Heavy Section Castings Research Articles

An Insight into the Defects-Driven Plasticity in Ductile Cast Irons.

The microstructure and tensile behavior of two heavy section castings that had chemical compositions typical of GJS400 were investigated. Conventional metallography, fractography, and micro-Computer Tomography (μ-CT) were employed, enabling the quantification of the volume fractions of eutectic cells with degenerated Chunky Graphite (CHG), which was identified as the major defect in the castings. The Voce equation approach was exploited to evaluate the tensile behaviors of the defective castings for integrity assessment. The results demonstrated that the Defects-Driven Plasticity (DDP) phenomenon, which refers to an unexpected regular plastic behavior related to defects and metallurgical discontinuities, was consistent with the observed tensile behavior. This resulted in a linearity of Voce parameters in the Matrix Assessment Diagram (MAD), which contradicts the physical meaning of the Voce equation. The findings suggest that the defects, such as CHG, contribute to the linear distribution of Voce parameters in the MAD. Furthermore, it is reported that the linearity in the MAD of Voce parameters for a defective casting is equivalent to the existence of a pivotal point in the differential data of the tensile strain hardening data. This pivotal point was exploited to propose a new material quality index assessing the integrity of castings.

Multi-domain optimization of cast iron components in wind turbines

The continuously rising demand for renewable energies leads to increased installations of wind turbines with higher power. While the current power-to-weight ratio of up to 20 metric tons of cast iron per megawatt is stagnating, cast iron components of modern wind turbines are facing new challenges in terms of weight, manufacturability, and castability. These challenges can be addressed by systematically using multi-domain optimization approaches to reduce component weight and increase local component utilization.In order to meet the requirements for modern cast iron components, this multi-domain approach must employ methods from casting simulation, micromechanical analysis, topology optimization, and strength assessment. Here, casting simulation is used to determine local microstructure descriptors, which are subsequently used in micromechanical shakedown analysis to estimate the local microstructure-dependent fatigue strength. In parallel to the fatigue strength estimation, topology optimization is performed iteratively in combination with a castability analysis. The component strength is evaluated using a strength assessment approach based on the previously determined local material properties in combination with the topology optimized component.In this study, the overall concept of the proposed multi-domain approach is presented and requirements for the application of such an approach are formulated. The use case of this study is a planet carrier of a wind turbine gearbox manufactured from austempered ductile cast iron ADI-GJS-1050‑6. For this use case, a weight reduction of 17% was achieved while maintaining the required stiffness, such that the microstructure variance along the component was significantly reduced. Furthermore, the potentials and limitations of the presented approach are outlined and discussed in the context of the design of heavy-section castings.

Multiaxial plain and notch fatigue strength of thick-walled ductile cast iron EN-GJS-600-3: Combining multiaxial fatigue criteria, theory of critical distances, and defect sensitivity

Multiaxial fatigue tests were carried out on plain and notched specimens extracted from EN-GJS-600-3 pearlitic DCI heavy-section castings. Two notches of different severities were combined for the determination of mode I and mode III critical distances, along with the related intrinsic fatigue strengths. SEM and CT clearly showed that fatigue cracks initiated from graphite nodules and shrinkage pores in notched and plain specimens, respectively. Different multiaxial criteria were compared, and best predictions were obtained with the Carpinteri et al. criterion with four parameters: axial and torsional intrinsic fatigue strengths, combined with the two different related critical distances.

Long solidification time effect on solution strengthened ferritic ductile iron fatigue properties

Nowadays, lightweight design, aimed at reducing the eco-impact of mechanical components, is considered imperative and it asks for ever novel materials. In this scenario, solution strengthened ferritic ductile irons, thanks to their good combination of strength and ductility, are a potential alternative to the more expensive light alloys. Their unique properties come from a fully ferritic matrix solution hardened by high amounts of silicon ranging from 3 wt% to 4.3 wt%. Moreover, the fully ferritic matrix makes the alloy less sensitive to the hardness variation through the thickness and therefore more easily workable, compared to a ferritic-pearlitic cast iron of the same strength. With reference to heavy section castings, wind energy industry could take considerable advantage from this material in reducing the weights of heavy section components, provided that their mechanical properties, compared to those of separately cast samples with relatively thin wall, are well assessed. Keeping this challenge in mind, this work is aimed at characterizing the static and fatigue strength of a heavy section solution strengthened ferritic ductile iron casting. Static mechanical properties were found to decrease slightly with the increase of the solidification time. On the other hand, the fatigue resistance was reduced by 33% when the solidification time increased from 10 min to 10 h. Reasons were attributed to defects induced by long solidification times such as degenerate graphite and shrinkage porosities acting as crack initiation sites. Finally, a ‘master curve’ linking the solidification time with the fatigue strength is proposed as a design tool for designers who decide to exploit the potentialities of the investigated cast iron grade.

