Science of war-metallurgy-iron-quenching

Grain size greatly affects the toughness of steel. Smaller grain size gives tougher or harder steel. When steel is heated and cooled quickly, as in quenching, smaller grain size is the result. This makes the steel harder but brittle.

When a force is applied to the surface of a metal, layers of atoms roll over each other. This rolling of layers of atoms over each other is hindered by grain boundaries because at the boundaries atoms don't line up properly. Therefore more grain boundaries there are, or the smaller the individual crystal grains, the harder the metal becomes.

Since the grain boundaries are areas where the atoms aren't in such good contact with each other, metals tend to fracture along grain boundaries. Increasing the number of grain boundaries not only makes the metal harder, but also makes it more brittle.

Science of Conflict Metallurgy-Iron Quenching
Pure iron is a soft metal that is useless for the construction of weapons. When iron is heated to just over 900^oC and then cooled, groups of atoms combine together to form grains, as shown on the right. Grains are separated by boundaries called dendrites.
Grain size greatly affects the toughness of steel. Smaller grain size gives tougher or harder steel. When steel is heated and cooled quickly, as in quenching, smaller grain size is the result. This makes the steel harder but brittle. When a force is applied to the surface of a metal, layers of atoms roll over each other. This rolling of layers of atoms over each other is hindered by grain boundaries because at the boundaries atoms don't line up properly. Therefore more grain boundaries there are, or the smaller the individual crystal grains, the harder the metal becomes. Since the grain boundaries are areas where the atoms aren't in such good contact with each other, metals tend to fracture along grain boundaries. Increasing the number of grain boundaries not only makes the metal harder, but also makes it more brittle.
Heating a metal and then letting cool slowly is known as annealing. Annealing tends to move the atoms into a more regular arrangement and decreases the number of grain boundaries by producing larger crystal structures, as shown in the animation on the right. This makes the metal softer. Banging the metal around when it is cold tends to produce lots of small grains. Cold working therefore makes a metal harder. To restore its workability, you would need to reheat it. You can also break up the regular arrangement of the atoms by inserting atoms of a slightly different size into the structure. *Alloys* such as brass (a mixture of copper and zinc) are harder than the original metals because the irregularity in the structure helps to stop rows of atoms from slipping over each other.
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