Grain Refinement of α-iron by Repeated Carburizing and Decarburizing Reactions

概要

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The grain refinement of α-Fe using repeated carburizing and decarburizing at 1073 K are investigated. It is found that the grain sizes are decreased by carburizing reaction but are increased by decarburizing reaction. Repeated α ↔ γ phase transformations make grain size smaller. However, without pinning elements, the reduction of the grain size has limitation. Sulfur does not have enough effects on the grain growth with the grain size of less than 30 μm. To improve the grain refinement process, Fe3C was introduced by annealing at 873 K along with the carburizing and decarburizing processing. Formed Fe3C can supply a lot of nucleation site for austenite nuclei during the reaustenitizing and retards ferrite grain growth during γ → α transformation by the decarburization reaction. The fine uniform grains of less than 10 μm have been produced by the carburizing and decarburizing processing at 1073 K along with adjusting the cementite size and the whole volume during the heat treatment at 873 K.