Nitrogen has a favorable effect on the formation of graphite and matrix when cast iron is solidified and cooled. The strength of cast iron is improved by the improvement of metallographic structure. The distribution of nitrogen in the metallographic structure is tested by experiment. The conclusion is: 1. Nitrogen makes cast iron solution. When the equilibrium and non-equilibrium primary crystallization temperature decreases, the crystallization polycooling increases, the temperature range of Eutectic Transformation increases, the nitrogen effect on Eutectic Transformation Temperature decreases, and the transformation temperature range increases. The influence of nitrogen on eutectic transformation temperature is related to the carbon content of cast iron. 2. Nitrogen affects the morphology, quantity and distribution of graphite. It shortens the length of graphite, increases the bending degree, blunts the end and decreases the aspect ratio. 3. Nitrogen has a significant effect on the matrix structure. It shortens the primary axis of primary austenite, decreases the secondary arm spacing, refines the eutectic group, increases the number of pearlite, and increases the microhardness of pearlite and ferrite. 4. The nitrogen concentration on the graphite surface after eutectic transformation was measured. It was found that there were several nitrogen adsorption layers with atomic thickness on the graphite surface. The nitrogen concentration in the graphite was obviously higher than that in the matrix, which hindered the growth of the graphite, thus refined the eutectic structure and distorted the lattice of the graphite during the growth, resulting in the bending and branching of the graphite. The lattice constants of ferrite and cementite increase obviously before and after nitrogen addition, which is caused by the distortion of nitrogen atoms dissolved in the ferrite and cementite, thus improving the strength and microhardness of the matrix.
Therefore, the suitable carburizing agent should be used to control the nitrogen content in a range which can not only improve the strength of cast iron, but also produce no nitrogen porosity. We control it under a safe upper limit. If the strength of cast iron is not satisfactory, we can use this kind of nitrogen content. Nitrogen strengthening inoculant can increase the content of nitrogen to improve the strength of cast iron. However, with the content of sulphur, high sulphur content can be allowed to ensure good inoculation effect of cast iron.