α→γ Transformation Behavior during Heating from(α+γ) Region in 0.18% Carbon Steel

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Abstract
α→γ transformation behavior during continuous heating at a rate of 3K/s from the (α+γ) region in 0.18% carbon steel has been investigated by dilatometric measurement and metallography. The transformation was delayed with increasing an initial amount of proeutectoid α before the heating. When the amount of proeutectoid α before the heating was same, the transformation kinetics was delayed with increases in initial γ grain size and in the γ→α transformation temperature in the (α+γ) region. Microscopic observation showed that the α→γ transformation proceeded only by the growth of pre-existing γ, namely the dissolution of proeutectoid α, without nucleation of new γ grains in the proeutectoid α particles. When the temperature (T0.9) for 90% γ in the α→γ transformation was plotted against the size of α particles before the heating, the experimental data lay on a straight line and T0.9 increased with increasing the α particle size, regardless of various factors such as the initial γ grain size, the γ→α transformation temperature, and the amount of proeutectoid α. The effect of the various factors on the α→γ transformation behavior could be well explained by considering the influence of those factors on the size of proeutectoid α particles.