Views: 57 Author: Site Editor Publish Time: 2018-04-11 Origin: www.fuchun-casting.com
When a casting solidifies, with the decrease of temperature, the casting will become solid in this process will occur phase transformation, as the casting into solid phase transformation process, phase transformation will also occur shrinkage or expansion process, due to the existence of a thicker outer wall of the casting, the temperature of the outer layer drops rapidly, but the temperature of the inner layer will drop very slowly. Therefore, the thicker the wall thickness is, the longer the cooling time will be.
This phenomenon results in different shrinkage of the inner and outer layers of a complete castings and different thicknesses. This phenomenon will affect the mutual restraint between the inner and outer layers of the castings, resulting in tensile or irreversible deformation of the castings. Thermal stress is a phenomenon that the thickness of the casting wall is not uniform in the same unit time and the degree of shrinkage is different.
These stresses can lead to fracture or plastic deformation depending on the other variables of heating, which include material types and constraints. Temperature gradients, thermal expansion or contraction and thermal shocks are things that can lead to thermal stress. This type of stress is highly dependent on the thermal expansion coefficient which varies from material to material. In general the larger the temperature change, the higher the level of stress that can occur.
Effects of casting stress:
The remnants of casting stress have a very bad effect on the quality of a casting.Residual stress is an important factor that causes deformation during the production and storage of castings. It will reduce some properties of castings after long-term accumulation.
For example, the direction of residual stress obtained by a casting and its direction of working stress are duplicated, then in this casting working time, the two forces will produce mutual effect, resulting in the direction of the force beyond the scope of the casting material, so that the casting deformation and even fracture.And in large-scale production process, if the castings are deformed, the castings can not be put into the pre-prepared mold, which leads to the failure of production line.In addition, for some castings with high precision requirements,if the residual stress in the castings causes the deformation of the castings, then it will greatly affect the accuracy, but also cause economic losses.