The pouring speed of valve castings directly affects the temperature uniformity of the molten metal before the solidification of the valve castings, and then affects the uniformity of the temperature fields in various parts of the castings. The casting speed is fast, the mold is filled quickly, the heat loss is less in the casting process, the temperature of the molten metal in the mold cavity is relatively uniform before solidification, and the temperature distribution of the various parts of the casting is relatively uniform during solidification. When the casting speed is slow, the temperature distribution curve of the valve casting is steeper than that of the later pouring part.
Annealing treatment of valve castings
1. Graphitization annealing: graphitization annealing of valve castings is to reduce hardness, improve processing properties, and improve the ductility and toughness of cast iron.
2. Stress-free annealing: in order to eliminate the residual stress of the casting, stabilize its geometric size and reduce or eliminate the distortion after cutting, it is necessary to anneal the castings without stress.
The chemical composition of cast iron must be taken into account in determining the annealing temperature. When the temperature of ordinary valve castings is over 550 ℃, the graphitization and granulation of some cementites may occur, which will reduce the strength and hardness. When alloy elements are contained, the temperature at which the cementite begins to decompose can be raised to about 650 ℃.http://www.wuxiorient.com/