Graphite Nodules Research Articles

Graphite Growth Morphologies in Cast Iron

Graphite growth morphology was studied by using InLense detector on FEG-SEM after performing ion etching on the samples. Star like and circumferential growth mechanism of graphite was observed in the graphite nodules. Pure ternary alloy of hypo eutectic and hyper eutectic composition was treated with pure Mg, Ca and Sr, to study the effect of O and S concentration in the melt, on the transition of graphite morphology from nodular to vermicular/compacted and flake graphite. The change in the melt composition between the austenite dendrites due to micro-segregation of S, O and inoculants and their possible effects on the transition of graphite morphologies as well as the nucleation of new oxides/sulfides particles is discussed with the help of thermodynamics.

Novel Approaches to Analyze Structure of Ductile Iron

The suggested approaches, including an automated SEM/EDX (Scanning Electron Microscopy/Energy Dispersive X-ray) analysis of graphite nodule nuclei and a special algorithm to convert two-dimensional to three-dimensional graphite nodule size distribution, were tested. The “soft” quenching technique was applied to develop small graphite nodules and increase probability to reveal non-metallic heterogeneous nuclei using automated SEM/EDX analysis. Ternary diagrams present experimental statistics of the graphite nuclei chemistry. A special algorithm for the conversion of diameters of two-dimensional sections of graphite nodules to the real three-dimensional distribution of spherical particles was developed. This algorithm is based on inverse simulation. The developed program calculates a three-dimensional nodule diameter distribution curve, the real average diameter, and volumetric nodule number. The examples of practical applications of these methods for spherical graphite characterization in different ductile iron castings are provided. Heterogeneous nucleation of graphite nodules is discussed based on the novel experimental data.

Effect of scanning speed during PTA remelting treatment on the microstructure and wear resistance of nodular cast iron

The surface of nodular cast iron (NCI) with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc (PTA) to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, microhardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization indicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidification. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3–3.1 times higher than the hardness of the substrate. The wear resistance of NCI was also significantly improved after the PTA remelting treatment.

Solution Strengthened Ferritic Ductile Cast Iron Properties, Production and Application

The ductile cast iron standard in Europe also considers new ductile cast iron grades with a unique combination of tensile strength with high elongation (e.g. Rm 72520 psi with A514%) achieved by the solution strengthening of the ferritic matrix by silicon with contents between 3% and 4.3%. The new grades tolerate a higher amount of pearlite and carbide stabilizing elements without the generation of embrittling pearlite or carbides in the microstructure. The presence of Cr, Mn, and V do not influence the elongation significantly. An advantage of the new cast irons is the cost efficient machining due to lower tool wear. The tool life with the machining of the fully ferritic grades GJS-500-14 and GJS-600-10 is about 50–60% longer in comparison to the ferritic-pearlitic grades. The investigated grades show very good cyclic mechanical properties but lower dynamic properties. The pre-condition to achieve optimized properties by well-shaped graphite nodules, achievable with special inoculation techniques, adjusted to the high silicon content and the solidification rate, otherwise threaten graphite degenerations. There is no increased danger of dross generation or of a worsening flowability (investigated with special test equipment). Also the supposed inclination to the generation of porosities with the higher Si content was not confirmed.

Effect of Section Thickness on the Microstructure and Hardness of Ductile Cast Iron

Abstract Ductile cast irons have several engineering and manufacturing advantages compared to other cast materials such as steels. These include high damping capacity, wear resistance, to and lower manufacturing cost. The combination of good mechanical properties and casting abilities of ductile cast iron makes its usage successful in structural applications especially in the energy and automotive industries. In this study, the influence of the section thickness on the microstructure and hardness of spheroidal graphite cast iron was investigated with a cascaded model using the full mold casting method. By the high linear correlation coefficient, R2 > 0.90, it was observed that the section thickness directly affected the microstructure and hardness. An increase in the cross-sectional thickness decreases hardness and number of graphite nodules per square millimeters, however, percentage of ferrite was increased.

Influence of silicon and addition elements on the mechanical behavior of ferritic ductile cast iron

An experimental study of mechanical properties and microstructural analysis was carried out on different compositions of ferritic spheroidal graphite cast iron. The influence of silicon on the ductility and strength of the material was noticed. The observation of microstructure and failure modes allowed us to explain the important role of chemical composition on the initiation and propagation of cracks. The high silicon segregation between the graphite nodules and the joints of eutectic cells decreases the mechanical material resistance. A metallurgical development was indispensable and necessary to ensure characteristics conformed to the industrial requirements.

Damaging Micromechanisms Characterization in Pearlitic Ductile Cast Irons

Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grains distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Considering ferritic DCIs, ferritic matrix – graphite nodule debonding is only seldom observed and the main ferritic DCI damaging micromechanism consists in cracks nucleation and propagation in the graphite external shell coming from the reduced carbon solubility in γ phase during cooling process (“onion-like” mechanism). A second damaging micromechanism is sometimes observed together with the “onion-like” mechanism and consists in cracks nucleation and propagation corresponding to the graphite nodule center (“disgregation” mechanism). The importance of this damaging mechanism is dependent on the loading conditions and is probably influenced by the graphite nodule nucleation mechanism during solidification.In this work, the damaging micromechanisms in a pearlitic DCI have been investigated by means of the Scanning Electron Microscope (SEM) observation of the specimen lateral surface during the tensile tests (“in situ” tests). Damaging micromechanisms development has been analyzed, focusing both the role of the graphite nodules and the cracks initiation and propagation in the pearlitic matrix.

