Common steels of both hypo- and hypereutectoid compositions form pearlitic or martensitic constituents when cooled from austenite temperature in air or in water. Here, we provide evidence that this is not the case in an ultrafine-grained hypereutectoid steel. In this system, when the grain size was reduced to a scale of 2 to 4 μm, normal pearlite could not be obtained when the steel was cooled in air; instead, nanometer-sized granular cementite and ferrite were formed in the eutectoid transformation. When the cooling rate was increased by quenching in saltwater, martensite was no longer formed; instead, fine lamellar pearlite was formed. This research indicates that these abnormal phase transformations were related to the rapid diffusion present in the ultrafine-grained steel, which changed the diffusive transformations.
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