Science and Processing of Cast Iron IX

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Authors: Adel Nofal
Abstract: This report is an overview on the cast iron research carried out at the Foundry Department of the Central Metallurgical R&D Institute (CMRDI) over the past three decades. CMRDI is an national industrial research institute, whose main objective is to serve the metallurgical industries in Egypt. This overview shows selected examples of the cast iron research programs covering fundamental research, optimizing melting techniques, developing cast iron alloys for special applications, investigating novel processing techniques of cast iron rather than sand casting, e.g. continuous casting, hot and cold rolling of ductile and austempered ductile irons as well as ausforming of ADI. The influence of elastic vibrations of mechanical and ultrasonic frequencies on the inoculation and structure formation of ductile as well as white high-Cr irons has been the subject of intensive research. The review illustrates that over 30 years, the foundry group at CMRDI has been striving to keep some sort of balance between fundamental and applied research, with the results of the latter being oriented towards developing the local foundry industry.
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Authors: Sarum Boonmee, Doru Michael Ştefănescu
Abstract: Sand-cast compacted graphite (CG) cast iron typically presents a surface layer (the casting skin) whose microstructure is significantly different than that of the bulk material. It is generally believed that the casting skin is the result on interaction between the metal and the mold moisture and atmosphere. This reaction results in a decarburized or graphite-free layer. The thickness of this layer is a function of process variables (e.g. cooling rate, pouring temperature, binder, coating, sand fineness, and Mg and inoculation level). The paper presents a summary of recent findings on the negative effect of casting skin on the static properties of CG iron through the use of the skin quality factor for tensile strength, defined as the ratio between the as-cast and machined tensile strength. A mechanism of casting skin formation is proposed and supported with experimental and computational data.
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Authors: Iulian Riposan, Mihai Chisamera, Stelian Stan, Eduard Stefan, Cathrine Hartung
Abstract: Specific investigations concerning La-contributions in graphite nucleation process in Ca,Al,La-FeSi inoculated grey cast irons were performed by using a scanning electron microscopy (SEM, EDS). It was re-confirmed that complex (Mn,X)S compounds act as major nucleation sites for graphite flakes. La was mainly concentrated in the first formed oxide-based micro-inclusions (similarly to Al), but also at an important level in the shell of (Mn,X)S compounds (accompanying Ca). It is assumed that La forms micro-inclusions later than Al, as La-reached phase surrounded Al-reached phase. Complex Al-La small micro-inclusions, as possible better nucleation sites for (Mn,X)S compounds and La-Ca presence in the shell/body of these sulphides, possible better nucleation sites for flake graphite, appear to be the peculiar effects of ferrosilicon based inoculants, which include these active elements, promoting type-A graphite, inclusively in low S-grey iron.
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Authors: Hideo Nakae, Kyohei Fujimoto
Abstract: The morphological transition from the A-type to D-type graphite (undercooled graphite) in cast iron has been studied using Fe-3.5%C-2.0%Si-0/0.1%Ti samples. The samples were prepared using a high frequency induction furnace flowing Ar atmosphere using 0.25% steel rods with or without Ti addition. The samples had Ti contents that ranged from 0 to 0.10% at 5 different levels by the addition of sponge titanium. The cooling curves of these melts were measured in a shell mold with an inside diameter of 30mm and 50mm height and in four BN-coated steel cup molds with a volume of 30ml each. The cooling curves were measured by CA thermocouples located at the center. The cooling curves were differentiated to determine the transition points, namely the onset and end points of the eutectic solidification. Three out of the four samples, solidifying in steel molds, were quenched during the eutectic solidification and their macro-structures and micro-structures were observed for the determining the solidification mode. The volume fractions of the D-type graphite area in the samples were measured using 30 microscope images of 50× magnification, and their eutectic temperature was also determined using their cooling curves. The volume fraction of the shell mold samples increased with the Ti addition from 5% to 55%, and if the Ti content was greater than 0.05%, the acceleration occurred with their maximum undercooling, ΔTMAX. The critical undercooling temperature, TA/D, and the critical solidification rate, RA/D, of the A-type to D-type graphite transition were determined by comparing the volume fractions to the solidification time. The ΔTMAX and TA/D values increased with the Ti addition. This is the main reason why the Ti addition accelerates the D-type graphite increase.
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Authors: Xiao Gang Diao, Zhi Liang Ning, Fu Yang Cao, Shan Zhi Ren, Jian Fei Sun
Abstract: Evolution processes of graphite morphology in ductile iron were investigated by quenching specimens during a long time holding of iron melt in a Ar atmosphere. Results show that spheroidal graphite is only observed at the early stage of melt holding. There are no evident changes in morphology of spheroidal graphite with increasing holding time up to 180 min. Subsequently chunky graphite precipitates directly after holding for 240 min as spheroidizing ability (Mg residual and RE residual) is insufficient. The number and size of eutectic chunky graphite cells increase with prolonged holding time. It should be noted that vermicular graphite forms around eutectic chunky graphite cells after holding for 360 min. When holding time reaches 420 min, graphite morphology is flake-like together with some chunky graphite. The graphite morphology in ductile iron changes from spherical to chunky, then chunky to vermicular, finally to flake with an increase in melt holding time. Both spheroidizing ability and numbers of effective nucleus decrease with prolonged holding time of melt, which affect graphite morphology.
