Analysis of metallurgical quality and the cause of porosity under nodular cast iron sheet

The most common defect in the production of nodular cast iron is the subcutaneous porosity, which is most likely to occur in the green sand mold, especially in the small castings with large specific surface area. Subcutaneous pores are usually located 0.5-1mm below the surface of the casting, which are small circular or elliptical holes, most of which are pinholes with an aperture of 0.5-2mm. The inner wall is smooth (sometimes with a graphite film on the inner surface), which are evenly distributed on the upper surface of the casting or far away from the inner sprue, but occasionally exist on the side and bottom of the casting. In as cast condition, subcutaneous pores are not easy to be found, but they are exposed after heat treatment or machining. The subcutaneous porosity affects the surface quality of the casting, and there are flake graphite in the place where the subcutaneous porosity occurs, so the mechanical properties of the place are deteriorated, and the casting will be scrapped seriously.

The casting material is QT450, the thinnest wall thickness is 6 mm, and the head arc and tail arc are non machined surfaces. According to the technical requirements, it is not allowed to have subcutaneous air holes with a depth of more than 1 mm on the surface. The mass of a single casting is 1.1 kg, and the total casting mass is 50 kg. It adopts wet sand casting and air pressure molding. The inner cavity of the casting is formed by coated sand hot core, which exhausts air from one end of the core head. At the initial stage, 5T cupola is used to melt the molten iron. The temperature of molten iron is about 1 430 ℃, and the bottom of the nodulation ladle is made into a pit. The nodulation method is used for spheroidizing. The spheroidizing agent is fesimg8re7, the inoculant is fesi75al0.5, and the total addition is 0.5% – 0.8%, which is added several times. The surface of the casting is smooth and smooth after rolling cleaning. After the second sand blasting treatment, a small round pit with a diameter of 0.5-2 mm is found on the surface of the casting, which is determined to be a subcutaneous air hole after dissection. In order to eliminate this kind of popular defect, in view of the characteristics that are easy to appear in the wet mold production of nodular iron castings, in addition to paying attention to the sensitivity of this defect to the “wall thickness effect” of castings, that is, the formation of this defect has an internal relationship with the solidification speed of castings, the reasons for its formation should be analyzed, and the corresponding countermeasures should be found out.

Subcutaneous porosity is a common casting defect of wet ductile iron castings, so it has a popular defect, and it is particularly sensitive to thin-walled castings. The reason is that the solidification speed of thin-walled castings is fast, the gas is too late to form bubbles, and the solidification speed of thick-walled castings is slow. After forming bubbles, the gas has enough time to float up and discharge, and will not stay on the surface of castings. The main factors affecting the formation of subcutaneous stomata are as follows:

① The amount of residual magnesium and residual rare earth increased significantly when the amount of residual magnesium was more than 0.06%. The statistical data show that when ω mg residue = 0.05%, the defect rate of subcutaneous porosity is 7%. When ω mg residue = 0.065%, the defect rate of subcutaneous porosity increases sharply, up to 33%. The higher the residual magnesium content, the greater the tendency of forming subcutaneous stomata. Rare earth elements can increase the surface tension of molten iron and effectively prevent the tendency of subcutaneous stomata. It is better that ω re residue = 0.025%.

② The higher the sulfur content in the molten iron, the more serious the situation is. This is because in addition to the hydrogen induced subcutaneous pores, the defects may be more serious due to H2S gas. Oxides and sulfides (including MGS) will be generated after spheroidizing treatment. If the slag is not cleaned and MGS flows into the mold cavity with the molten iron, the MGS floating up to the metal mold interface will react with the water gas at the interface, and the H2S gas will also form subcutaneous pores.

③ Under the influence of aluminum and titanium, when Al > 0.03% in the molten iron, the subcutaneous porosity increases. If both aluminum and titanium remain, the subcutaneous porosity suddenly increases. The production practice shows that when Al < 0.03%, the subcutaneous porosity generally does not occur, but if Ti > 0.01% at the same time, the subcutaneous porosity will occur in the casting.

④ When the moisture content of the mold is more than 5%, the subcutaneous porosity is easy to appear. Because of the chemical reaction between the molten iron and the water on the mold interface, H2 and H2S gas are produced. When the casting solidifies rapidly, it will stay close to the surface of the casting and form subcutaneous pores before it floats up.

⑤ The moisture absorption of charge with water, spheroidizer and inoculant and the increase of hydrogen content in molten iron will lead to more subcutaneous pores.

⑥ Excessive slag and oxidation inclusions in molten iron are the important reasons for the formation of subcutaneous pores (slag is catalyst which leads to the formation of subcutaneous pores).

⑦ It is easy to form subcutaneous pores due to the low temperature of hot metal discharging or pouring.

⑧ The pouring speed is slow, the molten iron is easy to oxidize, cool down, enter slag and form subcutaneous pores.

When the above factors appear alone, the subcutaneous stomata may occur, but it will not be very serious. If the infant factors act together, the tendency of subcutaneous stomata will increase greatly. According to the production practice of our company’s foundry, the above factors were compared, analyzed and checked one by one, and finally the influence of ②, ⑥ and ⑦ factors was determined to be more serious. At present, there is no systematic and exact theoretical basis for the influence mechanism of sulfur on the subcutaneous porosity of wet ductile iron castings, but it is an indisputable fact that the high sulfur content of the original molten iron is easy to produce subcutaneous porosity, because the high sulfur molten iron is easy to form more sulfide inclusions (MNS, FES, MES, etc.), causing subcutaneous porosity of slag, in addition, oxide slag (such as FeO, MnO) is formed during smelting and processing , alo, SiO, etc.), the oxide slag formed by the reaction of molten iron with the surface moisture of molding sand, etc., may become the substrate of gas nucleation. The adsorbed gas molecules gather and integrate into bubbles. The residual magnesium content in molten iron is too high. The more oxide and sulfide produced by magnesium during desulfurization and deoxidization, the subcutaneous pores of nodular cast iron will increase sharply, and the pouring temperature will be raised above 1380 ℃, which can In fact, the higher the temperature of molten iron is, the less inclusions are. On the one hand, oxidation reaction is not easy to carry out at high temperature; on the other hand, inclusions are easy to rise and exclude at high temperature.

In addition, the chemical composition, melting temperature and purity of molten iron are involved, which are three main indexes of metallurgical quality of molten iron. The more strictly controlled the chemical composition, the lower the sulfur content of harmful elements; the higher the temperature of molten iron, the less inclusions; the higher the metallurgical quality of molten iron, the less the possibility of subcutaneous porosity. Therefore, improving the metallurgical quality of molten iron is the key control link to prevent subcutaneous porosity in the production of wet ductile iron.

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