The effect of drawing velocity and phase transformation on the structure of directionally annealed iron

Undeformed iron bars were directionally annealed under various processing parameters. Columnar grains having an aspect ratio as high as 50 were produced at optimum processing conditions of a hot zone temperature of 850 °C, a 200 °C/cm temperature gradient and a drawing velocity of 1 μm/s. Equiaxed grains were formed both when the drawing velocity was low, and when it exceed the maximum growth rate of grains. The ferrite to austenite phase transformation hindered the columnar grain growth. A novel microstructure with columnar grains in the outer region, whose longitudinal axis parallels to the drawing direction, and fine equiaxed grains at the center of the specimen was observed. The skin effect of high frequency induction heating during the directional annealing was used to explain the formation of this novel microstructure.