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WORKING WITH ZINC COATED SCRAP IN CRUCIBLE INDUCTION FURNACES

Steel has its fiercest enemy in Aluminum. Steelmakers are reacting to the push of this metal with the development of steels that are increasingly technologically stronger (mechanical strength and resistance to oxidation). They have developed steels with a high elastic limit, which are more difficult to transform, reaching elastic limits of 30 M Pa. This type of steels has some elements such as Mn with contents of 1.4%, phosphorus 0.085%, etc.

When these types of steels are introduced into the Iron Foundry as part of the sheet package, "gifts" are added to the liquid iron. Mn (may be responsible for giving carbides in pieces), P (affects machinability), ..

One of the most common steels are those that have Zn coatings, as it is the best element to cover a steel sheet that resists corrosion.

For years, the iron foundry has been working with metal scrap from the automotive world and, therefore, with coatings with Zn. As a curiosity, a package of galvanized sheet, protected against corrosion on two sides, provides the liquid metal with the amount of 140g / m2, while an electrozinced Zn.Ni, 60g / cm2

PHYSICAL CHARACTERISTICS OF Zn

- It has a high specific gravity similar to iron: 7.13 g / cm3

- Melting temperature: 419 ºC

- Sublimation temperature: 910 ºC.

- The solubility of Zn in Fe reaches up to 2% in solid solution, that is, we will not see Zn in the microscope.

Zn PROCESS

Once the metal has sublimated, it oxidizes (it has a greed for O2) and ZnO is formed. It is a yellowish powder that we often see in foundry.

In the crucible induction furnace we can have values ​​of 0.150%. As this liquid metal is treated and remains in the casting furnace, this value decreases to values ​​<0.08%.

Due to the very physical characteristic of Zn, we will observe a curious phenomenon: the migration of Zn gas through the porosity of the coating until it liquefies, progressively turning into liquid and solid Zn.

CORELESS INDUCTION FURNACE

When, when we demolish the coating, we see solid Zn stuck to the coating that plasters the coil and / or inside the silicon coating itself, we are surprised and we have to understand that:

- Pure Zn is not detected by the oven's safety system, as it is a non-magnetic metal.

- We have a PROCESS problem with a double possible cause: (1) sintering that can be improved and (2) the way we load the zinc-plated scrap.

From the point of view of the QUARSIL sintering casting, we must do it with the cleanest possible loads (clean returns, ingot, Zn-free steel, etc ...). And these clean loads should be used, at least, in the next 3 washes. Only from the latter will we begin to introduce package or galvanized scrap.

And when galvanized packages are loaded, we must try to do it with a hot liquid bath (for example 1300-1350º) so that the Zn vapors formed as a result of their sublimation leave the bath at the highest speed and in the shortest time possible.

PRESS POUR & CASTING POURING FURNACES

The Zn that passes through the coating and is deposited in the boiler works, destroys it. As the months of the casting unit go by, the Zn gradually corrodes the sheet, converting thicknesses from 25mm to just over 5mm. What can we do? When this takes place, we must study well the thermal profile of the unit and use solutions that prevent Zn from being physically deposited in the furnace metallic shell.

Presence of Zn at the height of the first active furnace coil
Zn pure