Pressler's insights on mold cycle temperature and lifespan and mold quality

        Pressler independent research and owns the proprietary intellectual property rights: CN109021630A， CN108857300A, CN108857298A, CN108723208A

        For the hot forming mold, the hot forming mold is actually in an extreme working environment due to the environmental change of the alternating temperature and the friction of the high hardness metal compound from the product surface. The main early failure modes of this environment caused by hot pressing molds are plow-type abrasive wear and reticulated crack-type thermal fatigue cracking. In view of these failure methods, the main countermeasures are to adopt different mold life extension solutions for different products. These mold life extension programs are mainly for the material and process selection of mold materials and mold heat treatment and surface treatment. However, no matter which mold material is selected, the purity of the material should be strictly checked. The content of sulfur and phosphorus has an adverse effect on the generation and development of thermal fatigue cracks and should be controlled to a minimum.

       For the hot forming of bare board products, after the sheet is heated, it is inevitable that uneven oxide scale will be generated on the surface of the sheet. This kind of hard and highly easy-to-fall oxide scale provides grinding for the friction pair between the sheet and the mold surface The abrasive grains and surface unevenness of the grains cause the hot press mold to be prone to severe plow-type burrs and severe scratches on the surface of the product during early use. In view of this situation, three methods must be adopted to improve the mold life and product surface quality. 1. Minimize the generation of scale. Pressler® Coatit® technology and nitrogen protection in the heating furnace are used in the production line of Pressler respectively for the high temperature and oxidation resistance coating on the sheet surface. The high temperature and oxidation resistant coating on the surface of the sheet is a composite coating based on hexagonal boron nitride. This high temperature and oxidation resistant coating is applied before the sheet is placed in a heating furnace. By covering the surface, it prevents the combination of oxygen and iron atoms during the heating process, and greatly reduces the generation of scale on the surface of the sheet. 2. Add high temperature lubricant directly to the sheet and mold surface. The surface of the sheet used in the Pressler production line is resistant to high temperature and oxidation and is also a high temperature lubricant. It plays a role of high-temperature lubrication in the friction pair between the sheet and the mold surface, reducing the friction coefficient of the pair from 0.4 to 0.2 without lubricant. Greatly alleviates the severity of abrasive wear. Significantly improve the product surface quality and reduce the wear on the mold surface. At the same time, the low friction coefficient also promotes the formability of the sheet. In addition, due to the excellent thermal conductivity of HBN (hexagonal boron nitride), the heat conduction between the sheet and the mold is promoted (filling the tiny air gap between the two). HBN's soft texture does not bring abrasive particles, and it is an environmentally friendly material that is easy to clean. 3. Improve mold surface hardness and surface wear resistance. To improve the surface hardness of the mold, surface treatment technologies such as nitriding can be used, or hot-working mold steels with high carbon and chromium content such as D600, Cr7V, DE-HS, and heat treatment hardness can be increased to HRC54-56. However, these means for increasing the hardness have their own limitations. For example, the resistance of the nitrided layer to temperature changes is not high. The main reason is that the thermal expansion coefficient is different from that of the substrate, which may cause the early nitrided layer to peel off when the mold is used. In addition to the high carbon content affecting the performance of repair welding, excessive carbide is also one of the potential sources of thermal fatigue cracking. In addition to the high chromium content, in addition to reducing the thermal conductivity of the mold, forcing a longer holding time and reducing the production cycle, the high surface temperature of the mold surface is too long due to the low thermal conductivity. The mold surface is tempered and softened during mass production, which reduces the wear resistance and Causes thermal fatigue cracking. At the same time, the low tempering temperature of the mold material during heat treatment, such as below 620 °, helps to increase the hardness, but is not conducive to resisting the mold's ambient temperature above 650 °, which is likely to increase the tempering and softening of the mold surface in mass production. In this sense, a lower sheet entry temperature, such as 700 ° instead of the usual 750 ° -800 °, is beneficial for extending the life of the mold. Of course, this requires the press to go down fast enough. Special attention should be paid not to choose materials with poor abrasion resistance such as Dievar, QRO90, etc. under bare board conditions, especially when the scale is insufficiently controlled.

       For the hot forming of coated plate products, due to the protection of the aluminum-silicon coating, no scale is generated during the heating of the plate. At the same time, the aluminum-silicon coating reduces the coefficient of friction. Therefore, the main early failure form of the mold is thermal fatigue cracking, and the mold brushing is usually not very serious in the early stage. Therefore, from the mold materials, we must choose low-carbon, low-chromium, high-molybdenum materials with excellent thermal fatigue performance and lower hardness, such as Dievar, QRO90, HTCS, etc. At the same time, the heat treatment hardness should be controlled at HRC48-52. These materials have good thermal conductivity, and due to the low carbides, high heat treatment and tempering temperature are all conducive to the occurrence and development of temper softening and thermal fatigue cracking caused by the ambient temperature of the mold. However, due to the friction between the aluminum-silicon coating on the sheet and the mold surface during the forming process, it is inevitable that the coating will fall and adhere to the mold. These falling coating materials contain hard aluminum oxide inside, which becomes abrasive particles in the pair of friction pairs on the surface of the sheet and the mold. With the intensification of coating adhesion on the mold, plow-type wear will eventually occur on the mold surface and severe burrs will form on the product surface. In order to reduce the drop of the sheet coating due to friction, the surface finish of the mold becomes a decisive factor. However, because the mold surface hardness is not high, the initial good finish cannot be maintained for too long. PVD surface treatment can deposit a hard and wear-resistant coating on the mold surface at low temperatures, but because the coating is too shallow and is not a metallurgical combination with the substrate, the coating life is short. The nitriding treatment has a short life due to its low ability to resist alternating temperatures. High-temperature surface treatment methods such as TD or boronizing can not be used because the surface hardness is higher and the life is longer, but it is easy to cause deformation of the finished mold.

       Under the extreme conditions of hot stamping, the current hot stamping die materials and heat-treated surface treatment materials and processes cannot take into account wear resistance, high thermal conductivity, thermal fatigue resistance, and resistance to long-term tempering softening. As a result, the life of the mold is not as good as that of the cold stamping mold material. To this end, Pressler is developing a mold material that has both abrasion resistance, good thermal conductivity and good thermal fatigue properties, and its manufacturing process, namely Pressler® DieShell® technology. The main feature is the use of additive manufacturing to deposit low-chromium or chromium-free alloy materials on the water channel to the mold surface to increase the thermal conductivity and reduce the mold surface temperature. The core substrate is made of a low-alloy material to reduce the material cost. The outer surface of the weld is covered with a thin layer of a high-alloy coating with excellent thermal fatigue performance by laser cladding. The outermost layer is made of ion-nitriding to produce a high-hardness wear-resistant layer composed of chromium-molybdenum-titanium-aluminum-iron nitride. Additive manufacturing can directly manufacture the water channel and the water channel can follow the mold surface, making the water channel closer to the mold surface than the traditional manufacturing method, thereby increasing the part cooling rate and increasing the production cycle, while reducing the mold surface temperature and extending the mold life. . In addition, in order to further extend the life of the mold, Pressler is developing micro-nano surface treatment technology. It is characterized by femtosecond laser surface micro weaving technology to achieve self-lubricating and high temperature resistant coating deposition on the mold surface, and femtosecond laser surface detonation technology to achieve nanocrystalline alloy coating on the mold surface to improve the hardness of the alloy coating surface. Create more suitable conditions for nitriding and boronizing.