Slag inclusion casting defect refers to the casting defect formed by dry sand, coating and other inclusions as molten iron enters the casting during pouring. It is a common casting defect in lost foam casting production. On the surface of machined castings, white or black gray inclusion spots can be seen, which are distributed individually or in pieces. White is quartz sand particles, black gray is residue and other inclusions after pyrolysis of slag, coating and foam model. The casting defects of steel castings with slag are shown in the figure.
After the lost foam casting is cooled out of the box, it can be determined whether there are sand injection and slag inclusion casting defects according to the surface conditions of the casting and gating system. In production practice, if there is serious sand sticking or crack like sand sticking on the surface or connection of gate cup, sprue, runner, ingate and gate and on the surface of casting, it may cause sand feeding from gate cup, sprue, runner and ingate to all parts of casting. Among them, factors such as the lax closure of the sprue are the main reasons for the defects of sand feeding or slag inclusion casting. In addition, the size of net head of gating system, pouring temperature, negative pressure, dry sand particle size, pattern transportation process, packing operation and other process parameters are important reasons for casting slag inclusion and sand casting defects.
Slag inclusion casting defect is a difficult problem in lost foam casting production and a systematic project. Only by taking systematic measures and careful operation in these links can the slag inclusion casting defects of castings be reduced and basically eliminated, and high-quality castings be obtained.
(1) Coating with good comprehensive performance shall be selected
In addition to the properties generally required, steel casting coatings are also required to have a series of properties, such as high strength, high fire resistance, high erosion resistance, good explosion heat crack resistance, coating and hanging, suspension, non fluidity and so on. To prevent slag inclusion casting defects, first of all, the coating is required to have high strength and fire resistance, and the straight gate is tightly closed when the liquid steel enters the mold, and the coating layer on the surface of the casting and gating system will not fall off or crack. In addition, the coating hanging on the surface of the white mold is required to have sufficient room temperature strength and not crack during drying and transportation. In addition, the coating layer should have good high-temperature strength, that is, during the pouring process, the coating will not fall off and crack under the long-time scouring action of high-temperature metal, and the coating layer should have a certain thickness.
(2) Standardize packing operation
When packing, the pattern group shall be placed stably on the sand at the bottom of the sand box. It is not allowed to sprinkle sand and shake the shape when it is placed in the air, so as to avoid cracking the coating layer. Do not add sand to the pattern violently. First, use flexible sand to prevent the material layer from cracking. Close the straight gate tightly to avoid sand. The whole process of packing and molding should be very careful. Before pouring, the coating layer of pattern group must be free of any falling off, cracking and crack. Before pouring, it should be confirmed again that the gate cup is free of floating sand, dust and sundries.
(3) Reasonably set the pouring head, temperature and time
During pouring, the higher the pressure head is, the greater the scouring of the gating system and mold is, and the greater the possibility of sand entry caused by washing the coating is. The pressure head of different sizes of castings should also be different. Select the ladle with appropriate capacity, reduce the pouring height as far as possible, and the ladle nozzle shall be close to the gate cup as far as possible to avoid pouring small activities with large ladle. Select the appropriate pouring temperature, because the higher the pouring temperature, the higher the requirements for coating performance, and the easier it is to produce casting defects such as sand sticking and slag inclusion.
(4) Reasonably determine the negative pressure
The pouring process of lost foam casting is generally carried out under vacuum. The function of negative pressure is to compact dry sand, accelerate exhaust and improve mold filling capacity. The magnitude of negative pressure has a great impact on the quality of castings. Excessive negative pressure increases the possibility of absorbing dry sand and inclusions when liquid metal flows through cracks and cracks, and also increases the sand sticking casting defects of castings. At the same time, the negative pressure is too large and the mold filling speed is too fast, which increases the scouring intensity of the metal on the sprue and mold, which is easy to make the coating fall off and enter the metal, and also easy to damage the coating layer and cause sand entry. For steel castings, the appropriate negative pressure is generally: – 0.030 ~ 0.045mpa.
(5) Set slag retaining, skimming and slag collecting risers
During the design of gating system, the functions of slag blocking and skimming should be considered, and the setting of slag collecting riser on the casting can help to improve the defects of sand feeding and slag inclusion casting.
(6) Adopt molten steel purification technology
Purification must be paid attention to in molten steel melting, which is one of the key technologies of lost foam casting, including the whole process from molten steel melting, overheating, deoxidation to pouring into the mold.