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URN etd-0814114-095510 Statistics This thesis had been viewed 1561 times. Download 295 times. Author Yang-Yu Su Author's Email Address No Public. Department Materials Engineering Year 2013 Semester 2 Degree Ph.D. Type of Document Doctoral Dissertation Language zh-TW.Big5 Chinese Page Count 124 Title The Study of Microstructures and Properties in the Cold Work Tool Steel Keyword carbide retained austenite dimensional change residual stress lattice parameter wear wear lattice parameter residual stress dimensional change retained austenite carbide Abstract This study investigates cold work tool steels, SKD 11 and DC 53, have ferrite and carbides in the as-received specimen transformed to martensite, residual austenite and undissolved carbides after heat treatment. The mechanical propertied, residual stresses and dimensional changes related to the microstructure of cold tool steel have been investigated for both conventional and cryogenic treatments. This study compares the difference in determining the austenite amount in cold work tool steel using the micrographic method as opposed to the X-ray diffraction method. Calculating the amount of retained austenite in tool steel using X-ray diffraction analysis requires first taking off the primary carbide content. As trapped carbon atoms are released from the martensite, the martensite c/a ratios and dimensions of the specimens treated conventionally decreased at tempering temperatures above 673 K. However, these values increased owing to the increase in the c-axis when tempering was conducted at temperatures higher than 673 K. At higher temperatures, the retained austenite is converted into martensite. In the conventionally treated specimens, the compressive residual stress decreased with quenching after tempering because of the release of trapped carbon atoms. However, the retained austenite to martensite transformation and the precipitation of alloy carbides leads to increased martensite lattice parameters and dimensional residual stress changes. The residual stress is also changed to generate compressive residual stresses. The dimension change ratio in cryogenically treated and tempered specimens decreased as the tempering temperature increased through a reduction in the amount of trapped carbon. However, the magnitude of the compressive residual stress was reduced to zero. In order to investigate the wear effect of retained austenite, the hardened tool steel SKD 11 were subjected to be cooled to the cryogenic temperature with different cooling rate. The experiment results showed that slower cooling rate the more retained austenite can be transformed, and the greater the hardness can be obtained. Rather than merely to assess the transformation rate of retained austenite, the intention in this investigation has been to characterize the behaviour of wear mechanism associated with the amount of retained austenite existed. Advisor Committee Liu-Ho Chiu - advisor
Cheng-Hsun Hsu - co-chair
Fan-Shiong Chen - co-chair
Pee-Yew Lee - co-chair
Yong-Chwang Chen - co-chair
Files Date of Defense 2014-07-23 Date of Submission 2014-08-14