Formation Of Spheroidal Graphite Research Articles

The Role of Selenium on the Formation of Spheroidal Graphite in Cast Iron

Sulfur, an element that belongs to group 16 (chalcogens) of the periodic table, is an excellent promoter of nucleation substrates for graphite in cast iron. In ductile iron, sulfur favors a higher nodule count, which inhibits the risk of carbides and of microporosity. It is reasonable to expect that other elements from group 16, such as selenium or tellurium, play similar roles in the nucleation of graphite. The objective of this paper was to investigate the effect of selenium on the process of graphite formation. Thermal analysis cups were poured to evaluate the nodule count and size distribution. Some of the cups were not inoculated, while others were inoculated with a Ce-bearing inoculant, or with the Ce inoculant and additions of Se. Cross-shaped castings were also poured to quantify the microporosity regions by tomography. It appears that selenium additions modify the number and size of graphite particles, as well as the volume of microshrinkage. Direct correlations between these three parameters were found. Advanced Extensive Field Emission Gun Scanning Electron Microscope (FEG-SEM) techniques were used to identify the nature of the main nucleation compounds. Selenides, combined with Mg and rare earths, were observed to serve as nuclei for graphite. Their presence was justified by thermodynamics calculations.

Why is Graphite Spherical in Ductile Iron? A Study of Elemental Distributions at Interfaces in Ductile Iron Using Atom Probe Tomography and Transmission Electron Microscopy

Atom probe tomography (APT) with high sensitivity and spatial resolution was applied to study the elemental distributions in spheroidal graphite in ductile iron. A specimen of quenched ductile iron with some graphite nodules in direct contact with liquid phase was used in this study to understand the role of alloying elements in spheroidal graphite formation. Compositional analysis was performed at the graphite/matrix interface and graphite/nuclei interface using APT and EDX (energy-dispersive X-ray). Distributions of various alloying elements at interfaces were obtained. Compositional gradients were observed at both the graphite/liquid interface and the graphite/nuclei interface. The spheroidal graphite nuclei were differentiated as the complex compound made of oxide, nitride and sulfide. The graphite–nuclei interface was shown to be semi-coherent/incoherent using the transmission electron microscopy, with a high density of crystallographic defects and high curvatures in the graphite basal planes growing off the nuclei. Intercalation of large nodulizing elements within and in between the graphite basal planes may account for the spherical morphology of the graphite in ductile irons.

A NEW CERAMIC CASTING MOLD MADE BY GEL CASTING USING SILICA SOL AS A BINDER

This Paper presents a new ceramic casting mold made by gel casting using silica sol as a binder. The new ceramic mold is consisted of an alumina-mullite-zirconia matrix with the ratios of 38.332 wt. % alumina, 34.378 wt. % mullite, and 27.294 wt. % zirconia respectively, the slurry is mixed then the gelling agent is added and poured into the pattern. After gelation the mold is extracted and left to dry, then sintered. There are three main defects appear in the mold fabrication process. The 1st defect is the mold cracking, as a result of forced shrinkage of mold into pattern. The 2nd defect is the bad gelation behavior of mold, as a result of non equal gelling time. The last defect is mold surface cracks, due to increased silica ratio added to the mixture. As zirconia increased the bulk density and apparent porosity is increased, leading to higher mold permeability which is important to eliminate trapping of residual gases. The increased zirconia content decreases the micro hardness and the cold crushing strength, but increases the thermal shock resistance due to phase transformation during sintering. The ceramic mold is applicable for nodular cast iron so that the mold is hard enough to withstand the forces of spheroidal graphite formation when nodular cast iron is poured into the ceramic mold. The surface roughness no. of metal cast into ceramic mold is 0.582 μm, while of the metal cast into sand mold is 5.187 μm.

Influence of Melting Conditions on Graphite Morphology in Ni–C Alloy and Grain Number of Matrix

The influence of the melting conditions on the graphite morphology and the grain number of the matrix in a Ni–C alloy was studied. Two groups of parent alloys were prepared using high purity materials, one group containing the spheroidizing element of Ce, Mg or Ca, while in the other group, S was added as an anti-spheroidizing element.The spheroidizing element-added sample of 0.5 g was melted and held for 15 minutes at 1673 K in an Ar atmosphere and then cooled. At the high cooling rate of 1000 K/min, an irregular graphite appeared. When an Ar+3%H2 atmosphere and a 10-min holding time at 1673 K were used, the formation of the spheroidal graphite was confirmed at the cooling rate of 1000 K/min in both groups.The atmosphere and the holding time were then independently changed at the cooling rate of 1000 K/min. It was found that the 3%H2 did not significantly affect the spheroidal graphite formation. Moreover, the holding times of 1 min and 20 min also did not significantly affect the spheroidal graphite area fraction in the Ni–C alloy, while they affected the graphite morphology of the samples containing the spheroidizing elements like Mg.The grain number of the matrix around the graphite phase was also investigated in this study based on EBSD observations. The results showed that the matrix around the massive graphite morphology is polycrystalline, but is mainly single crystalline around the spheroidal graphite. The mechanism of the difference in the crystal structure of the matrix in the two different graphite morphologies was also discussed.

