Structure restoration and coarsening of nanocrystalline cementite in cold drawn pearlitic wire induced by low temperature annealing

Internal structure evolution of nano-scale cementite during annealing has exhibited a critical impact on mechanical performance for various heavily strained high-carbon steel materials. Through a combination of post-annealing and in-situ annealing transmission electron microscopy observations, structural evolution of heavily strained cementite during low-temperature annealing was investigated. During annealing, the morphology of cementite lamellae is stable when the temperature (Ta) is lower than 350 °C. Meanwhile, lattice structure restoration and coarsening of nanocrystalline cementite (θ- NC) occurred inside the lamellae. Starting from a nanocrystalline structure in the as-drawn state, the interiors of cementite lamellae were observed to transform into coarsened isometric shape θ-NC (140 °C < Ta < 210 °C) or elongated θ-NC (Ta < 350 °C). The coarsening activation energy of heavily strained cementite nanocrystalline in lamellae is estimated to be in a range of 37 ~ 50 kJ mol-1, while the coarsening behaviour is limited by the ferrite-cementite phase boundary.