Graphite Cast Iron Research Articles

A surface and sub-surface quality evaluation of three cast iron grades after grinding under various cutting conditions

Grinding is one of the most common finishing processes used in the manufacture of metal components that require a combination of both smooth surface finish and tight tolerances. Despite the abundant knowledge concerning this process, specific literature is still scarce regarding the grinding of different cast iron grades. These materials have a wide application in the automotive industry, notably in the manufacture of gears, crankshafts, and valve control shafts. In this sense, this paper presents an experimental study of the peripheral surface grinding of three grades of cast iron grades (gray, nodular, and compacted graphite) with two SiC abrasive grinding wheels. The input variables tested were two values for depth of cut (15 and 30 μm) and two worktable speeds (5 and 10 m/min). The output variables analyzed were surface roughness, microhardness, microstructures, and SEM images of the ground surfaces. The results showed that gray cast iron provided the best performance concerning surface and sub-surface integrity among the three cast iron grades tested, whereas the nodular cast iron exhibited both worst finishing and superficial texture. No microstructural changes were observed in the samples of gray and compacted graphite cast iron grades, irrespective of the cutting conditions investigated, unlike for the nodular cast iron grade in which microstructural change was detected. The ranking order for the grindability of the three cast iron grades in terms of roughness, microhardness and surface texture investigated in this paper is gray cast iron, compacted graphite iron and nodular cast iron.

Reassessment of Crystal Growth Theory of Graphite in Cast Iron

The problem of graphite crystallization and growth in cast iron has recently received increased attention. As most of the published literature describe analysis of room temperature graphite, there is a legitimate concern that the crystallization of graphite is concealed by recrystallization and growth in solid state occurring after solidification. To avoid confusion in the interpretation of room temperature graphite morphology, the authors used Field Emission Gun SEM on deep-etched interrupted solidification (quenched) specimens to reveal the morphology of graphite at the very beginning of solidification, when the graphite is in contact with the liquid. Information from related phenomena, such as crystallization of hexagonal structure snowflakes and metamorphic graphite, as well as of diamond cubic structure silicon crystals in aluminum alloys is incorporated in the analysis. Research discussing graphite produced through gas-solid and solid-solid transformations is also examined. Because the faceted growth of graphite is the result of diffusion-limited crystal growth in the presence of anisotropic surface energy and anisotropic attachment kinetics, a variety of solidification morphologies are found. The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets generated through the growth of graphene layers. As solidification advances, the platelets thicken through layer growth, and then aggregate through mechanisms that may include foliated/tiled-roof crystals and dendrites, curved-circumferential, cone-helix, helical, and columnar or conical sectors growth.

The Structure of Cast Irons

There appear to be two main growth mechanisms for graphite in cast iron i) Coupled eutectic growth forms of gray irons which are classical growth modes of simultaneous parallel growth of graphite and austenite, not reliant on a bifilm mechanism. These are necessarily fine structures as a result of their control by the rate of diffusion of carbon in the liquid. These structures are well understood. (ii) Uncoupled eutectic mechanisms which appear to be much less well understood, including (a) growth of graphite on silica bifilm substrates floating freely in the melt, forming such structures as A-type graphite flakes. This prediction appears to have now been confirmed by direct observation. The transition to (b) nodular morphology occurs by Mg eliminating the silica bifilms by an exchange reaction. In this way the substrates for flake growth are instantly removed, and graphite can now wrap completely around nuclei, thereby growing as a nodule. Graphite structures in heavy sections such as chunky graphite may now be understandable in terms of the reorganisation by flotation of bifilms and/or nuclei.

Effect of Preconditioning and Inoculation on Graphite Nodule Count and Their Size Distribution in Spheroidal Graphite (SG) Cast Iron: A Study to Minimise Rejection of Castings Due to Shrinkage Porosity

This paper attempts to investigate the effect of preconditioning and inoculation on nodule count and their size distribution to eliminate shrinkage porosity in spheroidal graphite (SG) iron castings. Studies were conducted on various heats of SG iron prepared for differential case casting with and without the addition of 0.1% Al, Zr, Ca–FeSi alloy as preconditioner and Ca–Ce–FeSi alloy was used as an inoculant. It was found that the combined effect of preconditioning and inoculation significantly improves nodule count and nodule size distribution which subsequently reduces the number of castings containing shrinkage porosity. The shrinkage quantity (Q) was found to reduce from 10 to 2.4%. Graphite nodule count and their size distribution were found to be important factors for controlling shrinkage porosity, and it was observed that preconditioning substantially improves the percentage of small size (5–15 µm diameter) graphite nodules. Higher nodule count reduces chances of shrinkage but simply having higher graphite nodule count of similar size does not help in minimising or elimination of shrinkage, it is necessary to have nodules of different sizes in a significant proportion.