Influence of Melt Holding Time on Microstructure and Mechanical Properties of Ductile Iron

Six groups of castings which solidified at different cooling rates with or without addition of 0.03%Sb were fabricated. The effects of the melt holding time, cooling rate, and Sb on the microstructure of ductile iron were studied. Results show that as for the other three blocks with addition of 0.03%Sb, reducing the length of eutectic plateau could improve the graphite morphology, and sufficient residual contents of spheroidizing elements REE and Magnesium could also lead to small and diffuse globular graphite particles no matter what the cooling rate is. As for the other three blocks with addition of 0.03%Sb, no matter what the cooling rate is, the nodularity index values are all about 90%. The main difference among the three blocks is just the size of nodular graphite. And even if two of the blocks solidified at different cooling rates, they own nearly identical NI value and globular graphite average diameter value, which is because they both possess the tolerated rare-earth elements/antimony ratio for spheroidal graphite formation.

Effects of Trace Elements on the Formation of Spheroidal Graphite in Ductile Iron

Spheroidal graphite was obtained by direct quenching iron melt atfter spheroidization and inoculation procedures. The morphology and the effects of trace elements on the formation of spheroidal graphite were investigated by scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy. Results indicate that all the graphite fixed by quenching, is nearly spheroidal with a diameter of approximately 20μm. In addition, the SEM image of graphite morphology confirms the screwed growth model of spheroidal graphite. The linear distribution of trace elements shows that the nucleus of spheroidal graphite consists of antimony, cerium, magnesium and sulfur element. Meanwhile, trace elements, such as antimony, cerium and magnesium, enrich around graphite nodule, which facilitates the formation of spheroidal graphite.

Heterogeneous Nucleus of Spheroidal Graphite and Mechanism of Spheroidal Graphite Formation

Heterogeneous Nucleus of Spheroidal Graphite and Mechanism of Spheroidal Graphite Formation

Influence of Melting Conditions on Graphite Morphology in Spheroidal Graphite Cast Iron Using Ni-C Alloys

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.

Microstructure Evolution of Heavy Section Ductile Iron

Ductile iron, Heavy section, Cooling curve, Microstructure, Cooling rate. Abstract. Two 250×250×250 mm cubic ductile iron castings solidified in sand and insulation mould were fabricated. The effect of cooling rate on graphite and matrix microstructure of heavy section ductile iron together with their cooling curves were evaluated. Results show that increasing the cooling rate leads to fine graphitization and favors spheroidal graphite formation. The matrix structure is fully ferrite structure at the edge of both castings, while pearlite can be seen near the eutectic cell boundaries at the center of two castings. Furthermore, the amount of pearlite increases with increasing solidification time. Cooling curves confirm that cooling rate affects solidification time of the eutectic transformation and characteristic temperature points on the cooling curves remain unchanged. Low cooling rate appears to significantly increase the eutectic plateau length. Besides, cooling curves show that eutectic temperature remains constant (about 1160°C), which allows for spheroidal graphite formation. Undercooling and inoculation fading during the long time eutectic solidification lead to pearlite formation in the center of cubic ductile iron castings.

Spheroidal Graphite Formation Process considered from Voids in Chilled Samples of Magnesium Treated Irons

Spheroidal Graphite Formation Process considered from Voids in Chilled Samples of Magnesium Treated Irons

Looking at Graphite Spheroids

Professor Carl Loper Jr. has been fundamental in the development of cast iron. He has taught numerous metalcasters and scientists the fundamentals of cast iron solidification and the mechanisms involved in the formation of spheroidal graphite. From the liquid state to the finished part, cast iron undergoes numerous transformations. Usually, during failure analysis of a cast iron part, one looks at the final process for answers but examination of the different solidification stages can often help to solve the problem. Areas to consider examining include: the use of conditioners before nodularizing treatment, the type of inoculants and the solid-state transformation can be elucidated using powerful tools such as Optical (OM) and Scanning Electron microscopes (SEM) and Energy Dispersive Spectroscopy (EDS) analysis.

Critical conditions for formation of spheroidal graphite

The influence of the interfacial energy between the Fe–C melt and basal plane of graphite, γ Gr/L, and the cooling rate on the formation of spheroidal graphite was investigated as a function of the sulfur content in cast iron using the sessile drop method in a He–3% H 2 atmosphere. The result showed that the γ Gr/L decreased from 2.62 to 1.48 J/m 2 with an increase in the sulfur. The corresponding surface energy of the graphite, γ Gr/V, also varied from 1.17 to 0.65 J/m 2. It is found that the spheroidal graphite is formed when the γ Gr/L is greater than 2.5 J/m 2, which is the critical interfacial energy of the spheroidal graphite formation. In addition, the cooling rate is also an important factor for producing the spheroidal graphite in the Fe–C alloy. The present study clearly shows that the critical cooling rate is a function of the sulfur content, namely γ Gr/L, in the Fe–C melt.

