2002 Volume 43 Issue 9 Pages 2205-2212
Nanocrystalline ferrite formation by ball milling in Fe–0.89C steels with initial pearlite and spheroidite microstructures and their annealing behaviors have been studied through microstructural observation and microhardness measurement. It was found that nanocrystalline ferrite first forms near the surface of powders due to localized severe deformation. The microhardness of nanocrystalline ferrite regions is much higher than that of work-hardening regions. The dissolution of cementite was observed together with the nanocrystallization of ferrite. The nanocrystallization rate of pearlite powder is faster than that of spheroidite powder due to higher work-hardening rate and smaller cementite size. After long time ball milling, the equiaxed nanocrystalline ferrite with less than 10 nm grain size forms in the whole powders of both pearlite and spheroidite structures, and the cementite dissolves completely. By annealing the milled pearlite and spheroidite powders, recrystallization was observed in the work-hardening regions, while continuous grain growth was observed in the nanocrystalline ferrite region. After annealing, microhardness of the former nanocrystalline ferrite region is always higher than that of the former work-hardening region when compared at the same annealing condition. The grain growth rate of nanocrystalline ferrite produced from pearlite structure is lower than that of spheroidite structure due to the finer grains.