Сучасні уявлення про структуроутворення у графітизованих чавунах (огляд)

Сучасні уявлення про структуроутворення у графітизованих чавунах (огляд)

Determination of pressure in the extradendritic liquid area during solidification

Complex-shaped lamellar graphite iron castings are susceptible to casting defects related to the volume change during solidification. The formations of these recurring defects are caused by the flow of the liquid in the intradendritic area, between the austenite dendrite arms, and in the extradendritic area between the austenite grains. The conditions for the liquid flow, in turn, are determined by the solidification behavior. The present study suggests a new measurement method and a novel calculation algorithm to determine the pressure of the extradendritic liquid during solidification. The method involves a spherical sample suspended in a measurement device, where the temperature and the volume changes are measured during solidification. The calculation algorithm is based on the numerical interpretation of the Clausius–Clapeyron equation where the temperature variation, the volume change and the released latent heat are processed to determine the local pressure of the extradendritic liquid area during solidification.

Effect of inoculation and nodularisation treatment temperature on recalescence and eutectic undercooling temperature in Spheroidal graphite (SG) cast iron

ABSTRACT In this study, the effect of inoculant composition and nodularisation treatment temperature on recalescence temperature (TER) and undercooling temperature (TEU) in SG iron has been studied by using thermal analysis. 0·1, 0·2 and 0·3 wt-% of three types of inoculants Ca,Ce,Al–FeSi, Ca,Sr,Al–FeSi and Ca,Ba,Al–FeSi were used as so called stream inoculation. Ca,Ce,Al–FeSi was found to be the most potent one in reducing both recalescence (∆T r ) and eutectic undercooling (∆T). The nodule count has also been found higher in Ca,Ce,Al–FeSi inoculated SG iron samples compared to other two inoculants. It has been observed that higher the nodule count lower is the ∆T r and ∆T and vice versa. The recalescence and shrinkage relationship of 24 heats of differential case castings has been established; it was observed that the number of castings containing shrinkage in respective heats increase as the recalescence and nodularisation treatment temperature increases.

Numerical modal analysis for vibration-damping properties of ductile cast

This paper deals with standard test method for the measurement of vibration-damping properties of materials, ductile cast iron - graphitic cast iron with the spherical shape of graphite (ductile cast iron). Investigation is focused on its mechanical properties, chemical composition and microstructure, which are necessary as input parameters for computational modeling and numerical analysis. The vibration or oscillating of experimental samples are closely connected with such parameters as natural shapes (eigenshapes) and natural frequencies (eigenfrequencies). Preparation of a geometric model of a homogeneous beam sample and numerical analysis was performed by finite element method in the ADINA software environment. This paper can be used for further development and growth from the aspect of application of computational tools in the area of damping and eigenfrequency properties for construction materials.

Mixed graphite cast iron for automotive exhaust component applications

Both spheroidal graphite iron and compacted graphite iron are used in the automotive industry. A recently proposed mixed graphite iron exhibits a microstructure between the conventional spheroidal graphite iron and compacted graphite iron. Evaluation results clearly indicate the suitability and benefits of mixed graphite iron for exhaust component applications with respect to casting, machining, mechanical, thermophysical, oxidation, and thermal fatigue properties. A new ASTM standard specification (A1095) has been created for compacted, mixed, and spheroidal graphite silicon-molybdenum iron castings. This paper attempts to outline the latest progress in mixed graphite iron published.

Quantitative Evaluation of the Shape of Graphite (Circularity) in Graphitic Cast Irons

Shape of graphite in graphitic cast irons is the most important microstructural parameter affecting mechanical properties. The paper deals with evaluation of the shape of graphite in cast irons by shape factor (circularity). Three specimens of graphitic cast irons with different shape of graphite (lamellar, vermicular and nodular) were used for experiments. The aim of this study has been to understand the influence of microstructure (especially shape of graphitic particles) on mechanical properties of graphitic cast irons. However, the evaluation of shape factor is a subject of interest in many fields of applications such as medicine and industrial processes, not only materials engineering. The shape factor plays an important role in materials science as a way to understand the relationship between microstructure and mechanical properties of technical materials.

