| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 99 === Different compositions of nickel free high nitrogen stainless steels (Fe-15Cr-12~22 Mn-2.8Mo-N) are prepared by low pressure arc melting process, nitrogen content is increased with Mn addition from 0.34wt% to 1.4wt%. Ingots are properly cut and hot-rolled under 1150℃ to 50% and 25% of their original thickness. The potentiadynamic polarization measurement were performed in a conventional three electrode cell with 3.5wt% NaCl medium. The result shows that pitting corrosion potential increased with nitrogen content. For 50% hot-rolled samples, pit morphology vary with their Mn content : in higher Mn content ones, pits growth beneath the surface toward some specific direction, left extensive lace-like cover region; in lower ones, pits tend to growth locally by etching grains, lace-like cover region is relatively small. Hence, the passivity and corrosion properties of stainless steels is sensitive to Mn content. For 75% hot-rolled samples, corrosion tends to start from grain boundaries, and in some cases, Cr2N precipitates from grain boundaries, it can be avoided be by carefully temperature control.
Nitrogen content varies with different sulfur level, surface active element (SAE) like sulfur should be removed before or during melting.
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