The source of nitrogen in cast iron

Nitrogen contained in cast iron mainly comes from atmosphere, various furnace charges and molds. Generally, nitrogen is not contained in lining materials and ladle lining materials.

1. Atmosphere

In the melting process of cast iron, the atmosphere in contact with molten iron is the source of nitrogen. However, the dissociation energy of nitrogen is very large (945 kJ / mol), it is not easy to dissociate, and its chemical properties are not active. Therefore, the nitrogen absorbed from the atmosphere by molten iron at relatively high temperature is very limited. When melting cast iron in cupola, the interface between molten iron and atmosphere is very large, and the distance between molten iron and atmosphere is very long. Generally speaking, compared with the induction furnace, the w (n) content of cast iron melted by cupola is slightly higher, but the difference is only about 20ppm.

Although there is no close contact between droplets and atmosphere in EAF melting, the high temperature of the arc can promote the dissociation of nitrogen, which is easy to be absorbed by molten iron. Compared with induction furnace, the w (n) content of cast iron with similar CE melted in EAF may be about 50 ppm higher.

2. Furnace burden

Generally speaking, nitrogen contained in various furnace charges is the main source of nitrogen in cast iron. The amount of W (c) and w (SI) in steel is low, and the solubility of nitrogen in it is high. Therefore, the amount of W (n) in steel is generally much higher than that in cast iron.

At present, there are many kinds of steels. Due to different smelting processes, the amount of W (n) in various steels may be quite different. For example, the amount of W (n) in EAF steel is about 60 ~ 120 ppm; The amount of W (n) in converter steel is about 100 ~ 200 ppm.

It is impossible to set the amount of W (n) in scrap as burden, but it should be considered that the amount of W (n) in molten iron will increase with the increase of the proportion of scrap in burden. When smelting in induction furnace, when the proportion of scrap increases, carburizing agent must be added correspondingly, which is the second factor that leads to the increase of W (n) content in molten iron. At present, there are many kinds of carburizers in the market, among which the amount of W (n) varies greatly.

3. Molding and core making materials

In the molding and core making materials used in the production of iron castings, all kinds of organic binders mostly contain nitrogen, and the problem of nitrogen absorption in the process of pouring and filling can not be ignored. The results show that various organic nitrides have different effects on increasing nitrogen in cast iron. At high temperature, ammonia nitrogen is easy to dissociate and release Primary Nitrogen, thus increasing nitrogen in molten iron.

Furan resin and resol resin used in self setting sand do not contain nitrogen originally, but in order to improve properties and reduce cost, they are often made into copolymer resin containing urea formaldehyde component. The nitrogen contained in these two resins comes from urea formaldehyde component and belongs to ammonia nitrogen. The second component of the resin used for urethane resin self hardening sand is polyisocyanate, w (n) content is about 6.0% ~ 6.6%, but it is not ammonia nitrogen and is not easy to dissociate; The hardener contained in the first component is tertiary amine material, belonging to ammonia nitrogen.

The resin used in the cold box core making process is also urethane resin. The two components do not contain ammonia nitrogen, but the nitrogen in the hardener triethylamine is ammonia nitrogen. The phenolic resin used in shell and core does not contain nitrogen, but the hardener Urotropine (hexamethylene tetramine) contains ammonia nitrogen.

From the above analysis, it can be seen that at present, all kinds of organic binders used in iron casting molding and core making contain ammonia nitrogen. In the process of pouring and filling, the liquid iron may increase nitrogen. The actual amount of nitrogen increase depends on the condition of the mold and sand core, the size and wall thickness of the casting.

J. orlenius of JNK Ping University in Sweden cooperated with the foundry to carry out the test. The results show that the w (n) content in molten iron can be increased by about 10 ppm after 390kg casting is poured with resin self setting sand.