| The effect of cooling rate on the morphology of the martensite and the mechanical properties of Fe-20% Ni martensitic alloy has been studied. The alloy was cooled from the austenite state at 1000℃ by the various practical cooling precesses iced brine, water, oil, air and furnace cooling Fe-20% Ni alloy is fully transformed to lath martensite even at very slow cooling rate such as 0.8℃/min. The size of block in the lath martensite structure increases with decreasing cooling rate. The mechanical properties such as hardness, tensile strength, elongation and reduction of area show nearly constant values, independent of cooling process However, although the impact energy shows a high and almost same vague at iced brine, water, oil and air cooling process, only the furnace cooling process shows a very low impact energy. Impact energy of the ail-cooled specimen, which was furnace-cooled again from a temperature just above M_s points is nearly same as or a little higher than that of the as-air cooled specimen. Hence, the reason that the furnace-cooled specimen shows a very low impact energy is considered to be due to packet/packet boundary segregation formed with assistance of movable dislocations during martensitic transformation between M_s and M_f temperature. |
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