Chunky graphite in spheroidal graphite iron: review of recent results and definition of an predicting index

Graphite degeneracy in heavy-section spheroidal graphite cast irons is mostly associated with the formation of chunky graphite which consists of large eutectic cells with interconnected graphite strings. At low level, appearance of chunky graphite is limited to its non-aesthetic effect on machined surfaces, while at higher level it is detrimental for mechanical properties of the components. Chunky graphite is often related to high silicon levels and too high cerium additions during the spheroidization treatment. The appearance of this defect may be limited by controlled additions of antimony that is thought to tight the excess of cerium, but other impurities and low level elements may have to be considered during melt preparation. This contribution proposes a review of recent results and approaches on chunky graphite appearance, primarily but not exclusively in the case of heavy-section cast irons. Based on this literature review and series of experimental data, a predictive index for evaluating the risk of chunky graphite appearance is proposed. Lines for further research work aimed at a better understanding of graphite degeneracy are finally suggested.

Effect of Si and Ni Addition on Graphite Morphology in Heavy-Section Spheroidal Graphite Iron Parts

Metallographic analysis is applied to the study of the chunky graphite morphology in heavy-section castings of spheroidal graphite cast irons. Three castings with different Si and Ni content were prepared. Three positions in casting from the edge to the centre, with different cooling rates, were chosen for microstructure observation. The effect of the Si and Ni content on the graphite morphology and mechanical properties of heavy-section spheroidal graphite cast iron parts were investigated. Cerium containing commercial inoculant was used for in-stream inoculation. Chunky graphite area was estimated in micro-and macrostructure. Mechanical properties were determined on tensile test bars taken from the centre of the casting. Macro-and microstructure examination showed that the castings with high Si-content and Ni addition had chunky graphite present, while the castings produced by use of low Si and Ni containing charge had no chunky graphite. High Si-content is strong chunky graphite promoter, especially in castings with slow cooling rate. Ni addition also promotes chunky graphite formation, but only in thermal centre of the casting (where the cooling rate is the lowest). The elongation is severely lowered when chunky graphite appears in the microstructure.

The Effect of Manganese Sulfide Inclusions and Zirconium Additions on the Mechanical Properties of Heavy Section Cast Steel

The aim of this study is to evaluate the effect of the addition of zirconium (Zr) as an inoculant to the microstructure and to examine the mechanical properties of test coupons and heavy section cast steel via a conventional sand casting method. The microstructural and mechanical properties of cast steel and Zr-added steel specimens have been studied in detail. Optical and scanning electron microscopes have been used to identify the inclusion morphology and microstructural changes. Tensile and Charpy V-notch impact test was carried out to determine the mechanical properties of test coupons and heavy section cast steel. The results show that the Charpy V-notch impact energy and elongation were improved by adding Zr. Fractographic studies show that the Type III manganese sulfide inclusions located on the dendrite arms evolve to Type I with the addition of Zr.

Effect of Shrinkage Porosity and Degenerated Graphite on Fatigue Crack Initiation in Ductile Cast Iron

Heavy-section ductile iron castings solidify under low cooling rates, giving the risk to form defects like non-metallic inclusions, microporosities and degenerated shapes of graphite that have a negative effect on fatigue resistance, favoring the crack nucleation. The aim of this work is to precisely identify the defects that most affect the fatigue behavior in heavy-section castings, in order to guide the foundry to take the proper countermeasures. For this purpose, fatigue rotating bending tests were carried out on specimens machined from small-scale samples opportunely cast to reproduce long solidification times. The fracture surface of broken samples were investigated by means of Scanning Electron Microscopy in order to identify crack initiation points and fracture mechanisms. Shrinkage porosities and spiky graphite were found to play the most important effect on crack nucleation, lowering the fatigue resistance of the castings, while chunky graphite just behaved as a preferential path for crack propagation.

Fatigue and Fracture Resistance of Heavy-Section Ferritic Ductile Cast Iron

In this paper, we explore the effect of a long solidification time (12 h) on the mechanical properties of an EN-GJS-400-type ferritic ductile cast iron (DCI). For this purpose, static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests are carried out on specimens extracted from the same casting. The obtained results are compared with those of similar materials published in the technical literature. Moreover, the discussion is complemented with metallurgical and fractographic analyses. It has been found that the long solidification time, representative of conditions arising in heavy-section castings, leads to an overgrowth of the graphite nodules and a partial degeneration into chunky graphite. With respect to minimum values prescribed for thick-walled (t > 60 mm) EN-GJS-400-15, the reduction in tensile strength and total elongation is equal to 20% and 75%, respectively. The rotating bending fatigue limit is reduced by 30% with respect to the standard EN-1563, reporting the results of fatigue tests employing laboratory samples extracted from thin-walled castings. Conversely, the resistance to fatigue crack growth is even superior and the fracture toughness comparable to that of conventional DCI.

