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Thermal processing of metals, depending upon the temperature and duration at temperature will produce changes in microstructure, alter residual stress patterns and may reduce composition variations throughout the structure in a process called homogenisation.
Localised and Bulk Heat Treatment
The process usually involves solid state reactions and can be applied to the total volume of metal being processed (bulk heat treatment) or confined to selected surface areas (localised or surface heat treatment).
Why Heat Treat Metals
In most cases, without heat treatment, alloyed metals will not provide the designed property combinations of strength and ductility. This could lead to premature failure in many operational situations.
Terms such as annealing, stress relieving, normalising, hardening, tempering, precipitation hardening, solution treating, ageing, homogenisation and so on relate to the physical conditions applied to metals undergoing heat treatment.
The Importance of Specifying Heat Treatments
It is vital that design engineers and metal fabricators include a specification for heat treatment requirements when drawing up manufacturing procedures. So often, problems late in the manufacturing cycle have been faced due to the lack of planning for heat treatment. Usually these problems culminate in unacceptable size changes, product distortion or surface degradation.
Specifying Properties for Heat Treated Metals
It is also important to recognise what are the important mechanical and physical properties that are to be obtained from the heat treatment process. These should be included in the product specification together with test procedures which enable verification of the results obtained.
Iron and Steel Heat Treatment
On a processed mass basis, iron and steel would make up the major share of heat treated product, be it stress relief of weld fabricated components, normalising or annealing quenching and tempering or surface hardening.
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