| Aiming at the low hardness of Ti(C,N)-based ceramic tool materials, high-performance Ti(C,N) composite cermet tool materials with high hardness were developed. The microstructure, mechanical property, sintering mechanism, hardening mechanism, high-temperature flexural strength and its reliability, and cutting performance of the developed cermet tools were deeply investigated.The Ti(C,N)-TiB2-WC and Ti(C,N)-TiB2-(W,Ti)C composite cermet tool materials were fabricated. The room temperature mechanical properties and microstructure of the composite cermet tool materials were examined. The results show that when the TiB2 content is about 20 wt.% and the additives of Ni and Mo are nano-scale, the Ti(C,N)-TiB2-WC composite cermet tool materials has the optimum mechanical properties which was sintered under a pressure of 32MPa and a temperature of 1600℃ in vacuum for 60 min. The sintering process of a higher temperature with a fast heating rate, or a lower temperature with a slow heating rate was found to be beneficial to the mechanical properties of the Ti(C,N)-TiB2-(W,Ti)C composites. The optimal sintering temperature range for fabricating the composites was from 1585 ℃ to 1620 ℃. At the sintering temperature of 1620 ℃ and the heating rate of 90 ℃/min, the composites TB20TW (Ti(C,N)-20wt.%TiB2-(W,Ti)C) displayed the optimum mechanical properties. It’s flexural strength, fracture toughness and hardness were 946 MPa,7.4 MPa-m1/2 and 22.9 GPa, respectively. At the sintering temperature of 1585 ℃ and the heating rate of 50 ℃/min, TB30TW (Ti(C,N)-30wt.%TiB2-(W,Ti)C) has a flexural strength of 1406 MPa, fracture toughness of 6.6 MPa-m1/2, and hardness of 21.9 GPa, exhibiting optimum mechanical properties. Two kinds of core-rim microstructures for Ti(C,N)-TiB2-(W,Ti)C composite cermet tool materials were found inside the TiB2 and Ti(C,N) grains. The boron and molybdenum atoms diffused mutually during the liquid-phase sintering process, which led to the formation of the core-rim microstructure of the B4MoTi solid solution rims. Both molybdenum and tungsten atoms diffused toward Ti(C,N) to form the (Ti,W,Mo)(C,N) solid solution phase during the sintering process.The sintering mechanism of the developed Ti(C,N)-based composite cermet tools was investigated and the sintering diffusion model was established. The results show that the growth rate of the rim structure increases with an increase in the sintering temperature, and the growth rate decreases after the first rise with the extended diffusing time.The hardening mechanism was studied and the residual stress model of Ti(C,N) composite cermet tool materials was built. The results show that the combined effect of the residual stress field contributes to the high hardness of the tool materials. Basically, the hardening mechanism attributes to the effect of core-rim structure for TiB2 and the high-hardness chemical bond between Ti(C,N) and TiB2.The variation of high-temperature flexural strength of the developed Ti(C,N)-based composite cermet tool materials was studied and the random distribution regularity was revealed. The distribution function of high-temperature flexural strength and the reliability function of the Ti(C,N)-based ceramic tool materials were built. Ti(C,N)-TiB2-(W,Ti)C composites were selected to be the cutting tool based on the high-temperature flexural strength. When the temperature was above 800 ℃, the high-temperature flexural strength of the Ti(C,N)-TiB2-(W,Ti)C composites decreased rapidly that attributed to microstructure damage caused by oxidative attack, the softening effect of the metallic phase at a high temperature, the reduction of interfacial bonding strength, the reduction of elastic modulus at a high temperature and the release of residual stress. The high-temperature flexural strength of TB20TW and TB30TW at 800 ℃ obeyed the Weibull distribution. The Weibull distribution module of the high-temperature flexural strength is greater than that of room-temperature flexural strength, then the distribution of the high-temperature flexural strength is less discreteness than that of room-temperature flexural strength.When the high-temperature flexural strength reliability was 0.9, the reliable flexural strength of TB30TW and TB20TW at 800℃ was about 720 MPa and 688 MPa, respectively.Based on a 3-dimensional FEM cutting model, the range of cutting parameters for martensitic stainless steel 17-4PH with Ti(C,N)-based ceramic tools under continuous wet-cutting conditions was optimized. For TB20TW and TB30TW composite ceramic tools used for continuously wet-cutting martensitic stainless steel 17-4PH, the cutting performance was studied. The results indicated that the cutting speed is the most contributory factor on tool life. With a cutting speed of 150m/min, a feed rate of 0.10mm/r, and a depth of cut of 0.25mm, TB20TW and TB30TW tools obtained the largest tool life, which was 140 min and 170 min, respectively. The wear patterns and mechanisms of the TB20TW and TB30TW composite cermet tools for continuously wet-cutting martensitic stainless steel 17-4PH were studied. The results showed that the main failure pattern of the two cutting tools was tool flank wear, and the wear mechanisms were oxidative wear, grooved wear, diffusive wear and adhesive Wear. |
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