Redistribution and Effect of Various Elements on the Morphology of Primary Graphite in Cast Iron (original) (raw)
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Graphite Nucleation in Compacted Graphite Cast Iron
International Journal of Metalcasting, 2020
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.
Manganese distribution and effect on graphite shape in advanced cast irons
Materials Letters, 2008
The manganese contribution to a change of the graphite shape (in nodular graphite cast irons) has never been revealed. We made obvious the negative action of this element on the nodularization of graphite. Using SEM, we observe a significant change of the shape of the precipitated graphite (decrease of the shape coefficient values), depending on the cooling rate, the nature and the quantity of the nodularizing elements. The distribution of manganese and silicon in the metallic matrix and in the nodular graphite of cast irons with various amounts of manganese was studied using Castaing Scanning X-ray Microprobe. The distribution profiles of (K α Mn and K α Si) show that polycarbides appear on the graphite periphery in the cast iron with manganese contents over 1.0% (confirmed by SEM-EDX analysis). These chemical compounds influence the slowing down of the expected growth of graphite during the austenitic decomposition. This allows us to suggest that manganese would be added to the referenced antinodularizing elements group (O, S, halogens and Pb, Te, Ti, Cu, Al, Sn, Sb, Bi).
Reassessment of Crystal Growth Theory of Graphite in Cast Iron
Materials Science Forum, 2018
The problem of graphite crystallization and growth in cast iron has recently received increased attention. As most of the literature data describe analysis of room temperature graphite, there is a legitimate concern that the crystallization of graphite is obscured by subsequent recrystallization and growth in solid state. To avoid confusion in the interpretation of room temperature graphite morphology, the authors used Field Emission Gun Scanning Electron Microscopy on deep-etched interrupted solidification (quenched) specimens to reveal the morphology of graphite at the very beginning of solidification, when the graphite is in contact with the liquid. Information from related phenomena, such as crystallization of hexagonal structure snowflakes and metamorphic graphite, as well as of diamond cubic structure silicon crystals in aluminum alloys is incorporated in the analysis. Research discussing graphite produced through gas-solid and solid-solid transformations is also examined. Because the faceted growth of graphite is the result of diffusion-limited crystal growth in the presence of anisotropic surface energy and anisotropic attachment kinetics, a variety of solidification morphologies are found. The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets generated through the growth of graphene layers. As solidification advances, the platelets thicken through 2-D nucleation or spiral dislocation growth. Depending on bulk composition, local supersaturation and undercooling, the platelets aggregate through a variety of mechanisms including foliated/tiled-roof crystals and dendrites, curvedcircumferential, cone-helix, helical (macro-spiral), and polyhedral columnar or conical (pyramidal) sectors growth.
A useful technique for studying graphite in cast iron
Materials Characterization, 1993
A new experimental procedure for studying eutectic graphite in cast iron is described. It consists of slow dissolution of the matrix, which is simultaneously replaced by a transparent one in which the graphite remains entrapped. This provides advantages for the study of nucleation using light microscopy. Afterward, the complete graphite lattice can be released through dissolution of the amorphous matrix, retaining unaltered the structure of the eutectic grains and the spatial distribution produced during the solidification process. In this way, the crystallography of the phase can be followed by different methods in zones with a welldefined morphology and also in zones where there are transitions between different morphologies. In the last case, the close bonding with the surroundings in which they were produced is maintained. Finally, the study of the particles included exclusively in the graphite phase can be optimized because of the elimination of matrix interference in microanalysis.
The role of manganese and copper in the eutectoid transformation of spheroidal graphite cast iron
Metallurgical and Materials Transactions A, 1997
The decomposition of austenite to ferrite plus graphite or to pearlite in spheroidal graphite (SG) cast iron is known to depend on a number of factors among which are the nodule count, the cooling rate, and the alloying additions (Si, Mn, Cu, etc.). This study was undertaken in order to deepen the understanding of the effect of alloying with Mn and/or Cu on the eutectoid reaction. For this purpose, differential thermal analyses (DTAs) were carried out in which samples were subjected to a short homogenization treatment designed to smooth out the microsegregations originating from the solidification step. The effect of various additions of copper and manganese and of the cooling rate on the temperature of the onset of the stable and metastable eutectoid reactions was investigated. A description of the conditions for the growth of ferrite and of pearlite is given and shows that these reactions can develop only when the temperature of the alloy is below the lower boundary of the ferrite/austenite/graphite or ferrite/austenite/cementite related three-phase field. The experimental results can be explained if the appropriate reference temperature is used. The cooling rate affects the temperature of the onset of the ferrite plus graphite growth in the same way as for the eutectic reaction, with a measured undercooling that can be extrapolated to a zero value when the cooling rate is zero. The growth undercooling of pearlite had values that were in agreement with similar data obtained on silicon steels. The detrimental effect of Mn on the growth kinetics of ferrite during the decomposition of austenite in the stable system is explained in terms of the driving force for diffusion of carbon through the ferrite ring around the graphite nodules. Finally, it is found that copper can have a pearlite promoter role only when combined with a low addition of manganese.
