Manufacturer of Castings, however, typically don’t cool uniformly throughout their internal structures. Outer areas and thinner portions will cool and shrink at different rates than inner areas and bulkier portions—often creating internal tension, or stress, that can only be alleviated through heat treatment. Steel is much more susceptible than iron to shrink stresses, and in some situations, these tensions can lead to significant internal and/or external voids, and possible eventual fractures.