Formation and annealing behavior of nanocrystalline ferrite in Fe-0.89C spheroidite steel produced by ball milling

Nanocrystalline ferrite formation by ball milling in Fe-0.89C spheroidite steel and its annealing behavior have been studied through microstructure observations and microhardness measurements. It was found that at the early stage of ball milling, the dislocation density increases and dislocation cells form due to plastic deformation. At the middle stage of ball milling, a layered nanocrystalline structure forms near the surface of the powder by localized severe deformation. The microhardness of nanocrystalline ferrite (10 GPa) is much higher than that of work-hardened ferrite (4 GPa). Together with the nanocrystallization of ferrite, the dissolution of cementite was observed. At the final stage of ball milling, equiaxed nanocrystalline ferrite forms from layered nanocrystalline ferrite by increasing the local misorientation. By annealing the milled powders, recrystallization was observed in the workhardened ferrite region, while in the nanocrystalline ferrite region, a slow grain growth was observed instead of recrystallization.