Ferritic Cast Iron Research Articles

Technological issues of manufacturing critical castings for the nuclear industry from ferritic cast iron with spherical graphite

The article describes technological solutions for casting blanks for containers for transportation of nuclear waste. As examples, calculations of external refrigerators are proposed to solve the issues of directional crystallization of the casting. Calculation of optimization of shrinkage processes is given. Specifically, the actual example shows a picture of the effect of shrinkage during the casting and cooling of the workpiece on the quality and density of the resulting casting material.

Effect of Electric Pulse Treatment on Hot-Dip Aluminizing Coating of Ductile Iron.

In this paper, hot-dip aluminizing of ferrite nodular cast iron was carried out after treating liquid aluminum with different electrical pulse parameters. Compared with that of conventional hot-dip aluminizing, the coating structure of the treated sample did not change, the surface was smooth and continuous, and the solidification structure was more uniform. When high voltage and large capacitance were used to treat the liquid aluminum, the thickness and compactness of the coating surface layer increased. The thickness of the alloy layer decreased, and, the compactness and the micro hardness increased, so the electric pulse had a certain inhibition on the formation of the alloy layer. The growth kinetics of the alloy layer showed that the rate-time index decreased from 0.60 for the conventional sample to 0.38 for the electric pulse treated sample. The growth of the alloy layer was controlled by diffusion and interface reaction, but only by diffusion. The AC impedance and polarization curves of the coating showed that the corrosion resistance of hot-dip coating on nodular cast iron was improved by electric pulse treatment.

Interaction of N with White-solidified Cast Iron Model Alloys: The Effect of Mn and Cu on the Formation of Fe and Si Nitrides

Surface remelting and subsequent nitriding improves the surface properties of cast irons. Upon remelting, a white-solidified surface layer forms, which contains coarse Si-free eutectic cementite (θ) and Si-enriched ferrite, pearlite or martensite in the intercarbidic regions between the eutectic θ. Nitriding produces a compound layer at the surface, which is composed of ε and γ’-iron (carbo)nitrides and enhances the corrosion resistance. Nitriding of white-solidified Fe-C-Si alloys, being model materials for remelted low-alloy ferritic cast irons, has shown that Si dissolved in α-Fe notably affects the formation of ε and γ’ in intercarbidic regions while Si simultaneously precipitates as amorphous nitride, X. Under process conditions only allowing for the formation of γ’ in pure Fe, Si dissolved in α-Fe promotes the formation of ε over the formation γ’, whereas Si-free eutectic θ transforms into nitride following the sequence θ → ε → γ’. The present work studies the nitriding of white-solidified Fe-3.5wt.%C-3wt.%-M alloys with additions of M = 1 wt.% Mn, 1 wt.% Cu or 1 wt.% Mn + 1 wt.% Cu, serving as model materials for remelted pearlitic cast irons. The presence of Mn and/or Cu causes notable deviations from the nitriding behavior known from Fe-C-Si alloys. Mn accelerates the precipitation of X in intercarbidic regions and obstructs the transformation of ε formed from Si-free θ into γ’. Cu promotes the formation of γ’ in Si-rich intercarbidic regions, surpassing the ε-promoting effect of Si.

Fatigue crack analysis of ferrite material by acoustic emission technique

Recently in various fields, numerous researches are going on for the assessment of material damage on the basis of crack initiation and propagation. Various methods are available in NDT for this purpose, among which analysis using released acoustic emission (AE) waves due to crack propagation is very effective due to its dynamic monitoring features. Various approaches are proposed for long time to make it an ideal method for accurate monitoring of crack behaviors in materials. In fragmentation theory there are some proportionality among the relations of AE event, AE energy, area and volume of cracks etc., which are calculated from the released AE waves from any dynamic crack. It has been found that the necessity of calculating the fractal dimension is important in verifying these relationships. This parameter is emphasized for determining the geometry of the irregularity in crack surface and crack volume. In this paper a novel approach based on image processing is proposed to find out the fractal dimension for analyzing the crack propagation characteristics. Finally, the proportionality relationships of AE parameters with crack propagation behavior in ferrite cast iron under fatigue loading are demonstrated experimentally.

