Graphite Nodules Research Articles

Effects of Image Analysis on Measured Graphite Nodule Count in Spheroidal Graphite Cast Iron

Effects of Image Analysis on Measured Graphite Nodule Count in Spheroidal Graphite Cast Iron

Effect of Solidification Time on Microstructural, Mechanical and Fatigue Properties of Solution Strengthened Ferritic Ductile Iron

Microstructural, mechanical, and fatigue properties of solution strengthened ferritic ductile iron have been evaluated as functions of different solidification times. Three types of cast samples with increasing thickness have been produced in a green sand automatic molding line. Microstructural analyses have been performed in order to evaluate the graphite nodules parameter and matrix structure. Tensile and fatigue tests have been carried out using specimens taken from specific zones, with increasing solidification time, inside each cast sample. Finally, the fatigue fracture surfaces have been observed using a scanning electron microscope (SEM). The results showed that solidification time has a significant effect on the microstructure and mechanical properties of solution strengthened ferritic ductile iron. In particular, it has been found that with increasing solidification times, the microstructure becomes coarser and the presence of defects increases. Moreover, the lower the cooling rate, the lower the tensile and fatigue properties measured. Since in an overall casting geometry, same thicknesses may be characterized by different microstructures and mechanical properties induced by different solidification times, it is thought that the proposed methodology will be useful in the future to estimate the fatigue strength of cast iron castings through the numerical calculation of the solidification time.

Effect of graphite morphology on the fatigue and fracture resistance of ferritic ductile cast iron

In this paper, four material variants of an EN-GJS-400-type ferritic ductile cast iron (DCI) are produced by varying solidification time and chemical composition. In this way, experimental batches with different size of graphite nodules and spheroidal graphite fraction are obtained. The material variants are characterized through static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests. It is shown that by increasing the mean diameter of the spheroidal graphite particles fatigue strength, tensile strength and total elongation decrease, while fatigue crack growth threshold and fracture toughness increase. On the contrary, reducing the spheroidal graphite fraction owing to degeneration into chunky graphite results in lower tensile and fatigue properties as well as fracture toughness, while the near-threshold fatigue crack growth resistance is improved. This effect is discussed by the light of crack closure, fracture surface roughness and fractographic analyses.

Influence of Tellurium Addition to Spheroidal Graphite Cast Iron on the Number of Graphite Particles

Influences of magnesium content on the actions of tellurium to graphite nodule count and chilling tendency in spheroidal graphite cast iron are investigated. Casting samples 30 mm in diameter were prepared by lab size induction furnace melting. On castings with two levels (0.020 mass% and 0.035 mass%) of magnesium content, tellurium content was changed from 0 to 45 mass ppm. With additions of a small amount of tellurium, the graphite nodule count increases and the chill ratio decreases. In the 0.020 mass% Mg samples, the deterioration of the spheroidal shape is intensified with excess content of tellurium, and the critical amount is 20 mass ppm. In the 0.035 mass% Mg samples, the chilling tendency become pronounced as the tellurium content is larger than the critical content of 24 mass ppm, even though the degree of spheroidization is sufficiently large. The cooling rate of the 0.035 mass% Mg sample with 10ppm Te decreases at the beginning of the eutectic reaction, and there was no chill in the microstructure with the maximum graphite nodule count. Although it is rarely found by electron probe microanalyzer (EPMA) analysis, inclusions in the center of a spheroidal graphite consist of Mg, Ca, S, and Te.

Problems and Improvements on the Production of Large Casting with Hi–Si Ductile Iron

We have studied the problems and improvement on producing large casting with solid solution strengthened ferritic spheroidal graphite cast irons (hereafter Hi–Si). As the matrix of Hi–Si material becomes hardened with silico-ferrite, hardness, tensile strength and yield strength increase. But elongation and impact value decrease conversely. If silicon content increases or if the cooling rate decreases due to heavy section, as silicon segregation progresses, the elongation and impact value are remarkably reduced. When increasing the graphite nodule count with optimum inoculation and setting chiller, the segregation of silicon decreases and then the mechanical properties are recovered. In addition, when Hi–Si reaches around 400 °C, its elongation decreases largely, which is called 400 °C embrittlement. This high-temperature embrittlement can be prevented with setting the ratio of Mg/P less than 1.5 and reducing the free Mg, because the grain boundary fracture occurs due to the free Mg not fixed as Mg3P2 at the grain boundary.

