced eutectic solidification of spheroidal graphite (SG) irons. The model adopts a previously proposed local solutal equilibrium approach to calculate the driving force for the growth of both graphite and austenite phases. The growth kinetics of graphite also includes the effect of the density difference between iron and graphite. The model is applied to simulate the microstructural evolution of both hypoeutectic and hypereutectic SG irons. The simulated microstructures, cooling curve and graphite nodule sizes at various cooling rates compare reasonably well with the experimental data, demonstrating the quantitative capabilities of the proposed model. The simulation results reveal some dynamic features of the divorced eutectic solidification, such as the interactive and competitive growth between austenite dendrites and graphite nodules, and the graphite growth controlled by carbon diffusion through the solid austenite shell.

Microstructure and Mechanical Properties of Ductile Cast Iron in Lost Foam Casting with Vibration

The microstructures and mechanical properties of the ductile cast iron (Dl) specimens obtained by lost foam casting (LFC) with and without vibration were investigated. The results indicate that the number of the graphite nodule increases from 173 mm−2 of the specimens produced by LFC without vibration to 334 mm−2 of the specimens produced by LFC with vibration, and the thickness of the ferrite shell increases. Meanwhile, the amount of the carbides decreases in the specimens produced by LFC with vibration and the granule structure then forms. These are mainly attributed to the “crystal shower” caused by the vibration. In addition, the tensile strength and elongation of DI specimens produced by LFC with vibration are improved due to the dispersion-strengthening of refined carbide and pearlite colony, uniform distribution of the graphite nodule, and increase of the amount of dimples and tearing edges.

Transformation process of weld heat-affected zone in solution-strengthened ferritic ductile cast iron

The purpose of this study was to examine the weldability of Si-solution-strengthened ferritic ductile cast iron (SSFDI: EN-GJS-500-14) compared with a traditional ferrite–pearlite type ductile cast iron of similar strength (JIS-FCD500-7). The α + graphite → γ reverse transformation characteristics on heating process and the transformation characteristics on cooling were investigated by the synthetic weld thermal cycle method. Heating SSFDI at 300°C s− 1 (200–800°C), the reverse transformation started at 837°C and finished at 1149°C. The austenite arose around graphite nodules and the fraction in the matrix moderately increased to 100% as the temperature became higher. In the ferrite–pearlite type ductile cast iron FCD500 with 2.27%Si, the reverse transformation occurred in the temperature range from 763°C to 1105°C. The pearlite rapidly transformed to austenite and then the ferrite moderately transformed. The austenitization rate of SSFDI was also lower than the ferrite-pearlite type ductile cast iron at 1000°C. In the case of the cooling parameter t8/5 = 25 s, the martensite area fraction increased as the maximum temperature rose. Less martensite appears in SSFDI, indicating arc welding would give rise to less brittle heat-affected zone un SSFDI.

Microstructure and Wear Behavior of TiC Coating Deposited on Spheroidized Graphite Cast Iron Using Laser Surfacing

Spheroidal graphite cast iron was laser cladded with TiC powder using a YAG fiber laser at powers of 700, 1000, 1500 and 2000 W. The powder was preplaced on the surface of the specimens with 0.5 mm thickness. Sound cladding and fusion zones were observed at 700, 1000 and 1500 W powers. However, at 2000 W, cracking was observed in the fusion zone. At 700 W, a build-up zone consisted of fine TiC dendrites inside a matrix composed of martensite, cementite (Fe3C), and some blocks of retained austenite was observed. In this zone, all graphite nodules were totally melted. In the fusion zone, some undissolved and partially dissolved graphite nodules appeared in a matrix containing bainite, ferrite, martensite and retained austenite. At 1500 W, the fusion zone had more iron carbides and ferrite, and the HAZ consisted of martensitic structure. At 2000 W, the build-up zone was consisted of TiC particles precipitated in a matrix of eutectic carbides, martensite plus an inter-lamellar retained austenite. The hardness of the cladded area was remarkably improved (1330 HV in case of 700 W: 5.5 times of the hardness of substrate).