Investigation of Grey Cast Iron Microstructure Influence on its Heat Resistance

Grey cast iron is one of the most common engineering materials used to make castings which work in thermal cycling. Under thermal cycling conditions, heat resistance is an important operational property of grey cast iron, which depends to the greatest extent on its structure – size and shape of the graphite inclusions, as well as the metal matrix composition. In this paper, the heat resistance of unalloyed grey cast iron with lamellar and compact graphite distributed in perlitic and perlitic-ferritic metal matrices was determined. The tests were carried out at temperatures of 500 – 800 °C. The results of the studies showed an insignificant difference of perlitic and perlitic-ferritic grey cast iron with lamellar graphite heat resistance. The heat resistance of perlitic ferritic grey cast iron with compact graphite is 16 – 48 % higher than the heat resistance of perlitic ferritic grey cast iron with lamellar graphite. Thus, when the shape of the graphite inclusions changed from lamellar to compact, a significant increase in the heat resistance of unalloyed grey cast iron occurred.

Fatigue Characterization and Optimization of the Production Process of Heavy Section Ductile Iron Castings

In heavy section cast iron components, anomalous microstructures can be formed especially at the core, because of low cooling rate, with a consequent decrease in mechanical properties. The matrix microstructure, the presence of surface defects, such as inclusions and porosities, and the size and morphology of graphite nodules are critical issues to ensure the adequate performance of the component in fatigue applications. Proper design of heavy section ductile iron castings requires a database of information about microstructures and mechanical properties that nowadays is missing or is very poor. In this paper, we presented the mechanical characterization of ductile irons with solidification times up to 20 h, carried out to identify the metallurgical and the process parameters that most affect the improvement in fatigue strength. An innovative production line was designed, aimed at reproducing on a small-scale sample the characteristics of heavy walled components. A standard ferritic ductile cast iron compared to a ferritic cast iron with high silicon content and a fully pearlitic ductile cast iron were studied by tensile, Brinell hardness and rotating bending fatigue tests. Optical microscope analysis and scanning electron microscope observations of the most significant fracture surfaces were also performed. It was concluded that the ferritic cast iron is more suitable for the production of large thickness castings undergoing fatigue loads, because the decrease in ductility and its effect on other properties is less pronounced.

Failure of malleable cast iron sprinkler pipe end cap due to freezing of water within

Abstract Failures in end caps of sprinkler firewater systems frequently occur due to the freezing of the water inside the sprinkler systems on cold winter days. In these failures, a flat top of an end cap of a sprinkler system fails and completely separates from the threaded body of the end cap. In this study, metallurgical investigations including fractography, metallography, EDS analysis, and hardness measurement were performed for the failure analysis of the end cap. Finite element analysis was employed to identify both the maximum stress location and the maximum stress magnitude resulting from the freezing of and interaction between the ice and end cap when temperature dropped below the freezing point. Microstructure observations confirmed the ferritic malleable cast iron of the end cap. The hardness of the end cap was significantly below the hardness range specified for standard ferritic malleable cast iron. Small cracks formed during the casting of the end cap and the low strength of the end cap caused the final fracture. Failure stress determined by finite element analysis simulating the freezing of the inside water could accurately predict the failure stress and failure temperature. High tensile stress and poor manufacturing quality caused the crack to propagate in the cleavage mode and at the final stage in the ductile mode.

Influence of Load and Temperature on Abrasion of Carbidic Cast Steel and Complex Alloyed Hardfacing

Steel and cement industries frequently experience from the failure of its core components due to high temperature (HT) operation at heavy loads causing high stress abrasive wear. In this work the effect of load and temperature on the abrasive wear behaviour is investigated for two Fe-based materials (a ferritic cast iron with Cr-carbide network and a carbide-rich complex alloyed hardfacing) in order to select materials for plant specific demands. Thereby the role of the carbide content and its distribution is of interest. A modified ASTM G65 setup was used for HT abrasive wear testing. The applied loads were 10, 45 and 80 N, and temperatures were room temperature (RT), 500 and 700°C. During testing coefficient of friction was measured and abrasive was collect­ed to characterise the wear behaviour (low stress/high stress condition).High stress abrasion was found to be the dominant mechanism at higher loads for all temperatures. A nearly linear increase of wear rate with raising normal loads was found for both the materials. Wear rates at RT were found to be similar for the two alloys, however the complex alloy showed increased wear at HT. The cast steel formed protective mechanically mixed layers (MML) by abra­sive embedding at HT. The hardfacing on the other hand showed brittle behaviour, which worsened with temperature. Based on these results it was concluded that very hard carbide-rich hardfacings performed unbeneficial at high stress conditions and MML-forming materials should be preferred for HT operation under these conditions.

