In order to ensure that there is no dead corner in the negative pressure inside the sand box, 5 air extraction sand boxes are used. In order to fill the axle housing cavity with dry sand in all directions, it must be matched by three-dimensional vibrating table and 30 ~ 40 mesh Baozhu sand.
The axle housing shall be poured in a single piece. Pay attention to the distance between the two pieces when packing.
Scrap steel with C < 0.2%, P, s < 0.025% shall be selected without oil stain, plastic and other sundries. When melting in a fast medium frequency furnace to the tapping temperature, deoxidize with silicon aluminum barium in the furnace and pure aluminum in the ladle. The total aluminum addition of the two is 0.1%, so as to ensure that the aluminum content of the casting is 0.04% ~ 0.08%. The pouring of each ladle of molten steel is completed within 10 minutes.
Because the molten steel in the vanishing model cavity is filled by negative pressure traction, the impurities in the molten steel will not float up, and all slag retaining measures of the gating system are invalid, including the filter screen. The only effective way to reduce the inclusion of castings is bottom leakage ladle pouring.
Argon blowing during pouring protects molten steel and reduces oxidation. When pouring, pour at the fastest speed without back spraying.
In the operation standard of lost foam casting process in the casting workshop, the pouring time and pouring speed have no practical significance for lost foam casting process operation. Pouring time and pouring speed are fuzzy concepts for lost foam steel casting. From the moment the liquid steel of lost foam casting enters the sprue, it is pulled by negative pressure, and its mold filling speed is much faster than that of normal sand casting, while the air and liquid steel are mixed and filled disorderly under negative pressure. After entering the mold cavity, the gas, impurities and other substances with low density in the molten steel are pumped away by negative pressure, and the impurities with large particles are blocked by the coating. In addition, the wall attachment effect of the molten steel, so the inclusions of the steel castings are generally about 5mm on the surface of the casting, while the internal compactness of the casting is high.
lost foam steel casting risers are generally open risers. Due to the different sizes of risers, the negative pressure in the cavity varies greatly, and the filling speed of liquid steel is greatly affected by the negative pressure. The real mold filling process can not be simulated. So far, there are few casting simulation software in the world, which has practical reference value for lost foam steel casting process.
Another key point of lost foam casting process is emptying. Although the carbon content of stmma is only 69.6%, the local carburization of low carbon steel castings can reach 0.2% ~ 0.4% or even higher without burnout. The gas output of stmma is 2 ~ 3 times that of EPS. If the thin-walled parts such as axle housing are poured quickly without being burned out, a large amount of gas cannot be pumped out by negative pressure through the coating, which will produce serious backinjection. Pouring after the white mold in the sand box is burned out is the only process measure to ensure no reverse injection and casting quality.
The negative pressure is controlled at 0.05~0.06 MPa before burning the foam. When the 3 risers are burned, the negative pressure should be 0.03~0.04 MPa and the negative pressure is less than 0.02 MPa. Burn the foam with oxygen cutting guns or special special guns, and stabilize the oxygen supply after lighting the foam. The flame should not be too large (sprayed outside the riser) or too small (easy to extinguish). Until the other two risers are completely burnt out, the oxygen can not be interrupted in the middle. At the same time, two risers can be fired at the same time. The middle riser will continue to blow oxygen until the flame is completely extinguished. The intermediate riser should not be ignited during the foaming process. It is impossible to determine whether the foam is completely burnt, and the intermediate riser can observe two. Edge flame combustion.
After the casting is completely burnt out, the negative pressure in the mold cavity is low, and the negative pressure in the dry sand outside the coating with a certain thickness is high. This equilibrium state provides sufficient stiffness for the middle sand core. The molten steel enters stably and quickly, and there is no smoke at the riser. The pouring temperature is controlled at 1600 ~ 1620 ℃. Remove the negative pressure 60 ~ 120 s after pouring.
High frequency vibration shall be implemented during pouring. The time for vibration is when the molten steel enters the riser and stops vibration before closing the negative pressure. Pay attention to the timing of high-frequency vibration, and do not advance it. Late vibration has no effect. The purpose of high frequency vibration is to avoid the formation of coarse grains at the hot spot, resulting in too large difference in overall mechanical properties.