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Metal Casting Technologies : December 2010
36 www.metals.rala.com.au BacktoBASICS Introduction ince the discovery of ductile iron, the principal treatment methods have all been centred on the use of magnesium and its alloys. Treatment methods have had to be developed to overcome some of the obvious difficulties in adding magnesium to liquid iron. Some of these include: ● Magnesium is lighter than iron and tends to float to the surface and burn in air. ● Magnesium has high vapour pressure and low boiling point at the normal iron treatment temperatures. This causes a violent reaction. ● Magnesium has low solubility and is difficult to dissolve in iron. ● Magnesium has high affinity for sulphur and oxygen causing excessive build-up in the treatment vessels. ● Magnesium fades over time due to reoxidation and evaporation resulting in loss of nodularity. ● Magnesium in high concentrations encourages the formation of carbides. Most of these drawbacks have been overcome by the use of alloys of magnesium with silicon and nickel and the development of treatment practices. Whilst treatment practices such as cored wire, porous plug and in-mould are in common use, the treatment ladle remains the most frequently used (figure 1). There are numerous designs of treatment ladle in use, some of the most common include the following. Open ladle pour-over process The open ladle pour-over process is the simplest of the ladle treatment techniques. A weighed amount of treatment alloy is placed to one side at the bottom of a pre-heated ladle after which the required amount of base iron is tapped into the treatment ladle as quickly as possible. The metal stream should be directed away from the treatment alloy. The ladle shape should be tall with a height to diameter ratio of 2.0 to 3.0:1, this allows the treatment alloy to be covered quickly and a head of metal established rapidly so as to improve recovery of magnesium. The ladle design should allow for 300 to 400 mm of freeboard to minimise splashing as the reaction can be quite vigorous. Typical magnesium recoveries by the open ladle pour-over process are 30 to 40% when using an alloy with 5-6% magnesium. Higher magnesium recoveries could be expected if denser treatment alloys such as Ni-Mg are used. Open ladle sandwich process The open ladle sandwich process is an improvement over the pour-over process in that a chamber is formed to one side of the base of the ladle to receive the treatment alloy. Small pieces of steel scrap such as punchings, or ductile iron turnings are then placed on top of the magnesium treatment alloy. This cover material is generally 1-2% of the weight of the metal to be treated and its purpose is to delay the time for the treatment alloy to react with the molten iron hence improving the recovery of magnesium. Ferrosilicon may also be added to this cover material so that some inoculation is done at the same time as the treatment. Ductile iron treatment ladles S J. F. Meredith, Casting Solutions Pty Ltd Figure. 1. Ductile iron production using a covered tundish ladle. Figure. 2. Schematic of open ladle as used in pour-over process.