Research On Thermal Stress Relief Mechanism And Methods Of Multi-metal Hot Forging Die

The service life of hot forging die is hot issue for a long time, which is more prominent in our country. The average service life of hot forging life in our country, only 3000-5000 forgings, has the big disparity with the international advanced level. The main problems existed are that high manufacturing cost, low service life, high consumption of die steel and backward design and manufacture technique.Hot forging die failure is a combination result of various factors mutual affecting. No matter direct affect factors or indirect affect factors, the root causes leading to hot forging die failure are only mechanical load and thermal load acting on the forging die. In order to improve hot forging die service life, it is not obvious by means of whole optimization method to reduce the mechanical load. Therefore, it is especially important of thermal load research. Because of the high temperature and stress in hot forging process, surface temperature of die cavity always exceeds tempering temperature of hot forging die in continuous forging process, which leads to high temperature softening of die cavity. And tremendous tensile stress exists on the surface of hot forging die resulting in cracks expansion in the lubrication and cooling stage. The existing hot-work die steels are not fully meet the requirements of high-temperature toughness and high-temperature hardness. It is contradiction of high-temperature hardness and high-temperature toughness in any homogeneous hot-work die steels. Although different heat treatment technologies can adjust the contradiction of high-temperature hardness and high-temperature toughness, adjust result can't change the status of rapid softening and fracture for die cavity surface of hot forging die. The traditional surface modification techniques have some role in improving service life of hot forging die, but still can not solve high temperature softening occurred in the surface and near surface of die cavity.Based on the above, this article aims to analyze the failure reasons of conventional forging die, establish a mechanical model, study the thermal stress relief mechanism and methods, determine temperature fluctuation zone, the location and method for the failure area of hot forging die, and explore materials composition design and preparation methods of heat-resistant layer of multi-metal hot forging die, Which provide the basis of design and manufacture of long life multi-metal hot forging die.Firstly, this article outlined the forms of hot forging die failures and influencing factors, introduced the study status and problems of hot forging die life, described the temperature and stress field of hot forging die, established FE model of conventional metal hot forging die based on the finish forging die of the car's front wheel hub to analyze temperature distribution and temperature gradient distribution in the first and continuous forging cycle of hot forging, defined the temperature fluctuations zone, which is most likely failure region, and the temperature gradient zone, established thermal load parameters for evaluating surface layer of hot forging die, put forward thermal fatigue and die wear index of hot forging die and design principles of long life hot forging die based on yield strength and fatigue strength, and discussed the relief failure methods of hot forging die.Secondly, this article established the material structure of multi-metal hot forging die and forward and reversion design methods of heat-resistant layer of multi-metal hot forging die material, determined the materials and preparation methods of the substrate layer, support layer and surface layer based on analyzing the existing hot-work die steels, proposed a FE model of multi-metal hot forging die, and analyzed and compared the temperature distribution and free expansion normal to the surface of hot forging die. The results indicated that the temperature distribution and free expansion of multi-metal hot forging die were lower than those of conventional metal hot forging die.Thirdly, this article had prepared W6Mo5Cr4V2 welding electrodes for overlying support layer to analyze the welding technological properties, microstructures, relationship between temper temperature and hardness, red hardness and tempering stability of 6542 welding electrodes. The results indicated that 6542 welding electrode, whose microstructure was tempered martensite matrix and white granular carbide distributing on matrix in the form of mesh, was worse in welding technological properties, better in red hardness and temper stability than those of D322.Finally, this article had prepared surface layers made from four cermet powders with different ratios on W6Mo5Cr4V2 high speed steel substrate. After compared various preparation methods (cladding, spray, remelting, and spraywelding), a variety of heat sources and the performance of various metal-ceramic powders for preparing surface layer of hot forging die, the results showed that spraywelding was better preparation method, ion beam heat source was the better heat source, and Ni60-SiC metal-ceramic powder was the better die material for surface layer of hot forging die.