There are a wide range of metals, each with their own working properties such as toughness, hardness, malleability etc. This means that not all enhancements are necessary for every metal. For example, Ferrous metals require a finish or coating to prevent them from rusting while Non-ferrous metals do not.
When a metal is ‘cold worked’ the crystals within the metal are distorted and changed leading to improved tensile strength and hardness. This is known as work hardening. The affects of work hardening can be removed by Annealing.
Annealing is used to make work hardened metal easier to work by making it less brittle and more ductile (able to stretch and bend). To anneal a piece of metal it is heated up and then slowly cooled back to room temperature. This softens the metal and it allows it to be bent and shaped more easily. In industry, the process is carried out in a specific temperature-controlled furnace.
Case Hardening is a process used for hardening the surface of steels with less than 0.4 % carbon. This makes the surface of the steel more resistant to indentation and abrasion. There are two stages:
Carburising which changes the chemical composition of the surface of low carbon steel so it can absorb more carbon. This is often done in a box with carbon powder where it is heated to around 950C degrees. The longer it is ‘baked’, the thicker the carbon layer.
Quenching where the hot metal is quenched in water. This fast cooling seals the hard surface without affecting the inner core.
Hardening is the process of heating medium and high carbon steels to alter the crystalline structure, holding them at a set temperature for a given time then quenching them in water, oil or salt baths. The hardening and subsequent 'quenching' greatly increases the hardness property but also increases the brittleness.
Tempering is a heat treatment for medium and high carbon steels that is carried out after hardening to reduce some of the excess hardness and increase the toughness. This reduction in hardness results in greater ductility and reduces the brittleness of the metal. To temper a metal, it is heated to below a critical point for a given time and then air cooled. The rise in temperature results in a reduction of hardness and an increase in toughness.
Metals have both physical and mechanical properties, but the mechanical properties can be enhanced through a variety of methods.
Different metals have different properties, but these can be altered through heating, cooling, quenching and case hardening.
Enhancements can be achieved through heat and chemical processes or through the addition of finishes. These are covered in greater detail in Finishes.