Graphite Cast Iron Research Articles

Guarantee of Mechanical Properties in Ferritic Spheroidal Graphite Iron Castings with Heavy Section

Guarantee of Mechanical Properties in Ferritic Spheroidal Graphite Iron Castings with Heavy Section

Eutectic cell and nodule count in cast irons

In the present work the predictions based on a theoretical analysis aimed at elucidating of eutectic cell count or nodule count N were experimentally verified. The experimental work was focused on processing flake graphite and ductile cast irons under various inoculation conditions in order to achieve various physicochemical states of the experimental melts. In addition, plates of various wall thicknesses s, were cast and the resultant eutectic cell or nodule counts were established. Moreover, thermal analysis was used to find out the degree of maximum undercooling for the graphite eutectic ΔTm. A relationship was found between the eutectic cell or nodule count and the maximum undercooling ΔTm. In addition, it was also found that N can be related to the wall thickness of plate shaped castings. Finally, the present work provides a rationale for the effect of technological factors such as the melt chemistry, inoculation practice, and holding temperature and time on the resultant cell count or nodule count of cast iron. In particular, good agreement was found between the predictions of the theoretical analysis and the experimental data.

Eutectic cell and nodule count in grey and nodular cast irons

In the present work the predictions of a proposed analytical theory were compared with the experimental outcome on metallographic determinations of eutectic cell count N and nodule count Nn. Accordingly, processing of flake graphite and ductile cast irons was carried out under various inoculation conditions in order to achieve a range of physicochemical states for the experimental melts. In addition, plates of various wall thicknesses s were cast and the resultant eutectic cell and nodule counts were established. Moreover, thermal analysis was employed for determinations of the maximum degree of undercooling for the graphite eutectic ΔTm. A relationship was found between the eutectic cell N or the nodule count and ΔTm. In addition, it was found that N and Nn can be related to the casting modulus M and hence to s in plate shaped castings. Finally, the present work provides insights into the effect of technological factors such as the melt chemistry, the inoculation practice and the holding temperature and time on the resultant N and Nn values. In general, it was found that the predictions of the present analysis are in good agreement with the experimental outcome.

Analytical investigations concerning the wear behaviour of cutting tools used for the machining of compacted graphite iron and grey cast iron

Compacted graphite iron (CGI) is the material for the upcoming new generation of high-power diesel engines. Due to its increased strength compared to grey cast iron (CI) it allows an increase in the cylinder-pressures and therefore a better fuel economy and a higher power output are possible. First examples of such engines are the 3.3 L Audi V8 TDI and the 4.0 L BMW V8. The reason why CGI is not used to a larger extent in large scale production up to now is its much more difficult machinability as compared to conventional CI, especially at high cutting speeds. In modern transfer lines high cutting speeds are used in the cylinder-boring operation. And especially in these continuous cutting operations the tool life decreased due to the change from CI to CGI by about a factor of 20. As was found out previously by us, the difference in tool lifetime can be explained by the formation of a MnS-layer on the tool surface in the case of CI. This layer cannot form when machining CGI because the formation of MnS-inclusions is not possible in this material due to the higher magnesium content which in turn is responsible for the formation of the graphite vermicles. The MnS-layer acts as a lubricant and prevents the adhesion of workpiece particles. This is the reason for the greatly reduced wear of CI in high speed machining operations. This MnS-layer is inspected closer by X-ray diffraction, X-ray induced photoelectron spectrometry, atomic force microscopy and secondary ion mass spectrometry in this work. Furthermore, available information on the performance of MnS as lubricant in PM-steels is comparatively discussed. This knowledge led to an economic solution of high productivity machining of CGI. The key was to reduce the cutting speed, replacing single insert tools with multiple insert tools. This allowed to increase the feed rate. By increasing the feed rate in the same amount as decreasing the cutting speed, the same productivity can be realized. This concept is leading to a number of multiple insert tools thus realizing a high productivity machining of CGI cylinder-bores with multi-layer-coated carbide tools.

