| The ductility of high-carbon high-alloy steels can be improved via refining the carbides by high-ratio rolling, and the service life of the mold fabricated by this type of steels could be prolonged consequently. However, high-ratio rolling leads to thickness limitation of the as-rolled sheet, which is hard to meet the requirement of large size dies. If weld the as-rolled sheet to medium-carbon alloy steel which is relatively inexpensive but possesses good ductility to prepare composite die steels, not only combines the strength and ductility of different materials and prolongs the service life of the die, but also lowers the cost. It is of great significance to the practical application especially to the large size cold work die steels.In order to overcome the disadvantages of serious carbide segregation and poor toughness of Cr12MoV and high speed steel that widely used in industry, a new type high-carbon high-alloy steel that is suitable to prepare composite cold work die steel was developed by reducing the content of C and Cr and increasing the content of Mo and V based on the composition of Cr12MoV, named after Cr10Mo1V1.The range of quenching temperature of this steel is wide (1000℃-1100℃). The hardness of Cr10Mo1V1 can reach to 63.5HRC after quenching at 1050℃and tempering at 150℃or 61.5HRC after quenching at 1150℃and tempering at 550℃. The mechanical properties of Cr10Mo1V1 such as the bending strength, the toughness and the abrasive resistance etc. are all better than Cr12MoV after conventional heat treatment. The working life of the mold die made of the newly developed cold work die steel Cr10Mo1V1 is more than five times longer than that of Cr12MoV.The finite element analysis of the stress distribution and it's variation of the puncher head during punching shows that the stress distributions of both the composition die steel and conventional die steel are the same. Therefore, the thickness of the edge and the type of base materials of a composite punching head can be determined by the evaluation of the stress distribution of the punching head according to the type and thickness of the materials being blanked.Considering that the heat treatment of composite die steel should be in accordance with the normal die steel, the feasibility of three kinds of mild carbon structural steel 5CrMnMo, 42CrMo and 60Si2Mn as base materials of composite die steel was studied mainly on the microstructure and mechanical property after heat treatment. The results show that though high temperature quenching results in the coarsen of austenite, plasticity, toughness and strength do not change too much because the plate martensite with poor toughness is eliminated notably, instead, lath martensite is obtained. After quenching at the high temperature followed low temperature tempering, the hardness is about 60HRC. in case the followed tempering temperature is 550℃,the hardness is 40HRC. The selected three medium carbon structural steels all can meet the different requirement of base materials of composite die steel according to stress analysisThe technique as well as the mechanism of vacuum diffusion welding using relatively higher pressure is studied, the results show that the welding quality is determined mainly by the deformation rate during the process. No matter how the temperature, pressure and the holding time change, same welding quality will be obtained as long as certain deformation rate is achieved to same extent.According to the above mentioned results, the parameters of vacuum diffusion welding using relatively higher pressure can be determined through procedures listed below:the pressure (P) is determined to ensureσ>RP under certain temperature while taking the conditions of equipment, the relationship between materials yield strength (RP) and temperature and the welding cross sectional area(S) into account.The holding time was determined based on the deformation rate and the microstructure requirements. Post welding annealing helps to eliminate the straight welding interface so that the welding quality can be improved further, meanwhile shortens the holding time. In case of dissimilar metals, the pressure (P) should be determined based on the yield strength of the material whose strength is lower. The working life of the screw plate made of composite die steel M2/5CrMnMo is more than two times longer than that made of Cr12MoV.
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