Heat Treatment of Heat-Resistant Ferrous Cast Alloys

Heat-resistant ferrous cast alloys are generally divided into four groups: ferritic cast iron, austenitic ductile iron, ferritic stainless steel, and austenitic stainless steel. There is an active debate about whether or not heat treatment should be imposed on these ferrous castings prior to placing them in service. In this investigation, a series of experiments were conducted to evaluate the effect of heat treatment on the microstructure, tensile properties, hardness, linear growth, and thermal fatigue life for alloys of each of the four groups. In general, heat treatment appeared to cause similar microstructural changes in these alloys such as the formation of precipitated particles adjacent to the cell boundary regions, but it also affected each material’s properties in a different fashion. In spite of a slight increase in tensile elongation at room temperature, annealing had no impact on the ductility minima at medium temperatures for ferritic cast iron. Heat treatment did not appreciably alter the tensile properties of austenitic ductile iron. The Brinell hardness of ferritic stainless steel samples was reduced, but the hardness was increased for austenitic stainless steel samples after heat treatment.

Damage mechanisms evolution of ductile cast irons under thermomechanical loadings

In this work, tensile and compression tests were carried out on ferritic cast iron samples in order to understand the influence of the stress state for temperatures until 800°C. Results show an asymmetric behaviour of ductile cast iron (DCI). The damage created during hardening was evaluated using local microstructural quantifications. Several damage differences are highlighted at room temperature. Therefore, when temperature increases, differences between damage mechanisms under tension and under compression are amplified. A relation between nodules deformation experimentally determined at different stress-strain stages and numerical simulation is proposed. The numerical predictions are in good agreement with the distribution of the aspect ratio (the ratio between major and minor axis of nodules) obtained experimentally. It was noted that nodules deformation can represent the total material strain at room temperature. In addition, it was shown that graphite nodules in cast iron cannot always be assimilated to voids during the strain.

The Effect of Cu on the Microstructure and the Elevated Temperature Properties of Ferritic Heat Resistant Cast Iron

The effects of copper addition on the microstructure and the elevated temperature properties of ferritic heat resistant cast iron were investigated. The as-cast pearlite formed due to the addition of Cu was fully eliminated by full annealing. As the content of Cu increased, the grain size of ferrite decreased. The grain refinement due to the addition of Cu enhanced the mechanical properties at room temperature, however, those at elevated temperature deteriorated. The addition of Cu diminished the volume change during α→γ transformation. The starting point of α→γ transformation increased with Cu contents under 1.15wt% Cu but this tendency was reversed above this point. This trend can be found also in the case of lattice parameter of ferrite matrix. It is inferred from Fe-Cu phase diagram that the addition of Cu enlarged the coexistence zone of α and γ, so it diminished the volume change during α→γ transformation.

High-alloy ferritic cast irons with different graphite microstructures for exhaust manifolds and turbocharger housings

Abstract It is well known that ferritic cast irons with 4 to 5 wt.% Si provide good service and low cost in many elevated-temperature applications. In terms of cast iron microstructures, both spheroidal graphite and compacted graphite cast irons are currently used to produce engine exhaust manifolds and turbocharger housings. Between the two graphite morphologies, the mixed graphite microstructure has been proposed and evaluated in the aspects of molten metal treatment, solidification characteristic, microstructural stability, standard tensile testing, long-term hot oxidation and thermal cycling testing. The effects of graphite morphologies on the material properties are presented. The samples with mixed graphite structure showed an improved ductility at medium temperatures and thermal fatigue resistance.

Ferrous High-Temperature Alloys for Exhaust Component Applications

There is a wide spectrum of cast ferrous heat resistant alloys available for exhaust component applications such as exhaust manifolds and turbocharger housings. Generally speaking, the ferrous alloys can be divided into four groups including: ferritic cast irons, austenitic cast irons, ferritic stainless steels, and austenitic stainless steels. Selection of a suitable alloy usually depends on a number of material properties meeting the requirements of a specific application. Ferritic cast irons continue to be an important alloy for exhaust manifolds and turbocharger housings due to their relatively low cost. A better understanding of the alloying effects and graphite morphologies of ferritic cast irons are discussed and their effect on material behavior such as the brittleness at medium temperatures is provided. The nickel-alloyed austenitic cast irons, also known as Ni-resist, exhibit stable structure and improved high-temperature strength compared to the ferritic cast irons. Austenitic cast irons with varying nickel contents were evaluated in terms of microstructure stability, heat treatment, oxidation and thermal fatigue resistance. For the most demanding applications requiring even higher elevated-temperature strength, creep, and fatigue resistance, ferritic or austenitic stainless steels may be selected. The chemical composition, microstructures, and properties of cast stainless steels are reviewed. Synthesizing this information on material behavior provides a guide to aid in the selection of ferrous alloys for high temperature applications.

