2020 – 13 – Grains and Strength

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When metal cools and crystals begin to form, grains start to grow in the metal. Grains grow quickly in one direction because of minor variations in the temperature of the cooling metal. Grains toward the surface branch forth like a tree, a process called dendritic growth. The surfaces where grains meet their neighbors are called grain boundaries.
A metal’s grain size depends upon the rate that is cools and the amount of nucleation sites that occur in the molten metal. The faster metal cools, the stronger and tougher both the metal and the smaller the grains will be. If metal cools slowly, it becomes easier to machine, but it may not be as strong.