PROCESS FOR PRODUCTION OF COMPACTED GRAPHITE IRON

Field of the invention

The invention relates to a process for production of cast iron with a structure predominantly consisting of compacted graphite shapes. The process is based on a combination of pre-treatment of a base iron and a final treatment in the mould. Background art

Compacted graphite iron is a cast iron alloy with a graphite structure between flake type graphite shapes and spherical shapes. The graphite shape is determined by the conditions in the liquid iron during the solidification. Treatment of a base iron, preferably with a carbon equivalent between 4.0 and 4.4 and with a sulphur content below 0.02%, with a ferrosilicon alloy containing 4-10% magnesium can be used to achieve the compacted graphite structure. The magnesium content must be kept within very narrow limits usually within +/- 0.003% and with a level of about 0.008 to 0.015% depending on conditions of the base iron and the cooling rate in the casting to be produced. As used herein percent refers to percent by weight. The treatment with magnesium is usually made in a ladle. Magnesium boils at 1090°C and since the temperature of the iron usually is higher than 1400 ⁰ C during the treatment some of the magnesium therefore vanishes as vapour, and some combines with the sulphur, oxygen and nitrogen in the iron. During holding of the iron before pouring further reduction of the active magnesium content occurs. This gradual reduction of active magnesium is called fading.

In order to avoid these problems the magnesium treatment can be made inside each mould. That technology known as in-mould treatment/technology is well-known for production of ductile iron. A special version of the technology as described in WO 01/54844 Al, is suitable

for production of compacted graphite iron. The in-mould technology is based on placing the magnesium alloy in a chamber in the gating system in the mould. During pouring, the iron flows into the chamber and gradually dis- solves the alloy. The treated metal then fills the casting cavity. The problems with fading of magnesium are eliminated when using this process.

One problem is that the sulphur level in the base iron often varies. Therefore the magnesium level must be adjusted. However, with the in-mould treatment this is practically not possible as the treatment chamber is the same in each mould. Another problem is that treatment with magnesium makes the structure sensitive to variation in cooling rate. With a high cooling rate e.g. in thin sections of the casting the graphite shapes tend to be more spherical. With long cooling rates i.e. in thick sections the graphite will precipitate as flakes.

It is known that treatment alloys containing both magnesium and cerium reduce these problems. However high levels of cerium can increase the risk for certain casting defects such as formation of primary carbides and shrinkages . Description of the invention

It is an object of the present invention to solve these problems.

The invention concerns a process for production of compacted graphite iron using in-mould addition of a magnesium alloy in accordance with claim 1. Preferred embodiments are defined in the dependent claims. The amount of cerium is adjusted in relation to the sulphur content in the base iron. The cerium level should be adjusted according to the formula:

%Cerium = (%Sulphur - 0.006)* 2.9 + A.

The value for A varies preferably between 0.01 and 0.03 depending on the configuration of the casting i.e.

variation in section dimensions and casting modulus. As cerium has a very high boiling point (3470°C) and a high density (6.14 g/cm ³ ) it does not show any fading effect. By adding cerium to the base iron it can be properly dis- solved and less magnesium alloy has to be added in the reaction chamber in the mould as cerium also has a structure forming effect.

The treatment alloy preferably contains 3-6% magnesium and 0.5-1.5% lanthanum. Lanthanum has a favourable effect in reducing defects such as carbides and shrinkages in the casting. The effect on shrinkages is highest just after treatment and therefore it is optimal to add the lanthanum as late as possible.

The alloys used can have various compositions since the paramount feature is the overall fraction of active metal. However, examples of compositions for commercially available alloys include:

For the magnesium alloy: 48% Fe, 45% Si, 5% Mg, 1.0% Al, 0.5% La and 0.5% Ca, and for the Cerium alloy: 65% Fe, 25% Ce, 7% La, and a balance of other rare earth elements .

According to a preferred embodiment of the invention cerium is added to the oven or the ladle (and not as a part of the magnesium alloy) and magnesium is added to the mould.

With the proposed process the magnesium addition can be reduced with at least 30% compared to a normal treatment in-the-mould without the pre-conditioning. The reduced magnesium level in the castings has also the advantage that casting defects such as dross and micro- shrinkage are minimized.