| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 92 === Modified 9Cr-1Mo alloy steel can virtually transform to lath martensite in continuous cooling heat treatment, since a high alloying content leads to a good harden-ability. Modified 9Cr-1Mo alloy steel has better creep resistance than other 9Cr-1Mo steels, because it contains 0.2 wt% V. In this study, we have performed isothermal and tempering heat treatment for the modified 9Cr-1Mo alloy steel, and the microstructural changes due to the phase transformation have been observed.
Isothermal heat treatments have been performed at 235℃, 285℃, and 335℃ for 15 minutes, then up-quenching at 450℃ for 15 minutes. The up-quench heat treatment is intended mainly to distinguish the different between the auto-tempered martensite transform from high temperature cooling down and the fresh martensite transform after isothermal heat treatment. In TEM microstructure analysis, it is found the amounts of auto-tempered martensite are majority, but the amounts of fresh martensite are less. Besides, it is observed the lath auto-tempered martensite contains M3C carbides which have different variants, and is also observed a long film morphology of retained austenite between two lath martensites, a few twins which are joined in order to release invariant-plane strain.
In tempering heat treatment at 650℃ for 30 minutes, it is found M3C carbides precipitate from martensite. In tempering heat treatment at 750℃ for 30 minutes, it is observed the M7C3 and M23C6 carbides precipitate from grain boundary. The evolution sequence of carbides is suggest to be M3C→M7C3→M23C6. Carbide could coarsen when increasing tempering temperature or time.
At a slow heat rate in a continuously heating treatment for tempering martensite, retained austenite could be decomposed quickly from long film morphology at 675℃, and produce the coarse M3C carbides.
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