There are often failures in the use of molds. Starting from the actual situation, we will focus on several failure methods.
There are mainly the following five types: plastic deformation failure; wear failure; fatigue failure; cold and heat fatigue failure; failure under the influence of comprehensive factors.
(1) Plastic deformation failure
The main reason is that the strength level of the mold material is not high; or the heat treatment process is not correct, and the maximum toughness of the steel is not achieved; the improper use of the mold causes local overload; the high temperature softening of the hot work mold will also cause the mold to undergo thermoplastic deformation and failure. Plastic deformation failure can cause the mold to deform, which ultimately affects the quality of the forging.
(2) Wear failure of mold
The root cause of mold wear failure is the friction between the mold and the blank. However, the specific form of wear and the wear process are related to many factors; the pressure, temperature, billet deformation speed and lubrication condition of the mold during the working process. For hot-worked molds, the wear resistance of the cavity surface is softened by high temperature, and the oxide scale itself also acts as an abrasive, which determines that the wear process is more complicated. The wear failure of the die mainly causes the wear of the forging die and affects the accuracy of the forging.
(3) Fatigue failure of the mold
The root cause of die fatigue failure is stress concentration and cyclic loading. Although the load on the mold is sometimes significantly lower than its yield strength, due to the local stress concentration, under low load, microcracks will still form at the stress concentration. The mold is usually served in a high-strength and low-plasticity state. When the micro-cracks are formed, under the cyclic load of the mold, the micro-cracks are easy to grow and eventually lead to fatigue fracture.
(4) Cold and hot fatigue failure of the mold
For hot work molds, the surface temperature of the mold often rises to the range of 600 to 900 ℃ due to the interaction with the hot blank during work. In order not to reduce the strength of the mold, it is necessary to spray a coolant to cool the mold after demolding. In this way, the surface of the mold repeatedly undergoes rapid cooling and rapid heating, and considerable cyclic thermal stress accumulates on the surface of the mold. This stress* is finally released in the form of thermal fatigue, forming thermal fatigue cracks. As for high-temperature oxidation, coolant corrosion, and friction between the mold and the high-temperature blank, it accelerates the process of thermal fatigue.
(5) Failure under the influence of comprehensive factors
The working conditions of the mold are intricate, so damage events occur from time to time. Once the damage is caused, each other may promote each other, which will accelerate the failure of the mold.