An Introduction To Tundishes
A tundish is a device used in the casting of metals. It is any open-topped vessel with holes in the bottom to deliver molten metal at a controlled rate into casting moulds. It is often used between smelting and casting to ensure a consistent and regulated flow, whilst enabling the switchover of casting moulds if needed. Tundishes are often made of steel and are lined with some kind of liner, which is always a refractory material, often in brick form. Such refractory materials employed as tundish linings, or of components in tundish linings, include anthracite, chromite and glass powders amongst others. Some 7% of all refractory materials in Europe are used for tundishes and in continuous casting(1,2), with a respectable 5% of this going to landfill after use(3). Tundishes are closely related to ladles, which are used to transport molten metal from the furnace to the tundish ahead of casting. These large, refractory lined, buckets are operationally very similar to tundishes and thus the principles for tundish lining design and materials largely apply to ladles as well.
Design Of Tundish Linings
Tundish linings are, in many cases, layered affairs. Layering of different refractory materials at different thicknesses is associated with longevity of the plant in question(4). A refractory material that is in contact with the molten metal is often magnesia based and can be of plaster (sprayed on) or brick form. Next is the ‘backup lining’, which is the largest lining by thickness and mass and provides most of the thermal insulation properties. It is often alumina based. Finally, there is a ‘safety lining’ between the alumina and the outer steel shell of the tundish to ensure it does not reach a safety critical temperature(5).
The major components of tundish linings are oftentimes made up of refractory bricks as opposed to monoliths. This is because a brick is easier/cheaper to produce than a monolith, and these bricks can be replaced when required, but overall a monolith is longer lasting than a brick. Tundishes are frequently not very long, and so the necessity to use a device such as a monolith to round a corner is a very rare occurrence. Tundish linings must be able to withstand thermal shock, be resistant to thermal loss and resist corrosion/oxidation - all over prolonged periods of time. It is imperative that molten metal cools and solidifies in the mold, therefore tundishes are insulated with several layers of refractories as mentioned previously(6).
Overall, it can be said that an ideal tundish lining will have the following qualities:
- Ability to withstand temperatures in excess of 1,500°C
- Maintenance of its shape at high temperatures
- Resistance to attack by slag
- Have no chemical or physical interaction with the molten metal
Materials Used For Tundish Linings
After a period of time, a tundish will need to be ‘deskulled’. That is, the hardened steel/slag residue that forms after lengthy periods of processing on the bottom of the tundish.
It is critical to consider not only the properties of the material itself when selecting refractories for tundish linings, but the properties that arise from the physical form it takes. Tundishes lined with layers of refractory bricks tend to need to be replaced sooner than linings made from monolithic or poured refractories(7), with data showing that bricks can last for fewer than five heats before needing to be replaced or reconditioned, compared to in excess 30 for a monolith. Early tundish liners were made of sodium silicate, formed from the reaction of silica and sodium hydroxide.
Common tundish lining materials for steel casting include silica, magnesia and alumina. One concern with using pure silica refractories for tundish linings is that it is able to oxidise certain materials within the molten steel, for example manganese in Hadfield steel(8). Reports suggest that under certain conditions, a manganese oxide layer could form at the molten steel-tundish interface, thus devaluing the steel and reducing the efficiency of the tundish itself.
Some other widely used components of - and additives to - tundish linings are discussed below.
In the tundish, silica and/or ground glass can be used to remove iron oxide from molten iron or steel(9). In removing this iron oxide, it can prevent downstream incursions and contributions to excess slag production which can be harmful to later-on tundish refractories. Aluminium killed steel production uses ground glass extensively as it is able to replace calcium silicide in for the conversion of leftover aluminium and aluminium oxide in the molten steel. As part of the tundish liner, ground glass cullet can be used and behaves as a flux, assisting in purifying the molten metal(10), affording thermal benefits and affords downstream benefits including improving ductility and machinability.
