| Summary: | 碩士 === 高苑科技大學 === 電子工程研究所 === 105 === In this study, SUS 420 ESR, hardness set within 49~51 HRC, processed in different tempering conditions and electric discharge machining (EDM) are compared. The different tempering conditions include low tempering(150〜250oC，LT) and subzero low tempering(-150oC，S) with low tempering (150〜250oC，S+LT). The surface treatment are improved by Aluminum lapping (ALP) and diamond lapping (DLP) and polishing. The electrochemical corrosion is embedding in 3.5wt% NaCl aqueous solution. The microscopic morphologies of the surface are observed by the stereomicroscope and the optical microscope. Scanning electron microscopy and energy dispersive analyzer analyze the remaining elements after corrosion.
The grain boundary of the low tempering is obvious than the subzero low tempering, but the difference of distribution of carbide is not clear. The amount and distribution of martensite and residual austenite are different in these two tempering methods. By subzero low tempering, residual austenite can transform into martensite, so the amount of the residual arenite are less than low tempering, then the corrosion is more worse.
The molten structure (white layer) and cracks are generated by electric discharge machining. Aluminum lapping and diamond lapping can remove white layer. By observing the cross section from optical microscope, aluminum lapping can removes effectively white layer effect than diamond lapping. After electrochemical corrosion, the pitting area of no lapping and diamond lapping are small, while it of aluminum lapping is larger. Because the white layer is hard to be corroded, corrosion is brought about from the cracks into the surface. No lapping and diamond lapping are with better corrosion resistance than aluminum lapping.
After EDM and electrochemical corrosion, the corrosion potential of low tempering and subzero low tempering is similar, either the sizes of pitting area are similar. EDM can promote the corrosion resistance, while more cracks on the surface are accompanied.
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