| Summary: | 碩士 === 國立臺灣科技大學 === 材料科技研究所 === 96 === We studied the phase transformations of a manganese steel in aging processes. The composition of the manganese steel is Fe-20 wt% Mn-0.5 wt% C. The alloy was undergone a solution heat treatment at 1100 oC and followed by the aging process at temperatures ranging from 900 to 450 oC.
After the heat treatment at 1100 oC, the alloy is composed of FCC grains and wicker-shape precipitates distributed within the FCC matrix. The wicker-shape precipitates are martensites which were identified as either HCP crystal structure or FCC twins. The martensite still existed in the FCC matrix after aging the alloy at low temperatures.
A new HCP precipitate formed in the FCC matrix after the alloy was aged at temperatures ranging from 800 oC to 650 oC. The size of the new precipitate is smaller than 5 �慆. The lattice constants of the HCP precipitate are a=0.320 and c=0.514 nm, where c/a =1.6037. This precipitate has not been found in Fe-Mn, Fe-C, or Fe-Mn-C systems, suggesting that it is the first observation in the alloy system.
When the aging temperature was below 600 oC, we observed a pearlite-like structure at the areas near the FCC grain boundaries. In TEM study, we found the carbides in the pearlite-like structure are composed of two kinds of carbides. One is M23C6 and the other M3C. Both of which have different composition as well as crystal structure. M23C6 has FCC structure with a= 1.057 nm, and M3C has an orthorhombic structure with a= 0.459, b=0.521 and c=0.712 nm. It is a hypoeutectoid transformation,γ → α + carbide, similar to that in common steels.
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