Solidification Of Iron Research Articles

Thermal Analysis to Optimize and Control the Cast Iron Solidification Process

The cooling curve and its derivatives display patterns that can be used to predict the characteristics of a cast iron. The effects of melting, superheating and holding in an acid lined coreless induction furnace were explored, as they affect the role of preconditioning and / or inoculation to restore solidification with low eutectic undercooling. Increased chill (iron carbides amount) in the experimental irons correlates well with certain thermal analysis parameters, such as the degree of eutectic undercooling. Preconditioning of the molten base iron before tapping led to improved solidification parameters in both untreated and inoculated irons as measured by the most significant thermal analysis cooling curve events. A double treatment incorporating preconditioning with inoculation improved the thermal analysis parameters, and consequently, the quality of the cast iron. If standard Ca-FeSi alloys do not have sufficient inoculation potential, the addition of the inoculant enhancing alloy (S, O and oxy-sulphides forming elements) will greatly enhance inoculation, well illustrated by changes to the thermal analysis parameters. A newly defined Inoculation Specific Factor [inoculation effect / inoculant consumption which led to that beneficial effect ratio] of different alloys is illustrated by thermal analysis, with good correlation with microstructural characteristics.

Effect of rapid solidification on the microstructure and microhardness of BS1452 grade 250 hypoeutectic grey cast iron

Containerless solidification of low alloyed commercial grey cast iron in two different cooling media (N2 and He) using a 6.5 m high vacuum drop-tube have been investigated. Both the conventionally cooled, as-cast alloy and the rapidly cooled drop-tube samples were characterized using SEM, XRD and Vickers microhardness apparatus. The estimated range of cooling rates are 200 K s−1 to 16,000 K s−1 for N2 cooled droplets and 700 K s−1 to 80,000 K s−1 for He cooled droplets (in each case for 850 μm and 38 μm diameter droplets respectively). Microstructural analysis reveals that the as-received bulk sample displayed a graphitic structure while the rapidly cooled samples display decreasing amounts of α-Fe as the cooling rate increases. At moderate cooling rates α is replaced with γ and Fe3C, while at higher cooling rates with α′. Microhardness increase with cooling rate but cannot be mapped uniquely onto cooling rate, suggesting undercooling also influences the mechanical properties.

Pretreatments of gray cast iron with different inoculants

Steel scraps were used as the main raw materials (including 70% steel scraps and 30% pig iron) to prepare the gray cast iron HT250 in a medium-frequency induction melting furnace. A new inoculant X, which mainly contains SiC, and a conventional one (FeSi75) were added into the molten iron, respectively. The effects of the inoculants were investigated through metallurgical analysis of the matrix microstructure and the mechanical properties of the cast test bar, and scanning electron microscope (SEM) observation of the tensile fractured surfaces. The properties of the graphite morphology, matrix structure, mechanical properties and fracture characteristics of the cast iron were examined. The results show that both the new inoculant X and the conventional inoculant FeSi75 resulted in improved properties of the molten iron over the original uninoculated molten iron. The inoculants produced smaller graphite particles, reduced tendency to form shrinkage cavities and porosities during the solidification of the molten iron, decreased supercooling degree, reduced formation of non-metal inclusions, and enhanced mechanical properties (of the cast irons). Most important, the new inoculants X displayed more significant effects than the conventional one because SiC had a high melting point and did not dissolve immediately after being added into the molten iron, which helped to form a large number of micro-zones with high carbon and silicon concentrations and induced heterogeneous nucleation of graphite.

