(1) Wear failure
Wear failure refers to the failure caused by the friction loss between the working surface of the die cavity and the blank, which changes the size and shape of the die cavity. The main reason of early wear failure of die is the increase of temperature and the decrease of die hardness, which aggravates the wear; The wear resistance of die material is not high; Poor installation and lubrication of forging die; The uneven gap between die blanks will cause local wear of the die; Surface oxidation of materials at high temperature.
(2) Thermal fatigue failure
The load borne by the die in the forging process is periodic load and has certain impact. If the fatigue strength of the working die is poor, or when it works close to its life limit, the die will produce thermal fatigue cracks or even fracture. Thermal fatigue cracks mostly originate in the weak links on the surface of die materials, such as stress concentration parts. In addition, the local temperature of the surface of the die cavity will rise sharply due to the flow and friction of the blank metal. Under the alternating action of cold and heat, the tensile and compressive stress on the surface of the die cavity will also change alternately, which may lead to thermal fatigue cracks in the die. In the failed die, once the internal fatigue crack appears, it will expand rapidly. Thermal fatigue cracks have various forms, which can be linear, radial, and some are connected into a network, which is often called “cracking”.
(3) Plastic deformation failure
Under high load, plastic deformation failure of die is easy to occur, such as local collapse of die cavity, out of tolerance of cavity size and longitudinal bending. The possibility of plastic deformation of the die can be reduced by improving the strength of the stressed part of the die. The means that can be taken include selecting high-strength die materials, improving the heat treatment hardness of the die, shot peening and rolling strengthening the working surface of the die cavity, etc.
(4) Fracture failure
Fracture failure is generally characterized by brittle fracture, which is the most serious failure form of die. According to the fracture mechanism, it can be divided into fatigue fracture and one-time fracture, and according to the fracture cause, it can be divided into crack propagation fracture and overload fracture.