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Solidification and cast structures

Segregation in castings and ingots includes microsegregation, on the scale of the spaces between dendrite arms, and macrosegregation, on the scale of the whole ingot. Which statement about the difference between them is correct?

  1. ABoth arise only from slow diffusion in the solid and are removed in the same way by homogenization annealing, but macrosegregation disappears in a shorter time
  2. BMicrosegregation arises from liquid flow and the settling of crystals, so heat treatment does not remove it, while macrosegregation arises from slow diffusion in the solid and is removed by homogenization annealing
  3. CMicrosegregation varies over short distances and can be reduced by homogenization annealing, while macrosegregation results from solute carried over long distances by liquid flow and the like, and heat treatment hardly removes it
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Answer: C. Microsegregation varies over short distances and can be reduced by homogenization annealing, while macrosegregation results from solute carried over long distances by liquid flow and the like, and heat treatment hardly removes it

Microsegregation is the uneven concentration in which a solute with k < 1 is lean in the dendrite cores and rich between the dendrites, over a scale of the arm spacing, some tens of micrometers. Once the diffusion distance √(Dt) reaches about half of this, the concentration evens out, so homogenization annealing at high temperature can reduce it. Macrosegregation forms when enriched liquid moves distances of several centimeters to several meters, through suction of interdendritic liquid by solidification shrinkage, convection driven by density differences, settling of crystals and so on. Centerline segregation in continuously cast slabs is a typical example; the distances are far too large to be undone by diffusion, so it is countered during solidification, for example by soft reduction near the end of solidification or electromagnetic stirring. The options that swap the causes and the ease of removal by heat treatment, or treat the two as the same mechanism, are wrong.

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More Solidification and cast structures questions

  1. Suppose Chvorinov's rule t = B(V/A)² holds (t is the solidification time, V the volume, A the surface area and B a constant). Under the same mold conditions, a cube-shaped casting of a certain volume solidified in 10 minutes. Which is closest to the solidification time of a spherical casting of the same volume?
  2. Which is a correct assumption behind the Scheil equation Cs = kC₀(1 − fs)^(k−1) (fs is the fraction solid), which gives the composition of the solid forming during solidification?
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