Question

In what context are terms columnar grains, chill grains and equiaxed grains used. Draw a neat picture showing the position of these grains in the above context.

Answer 1

Chill zone: Region within cast metals (usually closest to the mould wall) where the cooling rate is the fastest. Columnar zone: Region within cast metals (usually between the chill and equiaxed zones) where the grain structure is elongated in the solidification direction. Dendrites: Fine branches of solid material that project into the melt zone during solidification. Equiaxed zone: Region within cast metals (usually the core of the casting) where the grains are approximately the same size in every direction.

Columnar grains are long, thin, coarse grains created when a metal solidifies rather slowly in the presence of a steep temperature gradient. Relatively few nuclei are available when columnar grains are produced. Equiaxed and columnar grains are shown in Fig. 4.7. Note that in Fig. 4.7b the columnar grains have grown per- pendicular to the mold faces since large thermal gradients were present in those directions.

Columnar grains Equiaxed grains Mold (a) (b) Figure 4.7