Casting clean steel: induction furnace lining systems
20 Jan 2024
Key takeaways:
- Prevent refractory inclusion defects
- Reduce gas porosity defects
- Improve as-cast quality
How to prevent casting defects: furnace refractory linings
Coreless induction furnaces (CIF) comprise the vast majority of furnaces used in the steel foundry sector. While a range of factors play into the optimal running of these furnaces, the refractory lining system is the basis for success.
Refractory linings have two main jobs: firstly to protect furnace parts from the extreme heat and aggressive chemistry of the steel melt; and secondly, to keep as much of that heat within the process as possible, and so help maximise the thermal efficiency of the casting process. These roles expose refractory linings to significant thermal and chemical stresses. They are also vulnerable to mechanical abrasion during furnace charging.
These extreme conditions can cause the refractory lining to wear into and contaminate the melt, which may cause casting defects known as inclusions. From a clean casting perspective, the refractory lining needs to offer high levels of resistance to thermal, chemical and mechanical attack. This is particularly relevant for the working lining: the part that is exposed to direct contact with the melt.
How to prevent casting defects: purge plugs
Porous purge plugs (or gas diffusers) are optional lining components that allow the injection of an inert gas, such as argon, into the melt – with two major benefits.
Purging helps to degas the melt, which helps limit the formation of oxides, which are principally responsible for slag-line erosion of the refractory lining. This reduces the risk of chemical wear and refractory inclusions contaminating the melt, not only in the furnace but also in downstream processes where refractory linings are present (e.g., ladles, stopper rods, and nozzles).
Degassing also helps to reduce the occurrence of gas porosity defects. These are caused by the release of gas during solidification of the molten metal in the mould (solids being unable to hold as much gas as liquids). The lower the gas content of the melt, therefore, the less risk of gas porosity defects.
Secondly, purging improves mixing of the melt, creating a more thermally and chemically homogenous melt. This enables a more even distribution of alloying components and deoxidisers.