| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 88 === Carbon tool steel is material of low price, easy to quench, and high hardenability with cold workability. Therefore, it is frequently used for cutting tools and handiwork tools that need good hardness and wear-resistance, but not necessary to have high accuracy or good heat-resistance and etch-resistance. The knitting needle is one of its practical applications. However, the shallow harden depth, and the rapid softening during tempering or in high temperature environment and the low machinability of carbon tool steel become the drawbacks. A proper heat-treating process might be feasible to improve the mechanical properties of carbon tool steels. The ideal microstructure of carbon tool steels is a tempered-martensite matrix with evenly distributed diminutive carbides.
The purpose of the present experiment is to improve the mechanical properties of carbon tool steels used for knitting needles by modifying the conventional heat-treating processes. The proposed processes include:
(1)Using tempering parameter M to establish different tempering temperature and time, and austempering was added to the process.
(2)Cryogenic treatment.
(3)Double quenching. This process can improve the carbide precipitation and increase the grain-forming rate. Therefore, the grain can be refined and the strength and toughness can be improved simultaneously.
The present experiment data show that the specimen tempered at high temperature with short time period possesses better properties than those treated at low temperature with long time period. Meanwhile, the tempered soften-resistance of the specimen with additional austempering process is better than those with quenching process only. A double quenching process can obviously improve the mechanical properties too. After a double quenching process, the hardness value of the specimen was raised to Hv820 and the critical bending angle was among 75~100 degrees. In the present experimental environment, the optimal heat treatment conditions of carbon tool steels used for weaving needles are:
550℃×2 hr annealing → 820℃×10 min quenching →600℃ ×2 hr tempering → 810℃×1 min quenching → tempering
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