Nonwaste Recycling of Nonferrous Metallurgy Slags in Iron Casting

It is proposed to use waste slags of copper, nickel and titanium production at mining and metallurgical enterprises for their full recycling in graphite iron castings as a charge component, as well as an active additive in ladle and furnace inoculation of iron melt.

Investigation of Dendrite Coarsening in Complex Shaped Lamellar Graphite Iron Castings

Shrinkage porosity and metal expansion penetration are two casting defects that appear frequently during the production of complex-shaped lamellar graphite iron components. These casting defects are formed during the solidification and usually form in the part of the casting which solidifies last. The position of the area that solidifies last is dependent on the thermal conditions. Test castings with thermal conditions like those existing in a complex-shaped casting were successfully applied to provoke a shrinkage porosity defect and a metal expansion penetration defect. The investigation of the primary dendrite morphology in the defected positions indicates a maximum intradendritic space, where the shrinkage porosity and metal expansion penetration defects appear. Moving away from the defect formation area, the intradendritic space decreases. A comparison of the intradendritic space with the simulated local solidification times indicates a strong relationship, which can be explained by the dynamic coarsening process. More specifically, long local solidification times facilitates the formation of a locally coarsened austenite morphology. This, in turn, enables the formation of a shrinkage porosity or a metal expansion penetration.

Effect of Copper on the Crystallization Process, Microstructure and Selected Properties of CGI

Abstract The paper presents the results of the research on the effect of copper on the crystallization process, microstructure and selected properties of the compacted graphite iron. Compacted graphite in cast iron was obtained using Inmold process. The study involved the cast iron containing copper at a concentration up to approximately 4%. The effect of copper on the temperature of the eutectic crystallization as well as the temperature of start and finish of the austenite transformation was given. It has been shown that copper increases the maximum temperature of the eutectic transformation approximately by 5°C per 1% Cu, and the temperature of the this transformation finish approximately by 8°C per 1% Cu. This element decreases the temperature of the austenite transformation start approximately by 5°C per 1% Cu, and the finish of this transformation approximately by 6°C per 1% Cu. It was found that in the microstructure of the compacted graphite iron containing about 3.8% Cu, there are still ferrite precipitations near the compacted graphite. The effect of copper on the hardness of cast iron and the pearlite microhardness was given. This stems from the high propensity to direct ferritization of this type of cast iron. It has been shown copper increases the hardness of compacted graphite iron both due to its pearlite forming action as well as because of the increase in the pearlite microhardness (up to approx. 3% Cu). The conducted studies have shown copper increases the hardness of the compacted graphite iron approximately by 35 HB per 1% Cu.

Influence of heat treatment on the wear life of hydraulic fracturing tools

Wear phenomenon has caused severe damage or failure of fracturing tools in oil and gas industry. In this paper, influence of heat treatment on the mechanical properties and wear resistance of fracturing tool made of lamellar graphite grey cast iron were investigated. The surface composition and microstructure were characterized by X-ray diffraction (XRD) and metallographic microscope. Sliding wear tests were performed to study the tribological behavior. Tests results showed that wear rates of treated specimens decreased by 33 %. Besides, worn morphology and wear debris were analyzed using Scanning electron microscope (SEM) and Energy dispersive X-ray spectra (EDS). Wear failure mechanisms of specimens were identified. Furthermore, on-site experiment results indicated that wear loss of treated samples decreased by 37.5 %. The wear life of hydraulic fracturing tools can be improved obviously by the heat treatment.

Modelling and simulation of local mechanical properties of high silicon solution-strengthened ferritic compacted graphite iron

This study focuses on the modelling and simulation of local mechanical properties of compacted graphite iron cast at different section thicknesses and three different levels of silicon, ranging from about 3.6% up to 4.6%. The relationship between tensile properties and microstructure is investigated using microstructural analysis and statistical evaluation. Models are generated using response surface methodology, which reveal that silicon level and nodularity mainly affect tensile strength and 0.2% offset yield strength, while Young′s modulus is primarily affected by nodularity. Increase in Si content improves both the yield and tensile strength, while reduces elongation to failure. Furthermore, mechanical properties enhance substantially in thinner section due to the high nodularity. The obtained models have been implemented into a casting process simulation, which enables prediction of local mechanical properties of castings with complex geometries. Very good agreement is observed between the measured and predicted microstructures and mechanical properties, particularly for thinner sections.