Effect of Dynamic Strain Aging on Isothermal (473 K) Low Cycle Fatigue of Ferritic Ductile Cast Iron

The ferritic ductile iron (FCD400) is widely used as industrial material. As regards to its high temperature application, fatigue at elevated temperature, and advanced mechanical properties has been investigated and clarified. In low cycle fatigue (LCF), the S-N curve is presented by the relation of plastic strain to the number of cycles as fatigue life, which can be predicted by the Manson-Coffin model. In this model, the ductile index is appointed as a material constant of 0.5 and the ductile coefficient is related directly to the so-called plastic deformation capacity of the material. Namely, the low cycle fatigue life shall be dominated by the elongation rate. Concerned with it, one of authors reported that the elongation-to-fracture is 20% at room temperature, but reduces to about 9% at 473 K. This temperature is just within the range of dynamic strain ageing (DSA), which is the phenomenon of fluctuating stress due to mobile atoms in solid solution. So, in this study, strain controlled high temperature LCF-tests were carried out on ferritic cast iron at 473 K in air and the result is compared with those performed at room temperature, aiming to clarify the effect of elongation or DSA on the fatigue life. Particularly, this paper mentions about DSA effect to cyclic stress and cyclic plastic strain that causes contradiction of Manson–Coffin’s model in ‘‘the plastic strain range versus number of cycles’’ and ‘‘the stress range versus number of cycles’’ between room temperature (293 K) and 473 K. The fatigue life time at 473 K is 175 cycles lower than that at 293 K for all stress levels. However, DSA phenomena did not occur at high strain in 473 K cyclic fatigue test although it cannot be observed in tensile tests at 473 K. Based on the Manson-Coffin rule, the parameters (Cp ¼ 0:09 and 421, n ¼ 0:65 and 0.89 at T ¼ 293 and 473 K, respectively) for the fatigue life time prediction have been determined. [doi:10.2320/matertrans.MRA2008482]

Material Constants of Ferritic Spheroidal Cast Iron during Hot Deformation

The hot deformation behaviors of a ferritic spheroidal cast iron (FSCI) have been investigated by compression testing on a Gleeble 3500 machine of the DSI-YSU Joint Laboratory. The temperature rang was from 1073K to 1273K and strain rate from 10-3 to 1 s-1. The total true stain was 0.7. The result shows that the flow curves obtained are typical of dynamic recrystallization processes. The plots of either the natural logarithms of the corresponding temperature or the natural logarithms of strain rate against the hyperbolic of flow stresses satisfy straight line relationships over the experimental data, indicating that the hot compression of the FSCI is thermally activated. The material constants, including activation energy 0H as 240.8 kJ/mol, stress-level coefficient α as 1.352×10-8 Pa-1, stress exponential n as 3.9937, structural factor A as 5.64×108 s-1, are derived .

Fracture behavior of overmatched ductile iron weldment

Abstract The fracture behavior of welded ductile iron is discussed by means of experimental and theoretical studies. Ferritic nodular cast irons were joined by cold arc welding in order to obtain over-matched weldment in which the yield strength of the weld metal is higher than that of the base metal. As a result of the welding process, various regions with different mechanical properties formed in the welded specimens. In the experimental part of the study, the mechanical properties of the different regions in the welded specimens were identified and J-Integral toughness values were determined by fracture toughness tests for different crack locations including the base metal, the heat affected zone and the weld metal. In the numerical part of the study, finite element analyses were conducted by fixing 3-D models precracked at different locations for different crack lengths. In these models, after stress – strain analysis, stress triaxiality and plastic deformation characteristics around the crack tip were determined for each crack location and different crack sizes. It was found that stress triaxiality values increase with growing crack length. Different regions with different strength levels in the weldment affect the extension of plastic deformation in the models. Hindrance to the extension of plastic deformation at the heat affected zone and diffusion line causes an extra increase in stress triaxiality at the crack tip.

