Analysis of Causes of Overload Failure of Cold Work Die Processing

(1) Failure of insufficient material toughness

Because this type of failure has no macro signs and sudden fractures, it is the most dangerous accident in the failure of cold work molds. In the past, this type of failure has also occurred in personal accidents, causing great losses to production safety and economic construction. This kind of fracture failure under the unstable state is easy to occur in cold extrusion and cold heading dies, such as punch breakage, cracking, or even bursting. It is characterized by no obvious plastic deformation before failure occurs, and there is no shear lip on the macro fracture. And relatively flat, causing permanent failure of the mold irreparable.

This failure is related to insufficient toughness of the mold material and excessive stress. Analysis and calculation of the actual load-bearing capacity of the cold extrusion die shows that the working strain capacity of the punch before failure is thousands of times that of the material's fracture energy consumption, which shows that the punch bears high potential kinetic energy and low fracture resistance during work. According to the principle of conservation of energy, almost all energy of the punch fracture potential becomes expansion kinetic energy, and its expansion limit speed can reach 10 m/s. When there is stress concentration in the die structure, such as the transition zone at the tail of the hexagonal cold heading punch r≤1mm, the stress concentration factor Ktu003d2, when the cold extrusion punch step ru003d3mm, Ktu003d1.3, even machining tool marks , Grinding rough marks, etc. can become weak links and cause unstable fractures. (Guide: Surface treatment technology is an important means to improve the life of die-casting molds)

High-carbon, high-alloy cold work die steel, the use state is tempered martensite and secondary precipitates, contains more primary residual carbides, the material has high hardness, and the matrix absorbs energy and relaxes stress. The strain ability is low, and the uneven distribution of primary carbides seriously reduces the toughness of the material. Therefore, no macroscopic deformation can be seen in this type of failure fracture, and the size of the microscopic deformation is roughly equivalent to the carbide spacing.

(2) Insufficient strength failure

In the cold heading and cold extrusion punches, the material's compression resistance and bending resistance are insufficient, and the head is prone to sinking and bending failure. Such failures are prone to occur in the development of new products due to excessive working load and low mold hardness. Practical experience shows that the hardness of cold heading punch is less than 56HRC, and cold extrusion punch hardness is less than 62HRC. This type of failure is prone to occur; at the same time, it shows that the material strength is insufficient, plasticity is excessive, and the toughness potential can be used.

The empirical method to solve this kind of early failure is: brittle fracture failure reduces hardness (enhancement); deformation failure increases hardness (enhancement).

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