Heat Treatment in Design and Technology involves the use of heat to raise the temperature of a material (usually metal) to a point where changes in its crystaline structure occur and alter its working characteristics.
In a workshop situation, the source of heat is usually a Blow Torch but industrially, more consistency and accuracy is achieved by the use of temperature controlled ovens and kilns.
The purpose of Annealing is to increase Malleability. It is perhaps the most accessible Heat Treatment process in general workshops, using only a Blow Torch, but it differs slightly for different metals:
Steel is an Alloy of Iron and Carbon. Mild Steel has only a very small amount of carbon (less than 0.5%) and cannot be Hardened and Tempered but Tool Steel with a carbon content of around 5% or more can (i.e. 0.5% - 1.5%).
Tempering is a process whereby the hardened steel is heated again and cooled from a lower temperature. This increases the Toughness at the expense of losing some Hardness. In industry, the correct temperature would be set by the oven or kiln but, with experience, the changing oxide colours can be used to judge when it is reached sufficiently accurately for craft work.
Note: The water should be stirred before quenching to speed up cooling by taking the heat away and the heated steel plunged in vertically to avoid deformation and cracking.
Hardening and Tempering is therefore a two-stage process as follows:
Note: A skilled traditional Blacksmith could achieve Hardening and Tempering of a tool tip with a single heating operation. After initial quenching and hardening of the tip, the workpiece, still with residual heat, would be held against the edge of the anvil. The cold edge serves as a heat-sink to stop residual heat conducting down the tool tip as it is quickly polished. The workpiece is then raised to allow residual heat to conduct through and bring the tip up to the required temperature for Tempering.
The outer layer of Mild Steel must first have its carbon content enriched by covering it with a carbon-rich compound and ‘cooking’ it at Red Heat until sufficient carbon is absorbed into the surface. This process is known as Carburising.
The workpiece can then be quenched to harden the carbon-enriched outer layer to complete the Case Hardening process. Tempering is not necessary because the inner core remains Mild Steel and this is able to absorb any shocks.
This process is used to even out stresses and refine grain structure, especially in products which have been subjected to uneven heating and cooling, as in a welded structure for example.
The whole workpiece is raised to the temperature of the Critical Point and then cooled in still air. The cooling is therefore a little faster than for Annealing, which has to cool slowly within the furnace or kiln. This results in a finer grain structure and greater Toughness.
Because it is a faster process and requires less time in the costly furnace or kiln, Normalising is more economic than Annealing and the widely used method of increasing strength and toughness. Forgings and castings are Normalised after cooling to improve their condition prior to machining.
The effects of Heat Treatment on Steel are greatly dependent on its Carbon content. Low carbon steels can be almost unaffected but the properties of steels with only small amounts of carbon added are changed dramatically. In general, Hardness and Strength is traded for increased Toughness, Ductility and Malleability or vice versa.
The temperatures to which the Steel is heated, the time it is held at the temperature and the rate of cooling all affect changes in its Crystalline Structures in addition to controlling the type, size and distribution of Alloying elements.
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