Treating Metals

A ductile metal can be made stronger by preventing slip from occurring. In high carbon steel the presence of carbon atoms between layers of iron atoms stops them from slipping over each other. Strangely, the introduction of further dislocations can also strengthen metals such as copper or mild steel. This happens in a process called work-hardening, where the metal is repeatedly worked by hammering, stretching or bending while it is cold. The increased strength is attributed to a bottle-neck of dislocations preventing them from moving past each other.

Work hardening is not effective with metals which have low melting points such as lead (mp 328°C) and zinc (mp 420°C).

The size of the crystals or grains in a metal also affects its strength. Since dislocations cannot move past the grain boundaries, smaller grains lead to stronger metals. Heat treatment of a metal affects both the size of the grains and the concentration of dislocations. It can be of several types:

• annealing involves heating a metal until it is red hot followed by controlled cooling. This makes the metal more ductile and less prone to fracture.
• quenching involves heating the metal until it is red hot and then cooling it rapidly by putting it in cold water or oil. This makes the metal strong but brittle.
• tempering is the process of quenching followed by annealing at a lower temperature. This is done to produce a balance between strength and toughness.

When steel is heated to red heat, the grains enlarge. Slow cooling allows excess carbon to diffuse out of the crystals and results in the metal being annealed. Rapid cooling results in a brittle material as the excess carbon does not have time to diffuse out of the crystals.