球状黒鉛鋳鉄の硫酸水溶液中における腐食挙動に及ぼす黒鉛と基地の境界長の影響

球状黒鉛鋳鉄の硫酸水溶液中における腐食挙動に及ぼす黒鉛と基地の境界長の影響

Effect of silicon content and defects on the lifetime of ductile cast iron

In this work, the influence of microstructure on the mechanical properties has been studied for different grades of ferritic ductile cast iron. Mechanical tests were carried out and the effect of silicon on the resistance of material was well noticed. An increasing silicon content increases the strength and decreases the ductility of material. The lifetime and endurance limit of material were affected by the presence of defects in material and microstructure heterogeneity. Metallurgical characterizations showed that the silicon was highly segregated around graphite nodules which leads to the initiation of cracks. The presence of defects causes the stress concentration and leads to the initiation and propagation of cracks.

Modelling the Hardness Property of Produced Martempered Ductile Iron Through Interrupted Quenching Method

In this research, modelling the hardness property of produced Martempered Ductile Iron (MDI) through interrupted quenching method was studied. Martempered ductile iron was produced by the adoption of interrupted quenching in warm water (cheap, convenient and safe to operate – compared to the conventional method in salt bath furnace). The product (MDI) was found to possess unique high hardness values of 52.9Rc at a low tempering temperature and holding time having a predominantly martensite, graphite nodules enveloped with ferrite shells in its microstructures. A computer model was developed to predict the hardness values of the developed MDI and a model equation, Y = 62.504 – 0.079T ( o C) – 0.054t (minutes) was established, with a desirable degree of correlation and reliability for the hardness characterization. The model was tested using statistical tools and manual, and it was found to compare favourably well with the experimental results.

Effect of Amount of Products Suspended in Melt after Spheroidizing Treatment on Graphite Nodule Count in Spheroidal Graphite Cast Iron

Effect of Amount of Products Suspended in Melt after Spheroidizing Treatment on Graphite Nodule Count in Spheroidal Graphite Cast Iron

Effect of Number of Graphite Nodules on Fatigue Limit and Fracture Origins in Heavy Section Spheroidal Graphite Cast Iron

Effect of Number of Graphite Nodules on Fatigue Limit and Fracture Origins in Heavy Section Spheroidal Graphite Cast Iron

Undercooling, Cooling Curves and Nodule Count for Near-eutectic Thin-walled Ductile Iron Castings

A solidification model for ductile iron, including Weibull formula for nodule count, has been presented. The principal assumptions of the kinetic nature of growth, depending on undercooling in respect of the eutectic equilibrium temperature and austenite liquidus line, have been adopted, disregarding the diffusion processes, which was justified by the rapid course of the crystallization process in a thin-walled casting. From this model, the following parameters can be determined: cooling curves, kinetics of austenite and eutectic nucleation, austenite and eutectic growth velocity, and volume fraction.The correctness of the mathematical model has been experimentally verified by comparison with literature data in the range of the most significant factors, which include temperature field, the value of maximum undercooling, the value of minimum and maximum temperature on a cooling curve, maximum undercooling, the recalescence and graphite nodule count interrelated with the casting cross-section.

Experimental and numerical analysis of effect of cooling rate on thermal–microstructural response of spheroidal graphite cast iron solidification

This work presents an experimental and numerical study of the solidification process of an eutectic spheroidal graphite cast iron (SGI). The effect of the cooling rate on the thermal–microstructural response is particularly analysed. To this end, experiments as well as numerical simulations were carried out. The experiments consisted in a solidification test in a wedge-like casting such that different cooling rates were measured at specific positions along the part. A metallographic analysis was also performed in five locations of the sample with the aim of obtaining the number and size of graphite nodules at the end of the process. The numerical simulations were made using multinodular based and uninodular based models. These two models predicted similar results in terms of cooling curves and nodule counts. Besides, good experimental–numerical agreements were obtained for both the cooling curves and the graphite nodule counts.

Grain count in castings: theoretical background and experimental verification

In this work, a theoretical model is proposed for heterogeneous nucleation on substrates whose size distributions can be described by the Weibull statistics. In particular, the proposed model suggests that the size distribution for the various nucleation sites is exponential in nature. Measurements of grain count were carried out on experimental Al–1·3Si and Al–5·0Cu single phase alloys inoculated using an Al–5Ti–1B master alloy. In addition, experimental nodular and flake graphite iron castings were processed under various metallurgical conditions. In single phase alloys, the area of the equiaxed dendritic grain count was estimated from the electron backscattering diffraction analysis, whereas the graphite nodule and graphite eutectic cell count were estimated on polished cast iron surface sections by stereological means. In addition, maximum undercoolings were determined by thermal analysis. The experimental outcome indicates that the grain count can be properly described by a proposed exponential function of the maximum undercooling at the onset of alloy solidification. Finally, the magnitudes of the nucleation parameters were experimentally determined in this work.

