Nine factors of casting porosity formation and 5 improvement measures of process parameters

1. Genetic analysis of subcutaneous stomata

The subcutaneous blowhole is a common casting defect in wet-type Ductile Iron Castings, so it has a prevalent deficiency and is especially sensitive to thin-walled castings. The reason is that the thin-walled castings have a fast solidification speed. The gas can not form bubbles in time, and the thick-walled castings have a slow solidification speed after the gas bubble formation is enough time to float out, will not be stuck in the casting surface.

2.The main factors affecting the formation of subcutaneous stomata include:

(1) the amount of extra magnesium and the amount of rare residual earth increased significantly when the amount of residual magnesium was more than 0.06%. The statistical data show that when MG residue is 0.05%, the defect rate of subcutaneous blowhole is 7%. When Mg residue is 0.065%, the defect rate of subcutaneous blowhole increases sharply, up to 33%. The higher the amount of extra magnesium, the greater the tendency of forming subcutaneous STOMATA. Rare Earth elements can increase the surface tension of liquid iron, and can effectively prevent the tendency of subcutaneous STOMATA. Re residue = 0.025% is better.

(2) the higher the sulfur content in Molten Iron, the easier it is to cause subcutaneous blowhole. The higher the sulfur content, the more dangerous the situation. This is because, in addition to the hydrogen-induced subcutaneous pores, the defects may be aggravated by H2S gas. After spheroidizing, oxide and sulfide (including MgS) slag will be produced. If the slag is not cleaned, MgS will flow into the mold cavity with the molten iron, the MgS floating up to the metal-mold interface reacts with water and gas at the interface H2s gas produced by the reaction also formed subcutaneous pores.

(3) under the influence of aluminum and titanium, when Al > 0.03% in molten iron, the subcutaneous blowhole increases. When the residual aluminum and residual titanium are both present, the subcutaneous blowhole increases rapidly. The production practice has proved that when Al < 0.03% in ductile iron castings, the subcutaneous blowhole does not generally appear. If Ti > 0.01% at the same time, it will cause the cast to produce subcutaneous blowhole.

(4) When mold moisture is more than 5%, the subsurface blowhole is natural to appear in the wet ductile iron castings. H2 and H2s gas are produced by the chemical reaction between the liquid iron and the water on the mold surface. When the casting solidifies rapidly, it can not float up, so it stays on the surface near the casting surface and forms subcutaneous blowhole.

(5) the hygroscopicity of charge with water, spheroidizer, and inoculant and the increase of hydrogen content in molten iron will lead to more subcutaneous pores.

(6) the excessive slag and the inclusion of iron oxide are the important reasons for the formation of subcutaneous pores (slag is the catalyst leading to subcutaneous pores).

(7) the temperature of hot metal is low, or the pouring temperature is low, it is easy to form subcutaneous blowhole. the casting speed is slow, the liquid iron is easy to oxidize, lower the temperature, enter the slag, easy to form the subcutaneous blowhole.

(8) When the above factors appear alone, the subcutaneous STOMATA may be produced, but it will not be very serious. If all the factors act together at the same time, the subcutaneous stomata tend to increase greatly. According to the production practice of our foundry, we check, analyze and check the above factors one by one, and finally determine (2),(6)

(9) The influence of three factors is more dangerous. At present, there is no systematic and exact theoretical basis for the mechanism of the effect of sulfur on the subcutaneous blowholes of wet-type Ductile Iron Castings. Still, it is an indisputable fact that the subcutaneous blowholes are quickly produced when the sulfur content of molten iron is high, the reason is that more sulfide inclusions (MNS, FeS, MES, etc.) are easily formed in the high-sulfur liquid iron, which causes the slag to form subcutaneous pores. In addition, oxide slags (such as Feo, MnO, Alo, Sio) are formed during smelting and treatment, the oxide slag formed by the reaction of liquid iron with water on the surface of molding sand may become the substrate of gas nucleation, the adsorbed gas molecules aggregate into bubbles, and the amount of residual magnesium in liquid iron is too high, the more oxides and sulfides produced by magnesium in the process of desulphurization and Deoxidation, the more the subcutaneous blowhole of ductile iron will increase sharply, and the higher the pouring temperature will be, the more the subcutaneous blowhole can be effectively prevented, the less inclusion, the reason is: On the one hand, the oxidation reaction is not easy to carry out at high temperature; on the other hand, the inclusion is easy to rise and remove at high temperature.

The chemical composition, melting temperature, and purity of molten iron is involved, which are the three main indexes of the 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 fewer inclusions, and the higher the metallurgical quality of Molten Iron, the less likely it is to produce subcutaneous pores. Therefore, improving the metallurgical quality of molten iron is the key control link to prevent the subcutaneous blowhole in the production of wet ductile iron castings.