Metal Casting Technologies : MCT DEC 2017 (4TH QRT)
20 www.metals.rala.com.au METAL Casting Technologies 4th Quarter 2017 21 additions of 0.08 – 0.1%Sb did not benefit machinability. Sb additions were not recommended for stabilization of pearlite since, due to their damaging effect on graphite morphology, they gave small decreases in tensile strength and fatigue resistance. It was observed that the presence of Sb resulted in the formation of needle-like graphite spikes growing out from the surface of the compacted graphite forms . In flake graphite iron the fine needle-like form of graphite seen in Figure 1 and reported in the work on Sb in CGI  is often described as “Widmanstatten” graphite . It develops as single platelets as well as patches of fine plates set parallel to one another. The most common cause of such graphite is contamination with Lead or Bismuth, especially in combination with Hydrogen. Deleterious trace elements like Pb, Bi, and Sb have a synergistic effect when present together in irons and this effect can be more severe if Hydrogen has been picked up from wet refractories or moulds especially when excessive levels of Aluminium or Titanium are present. Antimony in ductile irons Contamination of ductile iron melts with Sb and other subversive (deleterious) elements such as Pb, Bi, Ti, Te, As, Al, etc. can result in the formation of sub-nodular and other undesirable forms of graphite especially in thick sections of castings. These forms include flake, quasi-flake, spiky, chunky and exploded graphite etc. [3, 14-16]. These elements tend to segregate to intercellular regions during solidification along with carbide forming elements such as Mn and Cr. As a result of such segregation there is a danger that intercellular regions may therefore contain not only eutectic and pearlitic carbides and non-metallic inclusions but also degenerate graphite forms, as shown in Figure 2. Normally in ductile irons to avoid any degenerate graphite forms the Sb level must be below 0.003%. The presence of subversive elements, which may arise from the use of impure low S pig irons or steel scrap in furnace charges, can be counteracted by including rare earth additions such as Ce in the nodularizing treatment. Up to 0.005% is normally added with the Magnesium alloy. Ce can also be introduced via Ce bearing inoculants. If furnace charges are based on pure pig irons which contain very low levels of trace elements then Ce is not normally included. Indeed, when such “pure” charges are used Ce must be avoided in nodularizing agents since, in the absence of other deleterious trace elements, Ce itself can give rise to undesirable graphite forms, notably “chunky” graphite [7, 17-19]. Chunky graphite, as shown in Figure 3, forms an interconnecting network in cells of up to 2-3mm in diameter causing significantly reduced mechanical properties. The formation of chunky graphite is still not completely understood although it is known that it occurs in the centre of heavy sections of irons produced from high purity charge materials which have been treated with Ce-containing Mg treatment alloy . Chunky graphite is more likely to form in irons with high carbon equivalent  or in the presence of Ni, Ce and high levels of Si. In high Ni austenitic nodular irons chunky graphite has been observed even in light sections , Likewise it is a problem in producing high Si-Mo ductile irons which are for cast turbocharger and exhaust manifold parts since graphite form in these compositions has been found to be highly sensitive to the presence of rare earth elements such as Ce . TECHNICAL FEATURE FIGURE 1. The effect of a 1%Sb addition on the microstructure of an inoculated flake graphite iron (3.3%C - 2.1%Si - 0.7%Mn - 0.05%S - 0.08%P) sand-cast as 30mm diameter bars. Large areas of carbide and spiky morphology of eutectic graphite. X500 . FIGURE 2. Ductile Iron: Degenerate graphite formed in intercellular regions due to the presence of subversive trace elements in the absence of Cerium. X100 . Introduction ron-founders often ask about the use of Antimony additions: “Why do some foundries use antimony? – We have always been told that it is a damaging trace element, especially in Ductile Irons”. This short review considers this question and outlines both the harmful effects and the potential beneficial uses of Antimony, as a trace element and as a deliberate addition in flake graphite and ductile irons. Antimony (Sb) can be picked up as a trace element in cast iron from charge materials such as steel scrap and any scrap containing vitreous-enamel coated materials or contaminated with white-metal bearing shells. It will also be present in foundry returns such as runner bars and feeding heads when used as a deliberate addition. Similar to contamination with Lead (Pb), Antimony can give rise to undesirable graphite forms, notably in ductile iron, resulting in reductions in mechanical properties [1-3]. Although its harmful effects can be offset by Cerium (Ce) additions in nodularizing agents. Antimony levels must normally be kept below 0.003% in ductile irons. Deliberate additions of Antimony can be used to form ferrite- free pearlitic matrices in flake and ductile irons [4,5] and to prevent the formation of chunky graphite in heavy section ductile irons [6-8]. Antimony in flake graphite irons During the 1960’s many reports were published, mainly by E. European workers [e.g. 9], on the use of antimony to improve anti-friction properties in grey irons, with increased wear resistance being claimed for Sb additions of 0.25-0.8%. In a review of this work the damaging effects of Sb on graphite form, level of eutectic carbides and on mechanical properties were also considered . At the mid-range of the levels recommended for improved wear resistance, namely 0.5%Sb addition, tensile and transverse strengths were reduced by 50% when compared to the base iron without Sb addition, while impact resistance was lowered by 70% with hardness increasing from 210 to 250HB. A 1%Sb addition reduced tensile strength from 275MPa, for no addition, down to 100MPa due to the presence of carbide-like constituents and “spiky” graphite in the microstructure as illustrated in Figure 1 . It has been suggested that Antimony can be used as a pearlite former if the level of addition is kept below 0.1%, but that above this level free eutectic carbide (chill) is formed . When limited to 0.1%, Sb can prevent ferrite rim at casting surfaces [10, 11] and can also improve growth resistance during thermal cycling . Additions of Tin (Sn), Antimony and even Arsenic (As) have been investigated, as melt additions and as mould washes, to reduce ferrite rim formation which is a particular problem in shell-moulded grey iron castings . It was concluded that additions are more convenient than mould washes with Sb proving to be more effective than Sn or As additions (Table 1). However, the addition of Sn was recommended since when Sb or As levels are above 0.05% it is believed that there is a greater chance of reduced tensile properties . This is a more conservative maximum than recommended elsewhere . Sb additions have also been investigated in work to improve the machinability of Compacted Graphite Irons (CGI) via reduction of ferrite content in the matrix . It was found that Antimony in Cast Irons By John Pearce I % Addition % Ferrite at Surface (1.3mm) Surface Hardness (Brinell) None 45 176 0.13 Tin 20 209 0.09 Arsenic 15 202 0.09 Antimony 5 216 AT THE MID-RANGE OF THE LEVELS RECOMMENDED FOR IMPROVED WEAR RESISTANCE, NAMELY 0.5%SB ADDITION, TENSILE AND TRANSVERSE STRENGTHS WERE REDUCED BY 50% WHEN COMPARED TO THE BASE IRON WITHOUT SB ADDITION, WHILE IMPACT RESISTANCE WAS LOWERED BY 70% WITH HARDNESS INCREASING FROM 210 TO 250HB. TABLE 1. Effect of additions on % ferrite formed in 13mm thick sections of grey iron castings produced in shell moulds. Results taken from Rickards .
MCT MAR 2018 (1ST QRT)