International Journal of Metalcasting, 2020
It is well-established that growth of spheroidal graphite occurs in several stages such as directly in the liquid, in the liquid through a solid austenite shell during the eutectic reaction, and in the solid because of decreased carbon solubility during cooling. The various mechanisms active during these stages of cast iron solidification have been the subject of much research over the recent years. To further understand graphite growth mechanisms, irons of similar composition but with three level of Mg (low-\ 0.010%, medium-0.017%, and high-0.047%) were cast in copper molds to produce small disks with white microstructure. The disks were annealed at 950°C to promote graphite growth, and then quenched in water after various holding times. The time evolution of nucleation and growth of graphite was analyzed through characterization of the microstructure by optical and scanning electron microscopy, on polished and deep-etched specimens. Unique structural features were observed in some unusual graphite aggregates, such as hemispherical graphite and hopperlike crystals. A mechanism for the growth of graphite in solid state is offered. The theory of multi-mechanism growth of graphite spheroids is confirmed.
Nucleation and Growth of Graphite in Eutectic Spheroidal Cast Iron: Modeling and Testing
Metallurgical and Materials Transactions A, 2016
A new model of graphite growth during the continuous cooling of eutectic spheroidal cast iron is presented in this paper. The model considers the nucleation and growth of graphite from pouring to room temperature. The microstructural model of solidification accounts for the eutectic as divorced and graphite growth rate as a function of carbon gradient at the liquid in contact with the graphite. In the solid state, the microstructural model takes into account three stages for graphite growth, namely (1) from the end of solidification to the upper bound of intercritical stable eutectoid, (2) during the intercritical stable eutectoid, and (3) from the lower bound of intercritical stable eutectoid to room temperature. The micro-and macrostructural models are coupled using a sequential multiscale approach. Numerical results for graphite fraction and size distribution are compared with experimental results obtained from a cylindrical cup, in which the graphite volumetric fraction and size distribution were obtained using the Schwartz-Saltykov approach. The agreements between the experimental and numerical results for the fraction of graphite and the size distribution of spheroids reveal the importance of numerical models in the prediction of the main aspects of graphite in spheroidal cast iron.
Study of the solidification structure of compacted graphite cast iron
International Journal of Cast Metals Research, 2016
This investigation focuses on the study of the solidification mechanism of compacted graphite cast iron (CGI). The solidification macrostructure was revealed in cast samples using a special technique known as direct austempering after solidification (DAAS). The microstructure was revealed by colour etching. The results were compared with earlier investigations of the solidification of spheroidal (SGI) and lamellar (LGI) graphite irons, and show that, similarly to other free graphite cast irons, the solidification of CGI is dominated by the presence of relatively large grains of austenite that can be observed with bare eyes. The CGI cast samples show a typical ingot structure, containing columnar and equiaxed grains, with a narrow columnar to equiaxed transition. The microstructure analysis showed that a dendritic substructure and a large number of eutectic colonies form the grains. Microsegregation is located inside the grains, mostly between secondary dendrite arms. The results indicate that the growth mechanism during solidification of CGI resembles that of LGI, but not the mechanism of SGI.
Kinetics of graphite expansion during eutectic solidification of cast iron
International Journal of Cast Metals Research, 2014
The paper introduces a new linear displacement analysis (LDA)/thermal analysis (TA) experimental device for measuring linear displacement during the solidification of cast iron. The experimental device comprises a sand mould encased in a steel shell that prevents mould wall movements. Thus, only the linear displacement caused by the shrinkage or expansion of the metal is recorded by the transducers. Two quartz rods introduced directly at different heights into the liquid metal and connected to two transducers record the linear displacement during the liquid-solid transformation and subsequent cooling. Two thermocouples positioned at the same height with the quartz rods allow for the concomitant TA and LDA and thus for the direct correlation between expansion/contraction and the temperature change during solidification events such as graphite formation. The LDA device was used to study the differences in the solidification mechanisms of irons with different graphite morphologies (lamellar, compacted/ vermicular and spheroidal) at carbon equivalent in the range of 3?7-4?4%. The analysis included the LDA and TA curves and full metallographic characterisation of the cast irons. In general, graphite expansion increased as the graphite shape changed from lamellar, to compacted and then to spheroidal. The most important process variables are the magnesium and carbon contents. Higher Mg residual and C in the iron produced more graphite expansion. Compacted graphite (CG) iron was particularly sensitive to the Mg residual. Indeed, the high Mg CG irons exhibited similar graphite expansion to that of spheroidal graphite (SG) iron, while the low Mg CG iron expansion was closer to that of the lamellar graphite (LG) iron. Graphite expansion increased for all data with the time interval over which graphite expansion occurred. It also increased with both carbon and carbon equivalent. The time for graphite expansion increased noticeably with the carbon content of the iron. It did not depend on the graphite shape. By combining TA and LDA, it was possible to plot the evolution of graphite expansion as a function of the fraction solid and thus to understand the kinetics of graphite expansion. The amount of expansion available at the end of solidification was quantified. Such data, when correlated with process variables, will be useful in decreasing microshrinkage and in producing riserless compacted and SG irons.