Technical Information 10

Elkem ASA, Silicon Division

Postal address: Office address: Telephone: Revision No. 2P.O.Box 5211 Majorstua Hoffsveien 65 B 47 22 45 01 00 14.03.1997N-0303 Oslo Oslo TelefaxNorway 47 22 45 01 52

Tundish Cover Ladle Nodularization

Magnesium treatment for nodularization of ductile iron can be carried out by several differenttreatment processes. The tundish cover ladle process will under most conditions, be aconvenient, effective and reliable process with good economy. The process provides goodconsistency and high recoveries of magnesium over a wide range of treatment sizes. A well-operated system will give between 60 – 80 % recovery of magnesium while the operating andmaintenance costs normally fall well below most other commercial processes. Tundish ladletreatments also offer virtually no flare, about 90 % fume reduction, no metal splashing andminimum carbon and temperature losses. The figure below shows an example of a tundishcover ladle.

Figure 1: Schematic representation of a tundish cover ladle with dual alloy pockets.

Calculation of Filling Hole Dimensions

The following formula can be used to calculate the diameter of the tundish lid filling hole:

htW

d⋅

= 07.0

Where d is the filling hole diameter in centimetres (cm), W is the liquid iron batch weight ingrams (g), t is the pouring time in seconds (s), and h is the ferrostatic height of metal in thetundish basin in centimetres (cm). Note: h is the height of metal, not the height of the basinitself.

Base metal sulphur contents should preferably not exceed 0.02% before treatment to ensuremaximum efficiency of the tundish ladle. If sulphur levels are higher, a desulphurizing step isrecommended prior to nodularization.

Magnesium Alloy Selection and Addition

The tundish cover process may be used with most magnesium ferrosilicon alloys containingbetween 3 to 12 % magnesium but generally alloys containing 4 – 6% Mg are used. Typicallyan alloy of the following composition:

45% Si 5% Mg 1% Ca 1% RE max. 1% Al Bal. Fe

would be used to treat base iron composed of a mixture of steel scrap, pig iron and returns.Alloy size grading of about 1 – 10 mm is most suitable for small treatments while sizes up to 4– 35 mm are preferable for larger treatments. See Elkem Technical Information Sheet No. 20for more details on selection of nodularizers in ductile iron.

The amount of alloy added usually lies between 1.2 and 1.8 weight % depending on the basesulphur content, the metal temperature, the magnesium content of the alloy and theconsistency with which the process is carried out.

Cover Material Selection

It is recommended to use a cover material over the magnesium alloy in the reaction chamberin order to obtain maximum treatment economy. The cover should retain the alloy in thechamber for as long as possible before the reaction starts. Covers commonly consist of cleansteel plate or clippings of a grade similar to that used in the melt charges. Cast iron coverplates can be cast from the spare metal left at the end of a cast thus avoiding the carbondilution due to the steel cover. However, the best results are obtained using a ferrosilicon alloyas a cover material.

Example of Recovery Improvements

Figure 2: Schematic representation of recovery improvements as a function of sulphur content,treatment temperature, and ladle design modifications. The initial case of 2.0 wt% addition raterepresents a situation with 0.03% S and 1520°C treatment temperature.