Oxygen and sulfur are the main surface active elements, which have an important influence on the change of graphite base surface and the growth speed of prism surface, making the growth speed of prism surface larger than that of base surface, hindering the formation of spherical graphite and promoting the growth of flake graphite. Sulfur is one of the important anti spheroidizing elements. Firstly, the spheroidizing elements are consumed in desulfuration and deoxidization. Some of the desulfuration products, such as MGS, enter the slags, and a small part of sulfide exists in the molten iron, which reduces the spheroidizing effect and shortens the incubation time. Generally, the content of oxygen and sulfur in molten iron is required to be as low as possible before spheroidizing treatment.
In the production process of thin-walled, the influence of oxygen and sulfur elements is different from that of ordinary ductile iron. The results show that for the ductile iron castings with a wall thickness of 2 mm, the sulfur content should be kept above 0.011% in order to effectively inhibit the white mouth of the structure. Pig iron with different sulfur content is used for smelting, and vacuum treatment is carried out under the same smelting process conditions. The influence of the oxygen content in the molten iron on the graphite quantity is analyzed. The results show that there is little effect on the number of nodular cast iron with a wall thickness of more than 6 mm, while for the thin-walled cast iron with a wall thickness of 2 mm, the number of nodular cast iron is the largest and the structure has no white mouth after vacuum treatment for 10 min. Under the condition that the structure is free of white mouth, the graphite quantity of the cast iron with sulfur content of 0.02% and wall thickness of 2 mm is the most, reaching 800 pieces / mm2. This result verifies one of the latest inoculation mechanisms: MGS, res and other sulfides can be used as the core of graphite nucleation, promote graphitization and increase the amount of graphite.
The existence of sulfide, which can not nucleate graphite due to oxidation, can often be observed at the grain boundary of the microstructure of thin-walled ductile iron. When the content of oxygen in the molten iron is high, the sulfide surface is oxidized, and it is difficult to play the role of graphite nucleation base. At this time, the number of graphite is small; when the content of oxygen is too small, the number of MgO and Al2O3 and other oxides with strong spheroidizing effect are reduced, and the spheroidizing effect is also weakened, the number of graphite is reduced, and the tendency of white mouth is increased. Vacuum treatment is used to improve the purity of the original liquid iron, which has been applied in the production of thin-walled ductile iron. It takes too much time to treat the original liquid iron by vacuum treatment, so the deoxidizing desulfurizer is added to the original liquid iron at the same time of short-time vacuum treatment, which has been applied in the production of thin-walled nodular iron. It is found that there is an optimal amount of deoxidizing desulfurizer with different vacuum treatment time.
In a word, reasonable vacuum treatment time can fully form oxide which can promote spheroidization as graphite nucleation base, and avoid too much oxygen to oxidize sulfide surface, and make sulfide lose spheroidization. Therefore, the reasonable vacuum treatment time can effectively increase the amount of graphite and inhibit the tendency of white mouth in the thin wall of nodular iron castings.