Improved analytical method for chemical analysis of cast irons. Application to castings with chunky graphite

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,

Influence of 1 wt-% addition of Ni on structural and mechanical properties of ferritic ductile irons

Two sets of ductile irons with and without Ni additions containing various low Si contents have been prepared in order to study the effect of Ni on structural and mechanical properties of thermal analysis cups and standard keel blocks. Because contradictory results appearing in literature, this work has been focused on the influence of this element on matrix structure and on impact properties at room temperature as well as at low temperatures. The structures of Ni free and Ni bearing alloys have been related to the features of cooling curves recorded on both casting types and to the tensile and impact properties of the materials

Effect of carbon equivalent on graphite formation in heavy-section ductile iron parts

The influence of post-inoculation and of cerium and antimony additions on the solidification process and on the formation of chunky graphite in ductile iron heavy-section parts have been studied previously in the case of near-eutectic alloys. It appeared of interest to complement these works by analysing the effect of carbon equivalent on graphite degeneracy. In the present work, hypo-, hyper- and near-eutectic melts have been cast in large blocks and standard cups. Analysis of the corresponding cooling curves recorded during solidification as well as microstructure observations on these casts have been carried out. A clear effect of carbon equivalent as promoter of chunky graphite formation is observed. The results have been added to the set of data already available and various correlations are discussed

Combined effects of copper and tin at intermediate level of manganese on the structure and properties of as-cast nodular graphite cast iron

Copper, manganese and tin are commonly used as pearlite promoter elements in cast irons. A number of studies have been aimed at quantitatively evaluate the effect of each of these elements, individually or at given levels of the others. As a matter of fact, while tin may be necessary for achieving a fully pearlitic matrix, it is known that when in excess it is detrimental for mechanical properties. As the pearlite promoting effect of each of those elements is totally different, it is of real interest to know the optimum combination of them for a given cooling rate. The present report is a first part of a work dedicated at characterizing the best alloying levels in terms of room temperature mechanical properties of as-cast pearlitic materials

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

Effect of mould inoculation on formation of chunky graphite in heavy section spheroidal graphite cast iron parts

The manufacturing process of heavy section ductile iron castings is strongly influenced by the risk of graphite degeneration under slow cooling rates. Appearance of this kind of defect is commonly linked to significant reductions in the mechanical properties of large castings. Studies on the effect of inoculation on chunky graphite formation in heavy sections have led to contradictory results in the literature and this triggered the present work. New experimental data are presented on the effect of mould inoculation on chunky graphite appearance during solidification of nodular irons which clearly demonstrate that mould inoculation increases the risk of chunky graphite formation in heavy sections. This is in agreement with some previous works which are reviewed, and it is suggested that the contradiction with other results could relate to the fact that these latter works dealt with chill casting

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. In large-size castings, chunky graphite generally appears away from the surface and its occurrence is made evident on a metallographic section by a darker contrast. Because of this sharp and clear transition between the non-affected and the affected zones, several authors looked for chemical heterogeneities at the scale of the part but all concluded to the absence of macrosegregation. This paper presents experimental evidence based on differential thermal analyses that metallurgical differences do exist in samples machined out of the affected and nonaffected zones of a single cast block

Experimental investigation on the effect of copper upon eutectoid transformation of as-cast and austenitized spheroidal graphite cast iron

Copper is known as a pearlite promoter in cast iron and has been used as such for a long time, most often together with low amounts of manganese. Literature data, however, has shown that these two elements act differently on the ferritic and pearlitic transformations. In order to provide more insight in the role of this element on the solid-state transformation of spheroidal graphite (SG) cast iron, this paper investigates the effect of adding copper in small step increments, from 0.11 to 1 wt. % Cu, to SG irons containing about 0.15 wt. % Mn. The characteristic temperatures for the stable and metastable eutectoid transformations as recorded during cooling after solidification in standard cups and after austenitizing are presented together with microstructure information. It is found that a copper content of about 0.6 wt. % is the upper limit over which only small amounts of ferrite could be obtained except at very low cooling rates. This is tentatively related to the lowering of the temperature for ferrite and the associated decreased kinetics for austenite decomposition in the stable system. Keywords: pearlite, eutectoid, metastable, ferrite, transformation temperatur

Microstructure Investigation of Small-Section Nodular Iron Castings with Chunky Graphite

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