Improved Analytical Method for Chemical Analysis of Cast Irons Application to Castings with Chunky Graphite

Abstract

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...

Primjena spektrometrijskih metoda u analizi metalurških uzoraka

“…1 Određivanje kemijskog sastava metalurških uzoraka ponekad zna biti vrlo složeno jer se ovisno o metodi određivanja zahtijeva prethodna priprema uzoraka. 2 Ta priprema u pojedinim metodama uključuje raščinjavanje metalurških uzoraka (npr. čelika, lijevova na osnovi željeza ili obojenih metala, troske itd.…”

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

“…Because of the sharp and clear transition seen in Figure 1 between the non-affected and the affected zones, several authors looked for chemical heterogeneities but all concluded to the absence of macrosegregation. [8][9][10][11] It should be recognized however that the analysis methods used are obscured by detection limits and accuracies that are not always clearly stated. Moreover, they all estimate global compositions and are not able to differentiate if a particular species exists in an atomic state or bound within a compound.…”

“…The higher nodule count observed in re-solidified material from the with-CHG areas as compared to no-CHG samples is considered to be due to metallurgical differences between the materials. As no significant chemical heterogeneities are expected, [8][9][10][11] this suggests that elements such as magnesium that favour spheroidal growth got bound as compounds in the with-CHG area when the block solidified. This could be in line with the observation of Källbom et al 9 who noticed the presence of numerous inclusions in the heavy section castings they studied, that contain Mg and S but not O in the chunky cells, while in areas with spheroidal graphite and at the with-CHG cell borders particles containing Mg, O and Si were observed.…”

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

“…3 where the arrows show the shift in CE from the no-CHG to the CHG areas for the hypo-eutectic and hyper-eutectic alloys. The greyed area around the eutectic CE value corresponds to ±U, where U is the uncertainty on the CE value estimated at 0.06 wt.% [11] from the average carbon and silicon contents of the alloys used in the present study. This could possibly be due to graphite flotation in the case of the hypereutectic alloys.…”