FeSiMg Alloy 6% 7% for Ductile Iron Foundry Magnesium Addition and Casting Process

Magnesium addition is the most expensive minute in a ductile iron foundry, because every kilogram that does not end up in the casting has been paid for twice: once at the bag and once in the scrap rate. FeSiMg alloy 6% and 7% is the usual choice for the middle of that problem. The 6% window gives enough magnesium to desulphurize and nodularize an average heat without leaving a surplus, while the 7% window carries reserve for heavy sections and slow pours. Both are supplied as screened lump or granules, so the treatment station can dose by weight rather than by guess.

Three treatment routes dominate ductile iron production. In plunger treatment, a bell pushes the alloy under the surface and holds it down while the iron reacts; magnesium bubbles rise through the melt and are partly absorbed. In sandwich treatment, alloy sits in a pocket at the ladle bottom and iron is poured over it. In stream or in-mould treatment, a measured granule dose meets the metal as it flows. Each route has a different recovery, and the same 6% alloy will behave differently in each.

Recovery is the number that decides cost. It moves with bath temperature, sulphur load, slag condition, the depth the alloy reaches and how long the heat waits afterwards. A station that keeps a heavy slag cover and pours quickly will recover more magnesium from the same bag than one that treats thin and holds long. ZhenAn supplies the two windows from the same grade list, which keeps the certificate format stable when a plant tests one level against the other. Fading is the other half of the calculation: 10–15% of the residual is gone in the first five minutes after treatment, which is why the pour window is usually held to 10–12 minutes. That is why the alloy choice and the practice have to be reviewed together rather than separately.

The comparison shows two grades built on the same carrier with different reserve. Silicon and the aluminium ceiling are identical, so a switch between them does not change the deoxidation behaviour of the carrier; only the magnesium and the calcium lines move. That is convenient for a plant that wants to change one variable at a time.

Two lots of the same grade still differ inside the window. A plant that weighs alloy per heat and logs the certificate figure will see the effect within a shift; a plant that doses by habit will read the drift as a furnace problem.

The 6% grade covers most jobbing work: pump bodies, gearbox housings, brackets, covers and general engineering castings poured from a treated ladle within a normal cycle. The 7% grade appears where the section is heavy enough to hold heat, where the charge carries high-sulphur pig iron, or where one ladle serves several moulds in sequence.

• Machine tool bases and counterweights, poured thick and cooled slowly.
• Valve bodies and pump casings with heavy flanges, where the last metal must still nodularize.
• Castings made from a charge rich in oily returns and rusty scrap.
• Multi-mould pours from a single ladle, where residual magnesium drops along the line.

A foundry that runs both windows should label the bags by grade at the bay, not only on the paperwork. Mixing them by accident is the most common cause of an unexplained residual change.

One more line belongs on the order even though it is not a product property: the treatment method. A supplier who knows whether the alloy goes into a bell, a pocket or a stream can flag a fraction that will not work before the container is loaded.