Simulation of feeding flow and prediction of macro porosity of SGI casting based on microstructure formation

Based on the model of microstructure formation of spheroidal graphite iron (SGI) casting which includes continuous nucleation and growth of graphite nodules, carbides and austenite, a method was proposed to calculate the feeding flow during solidification processes by considering mass conservation and using Darcy's laws. The method takes into account liquid shrinkage as well as shrinkage and expansion caused by different phase formation, and can calculate the flow in liquid and mushy regions. According to calculation of the flow, macro porosity formation could be described through treating free surfaces and boundary of the regions. The proposed method has been applied to an automotive wheel hub SGI casting with two different processing cases. The feeding flow during the casting solidification with two cases, especially in isolated regions, was investigated and discussed. With the flow calculation, expansion or shrinkage of liquid or mushy regions might be judged, which could offer important information for evaluating formation of macro porosity. The results of macro porosity prediction were compared with results of practical castings. It is indicated that two results were in quite good agreement, and the method could predict macro porosity formation more accurately.

Graphite Nodule and Eutectic Cell Count in Cast Iron: Theoretical Model Based on Weibull Statistics and Experimental Verification

In this work, a model is proposed for heterogeneous nucleation on substrates whose size distribution can be described by the Weibull statistics. It is found that the nuclei density, N nuc can be given in terms of the maximum undercooling, ΔT m , by N nuc = N s exp (–b/ΔT m ), where N s is the density of nucleation sites in the melt and b is the nucleation coefficient (b > 0). When nucleation occurs on all possible substrates, the graphite nodule density, N V,n , or eutectic cell density, N V , after solidification equals N s . In this work, measurements of N V,n and N V values were carried out on experimental nodular and flake graphite iron castings processed under various inoculation conditions. The volumetric nodule N V,n or graphite eutectic cell N V count was estimated from the area nodule count, N A,n , or eutectic cell count, N A , on polished cast iron surface sections by stereological means. In addition, maximum undercoolings, ΔT m , were measured using thermal analysis. The experimental outcome indicates that the N V,n or N V count can be properly described by the proposed expression N V,n = N V = N s exp (–b/ΔT m ). Moreover, the N s and b values were experimentally determined. In particular, the proposed model suggests that the size distribution of nucleation sites is exponential in nature.

Cover Picture: 3D Characterization of Graphite Morphologies in Cast Iron (Adv. Eng. Mater. 1‐2/2007)

AbstractThe cover picture shows the reconstructed 3D images acquired using FIB‐tomography of flake graphite in cast iron kindly provided by Halberg Guss GmbH. More information about the 3D characterisation of different irregular graphite morphologies can be found in the paper by A. Velichko et al. on page 39 of this issue.Design: Thomas Jörn

Development of Production Method of CV Graphite Iron Castings for High Power Diesel Engine Block

Development of Production Method of CV Graphite Iron Castings for High Power Diesel Engine Block

Contribution of dual phase (α, G, B) heat treatments to fatigue properties of SG cast irons

Ferritic spheroidal graphite (SG) cast irons are used to make parts that require high ductility and fracture toughness and good high-cycle fatigue resistance. Ferritising anneals have an all-ferritic matrix favouring high fracture toughness and ductility but this does not improve fatigue strength, and even lowers it in some cases. The purpose of the present study was therefore to optimise these heat treatments for the best compromise between fracture toughness and fatigue life. To attain this, the fatigue behaviour of cast irons was studied in the as cast and heat-treated conditions. A comparison of the lives and mechanisms of ferritisation of these metallurgical conditions made it possible to evaluate the influence of various parameters (characteristics of the initiation sites, properties of the matrix). From the present study, a method is proposed to optimise the heat treatments to produce ferrite in association with a stronger phase in the zones where microfissures are initiated. A study of heat treatments favouring the formation of a matrix made up of ferrite and bainite yielded encouraging results. The presence of bainite near the graphite spheroids and zones of porosity can increase resistance to crack initiation. This leads to a substantial improvement of fatigue life of the SG cast iron without at the same time embrittling the material.

Automatic Classification of Graphite in Cast Iron

A method for automatic classification of the shape of graphite particles in cast iron is proposed. In a typical supervised classification procedure, the standard charts from the ISO-945 standard are used as a training and validation population. Several shape and size parameters are described and used as discriminants. A new parameter, the average internal angle, is proposed and is shown to be relevant for accurate classification. The ideal parameter sets are determined, leading to validation success rates above 90%. The classifier is then applied to real cast iron samples and provides results that are consistent with visual examination. The method provides classification results per particle, different from the traditional per field chart comparison methods. The full procedure can run automatically without user interference.

