Magnesium treatment
Magnesium treatment describes a method in which the properties of the treated base iron melt are changed through a metallurgical intervention. The graphite in the casting is spherical instead of lamellar as is usual. This is necessary in order to be able to produce spheroidal graphite cast iron .
magnesium
Magnesium is a very reactive element with a high affinity for sulfur and oxygen. The solubility in iron is about 0.08%.
Due to the low melting temperature of 650 ° C and the low evaporation temperature of 1110 ° C, the magnesium vapor pressure at the usual treatment temperatures (around 1500 ° C) is approx. 10 bar. This fact makes treatment with magnesium difficult.
Treatment method
In the development of the treatment process, two different process paths were followed. On the one hand, the activity of the magnesium and thereby the reaction rate was reduced by dilution with a neutral material such as silicon, nickel or copper. On the other hand, processes have been developed that control the magnesium reaction through process and equipment technology and thus make treatment with metallic magnesium possible.
Treatment with master alloy
Because of the virulent reaction between the metallic magnesium and the treated iron melt, it is not easily possible to alloy the magnesium into the melt. A technically usable reaction rate was achieved by lowering the magnesium activity. It is proportional to the magnesium content in the master alloy. Master alloys are mostly based on ferrosilicon (FeSiMg) and have a magnesium content between 3 and 43%. The master alloys usually also contain rare earths, metals (RE) such as cerium, strontium, lanthanum and the like. The SE elements are intended to neutralize the effects of the interfering elements (lead, antimony, arsenic, etc.). Master alloys are also used in which nickel is alloyed with magnesium (NiMg) or copper with magnesium (CuMg).
The virulence of the treatment reaction is reduced by the use of master alloys to such an extent that it is possible to do without additional equipment for evaporation control. In principle, the master alloy is placed in a treatment vessel and poured over or mixed with the molten iron. There are numerous (more than 70) different devices and methods for introducing the master alloy into the melt. Depending on the process method, the magnesium output ranges between 15 and 90%.
Pure magnesium treatment process
Treatment with pure, metallic magnesium is not problem-free. Difficulties are caused by the high vapor pressure from the magnesium reaction. The pressure needs to be controlled. For this purpose, appropriate measures had to be taken on the treatment devices and in the process.The control was achieved on the one hand by controlling the pressure in a treatment vessel (autoclave or pressure pan), or on the other hand by limited and delayed contact between magnesium and the iron melt - Fischer converter, MAP immersion of coated magnesium, cored wire process.
Factors that influence the application and addition of the magnesium
GJS production in the pan (pouring over / tundish)
- S content in base iron
- Sulfur has to be neutralized in order to increase the surface tension of the iron. A high sulfur content in the base iron means an increasing addition of Mg.
- O content in base iron
- Oxygen has to be neutralized in order to increase the surface tension of the iron. As with sulfur, increasing oxygen also requires increasing Mg addition.
- Treatment temperature
- The treatment temperature should be kept as low as possible to avoid too violent a reaction. The higher the temperature, the more Mg evaporation and lower Mg output is achieved.
- Time between addition of master alloy and treatment
- The time between magnesium addition and treatment should be minimal to avoid preheating and oxidation of the alloy. At the same time, there should not be any liquid iron left over from the previous treatment in the pan, as this would lead to a reaction with the alloy.
- Slag from the smelting or holding furnace
- Furnace slag will react with magnesium and reduce the yield. A clean slag separation should take place in order to minimize slag transport from the furnace.
- Pan dimensions
- The ratio of the cup height to the diameter should be at least 2: 1. The treatment bag should have enough space for the alloy and cover material. The pan should be well insulated in order to minimize heat loss and thus the required treatment temperature. A tundish cover is recommended in order to increase the yield of the alloy and to minimize temperature losses.
- Covering material
- Covering the master alloy, for example fine-grain Fesi or steel flakes, delays the start of the reaction and ensures better Mg absorption in the liquid iron.
- Filling time
- The fill rate should be high in order to build up a high ferrostatic pressure in the pan before the reaction begins.
- Chemical composition of the master alloy
- A high Mg content in the master alloy will cause a violent reaction and reduced yield. A high Ca content will calm the reaction and increase the yield. However, the tendency to form slag is increased. Rare earths support a better yield because it is possible to work with a lower Mg content in the master alloy and a lower residual Mg.
- Grain of the master alloy
- A large grain distribution creates a dense packing in the pocket. The master alloy then dissolves more slowly and in a more controlled manner and reacts with a minimum of loss. Particles that rise and react on the surface are lost.
- Pouring time
- Long casting times require a higher residual Mg content at the beginning. This means a higher addition of master alloy and reduced Mg output due to a fading effect over time.
- Vaccinate
- With a good vaccination, even a small residual Mg content can result in a good nodal count. This in turn means that the addition of master alloy can be reduced and yield is better.
- Slag in the pan and in the treatment bag
- Slag formation in the ladle and in the reaction chamber leads to a reduced Mg output, caused by reactions between the slag and the magnesium. If the treatment bag is filled with slag, master alloy can get next to the bag. Pans should be tilted when emptied in order to avoid an accumulation of slag in the treatment bag.
- Storage of foundry alloys
- All foundry alloys will oxidize under the direct influence of moisture. Oxidized master alloys have a worse yield than new master alloys. The master alloys should be stored in a dry place and the packaging should only be opened at the treatment station.
Magnesium subside
Due to the reactivity of magnesium with other elements, especially with sulfur and oxygen, the effect of the magnesium treatment is limited in time. The decay is primarily a result of oxidation and evaporation. For this reason, the treated iron should be poured no later than 10 to 15 minutes after the treatment.
literature
- I. Henych, K. Regitz: Metallurgy and magnesium treatment of GGG melts. Kirkel-Limbach 2002.