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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.

RESUMEN:El uso creciente de fundiciones esferoidales en aplicaciones tecnológicas hace que se busquen nuevos métodos para mejorar su comportamiento frente a la corrosión. Entre las distintas maneras de abordar este problema, la modificación de su composición química tiene un doble interés dado que, junto con un aumento en la resistencia frente a la corrosión, se pueden conseguir propiedades mecánicas mejoradas. Todo ello se debe conseguir, además, sin aumentar de manera significativa los costes de producción. En este trabajo se presentan los primeros resultados del comportamiento frente a la corrosión de una serie de fundiciones grafí-ticas de composición modificada. En el caso de las fundiciones esferoidales se ha variado el contenido de silicio con el fin de conocer el efecto de este elemento. Los resultados encontrados muestran una ligera mejora en la resistencia frente a la corrosión en aquellas aleaciones con alto contenido de silicio respecto a las fundiciones convencionales durante las primeras horas de ensayo para, posteriormente, mostrar un comportamiento similar. No obstante, es importante destacar que estas modificaciones no comprometen las propiedades mecánicas de las aleaciones base, lo que señala un camino para la preparación de nuevas fundiciones con un mayor contenido en elementos de aleación. ABSTRACT: Corrosion behaviour of ductile cast irons partially modified with silicon in 0.03 M NaCl. The increasing demand of ductile cast irons with extensive technological applications leads to enlarge the corrosion resistance of this group of metallic materials. In this sense, the use of different chemical compositions on such cast irons becomes one of the most interesting aspects among the different ways to improve their behaviour against corrosion due to the extra opportunity for increasing the mechanical properties. Additionally such improvements have to be made without any increase of processing costs to keep the interesting competitiveness of developed cast irons. In the present work the preliminary results obtained from corrosion tests made on a group of cast irons with different chemical compositions are presented. Among ductile cast irons, silicon content has been varied in order to investigate the effect of this element on corrosion resistance of the alloys. The obtained results show a slight improvement of this property for the alloys with high silicon content with respect to the conventional ones though such effect was found in the first time period of the corrosion tests. Interestingly this

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Chunky graphite is a particular form of graphite degeneracy that appears in the centre of large iron castings, with a well-defined transition from the outer unaffected area and the inner affected one. All previous works that looked for macrosegration to explain the phenomenon concluded that there are no significant composition differences between the inner and outer parts of such castings. This was challenged again because the analytical methods generally used for chemical analysis are not efficient for low-level elements. Accordingly, an ICP-MS procedure has been developed and validated to replace the usual ICP-OES method. Together with the usual methods for analysis of C, S and Si, this ICP-MS procedure has been applied to characterize chemical heterogeneities in a large block with chunky graphite in its centre, and to a standard part for comparison. It could be concluded that no macrosegregation has built up during the solidification process of the block investigated, i.e. that chunky graphite appearance is not related to any composition changes at the scale of the cast parts, in particular of elements known to affect graphite shape such as Ce, Mg, Sb, S, · · · .KEY WORDS: nodular cast-iron; chunky graphite; chemical analysis; segregation; heavy section. measuring trace elements in small samples being about 0.1 g in weight is thus presented. Measurements of silicon, carbon and sulfur contents were performed with 0.5 g samples following the standard procedures that will be also shortly described for consistency. In the present work, these procedures have been applied to materials coming from the outer part (without CHG) and inner part (with CHG) of a heavy-section block. For purpose of comparison, measurements were also performed on material taken out from a light-section part that was cast with a melt of similar composition. This light-section part showed no CHG and presented a much larger nodule count. Experimental DetailsThe chemical analyses have been performed with three methods: -gravimetric method for silicon; -combustion with a LECO CS-200 facility for carbon and sulfur; -induction coupled plasma-mass spectroscopy (ICP-MS) with an Agilent 7500 CE apparatus for all other elements considered in this study. The first and third methods apply to solutions obtained by dissolving the material in acids having ACS analytical grade, i.e. containing less than 5 ppm metal atoms. These three methods are successively detailed below with more detail for ICP-MS; they all give results per weight percents that are recalculated, either automatically or manually, from the initial masses of material involved for analysis which were weighed on calibrated balances with the following requirements: -silicon analysis: 0.5Ϯ0.005 g; -carbon and sulfur analysis: 0.5Ϯ0.005 g; -trace elements: 0.1Ϯ0.0005 g. Measurement of SiliconSilicon is analysed along a procedure based on the E350 ASTM standard. The method consists in first dissolving the sample in a mixture of HNO 3 (68 mass%) and HCl (38 mass%). Silicic acid H 4 SiO 4 forms i...

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