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.

Fatigue design of heavy section ductile irons: Influence of chunky graphite

The influence of chunky graphite on mechanical properties, especially on fatigue resistance, of an EN-GJS-400 ferritic ductile iron, was investigated by tensile and rotating-bending tests. The samples for the experiments were machined from two heavy section castings. The first one was cast with the addition of a Ce-containing inoculant to promote chunky formation and the second one by using a standard Ce-free inoculant. The presence of chunky graphite in the samples decreased tensile strength and elongation at break, while no significant effect on the hardness and yield strength was measured. Also fatigue strength was negatively affected, with a decrease of fatigue limit of about 12%. This value was calculated also taking into account the different amount of microporosity in the two sets of samples and its effect on the fatigue strength by using the Murakami approach.Fracture analysis showed that chunky graphite has a limited influence on fatigue crack nucleation if compared to micro-shrinkages, while it behaves as a preferential path for crack propagation. A simple way to consider the presence of chunky graphite in the fatigue design of heavy section ductile irons is proposed on the basis of the present results and taking into consideration the published data too.

Microstructure and mechanical properties of heavy section ductile iron castings: experimental and numerical evaluation of effects of cooling rates

This paper focuses on the development of experimental relationships between the microstructure and mechanical properties of a sand cast ductile iron and on the design of a direct ‘in mould’ thermal analysis system for heavy section castings. The casting system was developed in order to obtain different thermal modules, and it has been studied by numerical simulation. The microstructure was estimated through numerical models implemented in a commercial casting simulation code and validated by comparing simulated and experimental data obtained from more than 2000 optical micrographs considering nodularity, density and size of nodules, fractions of graphite, ferrite and pearlite. Empirical relationships between the microstructure and mechanical properties were developed and then implemented in the simulation software in order to evaluate the local mechanical properties. The results of hardness and tensile tests carried out on samples extracted from the castings showed good accordance between predicted and measured mechanical properties.

Effect of convection and cooling on microstructure of mid-chilled solidified casting

In this study, the effect of convection on mid-chilled solidified castings was investigated. The heat transfer system employed was a cylindrical chilled mould sandwiched between two sand moulds; it was cooled by circulating water. The convection during solidification was investigated on the basis of cooling curve. A comparison of the cooling curves showed that convection reduced the temperature gradient and decreased the solidification time. EPMA analysis results revealed little variation in the stoichiometric ratio, M/C, of carbides. In contrast, M/C of Mo rich carbides varied considerably. The difference of convection in the upward solidified zone resulted in greater segregation, whereas the presence of convection in the downward solidified regions mitigated the problem of segregation. Therefore, shrinkage voids can be prevented and the solidification time can be decreased by placing chilling blocks on top of heavy section castings. In addition, grain coarsening and the degree of segregation can be reduced.

Effect of inoculant containing rare earth metals and bismuth on microstructure and mechanical properties of heavy-section near-eutectic ductile iron castings

The effects of inoculation sequence and inoculant chemical composition on heavy-section castings microstructure, with particular attention to chunky graphite, were investigated. The combinations of inoculant types with inoculation sequences were chosen basing on foundry experience.Thermal and chemical analyses were used to control the experimental foundry processes. Metallurgical analyses were performed by means of optical and field-emission gun scanning electron microscope and important microstructural parameters were measured and correlated with the results coming from tensile tests. In-stream inoculation with inoculant containing Bi and rare earths was found to drastically reduce the formation of chunky graphite. This result was attributed to the major fading resistance of such inoculant compared to the standard ones, confirmed both by nodule fraction measurements and sub-micrometric aggregates of Bi detected at the spheroid centre in the central part of the casting.

Statistical Analysis of the Influence of Some Trace Elements on Chunky Graphite Formation in Heavy Section Nodular Iron Castings

To study the formation of chunky graphite (CHG) in heavy section castings, 68 nodular cast irons of both pearlitic and ferritic grades were cast in cubic blocks 30 cm in edge. The volume fraction of the blocks affected by this degenerate graphite was quantified and related to the chemical analysis of the materials by means of a multivariate analysis. For the composition domain investigated, the effects of Ce, Cu, La, P, Sb, and Sn were statistically established with a high R 2 correlation coefficient.

Surface recrystallisation and oxidation of polycrystalline superalloy castings during isothermal exposure at 1100°C

The surface recrystallisation and oxidation of polycrystalline superalloy castings with different casting moduli were studied. Specimens exposed at 1100°C for 2 h in hydrogen, and at 1100°C for 2, 20 and 60 h in air were examined by optical and scanning electron microscopy and energy dispersive spectroscopy. No surface recrystallisation was observed on the surfaces of heavy section castings with moduli of 2·8 or 5·2 mm after exposure at 1100°C, and the oxidation scale was only 50 μm in thickness after exposure in air for 60 h. Surface recrystallisation was observed in the thin wall casting of 0·28 mm modulus; recrystallisation produced higher oxidation rates, owing to enhanced diffusion, and the oxidation scale thickness was 120 μm after 60 h. The mechanism of surface recrystallisation and its effect in accelerating surface oxidation are discussed.