Physico-chemical causes of the formation of chunky graphite in austenitic ductile iron

Chunky graphite (CHG) is a well-known phenomenon which occurs in heavy section spheroidal graphite iron (SGI) castings or SGI castings with higher alloying content. Chunky graphite results in a decrease of mechanical properties, in particular a decrease in elongation. Despite numerous studies, the root cause of the CHG formation remains unidentified. In publications analyzing the different stages of CHG formation based on quenched samples, it is assumed that carbon diffusion through the austenite halo is inhibited due to the size of the eutectic grain. Furthermore, due to the carbon supersaturation CHG nucleates near the liquid-austenite interface of the eutectic grain. Additionally, other studies show that partly incomplete graphite spheres can be interconnected with CHG where no nucleation is required. Using thermodynamic-kinetic material simulation two morphologies, the decoupled SGI growth and the coupled CHG growth, were modeled to simulate their impact on the solidification kinetics. In the present study, it was shown for the first time that with increasing Ni content conditions arise in the austenite halos which strongly reduce the diffusive flux of carbon towards the graphite spheroid and thereby decelerate the kinetics of SGI growth. With a content above 5 wt% Ni, the competing kinetics of CHG growth exceeds the SGI growth. Therefore, the preference in kinetics and the present thermodynamic constraints are critical to the morphology change from SGI to CHG.

Effect of the Annealing Temperature on the Microstructure and Properties of Ausferritic Nodular Cast Iron

Abstract The paper presents the microstructure and selected properties of ausferritic nodular cast iron annealed at the temperature 520 and 550°C. This choice was dictated by the temperatures used in the practice of nitriding. Nodular graphite in cast iron was obtained with use of Inmold process. Cast iron containing molybdenum and copper ensuring obtaining an ausferrite in the cast iron matrix without the use of heat treatment of castings was tested. The effect of annealing temperature on the microstructure and the kind of fracture of the ausferritic nodular cast iron was presented. The effect of an annealing temperature on hardness, impact strength and the microhardness of ausferritic nodular cast iron matrix was shown too. The lamellar structure of phases in the cast iron matrix after annealing has been ascertained. There has been an increase in hardness of an annealed cast iron and microhardness of its matrix. The reduction in the impact strength of the cast iron annealed at 520 and 550°C was approximately 10-30%. Both an increase in the hardness of cast iron as well as an decrease in its impact strength is probably due to the separation of secondary carbides during the heat treatment.

Effect of Different Molding Materials on the Thin-Walled Compacted Graphite Iron Castings

This article addresses the effects of six mold materials used for obtaining thin-walled compacted graphite iron castings with a wall thickness of 3 mm. During this research, the following materials were analyzed: fine silica sand, coarse silica sand, cerabeads, molohite and also insulated materials in the shape of microspheres, including low-density alumina/silica ceramic sand. Granulometric and SEM observations indicate that the sand matrix used in these studies differs in terms of size, homogeneity and shape. This study shows that molds made with insulating sands (microspheres) possess both: thermal conductivity and material mold ability to absorb heat, on average to be more than five times lower compared to those of silica sand. In addition to that, the resultant peak of heat transfer coefficient at the mold/metal interface for microspheres is more than four times lower in comparison with fine silica sand. This is accompanied by a significant decrease in the cooling rate of metal in the mold cavity which promotes the development of compacted graphite in thin-walled castings as well as ferrite fractions in their microstructure.

Effect of Nanocarbon Additions on Graphitization and Tribological Properties of Gray Cast Iron

The expediency of using nanostructured carbon materials (graphene, fullerene black) as effective additives to a cast iron modifier is substantiated. Experimental results for preparation of high quality iron castings with nanocarbon additions in the presence of a combined modifier are confirmed by improvement of cast iron structural and tribological properties. The promising nature of using fullerene black (FB) as a comparatively cheap by-product of fullerene nanotechnology exhibiting improved tribological properties is demonstrated. The mechanism of graphitization of cast iron modified with FB-additive is revealed on the basis of existing studies and results of research in the field of nanomodification.

A new approach to calculate the nodule density of ductile cast iron graphite using a Level Set

This paper proposes a new approach with digital image processing techniques to determine the number of graphite nodules on samples of nodular cast iron by following the NBR 6913 standard. Counting errors and excessive inspection time are common problems if the procedure is performed without the assistance of computational systems. To overcome these drawbacks, we propose here an algorithm for graphite nodule segmentation based on the Level Set technique. The proposed approach and two other computational methods, Watershed and Region Growing, are compared with the results given by experts using optical microscopy (OM). The results of the proposed method were closer to those of the experts using OM than the other two computational methods. The proposed method presented greater accuracy and faster execution time than the traditional method by visual inspection.