The role of rare earth magnesium in the production of nodular cast iron

Magnesium, rare earth and calcium can spheroidize graphite in cast iron. As a spheroidizing agent in practical application, magnesium is in the first place in terms of their respective comprehensive evaluation. The casting small knitting machine learned that under the condition of taking magnesium as the main nodulizing element, with appropriate rare earth, better nodulizing effect can be obtained.

1. Desulfurization and deoxidation

Magnesium, rare earth and calcium have strong desulfurization and deoxidization effects in molten iron.

As far as the free energy of elements reacting with sulfur and oxygen to generate sulfide and oxide is concerned, the desulfurization and deoxidation capacity of rare earth (cerium) and calcium are stronger than that of magnesium. However, the boiling point of magnesium is 1107 ℃, and it will be vaporized quickly after entering the molten iron, which has a strong stirring effect on the molten iron. At the same time, the gas dissolved in molten iron is easy to diffuse and precipitate into the bubbles, and then is carried out by the bubbles. Oxide and sulfide inclusions in molten iron are also easy to be adsorbed and discharged by bubbles. Considering the factors of reaction kinetics, the effect of magnesium on deoxidation and desulfurization in molten iron is actually stronger than that of rare earth (cerium) and calcium.

2. The influence on the white tendency of cast iron has two sides

The rare earth dissolved in molten iron has a strong role in promoting the formation of carbides, and is an anti graphitizing element. However, a small amount of rare earth in magnesium based nodulizing agent can greatly reduce the white tendency of cast iron; In the case of more addition, it will enhance the white tendency of cast iron. This seemingly contradictory phenomenon is actually not contradictory. The reason is that rare earths have a strong ability to combine with sulfur and oxygen. A small amount of rare earths soon react with sulfur and oxygen in molten iron, forming a large number of fine sulfides and oxides, providing a large number of heterogeneous crystal nuclei for graphitization of cast iron, greatly increasing the number of graphite balls, facilitating the diffusion of carbon in iron to the graphite balls, thus inhibiting the formation of white mouth. If more rare earths are added, in addition to those that interact with oxygen and sulfur, there is still excess dissolved in molten iron, and this part of rare earths will enhance the white tendency of cast iron.

3. Function of restraining interference elements

In nodular cast iron, some elements play the role of anti spheroidization, which are usually called interference elements. Interference elements can be roughly divided into two categories:

One type, some people call it consumable interfering elements, such as sulfur, oxygen, etc., which are easy to form compounds with various spheroidizing elements widely used at present. Magnesium, rare earth and calcium can be desulfurized and deoxidized to eliminate their negative effects. Of course, some of them will be consumed.

When the nodulizer contains rare earth, its sulfide and oxide have high stability, fine particle size, and are difficult to float in molten iron. Moreover, the mismatch between these compounds and the graphite lattice is very small, which can be used as the heterogeneous core of graphite precipitation. The oxide MgO of nodulizer magnesium has a high melting point, good stability, low solubility in molten iron, but its particle size is slightly larger, which can float to the surface and become scum, or be drawn into the casting to become slag inclusion. At the same time, the density of magnesium sulfide (MgS) is low, and it is easy to float to the surface of molten iron, but its stability is poor. After contacting with oxygen, it will form magnesium oxide, which will release sulfur back to the molten iron and react with magnesium in the molten iron again. The continuous occurrence of this reaction is one of the main reasons why the spheroidization is easy to decline when magnesium is used as the nodulizer alone. When rare earth (cerium) is used as nodulizer, the decline phenomenon is not so obvious, and the tendency to form slag inclusion is also small.

The other is the segregation type anti spheroidizing elements, such as titanium, antimony, lead, vanadium, bismuth, etc., which are enriched in the eutectic cluster when the cast iron undergoes eutectic transformation, thus distorting the shape of graphite. Magnesium has a poor ability to resist interference elements, while rare earth elements have a strong ability to inhibit the harmful effects of such interference elements. When the residual cerium content (mass fraction) in cast iron is 0.008%, the anti spheroidizing effect of interference elements can be effectively controlled. Therefore, the addition of rare earth to the nodulizing agent mainly composed of magnesium can slow down the nodulizing decline, and the tendency to form slag inclusions is also small.