Failure and prevention of cold work mold

The quality of cold work molds will directly affect the quality of its products. The life of the mold determines the production cost and economic benefits of the parts, and there are many factors that affect the cold work mold. Therefore, the failure modes of cold work molds are also various. According to actual production experience, effective preventive measures are proposed for several basic failure modes of cold work molds.

　　 For early failure molds, the main manifestations are the following three types of failure: ⑴ Fracture failure: including plastic fracture failure, fatigue fracture failure, creep fracture failure, low stress brittle failure, etc. (2) Deformation failure: including excessive elastic and plastic deformation failure. ⑶ Surface damage failure: including wear failure, corrosion failure, surface fatigue (pitting or peeling) failure, etc.

　　The harsh working conditions make the service life of cold work molds much shorter than other molds. Therefore, in order to prolong the service life of cold work molds, reduce costs, and improve economic efficiency, it is necessary to find out the root cause of mold failure and take effective measures to solve them.

　　1. fracture. This is the most common form of damage to cold work molds, mainly because the load exceeds the strength limit of the mold material or the mold produces stress concentration. For example: the punch is broken, the die and the fastening ring are cracked, the ejector rod is broken, and the insert is fatigued and cracked. (Guide: How to express steel grades)

　　2. Deformed. Because the load on the working mold that acts during cold forming is very large, the working mold that directly bears the pressure will produce a certain amount of elastic or plastic deformation, making the mold unusable and scrapped. For example: the elastic expansion of the concave mold cavity, the upsetting and bending of the punch, the upsetting of the ejector rod, the collapse of the center of the cushion and other deformations.

　　3. Wear. Due to the lubrication of the blank, the softening treatment, and the heat treatment strength of the mold, the cold work mold is the same as other molds, and the early wear and tear of the cavity surface often fails. Such as metal adhesion, pitting, spalling or sharp changes in size on the surface of the ligament working surface.

　　 When these defects occur in the mold parts, qualified fastener products cannot be formed, which will inevitably affect the company's production plan.

　　Precautions

　　 In order to prevent early failure of the mold and effectively extend the service life of the mold, corresponding preventive measures are mainly taken from the following aspects.

　　 One, mold material. Since the cold-worked mold bears a large unit extrusion force, high-performance mold materials with high strength, good toughness and good wear resistance should be selected; because the working conditions of the convex mold are more susceptible to the effect of eccentric force than the concave mold, the convex The wear resistance of the mold material should be selected higher than that of the concave mold material.

　　 2. Heat treatment. Die heat treatment includes: timely tempering of cold-worked die blank after forging to refine crystal grains, tempering after rough machining to eliminate stress, and stress-relieving low-temperature tempering after electric spark and wire cutting. Experience has shown that molds with a diameter of ≤80mm should be directly machined with bar materials, without forging, because improper forging will cause defects such as over-sintering, coarse crystals, segregation, etc.

　　 When the mold is quenched and tempered, attention should be paid to the heating rate, holding time, quenching temperature, cooling rate, number of tempering, surface hardness, etc. Excessively high temperature during mold quenching and heating can easily cause mold overheating and overburning, decrease impact toughness, and lead to early fracture. If the quenching temperature is too low, it will reduce the hardness, wear resistance and fatigue resistance of the mold, and easily cause plastic deformation and wear failure of the mold. When quenching and heating, it is necessary to take protective measures to prevent surface oxidation and decarburization. Decarburization will cause quenching soft spots and soft areas, reduce the wear resistance and fatigue strength of the mold, and affect its service life.

　　 Quenching cooling rate is too fast and the swimming temperature is too low, the mold is prone to quenching cracks. If the tempering temperature is too low and not sufficient, the residual stress in the quenching process will not be eliminated, the toughness of the mold will be reduced, and early fracture will easily occur.

　　3. Mold design The structure of the cold work mold must have sufficient strength, rigidity, reliability and good guidance, but the unreasonable structure of the mold will directly affect the service life of the mold. The working part of the mold should be designed with a large enough fillet radius to avoid stress concentration caused by sharp corner transition. The width of the ligament should be designed reasonably according to the metal flow condition to minimize friction, generally between 1.5-3.5.

　　 Fourth, mold processing. In the die cutting process, the following measures should be taken; improve the surface roughness, the micro-crack marks formed by grinding, the transition part should be smooth, and the knife marks and small defects cannot be processed organically to prevent stress concentration during use and cause cracks. .

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