Mechanical and microstructural characterization of Nodular Cast Iron (NCI) with Niobium Additions

Nodular cast irons (NCIs) involve the inoculation and nodulization of 0.03 to 0.06wt% Magnesium, as well as the addition of Molybdenum, Chromium, Nickel, Copper and Vanadium, although the presence of Niobium is not common. The literature reports that Niobium leads to better mechanical properties of the NCIs, and recent results indicate that 0.5wt% Niobium increases the strength and the Charpy notch toughness of NCIs, but decreases its hardness, in relation to a similar NCI without Niobium. The present report studies the addition of 0.23, 0.47, 0.67, and 0.85wt% Niobium to a NCI covering the effects on the microstructures, yield and tensile strengths, tensile elongations and Charpy toughness. The NCIs were prepared in a 120 kg medium frequency induction furnace, and samples were obtained following the ASTM A842 standard; the various test specimens were extracted from these samples according to specific standards for each test. Fracture analysis was performed in fractures specimens after the Charpy tests. It was shown that the volume fraction of pearlite increases as the Niobium content is raised, leading to increments in the yield and tensile strengths and hardness of these materials. Modest increment of tensile ductility and Charpy toughness are observed only for an addition of 0.23% Niobium. Fracture analysis revealed typical brittle cleavage surfaces and decohesion between grains and grain - graphite nodules interfaces, and very low porosity

Stress triaxiality influence on damaging micromechanisms in a pearlitic ductile cast iron

In the last decades, damaging micromechanisms in ductile cast irons (DCIs) have been widely investigated, considering both the matrix microstructure and the loading conditions influence. Considering the graphite nodules, they were initially considered as voids embedded and growing in a ductile metal matrix (especially considering ferritic ductile cast irons). Recent experimental results allowed to identify a more complex role played by the graphite nodules, depending on the matrix microstructure. In this work, damaging micromechanisms in a pearlitic DCI were investigated by means of tensile tests performed on notched specimen, mainly focusing the role played by graphite elements and considering the stress triaxiality influence.

Evaluación de propiedades elásticas de la fundición nodular empleando micromecánica computacional

Nodular cast iron is a Fe-C-Si metallic alloy whose microstructure consists of a matrix, composed in general of ferrite and pearlite, with graphite nodules embedded in it. Owing to its microscopical heterogeneity, the material response is governed by the properties, morphology and typology of the phases involved. This work reports on the evaluation of the elastic properties, i.e., Young's modulus and Poisson's ratio, of an equivalent homogeneous material that characterizes the macroscopic response of a nodular cast iron. Asymptotic homogenization is used to this end. This approach is applied to both 3D and 2D multiparticle cells simulated via the finite element method. Two different physically and geometrically-based criteria are considered to estimate the representative volume element (RVE), where the size of the RVE is found to be sensitive to the chosen criterion. The main microstructural features are obtained from a computational simulation of the solidification process of the material. The numerical predictions computed for the 3D and 2D cases are compared and discussed in terms of the resulting elastic properties. It is observed that the models employing 3D multiparticle cells require lower RVE sizes than the corresponding 2D models, where the latter present a stiffer elastic response.

Effect of Chills on the Microstructure and Mechanical Properties of Carbidic Austempered Ductile Iron

Carbidic Austempered Ductile Iron (CADI) is a recent addition to the Austempered Ductile Iron (ADI) family. The effect of chills on the microstructure and mechanical properties of CADI was investigated after Austempering. Three samples of chromium alloyed CADI, the first sample without chill, the second sample with bottom chill and the third sample with bottom and side chills were produced in order to evaluate the effect of chills on its mechanical properties. The samples were austenised for 2 hours at 925° C and then austempered at 325° C for 2 hours in a salt bath furnace. The microstructural features of the as-cast and the austempered CADI samples were analysed using Optical Microscope and Scanning Electron Microscope (SEM). The mechanical properties of the CADI samples (as-cast and austempered) were evaluated for hardness, impact and wear. By austempering at 325° C for 2 hours a typical microstructure of bainite was produced in all the three samples. Hardness and wear resistance of austempered samples produced using bottom and side chills were considerably higher than the corresponding values in samples produced without using any chill and also by using only bottom chill. This enhanced mechanical property in the bottom and side chill sample is attributed to the presence of bainite, carbides and more of uniform fine graphite nodules.