Transient Directional Solidification of Cast Iron: Microstructure Formation, Columnar to Equiaxed Transition and Hardness

A number of applications may require cast iron. Engine cylinder blocks, flywheels, gearbox cases, machine-tool bases may be manufactured by using grey cast iron while bearing surfaces with white cast iron. Thus, understanding the solidification behaviour of eutectic cast iron becomes an essential task, with certain points to be accomplished. Transient directional solidification may provide particular advantages in order to deal with these items, such as the large variation of growth rate (V) and cooling rate (T) values, which may allow a variety of microstructures and morphologies to be studied. The aim of this work is to examine the macrostructure regions, scale of the dendritic microstructure, proportions of the formed phases and hardness of samples obtained by transient directional solidification of a eutectic cast iron (Fe-3.5wt%C-2.5wt%Si). It was shown that a CET criterion should be based on a critical V value at the solidification front of about 0.6 mm/s. The effects of the formed phases, their proportions and λ2 on hardness of the cast alloy are assessed.

Solidification of superfine graphite iron revealed by interrupted solidification experiments

The tensile strength of near-eutectic grey iron can be increased from 230–300 to 300–345 MPa, without a significant increase in hardness, through 0.3–0.4%Ti addition to low sulphur (<0.01%S) iron. This is due to the combination of higher primary austenite/eutectic ratio and the precipitation of superfine-interdendritic-graphite (SIG), characterised by a fine (10–20 μm) and highly branched fibrous structure. To reveal the influence of the %Ti on graphite shape evolution during solidification and its relationship to the solid fraction, quenching experiments at successive solidification stages were carried out on hypoeutectic alloys with 0.18% and 0.32%Ti. The graphite shape factors were measured, and their evolution as a function of the titanium content and the solid fraction was analysed. SEM was used to evaluate the change in graphite shape during early solidification, as well as its nucleation and growth. The correlation between the oxygen in the melt and SIG formation was also explored. It was concluded that nucleation of graphite in SIG irons occurs on carbon rich regions at the austenite/liquid interface and, sometimes, on titanium carbides. Solidification velocity-undercooling curves were used to explain the transition from lamellar (type-A), to interdendritic (type-D), to SIG, and then to coral graphite.

Understanding compacted graphite iron solidification through interrupted solidification experiments

While the manufacture of compacted graphite (CG) iron castings has seen significant expansion over the recent years, the growth of CG during iron solidification is still not fully understood. In this work, effort was expanded to experimentally reveal the evolution of graphite shape during early solidification and its relationship to the solid fraction. To this purpose, interrupted solidification experiments were carried out on hypereutectic irons with three magnesium levels. The graphite shape factors were measured and analysed as a function of chemical composition and solid fraction. Scanning electron microscopy was carried out to establish the fraction of solid at which the transition from spheroidal graphite (SG) to CG occurs. It was confirmed that solidification started with the development of SG for all CG irons. The SG-to-CG transition was considered to occur when the SG developed a tail (tadpole graphite). The findings were integrated in previous knowledge to attempt an understanding of the solidification of CG iron.

Study of shrinkage porosity in spheroidal graphite cast iron

This study aims at identifying the relationship between the shrinkage cavities and the solidification structure in spheroidal graphite cast iron. Cast samples specially designed to contain shrinkage cavities were used. The solidification macrostructure was revealed using the Direct Austempering After Solidification method, while the solidification microstructure was revealed by using colour etching. At the midsection of the pieces, the shrinkage cavities and the solidification structure were observed jointly. The study showed that the classification of shrinkage porosity found in literature does not correspond to the ductile iron solidification model recognized by most of the scientific community. Early solidification models, and therefore shrinkage formation mechanisms, were proposed in instances when there was not a thorough knowledge of the morphology of the solid phases during solidification. Nowadays, defects formation mechanisms can be described with higher accuracy. Therefore, an updated classification of shrinkage porosity for spheroidal graphite iron is proposed.

Prediction of volume fraction of primary austenite at solidification of lamellar graphite cast iron using thermal analyses

Lamellar graphite cast iron was investigated with carbon equivalents varied between CE = 3.4 and 4.26, cast at various cooling rates between 0.195 and 3.5 °C s−1 covering the limits used for techni ...

