Metallurgical quality and technological measures to prevent subcutaneous porosity of nodular cast iron

① Desulfurization treatment. The so-called desulfurization is to transfer the sulfur in the molten iron to the slag as much as possible, and then remove it. The less FES in the molten iron, the higher the desulfurization efficiency. Properly increasing the basicity and temperature of the desulfurized slag and reducing the content of ferrous oxide in the slag are conducive to promoting the desulfurization. Therefore, increasing the slag quantity, improving the fluidity of the slag, strengthening the agitation, and increasing the temperature of the molten iron and slag are all available It is conducive to full desulfurization, timely slagging and preventing sulfur recovery. Using calcium carbide and calcium oxide as desulfurizer, the desulfurizing treatment was carried out by desulfurizing outside the furnace. The amount of calcium carbide added accounted for 0.5% ~ 1.5% of the mass of molten iron.

② In order to improve the purity of molten iron and reduce the slag, it is necessary to improve the purity of molten iron. The number of subcutaneous pores (the number of pores) has been reduced, but it has not been eradicated. The application of cryolite powder on the mold surface can eliminate the subcutaneous pores, but it is not ideal. After the casting is cleaned, there are still some small slag holes on the surface, sometimes they are very similar to the small pits formed by the subcutaneous pores, and the surface quality still does not meet the product quality requirements, which may be the reason why the reaction between cryolite and inclusions in the molten iron (mainly refers to solid slag) produces liquid slag, which is still stuck on the surface of the casting and cannot be excluded. Therefore, the design of the casting system for ductile iron castings Filter screen and slag retaining device shall be added as much as possible.

③ Adjust coke iron ratio and optimize smelting process. The improvement of cupola tapping temperature is beneficial to eliminate and reduce heredity and improve casting quality. Therefore, it can be said that the temperature of cupola tapping reflects the production level of modern cast iron. The temperature in front of cupola reaches over 1470 ℃, and the measures of desulfuration and slag collection are adopted at the same time, especially when adding commercial slag collectors, the quantitative cryolite powder (na3alp6, melting point 994 ℃) is taken as The fluxes are added to the surface of the molten iron and stirred and placed properly to improve the effect of slag collection, so as to prevent the hydrogen absorption of the interface molten iron layer, which can effectively inhibit the formation of the subcutaneous pores of the nodular iron castings, with remarkable effect and significantly improved process yield.

④ Control of chemical composition. Although the chemical composition is not taken as the basis for acceptance, it is the premise for the production of high toughness as Cast Ferritic Nodular Iron castings. Strictly control and calculate the burden ratio, control the final silicon content (Si < 3.0%), and minimize the silicon content under the condition of ensuring ferrite. In addition, if the manganese content is too high (Mn > 0.65%), it will also cause slag porosity. Controlling Mn < 0.30% can avoid the formation of cementite in the as cast state. At the same time, the use of low sulfur, low phosphorus pure charge, strictly limit the content of white mouth and anti spheroidizing elements, is also the principle that must be followed.

⑤ Improving the air permeability of the mold is conducive to reducing the subcutaneous pores.

⑥ Using 5T medium frequency coreless induction furnace to melt ductile iron, the subcutaneous porosity can be controlled. The composition is easy to control when melting in electric furnace, and the temperature of molten iron can be raised to 1530 ℃ ~ 1550 ℃. Because no flux is used, the purity of the molten iron is ensured, no coke (especially high sulfur coke) is used in the electric furnace, no sulfur is added, the metallurgical quality of the molten iron is greatly improved and stable, which creates hardware facilities for the production of high-quality ductile iron castings.

Under the condition that mold moisture, air content, nodulized element residue and other harmful elements are effectively controlled in melting nodular cast iron, the only effective measure to prevent subcutaneous porosity is to improve desulfuration and slag removal effect, ensure high purity of molten iron, with high melting treatment and pouring temperature, and improve metallurgical quality of molten iron. 3