| Summary: | 碩士 === 國防大學中正理工學院 === 兵器系統工程研究所 === 96 === The material characteristics and mechanical properties, such as yield stress, tensile stress, elongation, and hardness, of cast Inconel 718 Nickel base superalloy were investigated in this thesis. The optimal heat treatment conditions for each mechanical property of Inconel 718 were obtained using analysis of Taguchi method. In addition, this thesis also investigated the effects of adding Hafnium (Hf) on the mechanical properties and microstructure of cast Inconel 718. We hope, by way of adding Hf, to improve the mechanical properties of cast Inconel 718 without conducting forging or hot isothermal pressing (HIP) process.
At first, the heat treatment parameters including solid solution and aging treatment of remelted and cast Inconel 718 bought from Japan IHI corporation were investigated. The optimal condition for the process of solution and aging treatment was analyzed with Taguchi method. The appropriate heat treatment parameters, such as solid solution temperature, solid solution time, aging temperature and aging time, for each mechanical property like strength, elongation, or hardness were also analyzed using analysis of variance (ANOVA) and contribution analysis and then confirmed with experiments. Furthermore, using the IHI Inconel 718 as matrix, we investigated the variation of mechanical properties and microstructure of the specimens of cast Inconel 718, which were added with 1.0 wt%, 1.5 wt%, and 2.0 wt% Hf. The mechanical properties and microstructure between both the remelted and Hf-modified cast Inconel 718 that were heat treated with the aforementioned optimal parameters obtained from Taguchi analysis were also compared.
Our results indicate that, using the appropriate parameters of heat treatment, the cast Inconel 718 can achieve good result for each mechanical property, such as yield stress, tensile stress, elongation or hardness. The best mechanical performance of the cast Inconel 718 for each mechanical property was list as follows, the best yield stress was at about 999 MPa which has improved 68.7% than that of the reference specimen (592 MPa), the best tensile stress was at about 1144 MPa which has improved 34.2% than the 852 MPa of the reference specimen, the best elongation was at about 27.8% which has improved 122%, and the highest hardness was at about HRC 39 with 95% improvement. Compared to the literatures, these optimal mechanical properties were all superior. Moreover, when the heat treatment condition was solid solution at 1140℃ for 2 hr, air cooling to room temperature, raising temperature to 780℃and holding for 10 hr for the first aging, and then furnace cooling to 600℃and holding for 12hr for the second aging, we can obtain a well-balanced cast Inconel 718 which has good quality at all mechanical properties.
For the adding of Hf into cast Inconel 718, we have observed new phases precipitating in the Inconel 718 matrix. The phase identification of XRD and EPMA showed that these phases are Ni5Hf, and MC type carbides. When the adding amount of Hf was 1.5 wt% and the specimen was heat treated with the aforementioned optimal parameters obtained from Taguchi analysis, we found that the hardness of Hf-modified cast Inconel 718 has increased 20% to HRC 36. The yield stress and tensile stress has the highest value of 932 and 1108 MPa, which has improved 5% and 4.3%, respectively.
Finally, all the data including the data of Taguchi experiments, confirming experiments and Hf-modified experiments of cast Inconel 718 have been integrated and analyzed to implement the multiple characteristics analysis of mechanical properties to obtain the optimal heat treatment parameters to achieve good results for well-balanced multiple mechanical properties.
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