Árbók VFÍ/TFÍ - 01.06.2000, Side 326

324 Árbók VFÍ/TFÍ1999/2000
emptied and a protective water-based coating is applied to the mould surface before it is
used again in casting. IA experiences severe cracks in its moulds during tlic casting process.
Small cracks will generally form in the moulds just a few days after they have been intro-
duced to the process. These cracks will keep growing during use until the moulds fail or
become unusable due to deterioration. The reasons for mould failure are mainly large
cracks in mould side walls that cause the mould to split in two and mould comers that break
off. Mould deterioration and failure results in signifícant maintenance, operational and
replacement cost.
Casting of Ferro-Silicon
At IA two casting wheels are in operation. Figure l shows the layout of the casting
process.
Before the mould is put in use it is covered with protective coating at the surface of con-
tact between mould and melt (mould hot side) and heated for 6-8 hours (see [8]). After the
mould has been placed on the casting wheel it stays there until it becomes so deteriorated or
cracked that it is no longer usable or needs major repair. Cracked moulds are repaired with
glass fibers while in use, but smaller cracks are left unattended until they become a prob-
lem.
The Casting Process
Each mould is used over and over again so the casting process becomes a recurring cycle.
This cycle can be divided into six basic sub-cycles. The following text describes a one lay-
ered casting process monitoring a single mould on the casting wheel.
1. Casting: The melt in the crucible is poured into the mould at a temperature of nearly
1500°C (see [8]) at the ladle tilting stand. The melt cools in the mould.
2. Water cooling on the solidified crust: A spray system is used to shower the solidifíed
FeSi crust with water in order to decrease solidifícation time. Extensive water cooling is
important to decrease grain size in the solidifíed metal, thus improving the quality of the
product. It is therefore important to cool the FeSi as quickly as possible although the
trade-off is that mould edges are also cooled considerably, resulting in greater thermal
stress in the mould edges.
3. Air Cooling of mould and cast: Mould and cast are air cooled for a number of minutes
depending on the casting wheel workload. If the workload is high the process of solidi-
fication with water and air cooling can be as short as fíve minutes (Sigurðsson et. al. [8]).
The mould is then emptied at the mould tilting stand.
4. Mould air cooling: The mould is air cooled while waiting to enter mould preparation.
5. Coating (water-based) applicd to mould hot side: In mould preparation protective
materials are applied to the mould hot side during every casting cycle. Water is used to
attain an even coating distribution.
6. Mould air cooling: The mould is again air cooled while waiting for the next pour.