In many areas within the melting department, molten metal will come in contact with refractory material. It is important to pay attention to this, as a number of defects and inclusion occur when refractory material is eroded or surface reactions occur.
The use of carbon based blackings can effectively protect the refractory from attack by the molten metal. Blackings are usually applied to a hot surface, by brushing or spraying. They harden completely, and build a stable and abrasion resistant protective layer on different surfaces. This layer protects i.e. the refractory lining/furnace border area against adhesions of melt and protects pouring lips and troughs.
Special carbons are the main filler of blackings. Other additives are used to improve the workability are included. Therefore it is important that the mixing instructions are followed, to prevent any loss on performance.
For instance: It is essential to pre mix the Carboform/Carboform ZD in to a paste, the material should be allowed to stand for 24hrs before use to allow the product to reach full performance. This step is time consuming but will greatly improve the performance of the coating.
A further example is Trough blacking (i.e. our PS2), and also applies for the application of centrifugal casting. In this case material must ensure that remaining metal feather is removed completely after casting and turning down the launder so that a new coat (of a blacking) can be applied on the clean surface (if necessary). If material does not correspond to its requirement, it could come to casting defects what leads inevitably to higher extra costs.
Also in the area of roll casting moulds, Blacking is used (eg No.15 Chillroll Blacking), to facilitate removal of the casting from the metallic permanent mould after the casting process.
Requirements on blackings:
- High yield
- Easy to handle
- High abrasion resistance
- Extremely high adhesion
Thus, blackings are effective additives for many areas in a foundry and can prevent through their use wasting times for cleaning or problems during removal.