A graphite nodule growth model validated by in situ synchrotron x-ray tomography

An accurate prediction of ductile cast iron (DCI) microstructures is crucial for a science-based optimisation of cast component design. The number density and distribution of graphite nodules critically influence the mechanical performance of a component in service. Although models predicting nodule growth have been researched for many years, recent improvements have been impeded by lack of detailed experimental data on nodule growth kinetics for validation. This data has now been made available through in situ observations of the solidification of DCI using synchrotron x-ray tomography in combination with a high temperature environmental cell. In the present investigation, a new sphere of influence (SoI) model for spheroidal graphite growth is proposed. It inherently incorporates the competition for carbon between neighbouring nodules and the depletion of carbon in the matrix. Comparing simulation results to the in situ observations of graphite growth, the SoI model successfully predicts both growth of individual nodules as well as the size distribution of a large nodule population during solidification.

Influence of alloy element distributions on austempered ductile irons

ABSTRACTThe influence of alloy element distributions on austempered ductile iron microstructure and austempering treatment was analysed by a cellular automaton model that considers the ausferritic and martensitic transformations. The initial microstructure is modelled as spherical graphite nodules inserted in an austenitic matrix, in which the alloy elements are distributed in a uniform or non-uniform way. The study is performed for different chemical compositions and graphite nodule sizes. Delays in the development of ausferritic transformation are produced by the increment of graphite nodule size and the presence of alloy element microsegregations. Moreover, microsegregation reduces the final volume fraction of ferrite platelets. The predicted retained austenite volume fraction is in good agreement with the experimental measurements reported in the literature.

Analysis of Local Conditions on Graphite Growth and Shape During Solidification of Ductile Cast Iron

3D X-ray tomography recordings have been used to study graphite growth during solidification of ductile cast iron. Using data from such recordings, it is shown how local growth conditions influence growth rate and morphology of nodules during solidification. Experiments show that it is common for nodules to gradually change shape during solidification so that sphericity decreases. It is also found that different shaped nodules can evolve in direct contact with liquid iron and also after they are encapsulated in austenite. It is observed that a significant proportion of originally complete spherical nodules become less spherical via formation of protrusions on the surface; these new surfaces are observed to grow relatively faster. It is shown that encapsulation of the graphite nodule by austenite may be incomplete and that at the end of solidification, partial encapsulation and the effect of the number of nearest graphite nodules play a crucial role in determining the final graphite morphology.

A Finite Element Study of the Influence of Graphite Nodule Characteristics on a Subsurface Crack in a Ductile Cast Iron Matrix under a Contact Load

This paper describes a study of the effects of graphite nodule characteristics on a subsurface crack in austempered ductile iron (ADI). A representative specimen of ADI, subjected to sliding contact load, is modeled using finite elements aiming to obtain the shear stress intensity factor (KII). The parameters varied were (i) the nodule diameter (two different values were considered), (ii) the distance between the nodule and the tip of the crack and (iii) the position of the load relative to the tip of the crack. The results of the numerical simulations show that the smaller diameter nodule has a larger influence on KII, suggesting a higher contact fatigue crack propagation rate in the material with the smaller nodule. These results are the opposite of those observed in experimental studies and would appear to indicate that other factors should be also considered to ensure realistic estimates of the contact fatigue strength of ADI.

Revisiting the graphite nodule in ductile iron

The growth mechanism of graphite nodules in ductile iron was experimentally investigated using high-resolution 3D tomography of an individual graphite nodule in a near-eutectic ductile iron. The dual beam scanning electron microscopy (FIB-SEM) technique was used for this purpose. Iron particles elongated in the radial direction were observed inside a graphite nodule. Some micro-voids were detected inside the nodule, mostly located at the end of the iron particles. These observations were compared with established theories about the growth of graphite nodules and iron entrapment/engulfment in between the graphite sectors during solidification of ductile iron.

Effect of Silicon and Graphite Degeneration on High-Temperature Oxidation of Ductile Cast Irons in Open Air

