Papers by Keyword: Spheroidal Graphite

Abstract: The paper presents the results of the development of a special group of cast composite materials based on nodular cast irons. The alloys were based on alloying cast iron with copper in amounts that exceed its solubility in liquid Fe-C-Si systems (Cu>6 wt.%). A new component is obtained in the structure of castings in the form of a complex mixture based on the Cu-phase. Inclusions are relatively large (up to 200 μm) and globular in shape, with a uniform allocation in the volume of the castings. This favorably distinguishes the obtained alloys from the known cast irons and gives them high antifriction properties. With isothermal hardening, it is possible to obtain the lowest values of wear for structures of lower bainite. Developed cast composite materials are recommended to be used as tribotechnical materials working in conditions of poor lubrication and corrosion.

Abstract: Time-resolved and in-situ observations using synchrotron radiation X-rays were performed to observe solidification of cast iron (CE=4.5, 0.02mass%Mg). Morphology of graphite particles was influenced by specimen holder material. In the Al₂O₃ holder, graphite particles were spheroidal at the beginning and then deviated from the spheroidal shape. In addition, the coupled eutectic solidification of austenite and graphite occurred at the final stage. In contrast, the divorced eutectic solidification, in which graphite particles and austenite dendrites independently grew, was selected until the end of solidification in MgO holder. Spheroidal graphite particles were engulfed by austenite. Consequently, typical microstructure observed in ductile cast iron was reproduced in the in-situ observation. The results suggested that oxygen potential, which was determined by Al₂O₃ or MgO (specimen holder) in the observations, could be an important factor for the selection of eutectic growth mode and graphite morphology.

Abstract: A better understanding of spheroidal graphite growth is expected in a near future thanks to widespread use of transmission electron microscopy. However, common transmission electron microscopy is quite time consuming and new indexing techniques are being developed, among them is transmission Kikuchi diffraction in a scanning electron microscope, a recent technique derived from electron backscatter diffraction. In the present work, on-axis transmission Kikuchi diffraction in scanning electron microscope, completed by transmission electron microscopy, was used with the objective of producing new observations on the microstructure of spheroidal graphite. This study shows that disorientations between blocks and sectors in spheroidal graphite are quite large in the early growth stage, which may be indicative of a competition process selecting the best orientations for achieving radial growth along the c direction of graphite.

Abstract: Even using high inoculation levels, mottled structures are often obtained when casting Mg-treated cast irons in thin wall parts. For full graphitization of the cast components, this calls for a subsequent heat-treatment which is generally achieved in the austenite field. The aim of this work was investigating the impact of the process and the cooling rate on the graphite structure for two different casting conditions. The influence of the cooling rate on graphite degeneracy due to the presence of impurity was also investigated considering low-level additions of aluminium. Extensive metallographic investigation has been carried out from which it is concluded that the internal graphite structure is the same for the two studied cooling conditions. Accordingly, the growth mechanism of graphite should be the same when it precipitates from liquid, during eutectic reaction or else solid-state graphitization. Finally, microanalyses suggest magnesium and aluminium do not interact in the same way with graphite during its growth.

Abstract: In hypereutectic nodular cast irons, primary precipitation of graphite may lead to graphite flotation in thick section castings. Graphite degeneracy such as so-called exploded graphite is then often associated with this flotation phenomenon and it appears as precipitates where the nodular form is replaced by star-like or flower-like shape. It has been reported that exploded graphite develops after the primary spheroidal nodules have reached some tens of microns in diameter. In this contribution, a model for this transition is presented.

Abstract: Two S-type thermocouples connected to paperless recorder are used to record the temperature change during solidification of the wedge spheroidal-graphite casing. The cool-down time and eutectic temperature are obtained from the cooling curves which are calculated by Pro CAST software. Aided by the image-pro plus software, graphite nodules’ mean diameter and amount can be obtained. Carbon diffusion coefficient of graphite nodule is calculated according to the spherulite growth model. The result is in good agreement with the reference date which shows the validity of parameters. It could provide the parameters for simulation of casting solidification.

Abstract: The solidification of the austenite-spheroidal graphite (SG) eutectic is one of the most complex and therefore most difficult liquid-to-solid transformations to describe through computational modeling. This is because the iron-carbon equilibrium phase diagram is an asymmetric diagram that predicts that at relatively low solidification velocities it is possible to produce primary austenite in a casting of eutectic composition. In addition, the two phases of the eutectic solidify as a divorced eutectic, with only the austenite being in contact with the eutectic liquid. The paper proposes a computational model for the eutectic solidification of SG iron with visualization of the microstructure evolution using a cellular automaton technique. The model is an extension of the earlier Zhu-Stefanescu model for primary phase solidification and includes the growth of primary SG in the liquid, of primary austenite and of the austenite-graphite eutectic controlled by carbon diffusion through the solid austenite shell. The model outputs realistic images of the microstructure evolution from the beginning to the end of eutectic solidification of SG cast iron.

Abstract: The effects of the cooling rate, atmosphere and holding time on the graphite morphology of spheroidal graphite cast iron were studied using Ni-C alloys. Two groups of parent alloys were prepared using high purity materials, i.e., Group 1 containing the spheroidizing element of Ce, Mg or Ca, while in Group 2, S was added as an anti-spheroidizing element. For discussing the influence of the cooling rate on the graphite morphology, 0.5g of the Group 1 samples were melted and held for 15 minutes at 1673K in an Ar atmosphere, then cooled at 1000K/min or 20K/min. The results showed that perfect spheroidal graphite could not be confirmed, while irregular graphite appeared. The atmosphere was changed to Ar+3%H2 for preventing the oxidation, and the holding time was reduced to 10 minutes to prevent fading of the spheroidizing element. These results showed that the formation of spheroidal graphite was confirmed at the cooling rate of 1000K/min in both groups. Nevertheless, at the cooling rate of 20K/min, graphite morphology was only chunky or flake in both groups. In order to investigate which parameter is more important for the formation of spheroidal graphite, the atmosphere and the holding time were independently changed at the cooling rate of 1000K/min. It was found that the addition of the 3%H2 did not significantly affect the spheroidal graphite formation. Moreover, the holding times of 1min and 20min also did not significantly affect the spheroidal graphite area fraction in the Ni-C alloy, while they affected the ones containing the spheroidizing elements like Mg.

Abstract: The spheroidal graphite cast iron is widely used as a structural material in an industrial field. Possibility to be able to use by improving magnetic characteristic of spheroidal graphite as magnetic circuit material of product related to electromagnetism besides structure material. In this study, the influence that the amount of graphite precipitation , the matrix organization, and the structure gave to a magnetic characteristic was investigated in the spheroidal graphite cast iron that makes matrix ferrite by compounding C element and the Si element and heat-treatment. The graphite was completely precipitated at the heat-treatment temperature of 1173K or more, the organizatiom became a ferrite, and permeability rose. Moreover, Rough making by heat-treatment the size about the particle size of the ferrite and the appearance of the Fe-Si phase have raised permeability.