Perspectives on research on the formation of nodular graphite in cast iron

Current views on the mechanism of the formation of nodular graphite in cast iron are examined with respect to their explanatory power. The aim of this review is to find, if possible, a rationale for further studies. Arguments are presented in favour of a comprehensive model, encompassing the characteristics of both the nucleation and the growth of the graphite. A plea is made for focusing on the most salient features rather than on phenomenological aspects.

Quantification and modelling of 2,4-dinitrotoluene reduction with high-purity and cast iron

Cast iron has been used as a reactive material in permeable reactive barriers (PRBs) for site remediation. While reactions are generally believed to occur on the iron (oxide) surface, a recent study by [Oh, S.Y., Cha, D.K., Chiu, P.C., 2002a. Graphite-mediated reduction of 2,4-dinitrotoluene with elemental iron. Environ. Sci. Technol. 36 (10), 2178–2184] showed that graphite inclusions in cast iron can also serve as reaction sites for 2,4-dinitrotoluene (DNT). These authors also found that graphite-mediated reduction of DNT has a regioselectivity that is different from that for iron surface. In this study, we quantified the observations reported by Oh et al. and examined the role of graphite in cast iron through numerical modelling. Models containing one and two reaction sites were developed to evaluate the mass transfer, sorption and reaction rates for DNT reduction in batch systems containing high-purity and cast iron. Our simulations showed that the regioselectivity, defined as the ratio of the ortho- and para-nitro reduction rate constants, was 0.37±0.04 S.E. (S.E.=one estimated standard error) for iron surface and 3.59±0.76 S.E. for graphite surface. In the cast iron–water system, we estimated that at least 66±2% S.E. of the DNT was reduced on graphite surface, despite the low graphite content and the lower DNT reduction rate with graphite than with iron. Graphite played such an important role because of the rapid adsorption of DNT to graphite. In the batch experiments conducted by Oh et al., external mass transfer was not rate limiting. Surface reaction was the rate-limiting step for DNT reduction on the graphite surface in cast iron, whereas internal mass transfer and/or adsorption and surface reaction were important for high-purity iron.

The role of graphite morphology and matrix structure on low frequency thermal cycling of cast irons

Low frequency thermal cycling tests were carried out on four types of cast iron (viz., austempered ductile iron, pearlitic ductile iron, compacted/vermicular graphite iron and grey cast iron) at predetermined ranges of thermal cycling temperatures. The specimens were unconstrained.Results show that austempered ductile iron has the highest thermal cycling resistance, followed by pearlitic ductile iron and compacted graphite iron, while grey cast iron exhibits the lowest resistance. Microstructural analysis of test specimens subjected to thermal cycling indicates that matrix decomposition and grain growth are responsible for the reduction in hardness while graphite oxidation, de-cohesion and grain boundary separation are responsible for the reduction in the modulus of elasticity upon thermal cycling.

Modelling of microstructure and property distribution within industrial nodular graphite iron casting

A kinetic model that uses the internal state variable approach has been developed and implemented as a user-subroutine in a finite element model (FEM) program in order to relate the heat flow calculations to the micro-structure evolution within a complex shaped industrial nodular graphite iron casting. The model initially requires the input of the chemical composition and from there it is coupled with the heat flow calculations from which the evolution of the constituent phases are calculated utilising the constraint conditions imposed by the phase diagram and the kinetic equations. A further sophistication of the model involves the use of a separate set of response equations to relate the microstructure and the composition to the macro-mechanical properties, such as hardness, yield and tensile strength. In a calibrated form, the process model is capable of describing the microstructure evolution during primary and eutectic solidification as well as the subsequent solid-state transformations. IJCMR/368

Structural and operating aspects of rapid solidification of surface layer of spheroidal graphite iron castings