Repair welding of ductile cast iron by laser cladding process: microstructure and mechanical properties

The aim of this research was to determine the feasibility of a newly developed process in the repair of cracked gas turbine casings made of ductile cast iron. This study investigated the microstructural characteristics, metallurgy and mechanical properties of the repair weldments produced using fibre laser cladding. Optical microscopy, scanning electron microscopy and element probe microanalysis were used to investigate the microstructure at the cladding weld interface. The mechanical properties of the cladded specimens were evaluated after laser cladding. Our results revealed that the weldability of ductile cast iron can be enhanced by performing laser surface pretreatment to sublimate graphite nodules. Microhardness at the interface of the laser cladded weldments depended largely on the range of the heat affected zone and the degree of phase complexity. Under tensile loading, failures were limited to the base metal region of the weldments. Test results demonstrate that the impact toughness of the interface between the fusion zone and the base metal can be enhanced through the application of post-cladding heat treatment.

Fatigue crack tip damaging micromechanisms in pearlitic ductile cast irons

AbstractIn the last years, damaging micromechanisms in ductile cast irons (DCIs) were widely investigated: DCI microstructure (ranging from ferritic–pearlitic DCIs to austempered ones), graphite elements morphology (shape, dimension and distributions) and loading conditions were the main investigated parameters. Focusing on the role played by the graphite nodules, they were considered merely as voids embedded in a metal matrix. Recent analysis underlined that the nodules role in the DCIs damage evolution is more complicated and is influenced by the matrix microstructure. In this work, the crack tip damaging micromechanisms in a pearlitic DCI is investigated considering both fatigue loading conditions and the effects of overloads, focusing the interaction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules). On the basis of experimental results, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in pearlitic DCIs is critically re‐analyzed.

NANOTECHNOLOGY IN RAIL TRANSPORT

One of the main tasks of economic modernization of Russian railway transport is the introduction of innovative technologies for resource conservation and energy efficiency On railway transport daily consumed about 160 tons of steel shroud going to restore the wear of wheel sets of rolling stock. Annual losses from the depreciation account for more than 1 billion rubles. One of the main reasons for this excessive wear contact pair wheel-rail is the chemical affinity of the shroud and rail steels with different chemical composition of the individual components literally tenths of a percent This circumstance contributes to the most intense kind of destruction in nature gripe . The possibilities of improving the durability of the contact pair wheel-rail . The technology of electric- hardening ( ETL ), which refers to a combination of chemical and thermal treatment concentrated source of energy as generating structure in the surface layer of austenitic- martensitic composite papadayuschego in nanoscale range , while having high wear resistance. Change in the chemical composition during melting of the surface layer and doping eliminated the wear gripe and increase the resistance of the surface layer micro volumes Flaking Wear resistance bands increased by 2-2.5 times. Also increased the time and mileage between sharpening , regrinding decreased the number decreased by regrinding process losses , decreased frequency emitted residual thickness of the replacement exhaust shrouds Reducing losses of metal and no distortion profile geometry bandages made possible re hardening of the entire profile , without regrinding within tolerance on flange wear The expediency of removing machining after hardening , which is the source of origin of microcracks in the surface layer and the further destruction of the centers By combining operations ETL and hardening plastic deformation during operation can further improve the wear resistance due to the transformation of retained austenite. Studies of the structure of the hardened layer is doped during melting and quenching in weight shroud shown paradoxical combination of high hardness, ductility and toughness at the same time , which is unattainable by known methods. Electron microscopy has revealed the presence in the structure of the embryonic phase of graphite nodules in the austenitic- martensitic matrix , no carbides , all the carbon is in solid solution. Research on FT-IR spectrometer revealed the presence of a significant amount of fullerene C 60 and their derivatives. High fracture resistance of the surface layer is explained by the combined effect of all these differences structure, its high dispersion , transformation of austenite to martensite during mechanical impact energy absorption coming destruction. Implementation of scientific and industrial aspects have received an avalanche in the layering process of industrial tests, allows to predict that, with appropriate fine-tuning , improving durability bandages can be brought up to ten years