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Metal Casting Technologies : March 2007
20 www.metals.rala.com.au INOCULATION IN THE PRODUCTION OF DUCTILE IRONS A number of treatment methods have been devised to introduce magnesium into liquid irons of suitable composition, with Sulphur levels preferably below 0.02%, to ensure that the graphite phase grows in a nodular form [13-17]. Magnesium is volatile and extremely reactive at liquid iron treatment temperatures so it is more conveniently added in the form of a carrier alloy to avoid the dangers of explosive reaction, to ensure economic and consistent recovery of magnesium, and to minimize fume. The first alloys used were based on Nickel-15% Magnesium but foundries can now choose from a wide range of treatment alloys many of which are now based on Magnesium Ferrosilicons containing between 3-10%Mg with 50-70%Si . Unalloyed magnesium can be used for safe and efficient treatment if specially designed treatment equipment is employed, e.g. converter or pressurized ladle, cored wire, etc [14-16]. Treatment with pure magnesium is especially suitable for base irons of higher S content where combined desulphurization and nodularizing treatment is needed. Depending upon the purity of the base iron small amounts of cerium may be included with nodularizing treatments in order to inhibit the effects of subversive trace elements such as antimony, arsenic, lead, tin, etc. If cerium is not used the presence of such elements can influence nodule formation and can lead to imperfect nodular graphite structures resulting in irons with inferior mechanical properties. Most ladle treatment methods involve the use of magnesium ferrosilicon alloys in specially prepared ladles as in the sandwich and tundish (covered ladle) processes [16-18]. Compared to open ladle treatments, the use of a covered tundish ladle gives better Mg recovery with much less fume and glare . Commercial Mg ferrosilicons typically contain around 3-10%Mg, 44-50%Si, with up to 2.5%Ca, and up to 2.5% RE (Ce, La, etc.). Calcium reduces reactivity increasing Mg recovery and provides some inoculation effect. The rare earths (RE) neutralize deleterious trace elements, assist in nodularization, and like Ca, reduce reactivity and provide some inoculation. The lower Mg content ferrosilicons can be used very effectively, via flow through reaction chambers, tundish type ladles, or in the mould treatments, giving high levels of Mg recovery (due to low reactivity) together with some inoculation effect. Regardless of the form of Mg treatment employed, the treated iron must inoculated with a suitable inoculant to prevent eutectic carbides and to encourage a uniform distribution of well formed graphite nodules throughout the structure, the amount of inoculant used depending on the nature of the prior Mg treatment. For example, treatment with pure Mg or Ni-Mg type alloys requires the use of larger inoculant additions for effective nucleation. As mentioned earlier, treated iron must be poured into moulds as soon as possible after treatment so that the effects of both Mg loss and inoculant fade are minimized. There are a number of post-ladle or late inoculation treatment systems that have been developed to avoid these fading problems. These include metering measured amounts of controlled size inoculant from a dispensing unit to the pouring stream as it enters the mould, and the use of inoculant containing cored wire in a fed-wire arrangement, the latter being used particularly in automatic pouring systems. Cored steel wires from 5 -13mm in diameter are used to transport powdered treatment alloy or inoculant into the iron at the pouring station. Experience in the use of such systems has shown that they can replace all or part of the ladle inoculation resulting in savings in the amount of inoculant used and reduced Si increments in the iron. These treatments are consistent and reproducible but it must be remembered that they can only back up or replace ladle inoculation if there is sufficient residual Mg left in the iron. If Mg treatment has been ineffective (e.g. too high a treatment temperature giving low recovery) or if too much Mg has been lost because of delays in pouring then good nodularity cannot be obtained even if the degree of nucleation is high. An alternative approach is to use "in the mould" inoculation where the inoculant is placed in the pouring bush or in a cavity in the runner system, this latter technique also being used for Mg treatment. In this case a specially designed treatment chamber is incorporated into the filling system to make sure that the treatment alloy (Mg alloy +inoculant) dissolves uniformly, treating consistently all the metal that enters the mould. Ceramic filters are also used in the system such that only clean, correctly treated metal enters the casting cavity. Each mould is a separate treatment and this can present greater inspection problems than ladle treatment. The in the mould type treatment is most suitable for long runs of simple shaped castings where automatic assessment of nodularity can be easily made. PRE-CONDITIONING OF CAST IRON MELTS Pre-conditioning involves the treatment of liquid iron before nodularizing and/or inoculation treatments. The aim is to promote the formation of type A graphite in grey irons and the formation of nodular graphite in ductile irons by ensuring that melts have consistent and sufficiently high states of nucleation. Improvements in our understanding of the roles of Oxygen and Sulphur levels and associated oxy-sulphide micro-inclusions in graphite nucleation [11,19], and of the influences of recarburiser characteristics on nucleation potential [20-22], coupled with developments in interpreting the data from cooling curves during thermal analysis [20,21,23] have encouraged foundries to pay much