On the stable eutectic solidification of iron–carbon–silicon alloys

Extensive effort was expanded to elucidate the growth and morphology of the stable eutectic grains during early solidification of continuous cooled Fe–C–Si alloys. To this purpose, quenching experiments at successive stages during solidification have been carried out on five cast irons with various magnesium and titanium levels designed to produce graphite morphologies ranging from lamellar to mixed compacted–spheroidal.The graphite shape factors were measured on the metallographic samples, and their evolution as a function of the chemical composition and the solid fraction was analyzed. Extensive scanning electron microscopy was carried on to evaluate the change in graphite shape during early solidification, to establish the fraction of solid at which the transition from spheroidal-to-compacted-to-lamellar graphite occurs, and to outline the early morphology of the eutectic grains. It was confirmed that solidification of Mg containing irons started with the development of spheroidal graphite even at Mg levels as low as 0.013 mass%. Then, as solidification proceeds, when some spheroids developed one or more tails (tadpole graphite), the spheroidal-to-compacted graphite transition occurs.The new findings were then integrated in previous knowledge to produce an understanding of the eutectic solidification of these materials. It was concluded that in hypoeutectic lamellar graphite iron austenite/graphite eutectic grains can nucleate at the austenite/liquid interface or in the bulk of the liquid, depending on the sulfur content and on the cooling rate. When graphite nucleation occurs on the primary austenite, several eutectic grains can nucleate and grow on the same dendrite. The primary austenite continues growing as eutectic austenite and therefore the two have the same crystallographic orientation. Thus, a final austenite grain may include several eutectic grains. In eutectic irons the eutectic grains nucleate and grow mostly in the liquid. The eutectic austenite has different crystallographic orientation than the primary one.The solidification of the austenite/spheroidal graphite eutectic is divorced, with graphite spheroids growing on primary austenite dendrites. The eutectic austenite grows on the primary austenite and has the same crystallographic orientation. The result is large austenite (primary and eutectic) dendrites that incorporate numerous graphite spheroids. A eutectic grain cannot be defined.

The Influence of Pouring Temperature and the Filling Level of the Cup Samples on the Cooling Curves Parameters at Cast Iron Solidification

The main purpose of this study is to determine by experimental research the influence both of pouring temperature and cup samples size and their filling level on the cooling curves parameters at cast iron solidification. For the experiment it follows to determine the less sensitive parameters to pouring conditions both for reduced and normal cup samples. By using two cups with different cooling modulus it was established some correlation between some variable factors such as cooling modulus, sample filling level and casting temperature and the cooling curves parameters. The experimental results released a higher sensitivity of reduced size cup (lower cooling modulus) on pouring conditions (pouring temperature and mould cup filling level).

Effect of Chromium on the Solidification Process and Microstructure of Vermicular Graphite Cast Iron

Abstract The paper presents the results of studies of the effect of chromium concentration on the solidification process, microstructure and selected properties of cast iron with vermicular graphite. The vermicular graphite cast iron was obtained by an Inmold process. Studies covered the cast iron containing chromium in a concentration at which graphite is still able to preserve its vermicular form. The effect of chromium on the temperature of eutectic crystallization and on the temperature of the start and end of austenite transformation was discussed. The conditions under which, at a predetermined chromium concentration, the vermicular graphite cast iron of a pearlitic matrix is obtained were presented, and the limit concentration of chromium was calculated starting from which partial solidification of the cast iron in a metastable system takes place. The effect of chromium on the hardness of cast iron, microhardness of individual phases and surface fraction of carbides was disclosed.

Effect of Pressure and Temperature Factors on the Solidification of Cast Iron and Its Structure in Liquid Forging

This article examines the properties and microstructure of cast iron after fabrication of grinding balls by different kinds of casting and forging, with crystallization of the metal under pressure. A mathematical model of the process of solidification of a forging in a die is presented. Joint solution of two Fourier equations of heat conduction for the melt and for the solid skin is used to derive a kinetic equation of solidification and hence to determine the rate of solidification of the forging in the die. The effect of the pressure on the structure of the crystallizing metal and the quality of the forged grinding balls that are obtained is determined.

