Nitrogen element in cast iron

There are two forms of nitrogen in cast iron: one is dissolved nitrogen; The other is to form nitride with other elements. Nitrogen has a complex effect on the properties and quality of cast iron. It can be dissolved in liquid or solid cast iron, or precipitated as elemental gas in the solidification process of cast iron, resulting in porosity defects in cast iron. For example, there are nitride forming elements in cast iron, and stable and highly dispersed nitride can also be formed.

1. Solubility of nitrogen in cast iron

The solubility of nitrogen in pure iron and cast iron [w (c) 3.5%, w (SI) 2.3%] at 1 atm pressure and different temperatures is shown in Fig. At 1 600 ℃, the solubility of nitrogen in pure iron is about 460 ppm, and it solidifies to form nitrogen δ- The solubility of ferrite decreases to about 130 ppm. After that, with the decrease of temperature, the δ- The solubility in ferrite decreases slightly. In δ- When ferrite transforms to austenite, the solubility of nitrogen increases to about 220ppm. As the temperature of austenite continues to decrease, the solubility of nitrogen in austenite increases slightly.

At 1600 ℃, the solubility of nitrogen in cast iron with w (c) 3.5% and w (SI) 2.3% is about 75 ppm. Because cast iron has no δ- After ferrite precipitation, the solubility of nitrogen gradually decreases slightly with the decrease of molten iron temperature. During Eutectic Transformation, the solubility of nitrogen in austenite is much higher than that in eutectic liquid iron, and the solid solution amount can be 115 ppm. After that, the temperature of austenite continues to decrease, and the solubility of nitrogen in austenite increases slightly.

It can be seen that even if the amount of W (n) in the molten iron is slightly supersaturated, it is not easy for the casting to produce porosity defects due to nitrogen precipitation during solidification.

2. Effect of nitrogen on casting quality

If the amount of W (n) dissolved in the molten iron is very high, it will precipitate due to the decrease of solubility during the cooling and solidification process of the molten iron, which will lead to porosity defects in the casting.

During the cooling and solidification of molten iron, the dissolved nitrogen does not precipitate, but remains solid solution in cast iron. The dissolved nitrogen in cast iron can promote the formation of carbides. If it can be used properly, it is beneficial. If it is not used properly, it may also be harmful.

For gray cast iron, in terms of matrix structure, dissolved nitrogen can promote the formation of pearlite and make it stable; In terms of graphite morphology, the graphite sheet can be made shorter and thicker, and the end of the graphite sheet can be passivated to make it a little close to the appearance of vermicular graphite. Because of these two aspects, the strength of gray cast iron can be improved obviously by adding proper amount of dissolved nitrogen.

For nodular cast iron, the amount of W (n) in the molten iron can be greatly reduced due to the strong stirring effect in the process of nodularization. The amount of dissolved nitrogen in the cast iron is not high. Generally, nitrogen has no obvious effect on the nodularization of graphite and the matrix structure.

For malleable cast iron, the solubility of nitrogen is relatively high due to the low content of W (c) and w (SI). Generally, the content of W (n) is about 100 ppm. When the content of W (n) is higher, the pitting structure is not easy to appear, which is beneficial to malleable iron. However, if the amount of W (n) is too high, it will lead to the prolongation of the first and second stage graphitization annealing time, and it may also cause the residual pearlite, even cementite in the structure of the casting after graphitization annealing.

Nitrogen generally exists in all kinds of cast iron. In addition to the porosity defects caused by high content of nitrogen, the effect of dissolved nitrogen remaining in cast iron is various, and most of them are favorable.

In cast iron, nitrogen can also form nitrides with a variety of alloying elements and even trace elements, such as tin, cen, LAN, AlN, mgn2, Ca3N2, Si3N4, mn5n2, mn3n2, Fe4N (in order of nitride forming ability from strong to weak), etc. At present, little is known about the content of these nitrides in cast iron and their influence on the properties of cast iron. The reason for this situation is that there is no feasible method for the determination of W (n) in molten iron, only w (n) in solid samples can be determined. The total amount of W (n) in solid cast iron is melted in helium atmosphere, but the device is expensive. Kjeldahl method is used to determine the amount of dissolved w (n) in cast iron, which is cumbersome and time-consuming. It takes a long time to determine the total content of combined nitrogen by beeghly method, and it is not safe. At present, there is no method to determine various nitrides separately.

In this case, it is not easy to provide the target value to control the w (n) content for the production of iron castings, and the general production enterprises do not have the conditions to measure the w (n) content.

American Foundry Association has carried out a lot of investigation and research work, and carried out special research work. Some industrial countries have also carried out relevant research work. However, according to the data obtained under different conditions, the recommended values are often quite different, and the concept of W (n) quantity is also fuzzy. According to the recommended value of W (n) content of cast iron in the past, the author puts forward the fuzzy range of W (n) content control, which can be used as a reference for selecting burden and arranging burden ratio.

Scroll to Top