Primary α phase and its effect on the impact ductility of a high Cr content cast Ni-base superalloy

A high Cr content cast Ni-base superalloy, K4648, which contains 32–35wt.% chromium, was prepared by vacuum induction melting, and poured into testing bars or heavy section specimens. The as-cast, heat treated and fractured specimens were investigated by optical metallography, quantitative metallography, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The isothermal solidification followed quenching (ISQ) technique and differential scanning calorimeter (DSC) experiments were also carried out. The result indicated that the primary α phase precipitated from the residual liquid in interdendritic region at 1190°C near the solidus temperature. This phase can be represented as α-(Cr,Ni) in K4648 alloy because it solid solutionized by about 30at.% of element Ni. The Vickers microhardness value of primary α phase is 6.3GPa relative to 1.9GPa of the γ matrix at the load of 0.2N. This phase is brittle and tends to crack due to stress concentration during the solidification process. The primary α phase solutionized or transformed into M23C6 carbides during solid solution treatment in the range of 1180–1220°C. However, small amounts of α phase remained even at the obvious incipient melting temperature of 1200–1220°C. Therefore, primary α phase must be controlled during solidification process. The faster cooling during solidification process can decrease the amount of primary α phase in both as-cast and heat-treated K4648 alloys. Specimens cut from slow cooling heavy section castings with large amount of primary α phase possessed an impact ductility of 13.9J/cm2, which is much lower than 35.4J/cm2 of samples from faster cooling thin section testing bars. The extensive cracked α phase or transformed M23C6 carbides can be observed at the fracture surfaces and longitudinal sections of impact specimens. Large blocky primary α phase with a network-like distribution, forming as a result of slow cooling, has detrimental effects on the impact ductility of K4648 alloy and it is difficult to completely remove by heat treatment. Proper parameter to obtain faster cooling in the solidification of the alloy must be considered to avoid the detrimental primary α phase precipitation.

Experimental Evidence for Metallurgical Modification Associated to Chunky Graphite in Heavy-Section Ductile Iron Castings

Heavy-section castings made of spheroidal graphite cast irons are known to be prone to graphite degeneracy. Upon the various degenerate forms of graphite reported in the literature, chunky graphite is the most common and detrimental one. A great deal of effort has been made for many years to prevent its formation but no convincing description of the factors controlling its appearance has been given so far.

Formación de grafito chunky en piezas de pequeño espesor fabricadas utilizando fundición de hierro con grafito esferoidal

Chunky graphite is a degenerated graphite form which can be found in the thermal centre of ductile iron heavy section castings. Previous studies made on cubic blocks (300 and 180 mm in side) manufactured using alloys with fully ferritic matrix structures show that low cooling rates, excessive post-inoculation and high silicon and/or cerium contents in the melts are the most important factors that promote this kind of defect. The enhancement of these critical factors led to obtain chunky graphite in sections lower than 50 mm. Different experimental conditions have been used in order to establish the main parameters that affect this graphite malformation. The use of cutting-edge techniques in the analysis of chemical compositions has revealed that no significant differences can be found when comparing chunky areas and well-formed spheroidal graphite areas. On the other hand, it has not been possible to establish any correlation between the oxygen contents and the scale of the defect. However, it is noteworthy that the oxygen content is related to the use of magnesium or cerium as nodulizer agent.

Damage tolerant design of cast components based on defects detected by 3D X-ray computed tomography

This paper presents the application of 3D X-ray computed tomography (XCT) in deterministic fatigue strength analysis of relatively large cast components and also probabilistic fatigue design of heavy-section castings. To this end, more than 100 fatigue specimens were cut from a rejected wind turbine hub casting. XCT was used to find the 3D map of defects in selected fatigue specimens. All fatigue specimens were axially tested at R=−1 at room temperature and fatigue life scatter was established. Fractography analysis of broken specimens was performed. For CT scanned specimens, the defect sizes and positions obtained by XCT, in conjunction with the (nearly homogeneous) stress distribution of the specimens were used to predict the fatigue life and also the position of the fatigue crack initiation site. The predicted fatigue life and crack initiation position for CT scanned specimens were compared with experimental results. The obtained defect distribution by XCT was used in a random defect analysis to establish the scatter of fatigue life for the analyzed specimens. The predicted fatigue life scatter was compared with experimental results. The comparison showed that random defect analysis can successfully predict the scatter of fatigue life and may be used to find the probability of failure of cast components, based on the given defect distribution.