Bidirectional impact of undercooling on eutectic structural formation of hypoeutectic grey iron and its physical connotation

Bidirectional impact of the undercooling during eutectic solidification of hypoeutectic grey iron on the number of eutectic cells under different cooling rates and nucleation conditions was studied. The causes of this experimental phenomenon were explained by melt heterogeneous nucleation theory combined with the features of actual measured cooling curves. By deeply revealing the physical connotation of solidification undercooling, the undercooling nature and its impact on the solidification parameters were discussed with a wider view of phase transformation driving force and resistance. This discussion not only gives a reasonable explanation to the specific experimental phenomena in more depth, but also applies to a wide range of analysis.

Combination of microscopic model and VoF-multiphase approach for numerical simulation of nodular cast iron solidification

The ongoing increase in the size and capacity of state-of-the-art wind power plants is highlighting the need to reduce the weight of critical components, such as hubs, main shaft bearing housings, gear box housings and support bases. These components are manufactured as nodular iron castings (spheroid graphite iron, or SGI). A weight reduction of up to 20% is achievable by optimizing the geometry to minimize volume, thus enabling significant downsizing of wind power plants. One method for enhancing quality control in the production of thick-walled SGI castings, and thus reducing tolerances and, consequently, enabling castings of smaller volume is via a casting simulation of mould filling and solidification based on a combination of microscopic model and VoF-multiphase approach. Coupled fluid flow with heat transport and phase transformation kinetics during solidification is described by partial differential equations and solved using the finite volume method. The flow of multiple phases is described using a volume of fluid approach. Mass conservation equations are solved separately for both liquid and solid phases. At the micro-level, the diffusion-controlled growth model for grey iron eutectic grains by Wetterfall et al. is combined with a growth model for white iron eutectic grains. The micro-solidification model is coupled with macro-transport equations via source terms in the energy and continuity equations. As a first step the methodology was applied to a simple geometry to investigate the impact of mould-filling on the grey-to-white transition prediction in nodular cast iron.

Modelling and simulations of ductile iron solidification- induced variations in mechanical behaviour on component and microstructural level

The mechanical behaviour and performance of a ductile iron component is highly dependent on the local variations in solidification conditions during the casting process. Here we show a framework which combine a previously developed closed chain of simulations for cast components with a micro-scale Finite Element Method (FEM) simulation of the behaviour and performance of the microstructure. A casting process simulation, including modelling of solidification and mechanical material characterization, provides the basis for a macro-scale FEM analysis of the component. A critical region is identified to which the micro-scale FEM simulation of a representative microstructure, generated using X-ray tomography, is applied. The mechanical behaviour of the different microstructural phases are determined using a surrogate model based optimisation routine and experimental data. It is discussed that the approach enables a link between solidification- and microstructure-models and simulations of as well component as microstructural behaviour, and can contribute with new understanding regarding the behaviour and performance of different microstructural phases and morphologies in industrial ductile iron components in service.

On the Effect of Pouring Temperature on Spheroidal Graphite Cast Iron Solidification

This work is focused on the effect of pouring temperature on the thermal-microstructural response of an eutectic spheroidal graphite cast iron (SGCI). To this end, experiments as well as numerical simulations were carried out. Solidification tests in a wedge-like part were cast at two different pouring temperatures. Five specific locations exhibiting distinct cooling rates along the sample were chosen for temperature measurements and metallographic analysis to obtain the number and size of graphite nodules at the end of the process. The numerical simulations were performed using a multinodular-based model. Reasonably good numerical-experimental agreements were obtained for both the cooling curves and the graphite nodule counts.