Coal Dust/Powdered Anthracite
Magnesia carbon is a refractory material that is made of magnesia and a source of carbon, such as coal dust, anthracite or graphite(11). One of the major advantages of magnesia-carbon is that the carbon present modulates the expansion of the magnesia at high temperatures, rendering it a more stable long-term material. It has also been reported that thermal spalling (cracking of the refractory surface at temperature, with the possibility of parts breaking off) is decreased with increasing graphite/anthracite content(12). The Young’s modulus of the material is also said to be increased with additional carbon(13). In iron and steel production, degradation of the magnesia carbon tundish lining can be accelerated by the oxidation of the carbon by iron oxide(14), the impact of which can be reduced by the addition of small quantities of aluminium. Anthracite can be included in quantities of up to 15%.
Magnesia chrome is a popular choice of refractory for tundish linings for ferrous and non-ferrous metals. It is comprised of magnesia and chrome sand that have been cured into a porous refractory brick, with the addition of chromite being responsible for the enhanced thermal conductivity properties relative to pure magnesia(15).
In addition to conventional refractory bricks and linings, chrome finds use in the continuous casting space as part of improved magnesia plaster. This plaster is often applied as the top layer (in contact with molten metal) is a ‘conventional’ refractory, or to cover over the joints between refractory bricks. It is traditionally comprised primarily of porous magnesia, which has a tendency to be destroyed by the presence of calcium oxide or silica in the slag at high heat. In modern, improved plaster, large amounts of magnesia are replaced with chromite and olivine, to a lesser extent. The presence of chromite decreases the basicity gap, therefore preventing penetration of the refractory plaster with slag(16).
Safety With Regard To Chrome-Based Refractories
Hexavalent chromium is toxic to humans. Whilst the manufacture of chrome-type refractories typically use only chromite (containing only trivalent chromium, Cr(III)), transformation via oxidation has been observed(17) in magnesia-chrome refractories and tundish linings. The likelihood of more Cr(VI) is greater with more chromite being used in the refractory, but the addition of titanium dioxide can prevent large scale transformations to Cr(VI). Such treatments have been reported as reducing Cr(VI) content to well below US - but above European - standards. This phenomenon is of particular concern in copper casting, where magnesite-chrome refractories are most common(18).
Impact Of Refractories As Tundishes On Casted Metal Quality
With respect to steel, the identity and condition of the tundish refractory plays a role in the quality of the end steel product, as part of the overall interactions of metal, slag, refractory and atmosphere(19). Therefore, it can be said that the tundish can impact on the physical defects of the finished steel, the amount and nature of non-metallic incursions into the steel, and keeping the steel within the predefined chemical space. Tundishes themselves are increasingly used to aid in purification of the molten steel to prevent oxidation and absorb non-metallic impurities via their typically porous structures(20). Additionally, incursions can be removed by physical processes such as integrated filtration within the tundish, or treatment of the molten metal with gas blown through the tundish body(21).
Degradation Of Tundish Linings: The Influence Of Slag
Slag is a largely unavoidable part of the ferrous metal casting process and it has complex interactions with refractory tundish linings, which generally do not benefit the overall casting regime. Some of the impacts of slag have been previously touched upon, and refractory-slag interactions are regarded as one of the most detrimental parts of casting(22). Reports suggest that the optimal way of ensuring no build up of tundish slag, slag from smelting and from the ladle should be removed ahead of reaching the tundish by physical or chemical means(23). Furthermore, the use of a suitably high temperature refractory such as chromite in contact with the molten metal can decrease the potential effects of tundish slag impregnating the refractory rendering it inefficient under operating conditions(24). High alumina content refractories are reportedly less prone to slag penetration(25).
- Tundishes are an essential and important part of the modern metal casting industry
- Tundish linings are formed of layers of refractory materials, often alumina and magnesia based, frequently having other refractories included in their manufacture such as chromite, ground glass and anthracite
- Refractories in the tundish provide not only temperature control benefits, but also they can increase the quality of the finished cast metal by influencing errant incursions
- Slag is an ongoing concern in ferrous metal castings, as it can interfere with refractories in the lining rendering them less effective, but this effect can be mitigated through optimal refractory composition and physical intervention
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