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

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.

Chunky graphite formation in ductile cast irons: effect of silicon, carbon and rare earths

“…For checking if such an optimum amount of cerium or RE exists also for chunky graphite appearance, the present results were compared with previous work when Y2 keelblocks were used [24]. A general trend is obtained for this casting size with a minimum area affected by chunky graphite for a cerium content of 0.004 -0.005 mass%.…”

Structure of graphite precipitates in cast iron

“…Microstructural investigations were first carried out on a commercial spheroidal graphite cast iron kindly provided by Azterlan Metallurgy research center. The composition of this material is reported in Table 1 and details about alloy preparation can be found elsewhere [16].…”

Crystallography of graphite spheroids in cast iron

“…The alloy was cast in chemically bonded sand moulds that contained standard keel blocks as described elsewhere. 17 Inoculation was carried out by adding 0.15 wt-% of a mm alumina suspension at 400 mm 22 and their mean diameter at 19 mm. Automated crystal orientation mapping (ACOM) was carried out in a JEOL 3010 TEM at the SIMaP laboratory, operated at 300 kV, equipped with a DigiStar external source with step size of 17 nm.…”