| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 99 === This article of this study is to investigate the effect of different heat treatment parameters and different alloy elements on the microstructure and mechanical properties such as hardness and impact value of SKD61 and its modified steels. The results show that the modified steels containing higher alloy elements have higher hardenability and possess higher hardness after quenching at different temperatures and in different holding time. Steels with higher hardness after quenching will not always have higher hardness after tempering. It is more relevant to the elements of alloy. The tempering temperature of Z steel, which contains lower alloy elements, does not increase significantly with the quenching temperature and heating time. Its tempering time also changes little for all different quenching temperature and heating time. On the other hand, the modified steel B and C contain much more Mo in comparison to Z steel. The amount of Mo dissolved in the matrix will increase with the raise of quenching temperature, and increase the resistance of softening while tempering. It will take three or more times to temper to target hardness of 48HRC rather than twice if the temper temperature remains unchanged.
In the performance of impact value, high alloy containing steel B and C performed better at the lower quenching temperature of 1000℃ and shorter heating time of 30 minutes. The relevance between hardness and impact value of modified steel B and C is weak, because the materials always possess rather stable impact value. As for the modified steel A, the heating time of quenching affects the impact value markedly especially at the lower quenching temperature of 1000℃
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