Effect of Large-strain Warm Deformation Prior to Austenitization on γ Grain Size in 0.3%C-9%Ni Steel

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Abstract
γ grain size refinement is one of the most important factors for microstructural control in heat treatment. Cold or warm deformation prior to austenitization is known to have a beneficial effect on γ grain size reduction, and in this paper, the effect of large-strain deformation at a temperature range just below Ac 1 transformation temperature on γ grain size after austenitization was studied using 0.3%C-9%Ni steel. Cylindrical specimens were deformed up to 70% in a single pass by compression using laboratory deformation simulator. Increase in reduction ratio up to 50% continuously refined γ grain size down to 2 μm after subsequent reheating to austenitization temperature: 700°C. Drastic change in α→γ transformation behavior was found for the reduction above 70%. Spontaneous reverse transformation was induced by such a large strain deformation even without subsequent reheating. It was revealed by TEM observation that γ grain size of the specimen was remarkably refined down to around 0.5 μm. Adiabatic heating due to deformation likely contributes to such a spontaneous transformation. Transformation mechanism for the spontaneous transformation was discussed.