During the continuous cooling process after solidification, if the solid shrinkage of the casting is obstructed by Yin, the internal stress will be generated in the casting, which is called the internal stress of the casting. This kind of internal stress is sometimes temporarily stored in the cooling process and sometimes remains until room temperature, which is called residual internal stress. The internal stress of casting is the main reason for the defects such as deformation, cold crack and hot crack. According to the causes, it can be divided into thermal stress and shrinkage stress.
1.The thermal stress is caused by the uneven wall thickness of the casting and the different cooling rate of each part, so that the contraction of each part of the casting is not consistent in the same period. In order to analyze the formation of thermal stress, it is necessary to understand the change of metal state from high temperature to room temperature, that is, to distinguish the plastic state and the elastic state.
2.Plastic state: the metal is in plastic state from solidification termination temperature to recrystallization temperature (620-650 ℃ for steel and castings). At this time, the elongation is very high and the plasticity is good. Under a small external force, plastic deformation (permanent deformation) occurs, and the internal stress is eliminated by itself.
3.The metal with elastic state lower than recrystallization temperature is in elastic state. At this time, under the action of external force, the metal has elastic deformation, and the stress continues to exist after deformation.
It is the internal stress formed by mechanical obstruction of contraction, so it is also called mechanical stress. There are many reasons for the formation of mechanical barriers, such as too high temperature strength of molding sand or core, poor yielding, too little sand consumption of sand box belt or core bone, etc.
Shrinkage stress generally causes tensile or shear stress in the casting, which is temporary. After the casting is boxed or the core is taken out, the stress will disappear. However, the shrinkage stress can work together with the thermal stress in the mold, which enhances the tensile stress and causes the hot crack or cold crack in the casting at high temperature or low temperature.