The initiation of hot cracks in most steel castings comes from the involved double-layer film, which expands our thinking for analyzing cracks. When casting steel castings, Xinyuanzhu Group found that during the filling process of castings, splashes or fluctuations of the liquid surface often occur. Therefore, the involvement of oxide film is inevitable. However, the involvement is not only caused by the strong motion of the rupture wave, but also easily occurs when the free surface of the liquid shrinks. When the surface of the liquid shrinks, the area of the oxide itself cannot shrink. Therefore, the excess area is folded, and it is involved. Inside the molten metal, the involved oxide film is always double-layered, so it is called a double-layer film. The double-layer film is often wrapped with sand, oxide slag, pores and other inclusions.
It is generally believed that the occurrence of hot cracking is caused by the solidification shrinkage and shrinkage of liquid metal, and it is related to the properties of the casting alloy itself, the properties of the mold, and the structure of the casting. Based on its own casting experience, xinyuanzhu group takes T-shaped hot section as an example, and summarizes the causes of hot cracks as follows:
1. The molten steel is poured into the mold to fill the cavity. Due to the turbulence of the flow and the shrinkage of the molten steel, the oxide film is folded and involved. The molten steel is especially embedded with a double layer of oxide film on the upper surface.
2. The surface of the mold and the molten metal interact at high temperatures, and the generated gas or inclusions are carried by the oxide film and penetrate into the surface of the molten steel to form an interface layer. For resin sand molds, the sulfide decomposed by the hardener infiltrates into In the molten steel, it stays 1.5~2.0 mm away from the surface of the casting, where the fine crystals are chilled
3. With the shrinkage of the dendrite framework during the solidification process, small deformations are generated, and stress is generated by the obstacle of the core. When the generated stress is greater than the breaking strength of the metal material at the current temperature, the metal will first be in the double-layer film containing the inclusions. The dense areas are pulled apart, forming tiny "Z"-shaped "micro cracks"
4. The corners lag behind the other parts to solidify, and the solidified case grows slowly, especially the resin sand mold contains a low-melting phase equivalent to type II manganese sulfide, which worsens the delayed solidification at the rounded corners, and the overall shape of the casting is slightly deformed. The resulting stress is concentrated at the rounded corners, causing micro-cracks to grow, forming deeper or even open "Y"-shaped "pull cracks"; in addition, the bottom of the "T"-shaped hot joint will cause the surface under the action of greater tensile stress. The micro-cracks continue to extend inward, and if the molten steel cannot be fed in the end, an open "X"-shaped "shrinkage crack" will be formed.
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