2002 Volume 42 Issue 12 Pages 1430-1437
Microstructural evolution and nanocrystallization in various carbon steels by ball milling and ball drop test has been studied. In ball milling, nanocrystallization was observed in all the carbon steels irrespective of the carbon content (up to 0.9 mass% C) or starting microstructure (ferrite, martensite, pearlite or spheroidite). In ball drop test, nanocrystallization was observed in high carbon steels or ultrafine grained low carbon steels. It is realized that high strength before ball drop test is required for the nanocrystallization. The nanocrystalline structure obtained by ball milling and ball drop test has similar microstructure with dark smooth contrast. The morphologies such as pearlite lamellar, spheroidite cementite, ferrite grain boundary disappeared by nanocrystallization. The boundary between the nanocrystalline and work-hardened regions is quite sharp. The hardness of the nanocrystalline region is about two times higher than that of work-hardened region. The annealing of nanocrystalline region shows substantially slow grain growth and re-precipitation of fine cementite. This annealing behavior is quite different from the work-hardened region which is characterized by recrystallization and fast grain growth.
From the present study, it was confirmed that the nanocrystalline structure produced by ball milling and ball drop test has quite similar in structure, hardness and annealing behavior although the number of deformation applied is substantially different.