The use of high silicon ductile irons is increasing as they offer some advantages with respect to conventional pearlitic–ferritic grades such as high elongation at rupture for a given tensile strength value and a fully ferritic matrix. Besides addressing mechanical requirements, some castings must fulfil corrosion and high-temperature oxidation requisites. Two different ductile cast irons with silicon contents of 2.04 and 5.21 wt% were used so as to comparatively study their mechanical properties and high-temperature oxidation responses. The structure of both alloys contains nodular graphite and a fully ferritic matrix. Some samples of the alloy with 5.21 wt% silicon also contain abundant degenerated graphite which was identified as Chunky graphite. Oxidation resistance of both materials was evaluated by exposures to air at 650 °C for 720 h using a tubular furnace. The alloy with 5.21 wt% silicon showed an oxidation resistance about three times higher than the low silicon alloy. Although both alloys showed similar oxidation mechanisms, the oxidation scale formed on the high-silicon alloy stands out for having lower thickness, higher silicon content in the metal/oxide interface and more compact and adherent layers. Samples with Chunky graphite showed a similar evolution to those with graphite nodules, so no negative effect from this graphite degeneration on oxidation process was observed. The analyses performed by XRD revealed the presence of fayalite in the 5.21 wt% Si alloy, which is responsible for the better oxidation resistance.

Nucleation and growth of graphite particles in ductile cast iron

Samples taken from three commercial runs of nodular cast iron, obtained from the same base metal, same nodularizing alloy, but different inoculants were studied. The percentages by weight of inoculant and nodularizing alloy were, respectively, 1.0% and 1.3%. In this way the nucleation and the growth of graphite nodules were studied by microstructural analysis using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). In this way, we observed that the growth sequence of the graphite nodules was associated to the formation of complex silicates rich in aluminum, calcium and barium around the graphite cores, generating an interface favorable to the heterogeneous graphite solidification from these particles of size larger than critical radius. Therefore, the race 1 sample inoculated with FeSi75% presented the lowest amount of nodules/mm2 and, consequently, less elongation, since there is a direct relation between the elongation and the number of nodules/mm2.

Effect of machine scatter on the rotating bending fatigue life of materials

AbstractRotating bending fatigue test results vary from specimen to specimen even if materials are tested in a standard laboratory set up because rotating bending fatigue life or fatigue strength depends upon various factors due to its dynamic action. One method of testing may be machine scatter (offset and angular misalignment). Angular misalignments are unavoidable in dynamic machines due to continuous operation leading to transverse vibrations in specimens. The magnitude of transverse vibrations was measured manually by dial gauge arrangement. Experiments were carried out until fracture either with smooth running conditions or with the effects of offset and angular misalignment. The enhanced effect of misalignments on fatigue life was done through the histogram normal distribution technique. There was a clear distinction between the appearances of the fracture surfaces between the specimens subjected to smooth running conditions and specimen vibration, which reveals general fatigue fracture phenomena and intergranular fatigue cracks, larger decohesion of graphite nodules and isolated cleavage facets. The manual technique used without any resistance strain gauges, as they were not efficiently sustainable under repetitive loads, helped in understanding the dynamic machine scatter on fatigue life in an economical way where there were lesser concentrations in these areas.

Automatic quantification of spheroidal graphite nodules using computer vision techniques

The microstructure of nodular cast irons is characterized by the presence of spheroidal graphite nodules. Metallographic tests may show irregular or degenerated graphite nodules that indicate a reduction in tensile strength of the material as well as its yield limit. This work proposes a computer vision algorithm to estimate the amount of degenerated graphite nodules as well as image analysis necessary to determine the relationship between this quantity of degenerated nodules and the loss of mechanical properties of the nodular cast iron. The proposed algorithm was tested using two cast iron samples, by measuring their microhardness and tensile strength. The results show that the amount of degenerated graphite nodules is inversely proportional to the limit of traction resistance. Sample “A”, of the two samples tested, presented more degenerated nodules than “B” and a lower limit of traction resistance; therefore, it needs less strength to break.

A cellular automaton-finite difference simulation of the ausferritic transformation in ductile iron

ABSTRACTThe development of the ausferritic transformation of a ductile iron was analysed using a novel cellular automaton-finite difference model, which considers geometrical details of the microstructure, nucleation of the new phase at graphite nodule surface, contact between growing phases, and carbon diffusion in austenite. The role of nucleation, austenite carbon enrichment, and contact between phases in the different stages of the growth kinetics was studied. Moreover, a parametric study was performed to investigate the influences of graphite nodule size, and austenitizing and austempering temperatures on the required time to end the transformation and final phase fractions. The obtained results are in agreement with experimental data reported in the literature.