Study of voids in chilled samples of magnesium treated irons

Chilled samples of base iron and magnesium-treated iron were surveyed using optical microscopy, SEM and computer assisted microanalysis. Numerous voids, 0.5–1 m m in size, were observed in magnesiumcontaining chilled samples. Graphite spheroids at an early stage of growth, 3–5 m m in size, and inclusions were also observed. Voids were not found in the base iron samples but inclusions were observed. Magnesium enrichment was detected at the voids, graphite and inclusions of the magnesium treated samples. It is proposed that the voids represent the traces of magnesium vapour bubbles formed in the melt. Some graphite spheroids consist of a thin graphite layer at their surface and a hollow core, a morphology that supports nucleation on the void walls followed by growth towards the centre. Although inclusions were occasionally observed in the centres of graphite spheres, they do not appear to be essential for spheroidisation to occur. It is concluded that gas bubbles formed by free magnesium contribute to the formation of spheroidal graphite by supplying nucleation and growth sites.

Inhibitory Effects of Various Elements on Spheroidal Graphite Formation in Cast Iron

Inhibitory Effects of Various Elements on Spheroidal Graphite Formation in Cast Iron

Internal features of graphite in cast irons: Confocal microscopy: useful tool for graphite growth imaging

Spherulitic crystallisation is a mode of growth of crystals from the melt. Considerable attention has been given to spheroidal graphite formation, providing detailed information about the internal microstructure of the spherulites in spheroidal (SG irons) and compacted graphite irons (CG irons) ( Stefanescu, D., 1990. Cast Irons. ASM Handbook, 10th ed., vol. 1). Nevertheless, the mechanisms responsible for this mode of crystallisation are not fully understood. This study deals with the inoculation mechanisms, with particular emphasis on the study of the inclusions for the heterogeneous nucleation of graphite. It is shown that the graphite nuclei are sulfide products of the nodularizing treatment. It has been observed that when rare-earth treatment is applied, the central nucleus consists of a core and an envelope from which the graphite grows. Confocal Scanning Laser Microscopy (CSLM), in reflection mode, was used to study the internal features of the spheroidal graphite growth. Confocal reflection imaging, which has a capacity for optical sectioning of the sample, provides high-resolution images of surface and subsurface regions of interest contained within a semi-transparent sample. Furthermore, three-dimensional reconstruction of these optical sections can provide insight into the mechanism of graphite growth mechanism interpretation. With CSLM the radial growth of graphite was seen. Other techniques, such as TEM, SEM–EDS, WDS, AES and SAM were also used to corroborate the results.

The Role of Rare-earth Metals in the Process of Spheroidal Graphite Formation

Master alloys containing rare-earth metals (REM) are often used along with magnesium in ductile-iron casting production as nodularising agents. The amount of the additions made in both cases is generally arrived at empirically, which often gives rise to unstable structures and the formation of chill in the iron. This paper deals with a thermodynamic approach to the choice of REM and the estimation of the optimum amount. On the basis of thermodynamics of Ce and Y reactions with sulphur and oxygen, it is shown that the type of the compounds formed is determined by the ratio of their concentrations in the melt. When the level of impurities are those normally present in the iron, then oxisulphides and REM sesquisulphides are mainly formed. These compounds may be found to have been formed in the melt before the iron commences to solidify and they can serve as substrates for heterogeneous nucleation of graphite. Stoichiometric relationships to calculate the amount of REM which will provide a stable formation of spheroidal graphite are suggested.

Neutralization by Cerium of Detrimental Effect of Sulfur and Selenium on Spheroidal Graphite Formation in Cast Iron

Neutralization by Cerium of Detrimental Effect of Sulfur and Selenium on Spheroidal Graphite Formation in Cast Iron

Formation of spheroidal graphite in cast iron

1. Refractory particles of Polirit introduced into a cast iron melt are passive graphitizing centers around which graphite inclusions form. Growth and configuration of these inclusions is regulated with a reduction in melt temperature by precipitation on their surface of surface-active elements present in the melt. There is no cast iron melt refining by such surface-active elements as sulfur, oxygen, etc. 2. In order to obtain spheroidal graphite in cast iron it is necessary to introduce into the melt modifying elements forming with surface-active elements, primarily sulfur and oxygen, refractory compounds which are active graphitizing centers, and lead to removal of these elements from participation in the crystallization process.

Reaction Products in Neutralization by Cerium of Detrimental Effect of Antimony on Spheroidal Graphite Formation

Reaction Products in Neutralization by Cerium of Detrimental Effect of Antimony on Spheroidal Graphite Formation

The Neutralization by Cerium of the Detrimental Effect of Antimony on Spheroidal Graphite Formation in Cast Iron

The Neutralization by Cerium of the Detrimental Effect of Antimony on Spheroidal Graphite Formation in Cast Iron