Picture 1 - Dendrite crystal

Grains Dendrites grow to a point where they begin to bump into one another, and eventually, they can not grow any larger because of being restricted by other dendrites around them. At that point, the dendrite has reached its maximum size and is referred to as a grain.

The space that exits between grains, usually just a few atoms wide, is referred to as a grain boundary. See picture 2.

Grain size greatly affects the toughness of steel. The smaller the grain size, the more ductile the steel. See Picture 3.

Cooling rate If molten steel is cooled slowly, dendrites have a longer time to grow before they begin to bump into neighboring dendrites. Thus, a large grain size is formed.

When steel is cooled quickly, the number of dendrites that begin growing increases, and the dendrites form quickly and begin to be restricted by other dendrites. Smaller grain size is the result.