The relationship between the surface layer microstructure of iron castings after gas tungsten arc weld (GTAW) surfacing and their service properties (friction wear resistance and microhardness) has been investigated. A structural analysis of the fused (remelted) zone, in particular evaluation of the interphase spacing in the cementite eutectic λav, is presented. Relationships between the GTAW process parameters, microstructure and microhardness of the fused zone have been developed and are presented. Formulae relating hardness to λαv and relating friction wear rate to λav and hardness are presented. IJCMR/426

Prediction of porosity defect in spheroidal graphite iron castings

Although the volume expansion due to graphite formation in spheroidal graphite (SG) cast iron greatly affects the porosity defects, conventional methods Tor predicting the defects don't directly consider it. In this paper we propose a new method considering the effect in order to predict more accurately the porosity defects, in particular, the porosity defect caused by casting volume change. In this method, the graphite and austenite volume changes during solidification are calculated under the assumption of one nucleation event using experimental data and quasi-steady state diffusion of carbon through the austenite shell. An equivalent expansion coefficient change is calculated from the volumes of graphite and austenite for each mesh, and used to estimate the deformation and stress distribution. For the stress analysis the voxel method is used to save the computational time and memory. This method suggested that the main mechanism of the porosity formation is the larger expansion in the surface region than in the inner region of casting due to temperature difference, causing tensile stress or negative pressure in the inner region. The comparison of the simulated and experimental results showed that the proposed method is promising for practical estimation of the porosity defect in SG cast iron castings.

Cast iron melting and solidification studies by advanced thermal analysis

Thermal analysis in foundry is performed by measuring the melt temperature during the free cooling in cups. Information on liquidus, eutectic, solidus temperatures and recalescence effects is collected. Advanced analyses in the laboratory use differential devices where a temperature program is imposed to both the sample and an inert reference. The sensitivity in the detection of temperature points is enhanced and quantitative determination of the heat release becomes feasible.In this work we have employed a high temperature differential scanning calorimeter to follow melting, solidification and eutectoid transformations of master alloys for the production of lamellar, compact graphite and spheroidal cast irons under protective helium atmosphere.A high silicon ductile iron alloyed with 2% wt. Al, devised for application in engine components, was analysed as well.The correlation between thermophysical data obtained by calorimetry, free cooling in cups and microstructure is shown.

Simulation of microstructure formation of spheroidal graphite iron casting

In this paper, the mathematical model of microstructure formation as well as gray- to-white transition of spheroidal graphite cast iron was established, and the microsegregation of alloy elements in both stable and metastable eutectics was considered as well. The simulation and quantitative experimental analyses of a step-shaped casting were carried out. The comparison of the simulation and experiment results indicated that they agreed quite well on nodule counts, and that the growth of white eutectics and carbide distribution could also be predicted by solidification simulation. Moreover, the simulation of a real brake housing SG iron casting was verified with the experiment results, and they were in good agreement not only on nodule counts but also on pearlite content and hardness.

Macro porosity prediction of SG iron castings considering microstructure formation

Some macro- and micro- phenomena and their interaction during solidification of spheroidal graphite (SG) iron castings, including the nucleation and growth of microstructure, gray-to-white transition, latent heat and volume change from various phase formation, and so on, were considered in the porosity prediction of the castings. With modeling microstructure formation, the volume expansion and contraction resulted from temperature drop and phase formation of graphite, austenite, and carbide were calculated. Based on the judgment of the hot spots and feeding relationship, a method to predict macro porosity of the castings has been developed and has been applied to a real hub and brake housing SG iron castings. The results were verified with the production, and compared with commercial softwares. The process of the hub casting was improved as well. It seems that the proposed method is applicable for predicting macro porosity of real SG iron castings.

Wear behaviour of graphitic aluminium composite sliding under dry conditions

Abstract A modified mixing and casting procedure was used for the preparation of Al-1.2 Si-0.8 Fe–Gr composite (wt.%). Graphite and gray cast iron powders were blended and dispersed in Al-1.2 Si matrix alloy melt and poured in a cast iron mould. Gray cast iron powder worked as carrier for graphite particles. The presence of graphite improves the ultimate tensile strength, tensile and compressive stress but beyond 1 % of graphite all properties deteriorate. However, hardness continuously increases and percentage elongation reduces with graphite. In wear studies, different mechanisms: mild-oxidative, oxidative-metallic and severe-metallic, are operative in the different combinations of sliding velocities and applied loads.