Modeling of Stable and Metastable Eutectic Transformation of Spheroidal Graphite Iron Casting.

Abstract

In this work, a mathematical model for stable and metastable transformation of SG cast iron, coupling with microsegregation of some elements and latent heat resulted from different phase formation, was presented. The quantitative experimental results of specimens obtained from a step-shape sample casting were compared with the simulation, and two results of nodular counts and sizes agreed quite well. For white eutectic growth in mottled SG iron, the frequent impingement of white eutectic with austenitegraphite bulks might suppress its growth remarkably and result in quite lower growth rate than that from the melt exclusively. Meanwhile, the relationship of morphology of carbides and solidification was discussed, and it was indicated that the growth of white eutectic and carbide morphology could also be predicted by solidification simulation. It was found that neglecting silicon microsegregation, if metastable transformation was advantageous to stable, the simulated carbide fraction was higher, and vice versa.KEY WORDS: spheroidal graphite cast iron; stable and metastable microstructure; eutectic transformation; modeling and simulation.ously, and may grow first freely and then surrounded by austenite shells. Considering the carbon diffusion and limited convection of liquid in front of liquid and graphite interface of Fig. 1, the growth rate of graphite spheres without austenite shell can be computed as follows:.... (5) where R t G is radii of graphite spheres (m) (superscript t and tϪdt are the calculation time) r l , and r G are densities of liquid and graphite (kg/m 3 ), D l C is diffusion coefficient of carbon in liquid (m 2 /s), C ළ l and C G are carbon content of initial liquid and graphite (%), C l * is carbon content of liquid in equilibrium with graphite and equals to C l/G approximately (Fig. 1), and d is thickness of carbon diffusion boundary layer in front of liquid-graphite interface (m).After the nodule was encapsulated by the austenite shell, its growth is dominated by the diffusion. 9) On the basis of carbon diffusion through the shell, the growth rates of bulks and graphite spheres were given as follows 10) :........... (6) ....... (7) where R g is radii of austenite shell of eutectic bulk, D C g is diffusion coefficient of carbon in austenite, r g is density of austenite, C g/l and C l/ g are carbon concentration of austenite and liquid in liquid-austenite interface respectively, and C g/G is carbon concentration of austenite in equilibrium with graphite.The growth of graphite spheres during eutectoid transformation could be determined by calculating the growth of ferrite shells with the model in the former research. 11)The growth of lamellar eutectic structure, such as ledeburite eutectic, was characterized by the well-known relationship 12,13) : where KЈ and KЉ are constants related materials properties, V is solidification rate (m/s). Because of crystal defects, the white eutectic grain often grows bending, and eventually becomes spherical. Therefore, the white eutectic was usually simplified ...

Influence of Chemical Composition and Solidification Rate on the Abrasion and Impact Properties of CADI

“…Accordingly, the size and composition of carbides may vary, from low alloyed cementite to high alloyed carbides, depending on the chemical composition of the heat and the cooling rate imposed during solidification. [6][7][8][9] As already demonstrated, 10,11) ledeburitic carbides, produced by controlling either the cooling rate or the silicon content (non-alloyed carbides), have a great tendency to dissolve during the austenitizing stage of the heat treatment and are less stable than alloyed carbides are. Therefore, if the cast parts need to be heat treated, carbide dissolution during the austenitizing step must be taken into account.…”

Simulation of feeding flow and prediction of macro porosity of SGI casting based on microstructure formation

“…For white eutectic growing in molten SGI, the parameter k is approximately 1610 26 -2610 26 m s 21 uC 22 (Ref. 6). Hence, the volume fraction of carbide in white eutectic could be determined according to the volume ratio of carbides y48?5% in the eutectic.…”

“…Under the condition that the melt is spheroidised with Mg-Re alloys and inoculated with Fe-Si alloys, A and n may be set to 1?8610 10 m 23 K 22 and 2 respectively for the nucleation of graphite nuclei. 6 Because the nucleation potential of white eutectic is greater than that of grey one, the parameter A for white nucleation was often several times as grey one. 7,8 In proeutectic transformation of hypereutectic SGI, the primary graphite spheres grow directly from the melt.…”

“…10 The latent heat resulted from different phase formation and the microsegregation of silicon during the stable and metastable eutectic transformation are considered according to former research. 6 Calculation of feeding flow During solidification process of SGI castings, the mass conservation in the liquid and mushy regions may be given by…”

Development and Characterization of a New Type of Ductile Iron with a Novel Multi-phase Microstructure

“…The lower the cooling rate, the greater the microsegregation effect, and so the probability of carbide precipitation at the Last To Freeze (LTF) zones increases and promotes alloyed carbides formation. As a consequence, the size and composition of carbides may vary significantly, from typical unalloyed ledeburitic carbides to high-alloyed carbides, [9][10][11][12] depending on the cooling conditions. It has been demonstrated that non alloyed carbides (produced either by controlling the cooling rate or the silicon level) are less stable than high alloyed carbides, and have a high tendency to dissolve during the austenitizing stage of heat treatments.…”