Reducing residual stress of high strength cylinder head casting

The residual stress of cylinder head castings has always been an important problem in cylinder head casting. After casting, due to the different cooling rate of each part of the casting, the shrinkage of each part is different, which will restrict each other and produce stress. This kind of thermal stress caused by the thermal hindrance of linear contraction is usually the residual stress of casting. The greater the difference of wall thickness, the greater the residual stress. Casting stress is the main cause of casting deformation, cracking and other defects.

The elimination of residual stress in casting mainly includes natural aging and thermal aging. The natural aging method is to expose the casting with residual stress outdoors for a long time, so that the stress is released and disappeared slowly. Although the cost of this method is low, it takes a long time and the efficiency is low, so it is not suitable for continuous production of large quantities of castings; At the same time, the effect of natural aging method to remove residual stress is very limited, generally about 10% reduction at most.

The hot time effect method is to heat the casting to a certain temperature in the range of 550 ~ 650 ℃, keep it at this temperature for a period of time, so as to eliminate or reduce the stress, and then slowly cool it to room temperature. Hot time effect is a reliable and mature method to remove residual stress. At present, this method is widely used in casting production to eliminate residual stress. But it has the disadvantages of high energy consumption and high cost.

By reducing the casting stress, the machined castings have better dimensional stability and precision durability. Because of the thin wall and complex structure, the cylinder head parts are easy to produce large residual stress, which is the focus of the main engine manufacturers and the difficulty of the cylinder head manufacturers. In recent years, with the continuous development of engine technology, cylinder head parts tend to develop in the direction of high strength and complex structure. To solve the problem of high residual stress in the casting production of high strength cylinder head has become a problem that every foundry has to face.

Due to the high material requirement and complex structure of high grade gray cast iron cylinder head, the residual stress of castings after pouring is large, so the forced stress relief method must be used to reduce the residual stress. It is necessary to consider the casting characteristics, cost, efficiency and other factors in order to release the internal stress of the casting as much as possible and meet the technical requirements of the casting; If the cooling condition of the factory allows, the cost and effect of in mold annealing are lower and better.

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