Analysis of cracks in lost foam steel casting parts

Open riser empty shell pouring can effectively solve the problem of casting crack. The occurrence of casting cracks in the new foundry can be attributed to two aspects: composition and slag inclusion porosity defects. The reasons for the existence of casting defects are never unilateral. To solve the problem, we must have an overall view, comprehensively study multiple variables and think about the problem dialectically.

A conclusion can be drawn by comparing five batches and 15 heats of zg27simn castings in two stages. The white mold is the fundamental source of carburizing. To overcome the carbon carburizing of castings, we should reduce the carbon residue in castings from two aspects, and adopt the foam model with qualified density. Optimizing carbon emission during casting and adopting open riser design process are conducive to slag collection and exhaust. As the best process scheme, shell casting can fundamentally solve the problem of low steam steel casting carburization of low carbon steel and solve the cracks caused by uneven composition of castings. Compared with the dark riser, the open riser is more conducive to the discharge of gas and the floating of impurities in steel water, reduces the existence of slag inclusion pores and other defects in the casting, and solves the cracks caused by slag inclusion pores.

However, empty shell casting has its limitations in the experimental process. The following aspects need to be considered:

(1) The requirements for lost steam steel casting coating are relatively strict for empty shell casting. During the experiment, there are network lines and sand sticking on the casting surface. The reason is that cracks appear in the coating layer during the model emptying process, which leads to the external drilling of molten steel and increases the difficulty of later cleaning. Therefore, the coating layer must have sufficient strength to ensure that there is no cracking in the burning process. This requires that the thickness of the coating layer is thicker than that of the traditional lost foam steel casting, and the coating layer must be completely dried. However, too thick coating is not conducive to the emission of gas on the surface of castings. It is very important to study and find a suitable coating.

(2) Dry sand molding must also have appropriate negative pressure. After the mold shell is burned out, the role of the coating layer supporting the mold cavity approaches zero. The key to whether the box collapses or not is sufficient negative pressure, which has high requirements for vacuum equipment. Reasonable holding time after casting is also the key to the quality of castings. If the holding time is short, the casting can not be completely solidified, resulting in deformation. If the holding time is too long, it is not conducive to the release of internal stress of the casting. The gas flow channel under vacuum condition and the reasonable pressure holding time of castings with different wall thickness must be considered in process design, which has high requirements for process design.

(3) The mold burning personnel must cooperate with the casting personnel and the production line operator. Any problem in these links will lead to box collapse and scrap of castings.

(4) The open riser has limited thermal insulation effect and poor feeding capacity. Although the factory adopts the top casting process and covers it with thermal insulation covering agent after casting, try to ensure the final cooling of the riser, increase its feeding capacity, and eliminate shrinkage porosity and other factors that are easy to cause casting cracking. The riser is relatively large, and the casting yield is still relatively low.

(5) Reasonable slag avoidance means in casting is also very key. This requires comprehensive consideration of a series of problems, such as electric furnace smelting, ladle materials, heat preservation and slag collection means, casting slag retaining means and so on.

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