One property of metals that affects their strength and durability is their grain size. But what is a grain, and where do these grains come from? A grain is an individual metal crystal. If you have been watching this series in order, you know that metals are crystalline in nature, that is, they are made up of repeating patterns of metal atoms. But with a few exceptions, metals don’t look like crystals when you look at them. That’s because the metals are not a single crystal, but rather a collection of crystals that have grown together. This means metals tend to be what are known as polycrystals.
You see, as a metal solidifies the atoms begin to group up and cling together. This happens at different locations in the metal puddle. The points where this happens are called “nucleation points.” Each of these nucleation points will grow to become a metal grain. The speed at which these nucleation points form is called the nucleation rate. The faster the nucleation rate, the smaller the grains will be.
Of course, the speed at which the grains grow also affects the grain size. If the grain growth rate is fast, the crystals and therefore the grain size will be large.
When the grains have grown so large that they bump up next to their neighbors, a boundary is formed. Since the atoms along these boundaries are not fully connected like the internal atoms, the grain boundaries have properties that are a little different from the material inside the grains.
The ideal grain size will depend on the intended purpose of the workpiece. As a general rule, smaller grains result in stronger metals, and larger grains allow for a little more ductility. The alloy composition of the metals and the speed at which the metal is cooled are the most important factors affecting grain size.
It’s important to point out that metals don’t need to be solidifying from liquid to solid for grains to grow. Many metals will change their structure when heated, even well below their melting point. For example, pure iron melts at about 1500 degrees Celsius, but its grain structure will begin to change well below this level. The temperature where this can happen thoroughly is called the recrystallization temperature. When a metal reaches its recrystallization temperature, its crystals change structure and new grains grow. This allows grains that may have been distorted by bending or hammering to be restored to fresh, smaller, undistorted grains. But if the metal is heated for an extended period of time, the smaller grains will tend to grow together to form larger grains. So these times and temperatures must be carefully controlled.
For most metals, a microscope is required to see the grain structure, but in some cases, such as this zinc galvanized steel, the grains are clearly visible to the naked eye.
So in conclusion, metals are composed of grains that are individual metal crystals. They affect the strength and ductility of metals. Metal grains form as a metals solidifies, but they can also form when a metal is heated to its recrystallization temperature.