The tensile fracture behaviour of intercritically annealed and quenched + tempered ferritic ductile iron with dual matrix structure

The tensile fracture behaviour of oil quenched from intercritical annealing temperature (ICAT) range (α + γ) and then tempered ferritic ductile iron having dual matrix structure (proeutectoid ferrite + martensite) with different martensite volume fraction and its morphologies have been studied for a ductile ferritic cast iron containing 3.5 C wt%, Si wt% and 0.318 wt% Mn. In both fine and coarse dual matrix structures microvoids formed at the interface between the graphite nodules and the surrounding structure, martensite particles, inclusions and martensite/ferrite interfaces in the necked region. The martensite morphology had an influence in determining martensite cracking. Coarse and interconnected high C martensite distributed along the intercellular boundaries cracked easily. Martensite cracking was less frequent and the microvoids were smaller in the fine structure than the coarse ones. Microvoid coalescence was the dominant form of fracture in both structures. The specimens with fine structure had higher densities of voids. In these samples, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite.

Self-consistent one-particle 3D unit cell model for simulation of the effect of graphite aspect ratio on Young’s modulus of cast-iron

The graphite phase of ferritic cast-iron was assumed to consist of randomly oriented, rotationally symmetric, ellipsoidal inclusions of same dimensions. Accordingly, a self-consistent one-particle 3D unit cell model was developed to simulate the effect of graphite aspect ratio on cast-iron elastic constants. The cube shaped unit cell is made up of an inner rotational ellipsoid of graphite surrounded by α Fe–2.5%Si matrix in a concentric outer ellipsoid of the same aspect ratio as the inner graphite ellipsoid in order to model the desired graphite content. The remainder of the unit cell is filled up by the cast-iron compound, the elastic behaviour of which is determined self-consistently. In order to obtain elastic properties, the unit cell was subjected to uniaxial loading. Calculations of stress and strain distributions, in dependence on the orientation of the graphite ellipsoid, were carried out by the finite element method using 3D elements. Finally, updated values of Young’s modulus and Poisson’s ratio were derived by employing Hooke’s law. This procedure was repeated, using the updated elastic constants as new input, to get a self-consistent convergent solution. The results are compared with finite element calculations using a conventional one-particle 3D unit cell model with multiphase elements, and an analytical solution given in the literature. Comparison with experimental data shows the relatively wide range of validity and the superiority of the self-consistent method.

Spheroidal graphite ferritic cast iron—an ideal model material to examine the deformation a single phase matrix containing soft-spheroidal inclusions

Our earlier work has shown that the near spherical pockets of graphite in a spheroidal graphite ferrite cast-iron offer little or no restrain to the manner in which the matrix deforms during working. However, the changes in shape of the soft graphite spheroids may be used to indicate the local strain to which the region containing any particular spheroid has been subjected. This study represents an attempt to test the presumption that the plastic deformation of a soft inclusion-containing material usually results in mechanical anisotropy due to the non-uniform deformation of the matrix. This is done by studying the plastic deformation of model material containing soft spheroidal inclusions. This permits observation of the relationship between the local matrix deformation, the inclusion deformation, and the gross mechanical shape change of the composite material. Cylindrical specimens of spheroidal cast iron were compressed and the deformation of the inclusions and the overall matrix deformation were measured from observations on longitudinal sections of the specimens using image analysis. A deformation ratio (K) of the inclusions to the matrix was derived. The measured deformation values agreed well with those predicted, thus validating the earlier presumption.

Effect of Nodular Graphite on the Deterioration of Vibration Fracture Resistance of Ferritic Cast Iron under an Aqueous Environment

An intergranular fracture feature could be observed when ferritic spheroidal graphite cast irons were tested under continuously wet conditions. The maximum depth of intergranular fractures was closely related to the microstructural feature and ambient environment that caused the deterioration in vibration fracture resistance. Experimental evidence has confirmed the overall D-N curves can be generalized into three characteristic regions. Intergranular fractures occurred in the vicinity of the surface; the existence of noular graphite act as porosity should be considered as a dominant microstructural factor on the initiation of intergranular fractures. On the other hand, intergranular fractures can be prevented resulting in better vibration fracture resistance when a specimen of ferritic SG cast iron is covered with oil film, or if an identical test is performed using the Fe-3Si steel specimen containing no nodular graphite.