Structure Characteristics of Silicon Alloyed Ductile Cast Iron

Experiments deal with the solidification pattern of FeSiCaMgRE-treated and un-inoculated and inoculated [Ca,Ce,S,O-FeSi] three ductile iron compositions [2.5%Si; 4%Si and 4%Si-1.6%Mo (Si:Mo = 2.5)]. Structure analysis is performed in 20mm diameter bar, resin sand mould solidification. Increasing of the Si content leads to increasing the graphite amount. Mo addition in high Si-iron limits the graphitizing effect of Si, but the graphite amount remains higher compared with conventional iron. A similar evolution is registered for graphite nodule count, while Si-Mo iron has lower nodule count compared with referred iron. For the same production conditions, graphite nodularity [according to ISO 945 evaluation] is negatively affected by Si and Si-Mo: it decreases from 95% up to 80% value, but remains at the accepted level, which means that the selected inoculant seems to be effective in these irons. For high Si-ductile irons, the nodularity evaluation must be more carefully considered, by including a minimum limit of the accepted sphericity shape factor. Increasing of the Si content leads to a prevalent ferritic matrix. Supplementary addition of Mo at the upper limit in Si-Mo affects the ferritic effect of Si, resulting an intermediary ferrite and pearlite mixture. It is found that inoculation is important for high-Si, and particularly for Si-Mo alloyed irons, requiring a high efficiency inoculation procedure.

Primary Structure and Graphite Nodules in Thin-Walled High-Nickel Ductile Iron Castings

This paper considers the most important quality factors in processing spheroidal graphite cast iron; namely, primary grains and graphite nodules in thin-walled ductile iron castings (TWDI). In the present study, the effect of grain refinement (by means of Ti, Nb and Zr) and of the holding time after spheroidization and inoculation on effecting the primary grains and eutectic structure in TWDI castings was investigated. Moreover, metallographic examinations (including electron backscattering diffraction, EBSD) were carried out to reveal the macro- and micro-structural features during the primary and eutectic solidification of the cast iron. EBSD results indicate that, within a single dendritic grain, there are numerous boundaries that split the grain into numerous smaller areas. In particular, it is found that the graphite nodules are in contact with the boundaries inside the primary dendritic grain. In turn, crystallization of highly branched dendrites is observed, which seems to “push” the graphite nodules into the interdendritic regions during their growth. The present work investigates the dominant mechanism that gives rise to the primary spheroidal graphite cast iron (SGI) structure. In addition, this work shows that the melt quality is closely associated with the resultant morphology and number of austenite dendrites, graphite nodules, and matrix structure.

Determination of the Effective Elastic Modulus for Nodular Cast Iron Using the Boundary Element Method

In this work, a multiscale homogenization procedure using the boundary element method (BEM) for modeling a two-dimensional (2D) and three-dimensional (3D) multiphase microstructure is presented. A numerical routine is specially written for modeling nodular cast iron (NCI) considering the graphite nodules as cylindrical and real geometries. The BEM is used as a numerical approach for solving the elastic problem of a representative volume element from a mean field model. Numerical models for NCI have generally been developed considering the graphite nodules as voids due to their soft feature. In this sense, three numerical models are developed, and the homogenization procedure is carried out considering the graphite nodules as non-voids. Experimental tensile, hardness, and microhardness tests are performed to determine the mechanical properties of the overall material, matrix, and inclusion nodules, respectively. The nodule sizes, distributions, and chemical compositions are determined by laser scanning microscopy, an X-ray computerized microtomography system (micro-CT), and energy-dispersive X-ray (EDX) spectroscopy, respectively. For the numerical model with real inclusions, the boundary mesh is obtained from micro-CT data. The effective properties obtained by considering the real and synthetic nodules’ geometries are compared with those obtained from the experimental work and the existing literature. The final results considering both approaches demonstrate a good agreement.

Characterizing Graphite Nodule in Ductile Iron using TKD and TEM

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

A New Approach to Modeling the Ductile Cast Iron Microstructure for a Finite Element Analysis

In this article, a methodology for the generation of geometric models representing the microstructure of a ductile cast iron (DCI) is presented. This methodology is based on a series of image processing algorithms to extract the graphite nodules contours and the utilization of the Bezier curves to smooth the geometric models curves. For the contours obtained by the image processing stage and generated geometric models, the circularity is calculated using the circular shape factor index, in order to analyze the induced error through the discretization process by the image processing stage and the variation of the circularity as the design parameters of the geometric modeling change. On the other hand, the design parameters effect of the geometric models on the stress behavior in the microstructure, through a finite element analysis, is also analyzed. It is shown that as the Bezier curve degree increases, the circularity of the geometric models decreases, thus increasing the maximum stresses produced in the DCI microstructure. Further, it is also found that the number of interpolation points has a significant effect on the mechanical properties when the Bezier curves degrees are equal to 10 and 12 than for the lower degrees.