The samples as cast and normalized were observed by Kearns vh-5000 ultra depth of field optical metallographic microscope. The metallographic microstructure of the samples as cast and at different normalizing temperatures is shown in the figure. Figure a shows as cast nodular cast iron, which is composed of pearlite, ferrite, graphite and a small amount of cementite, and a large amount of graphite is surrounded by ferrite in the as cast state, showing an ox eye shape. Fig. B shows the microstructure of the metallographic phase at the normalizing temperature of 870 ℃. Compared with as cast nodular cast iron, there are a lot of fine pearlite around the graphite ball, and the ferrite becomes less. In addition, massive cementite is mixed in the middle of pearlite structure. Fig. C shows the metallographic microstructure with normalizing temperature of 900 ℃. Compared with 870 ℃, there are more fine-grained pearlite, but there is no obvious change in cementite content. It can be seen from figure d that when the normalizing temperature increases to 930 ℃, there is almost no massive cementite structure, and the granular pearlite structure continues to increase and the ferrite structure continues to decrease.
When the temperature rises, the hot pressing stress occurs in the material. When the hot pressing stress reaches a certain value, it will deform the austenite and plug the dislocations in the austenite around the cementite. These dislocations produce local stress around the cementite, making the cementite unstable and easy to decompose. In addition, under thermodynamic conditions, cementite is a metastable phase, which will change from high temperature to low temperature (Fe3C → 3Fe + C). However, the diffusion of carbon atoms will increase with the increase of temperature, so as to promote the decomposition of cementite. Theoretically, the nucleation and growth temperature of graphite in austenite is 900 ~ 950 ℃, which is consistent with the reduction of cementite seen in the experiment when the normalizing temperature is 930 ℃.
Most of the carbon in nodular cast iron exists in the form of graphite balls, which can absorb or release carbon atoms, and the decomposition of cementite will inevitably produce carbon atoms, which will form graphite core at the austenite grain boundary. According to the colloidal equilibrium theory, the solid solubility of graphite core is large. There is a high to low solute concentration gradient between graphite core and graphite ball, which makes the carbon in graphite core diffuse around graphite ball. Therefore, under the normalizing condition of 930 ℃, cementite decomposes and granular pearlite becomes more in the microstructure of nodular cast iron.