On the Eutectic Solidification of Spheroidal Graphite Iron: An Experimental and Mathematical Modeling Approach (original) (raw)
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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.
Solidification of spheroidal graphite cast irons—II. numerical simulation
Acta Materialia, 1998
AbstractÐA model of the solidi®cation of spheroidal graphite cast iron (SGI) has been detailed in Part I. It was designed to allow the description of the successive steps of solidi®cation of hypereutectic as well as hypoeutectic alloys, i.e. the pro-eutectic deposits and the eutectic reaction. In the present paper, this model is used ®rst as a tool to analyse the main features of the solidi®cation of SGI as experimentally established. Calculations are then described which were carried out to simulate a series of test castings from which suf-®cient experimental information was obtained to discuss the validity and the limits of the present modelling and simulating approach. Particular emphasis is put on the nucleation and growth of graphite nodules and on the precipitation of non-eutectic austenite during the eutectic stage.
Solidification macrostructure of spheroidal graphite cast iron
International Journal of Cast Metals Research, 2001
Methods to reveal the macrostructure of spheroidal graphite cast iron (SGI) are not available. This has caused a number of controversies involving the solidification mechanism of this metallic alloy. The objective of the present work is to examine the use of a novel technique (DAAS) reported recently, to reveal the solidification macrostructure of SGI made using conventional melting and pouring practices, and cast in sand moulds. The results show that the DAAS technique successfully reveals the macrostructure. The basic features of the solidification macrostructure of SGI are similar to those of most metallic alloys. Nevertheless, some particular features have become evident. The grain size of SGI is considerably larger than previously estimated. The grain size of hypereutectic SGI is consistently smaller than that of hypo and eutectic irons cast in similar conditions. The observation of the actual solidification macrostructure of SGI led the authors to propose a description of the solidification mechanism that takes into account the actual size and morphology of the solidification units. The new DAAS technique is a very valuable tool which can be used to investigate a number of issues about SGI solidification still under discussion.
ISIJ International, 2009
Analysis of cooling curves recorded at the centre of large blocks cast with near-eutectic spheroidal graphite cast irons prone to give chunky graphite has been checked against microstructure observations. It has been observed that solidification proceeds totally at temperatures lower than the stable eutectic temperature and the following solidification sequence could be proposed: 1) nucleation of primary graphite in the liquid; 2) initial eutectic reaction processing by growth of austenite-like dendrites encapsulating the primary nodules; 3) bulk eutectic reaction related to nucleation and then growth of CHG cells and of secondary nodules, these latter giving spheroidal graphite eutectic cells. It was found that the maximum recalescence during the eutectic reaction first increases with the volume of the block affected by chunky graphite, and then decreases when most of the material is affected. Interestingly enough, a relationship between the volume of the blocks affected by CHG and the recalescence measured on TA cups has been observed.
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.
On the growth of spherulitic graphite in nodular cast iron
Carbon, 1975
Considerable evidence exists in the literature on graphite to indicate that it produces curved crystals bounded by basal planes but formed by growth in the basal plane. This evidence is supplemented by recent observations on pure Fe-C-Si alloys which show that graphite in cast irons can also undergo curved crystal growth. A model of graphite spherulite growth in cast iron is presented. The model is based upon curved growth of graphite during solidification. and indicates that spherulitic growth is circumferential rather than radial.
International Journal of Cast Metals Research, 2013
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