Macro and microscopical approach to the damaging micromechanisms analysis in a ferritic ductile cast iron

The influence of the graphite nodules morphology (shape, dimension and distribution) on ductile cast irons (DCIs) mechanical properties is experimentally confirmed both in static, quasi static and cyclic loading conditions. According to the most recent results, these graphite elements cannot be merely considered as “microvoids embedded in a metal matrix”, but their presence implies a modification of the damaging micromechanisms, depending on the metal matrix microstructure.In this work, a fully ferritic DCI has been investigated: tensile tests have been performed on notched mini tensile specimens. Specimens lateral surfaces have been observed by means of a Scanning Electron Microscope (SEM) during the tests (“in situ” tests) at different magnifications. Considering the results obtained at low magnification, quantitative image analysis has been applied in order to investigate the triaxiality evolution during the test.

Microstructure feature of friction stir butt-welded ferritic ductile iron

This study conducted friction stir welding (FSW) by using the butt welding process to join ferritic ductile iron plates and investigated the variations of microsturcture in the joined region formed after welding. No defects appeared in the resulting experimental weld, which was formed using a 3-mm thick ductile iron plate and tungsten carbide alloy stir rod to conduct FSW at a rotational speed of 982rpm and traveling speed of 72mm/min. The welding region was composed of deformed graphite, martensite phase, and dynamically recrystallized ferrite structures. In the surface region and on the advancing side (AS), the graphite displayed a striped configuration and the ferritic matrix transformed into martensite. On the retreating side (RS), the graphite surrounded by martensite remained as individual granules and the matrix primarily comprised dynamically recrystallized ferrite. After welding, diffusion increased the carbon content of the austenite around the deformed graphite nodules, which transformed into martensite during the subsequent cooling process. A micro Vickers hardness test showed that the maximum hardness value of the martensite structures in the weld was approximately 800HV. An analysis using an electron probe X-ray microanalyzer (EPMA) indicated that its carbon content was approximately 0.7–1.4%. The peak temperature on the RS, 8mm from the center of the weld, measured 630°C by the thermocouple. Overall, increased severity of plastic deformation and process temperature near the upper stir zone (SZ) resulted in distinct phase transformation. Furthermore, the degree of plastic deformation on the AS was significantly greater than that on the RS, and relatively complete graphite granules and the fine ferrite grains resulting from dynamic recrystallization were observed on the RS.

A FE based machining simulation methodology accounting for cast iron microstructure

In the present paper the effect of graphite nodularity in the microstructure of cast iron is assessed with respect to its machinability at orthogonal cutting. A micro-structure based model of the cast iron material has been developed based on analyses of micro-graph images. The image analysis combines pearlitic grains with graphite nodules to produce the micro-structures. Continuous deformation behavior of pearlite and graphite phases is described using the Johnson–Cook (JC) viscoplasticty model including temperature dependence. Of particular concern is the modeling of the chip formation, induced by the micro-structure morphology and the mechanical behavior of the work piece material in the vicinity of the insert. This is described by the element deletion methodology along with the Johnson–Cook failure criterion to realize the fracture kinematics. A parametric study is carried out by varying the nodularity of the graphite. To validate the model, the finite element results are compared to machining test results carried out on a Compacted Graphite Iron (CGI) material in a related paper. It turns out that the FE-simulated and experimentally observed cutting force and chip formation compare fairly well.

Ductility Loss in Ductile Cast Iron with Internal Hydrogen

Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone, and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile because of the accompanying interconnecting cracks between the adjacent graphite nodules. Inside the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss.

Rolling contact fatigue resistance of PVD CrN and TiN coated austempered ductile iron

This work studies the rolling contact fatigue (RCF) behavior of PVD CrN and TiN coated ADI samples. The effect of the substrate surface finishing method (grinding and polishing) and coating material on RCF life is also evaluated. Coatings were applied in an industrial reactor. Deposition times were adjusted to obtain similar coating thicknesses in both coating materials. RCF tests were performed in a flat washer type testing rig, using a thrust ball bearing as a counterpart. The maximum contact pressure (p0) was set at 1000 and 1400MPa.The results obtained indicate that the RCF tests performed at p0=1000MPa cause no failures on the samples analyzed, while those performed at p0=1400MPa cause failures in both uncoated and coated samples. Failures in coated samples are mainly characterized by coating delamination and, to a lesser extent, by spall formation in the substrate. Graphite nodules present on the substrate surface act as preferential sites for coating fracture and subsequent delamination. The RCF resistance of coated samples is lower than that of uncoated samples regardless of the substrate surface finishing method employed. The surface hardening produced by the abrasive cutting of grinding improves the RCF resistance of the uncoated and coated samples.