According to the causes, the pores produced by lost foam casting steel castings are divided into the following four categories. The pore defects of steel castings are shown in the figure below.
(1) Pores caused by foam like cracking products being sucked into molten metal
Turbulence occurs in the process of pouring and filling, or part of the pattern is surrounded and cracked by metal liquid in the process of gasification, and the generated gas cannot be discharged from the metal liquid, which will form pores. The pores are large and numerous, and there is carbon black on the inner surface.
Preventive measures: improve the process to make the liquid steel fill the mold smoothly without turbulence; Reasonably increase the pouring temperature and increase the negative pressure (if turbulence causes pores, reduce the negative pressure); Improve the permeability of coating and molding sand.
(2) Pores caused by foam appearance and poor coating drying
When the coating of foam pattern is not dry well and the content of foaming agent is too high, a large amount of gas will be generated during pouring, resulting in pores.
Preventive measures: fully dry the foam pattern (operate according to the mold material foaming and molding process characteristics); The coating must be dry and the addition of foaming agent shall be strictly controlled.
(3) Air holes caused by excessive adhesive
The pattern binder has large gas generation, excessive use and slow gasification, which is easy to draw the gas into the liquid steel to form pores.
Preventive measures: select the model adhesive with low gas generation. On the premise of ensuring the adhesion, the less the amount of adhesive, the better.
(4) Air is drawn in during pouring to form pores
When pouring, the sprue is not full, and it is easy to draw in air. If these gases cannot be discharged in time, it is easy to cause pores.
Preventive measures: a reasonable gating system shall be designed to ensure that the liquid metal flows smoothly without air entrainment during mold filling; When pouring with closed gating system, ensure that there is a certain amount of liquid metal in the sprue cup to ensure that the sprue is full; The use of hollow sprue formwork can reduce the air generation and prevent air holes.
(5) Blowholes caused by molten steel melting quality problems
Carbon steel smelting process contains gas, which needs pre deoxidation treatment and final deoxidation treatment. If deoxidation is not sufficient, oxygen will remain in the molten steel and form pores.
Preventive measures: in the smelting process, strictly follow the smelting process, deoxidize before pouring and purify the molten steel.