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Authors: Yuji Kato, Ying Zou, Hideo Nakae
Abstract: The effects of the cooling rate, atmosphere and holding time on the graphite morphology of spheroidal graphite cast iron were studied using Ni-C alloys. Two groups of parent alloys were prepared using high purity materials, i.e., Group 1 containing the spheroidizing element of Ce, Mg or Ca, while in Group 2, S was added as an anti-spheroidizing element. For discussing the influence of the cooling rate on the graphite morphology, 0.5g of the Group 1 samples were melted and held for 15 minutes at 1673K in an Ar atmosphere, then cooled at 1000K/min or 20K/min. The results showed that perfect spheroidal graphite could not be confirmed, while irregular graphite appeared. The atmosphere was changed to Ar+3%H2 for preventing the oxidation, and the holding time was reduced to 10 minutes to prevent fading of the spheroidizing element. These results showed that the formation of spheroidal graphite was confirmed at the cooling rate of 1000K/min in both groups. Nevertheless, at the cooling rate of 20K/min, graphite morphology was only chunky or flake in both groups. In order to investigate which parameter is more important for the formation of spheroidal graphite, the atmosphere and the holding time were independently changed at the cooling rate of 1000K/min. It was found that the addition of the 3%H2 did not significantly affect the spheroidal graphite formation. Moreover, the holding times of 1min and 20min also did not significantly affect the spheroidal graphite area fraction in the Ni-C alloy, while they affected the ones containing the spheroidizing elements like Mg.
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Authors: Edis B. Ten
Abstract: Oxygen influence on the structure and properties of cast iron is contradictory and depends on its state. It is supposed that the chemically combined oxygen exists into molten metal as Non-metallic inclusion, and it acts as a Graphite crystallization substrate and also increases graphitization degree of the cast iron. Dissolved oxygen deactivates the potential graphitization centers that are why cast iron chill tendency is increased. At the same time, according to experimental results, the more cast iron is saturated by oxygen, the higher its tendency for graphitization modification. In this study, the influence of the oxygen state on graphite forming process during the cast iron crystallization is considered and the details of its action on the graphite crystallization at different stages of casting solidification process are also represented. It is shown that oxygen, initially present in molten melt in chemically combined state, does not have a significant effect on the Cast Iron crystallization. At the same time, the dissolved oxygen in non-modified cast iron causes chill, but in modified Cast Iron the over-equilibrium dissolved Oxygen content promotes graphitizing crystallization by forming dispersed pre-crystallizing (“fresh”) non-metallic inclusions in liquid cast iron. On the basis of these concepts, the original interpretation of several well-known facts concerning graphitization modifying is presented. These facts have been widely discussed at the present time, and there is a great number of contraversal opinions on this account. Particularly, the new aspects of the damping effect of the graphitization modifying of the cast iron have been discussed. The causes of ferrosilicon graphitizing power increase by addition of the aluminum, calcium, barium, rare-earth metals and other powerful elements-deoxidizers to it are shown. The additional factors of high effectiveness of the in-mold graphitization modifying of cast iron are presented also
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Authors: Vladimir L. Naydek, Vladimir P. Gavrilyuk, Ivan G. Neizhko
Abstract: The paper reviews the theories of graphite formation in iron. The hypothesis of nodular graphite formation in pure iron-carbon alloy is proposed and discussed.
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Authors: Iker Asenjo, Jacques Lacaze, Peio Larrañaga, Susana Méndez, Jon Sertucha, Ramón Suárez
Abstract: Parameters that affect chunky graphite formation in heavy-section castings have been studied in previous works which showed that inoculation and cerium addition both increase the tendency for this degenerate graphite. This suggested that laboratory study on chunky graphite formation could be performed on small castings by over-treating the melt. Though the role of silicon was not ascertained, it appeared of potential interest to also investigate its effect in relation with the carbon equivalent of the iron and the nucleation potential of the melt. Keel-blocks were thus cast using Ce or Ce-Mg treated melts, with increased silicon content (up to 4.0 wt.%) and inoculation rate as compared to usual practice. It was observed that chunky graphite systematically appeared in more or less extended areas centred on the upper part of the keel-blocks. The as-cast microstructure (graphite shape and distribution) has then been studied in relation to melt composition and additions (Ce treatment and inoculation) in both affected and non-affected areas. Finally, microanalysis of oxides and other minor phases showed them to be similar to those appearing in heavy-section castings. It may then be concluded that chunky graphite appears in light-section castings in the same way than in heavy-section castings when using over-treated melts.
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