| With the development of equipment manufacturing industry, high-strength steels have been widely used in national defense, aerospace, automotive, chemical industry, and so on, because of their high strength and good mechanical properties. However, due to the low thermal conductivity, high strength and hardness, the machinability of high-strength steels is poor, and tool life is shortened. In this dissertation, in order to accomplish the efficient machining of difficult-to-machine materials of high-strength steels, the performance-driven design method for tool materials was proposed to enhance the pertinence of tool material design and development, and to improve tool cutting performance. Under its guidance, the Ti(C,N)-based cermet tools with good cutting performance in the machining of high-strength steels were developed successfully. And the microstructure, mechanical properties and cutting performance of Ti(C,N)-based cermet tool materials were also systematically studied.The performance-driven design method for tool materials used in machining of high-strength steels was proposed. Based on the fuzzy comprehensive evaluation method, the cutting performance of cemented carbide tools, ceramic tools and cermet tools was comprehensively evaluated with considerations of multiple evaluation indicators, namely machined surface roughness of workpiece, average flank wear of tool and tool life. And cermets were chosen as the optimum tool materials for the finish machining of 42CrMo high-strength steel under continuous dry turning conditions. The relationship between mechanical properties and cutting performance of cermet tools was studied by the entropy method and index model method. And the subjective and objective weights of mechanical properties of cermet tools were respectively assigned, by which it can be obtained that the comprehensive weights of transverse rupture strength, hardness and fracture toughness were 0.171,0.240 and 0.589, respectively. The Ti(C,N)-based cermets were chosen as tool materials for the machining of high-strength steels. And the hard phase was Ti(Co.5,No s), the binder phases were Ni and Co, and the additive phases of carbides were WC and Mo2C.The Ti(C,N)-based cermets prepared by hot-pressing sintering process and pressureless sintering process were compared, and it was revealed that the pressureless sintering process was more suitable for the fabrication of cermet tools used in machining of high-strength steels. The preparation process of Ti(C,N)-based cermets was also optimized, and it was concluded that with the preparation process parameters of oleic acid as dispersant,2.0 wt.% paraffin as forming agent, a pressing force of 175MPa, a holding time of 150s and a heating rate of 3℃/min in liquid phase sintering, the fabricated Ti(C,N)-based cermets showed bright surface and good shape without obvious pore, burr, crack and camber, and can meet the requirements for the fabrication of Ti(C,N)-based cermet tool materials.The effects of material compositions and sintering parameters on the microstructure and mechanical properties of Ti(C,N)-based cermets were studied, and under the guidance of weight allocation of mechanical properties for cermet tools, the material compositions and sintering parameters were optimized by the method of fuzzy vector uniformization. It can be found that the tool material of TMWNC with 50wt.% Ti(C,N),25wt.% Ni-Co and a Co/(Ni+Co) ratio of 0.2, and 25wt.% Mo2C-WC and a Mo2C/(Mo2C+WC) ratio of 0.4, had the optimal comprehensive mechanical properties when the sintering temperature was 1430℃ and the holding time was 60min. In addition, the tool material of TMWC with 50wt.% Ti(C,N),25wt.% Ni-Co and a Co/(Ni+Co) ratio of 1.0, and 25wt.% Mo2C-WC and a Mo2C/(Mo2C+WC) ratio of 0.2, can possess the optimal comprehensive mechanical properties when sintered at 1430℃ for 45min.Finally, the cutting performance of self-made Ti(C,N)-based cermet tools of TMWNC and TMWC in the continuous dry turning of 42CrMo high-strength steel was studied, and the comparative study with other tools of different materials on the finish machining of high-strength steel was conducted. The results indicated that at the same cutting speed, the tool life of TMWC was longer than TMWNC. For the tool of TMWNC, when the cutting speed was low, the tool failure was caused by the abrasive wear. And when the cutting speed increased, the surface flaking and nose breakage occurred due to the comprehensive effects of adhesive wear, abrasive wear, mechanical fatigue and thermal fatigue. And for TMWC, the tool wear was severe, accompanied by chipping as a result of adhesive wear and abrasive wear. The finish machining of high-strength steel with tools made of different materials showed that the tools of TMWNC and TMWC, which were developed based on the performance-driven design method, presented better edge chipping resistance and notch wear resistance than the commercial cermet tool. When the cutting speed was 150m/min, the comprehensive cutting performance of TMWNC and TMWC was better than that of cemented carbide tool and ceramic tool, also better than that of commercial cermet tool which contained NbC.The above research results show that the fuzzy comprehensive evaluation method is feasible and accurate for the optimum selection of tool materials. Meanwhile, the weight allocation of mechanical properties for tool materials can help to improve the comprehensive cutting performance of tools. The performance-driven design method and fabrication process for Ti(C,N)-based cermet tool materials can not only meet the requirements for tool materials in the machining of high-strength steels, but also provide reliable theoretical guidance and technical support for the design and development of tool materials. |
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