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Metal Casting Technologies : March 2007
18 www.metals.rala.com.au TECHNICAL FEATURE • For ferrosilicon to be an effective inoculant then it must contain small amounts of minor elements such as calcium, aluminium, zirconium, cerium, barium, manganese and strontium. • Lack of control in the use of inoculants can give rise to other problems such as shrinkage defects caused by excessive mould dilation, pinholes due to Al pick up, and inclusions of undissolved inoculant and slag. • The rates of fading of inoculation treatments are most rapid during the first few minutes after treatment and the effects of the treatment are halved after about 5 minutes of holding. • Barium containing ferrosilicons tend to be more persistent and can show a reduced tendency to fade in ductile irons. • Graphitic carbons with suitable crystal structures can inoculate flake irons but not ductile irons. Amorphous carbons do not act as inoculants. • It is difficult to effectively inoculate grey irons with sulphur contents below 0.05%, especially below 0.03%, using conventional ferrosilicon inoculants. Most commercial ferrosilicon based inoculants therefore contain up to around 5% of carefully controlled levels of elements (Ca, Al, Sr, Ba, Mn, Zr, Ce, etc.) that are capable of forming micro-inclusions of complex oxides or oxysulphides having suitable surface and crystallographic characteristics to heterogeneously nucleate graphite [11-12]. The problem of inoculation of low S grey irons has grown as increasing numbers of foundries have replaced cupolas with electric induction melting, and in turn are using less pig iron and greater proportions of steel scrap in charges. For example, many automotive iron foundries in Thailand base their charges on very low S steel scrap from their automotive pressings neighbours. These foundries do not want to add S to their grey iron melts (allowing the use of normal levels of conventional grade ferrosilicons) since they also produce ductile irons and do not want to separate foundry returns. To meet the needs of such foundries the major inoculant producers have developed special ferrosilicon inoculants containing Sr and rare earth (RE) elements such Ce and La that can be used to treat very low S grey irons at relatively low addition rates. Likewise an inoculant containing zirconium has also been developed to gather nitrogen as zirconium nitrides and so prevent N related blowhole defects. Inoculant Type %Si %Al %Ba %Ca %Mn %RE %Sr %Zr Foundry Grade FeSi 75 1.2 - 1.0 - - - - FeSi - Sr 50 or 75 <0.5 - <0.1 0.8 FeSi - Ba1 75 1.0 1.0 1.0 - - - - FeSi - Ba2 75 1.0 2.5 1.5 - - - - FeSi - Zr 75 1.2 - 2.0 - - - 1.5 FeSi-Mn-Zr-Ba 65 1.2 0.8 1.2 4.5 - - 4.5 FeSi - RE 75 1.0 - 0.8 - 2.0 - - FeSi-Sr-RE 75 <0.5 - <0.1 - 2.0 0.8 - FeSi - Sr -Zr 75 <0.5 - <0.1 - - 0.8 1.2 Table 1. Approximate compositions of some typical Ferrosilicon based inoculant materials, in each case the balance is Iron. The FeSi-RE type can also contain small controlled amounts of Oxygen and Sulphur to boost nucleation where high nodule numbers are needed in producing ferritic ductile iron. Magnesium ferrosilicons used as nodularizing agents normally contain around 45%Si and have a range of Mg levels from 3 - 10%, some grades may contain up to 3%Ca, 1%Al, and 3%RE. Nodularizing agents and inoculants are supplied in controlled size ranges to suit their intended modes of application e.g. 3-25mm for MgFeSi, 2-6mm for ladle inoculant, and 0.2-0.6 for late in stream inoculant.