Cooling Rate Analysis of Thin Wall Ductile Iron Using Microstructure Examination and Computer Simulation

Cooling rate plays an important role in thin wall ductile iron solidification, due to their thickness. Casting simulation is use as a tool to estimate the cooling rate. In the other hand, every microstructure has its own cooling rate. This paper explores the similarity of solidification mechanism between simulation and graphite characteristics. Three types of casting design simulated and produced. Solidification mechanism is analyzed based on cooling rate sequence and trend line matching. Temperature gradient and thermocouple function represent simulation while graphite characteristic represent experiment. The result shows that similarity in solidification mechanism is not found between simulation with experiment due to lack of parameters in both sides.

The Influence of Sulphur and Oxigen Additions in Mg-Treated Iron on the Cooling Curves Parameters

The main objectiv of this experimental reserch is a comparative analysis of sulphur and oxygen effects on the cooling curves parameters at different iron melt modifying potential (residual Mg content). For the experiment, three irons with different modifying potential (0.0014, 0.0213 and 0.033 wt % residual Mg content respectively) were developed. After Ca-Ba inoculation, the three irons were additional treated by stoichiometric equivalent additions of sulphur or oxygen as FeS2 and Fe2O3 respectively. Both FeS2 and Fe2O3 sources were placed on the bottom of standard Quik-Cup moulds,usualy used for iron melt thermal analysis. The effect of sulphur and oxygen on the standard and nonconventional cooling curves parameters were evaluated to a more deep understanding of the cast iron solidification mechanism. The cooling curves parameters are strongly influenced both by the initial residual Mg level of iron melt and afterwords sulphur/oxygen additions. A very complex and sometimes confused variation of the cooling curves parameters was recorded because of different actions of the three factors (iron melt modifying potential, sulphur and oxygen). As a previous paper highlighted, sulphur addition has a strong graphite decompactizing effect while oxygen addition has mostly an inoculant effect.

Improved CFD Model to Predict Flow and Temperature Distributions in a Blast Furnace Hearth

The campaign life of a blast furnace is limited by the erosion of hearth refractories. Flow and temperature distributions of the liquid iron have a significant influence on the erosion mechanism. In this work, an improved three-dimensional computational fluid dynamics model is developed to simulate the flow and heat transfer phenomena in the hearth of BlueScope’s Port Kembla No. 5 Blast Furnace. Model improvements feature more justified input parameters in turbulence modeling, buoyancy modeling, wall boundary conditions, material properties, and modeling of the solidification of iron. The model is validated by comparing the calculated temperatures with the thermocouple data available, where agreements are established within ±3 pct. The flow distribution in the hearth is discussed for intact and eroded hearth profiles, for sitting and floating coke bed states. It is shown that natural convection affects the flow in several ways: for example, the formation of (a) stagnant zones preventing hearth bottom from eroding or (b) the downward jetting of molten liquid promoting side wall erosion, or (c) at times, a vortex-like peripheral flow, promoting the “elephant foot” type erosion. A significant influence of coke bed permeability on the macroscopic flow pattern and the refractory temperature is observed.

Thermal Microstructural Multiscale Simulation of Solidification and Eutectoid Transformation of Hypereutectic Gray Cast Iron

Although the gray cast iron solidification process has been the subject of several modeling studies, almost all available models appear to deal with only the more widely used hypoeutectic compositions. Models related to hypereutectic gray iron compositions with lamellar (or flake) graphite, and in particular for the proeutectic and eutectoid zones, are hard to find in the open literature. Hence, in the present work, a thermal microstructural multiscale model is proposed to describe the solidification and eutectoid transformation of a slightly hypereutectic composition leading to lamellar graphite gray iron morphology. The main predictions were: (a) temperature evolutions; (b) fractions of graphite, ferrite, and pearlite; (c) density; and (d) size of ferrite, pearlite, and gray eutectic grains; (e) average interlamellar graphite spacing; and (f) its thickness. The predicted cooling curves and fractions for castings with two different compositions and two different pouring temperatures were validated using experimental data. The differences between this model and existing models for hypoeutectic compositions are discussed.