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Internal residual stresses may arise in the production process of a workpiece. This may have been caused by a combination of e.g. welding, bending or casting. Should permanent deformation (plastic deformation) take place within the process, dislocation density occurs. The structure of the material is distorted locally which brings about tensions. Whenever a workpiece has been welded on, the weld seam cools from melting temperature to room temperature. The welding material wants to contract, but the adjacent material being much colder, stops it from doing so. This causes residual stresses in the adjacent area of the weld.
Stress-relief annealing to reduce strain and yield strength after welding
The principle of stress-relief annealing is based on reducing the strain and yield strength by means of temperature increase. This can be performed in 2 temperature ranges. For standard carbon steel the most common temperature is between 580°C and 620°C. So-called low stress-relief annealing is executed at temperatures between 300°C to 400°C. In this temperature range at least peak tensions are reduced. This range can be selected whenever dimensional stability is critical. Also this range can be preferred because materials have been used whose properties are susceptible to higher temperature ranges.
Some types of stainless steel are often annealed at a sustained period of time at a temperature of 420°C. The temperature and time used in stress-relief annealing procedure are usually determined by their norm or designcode.
As a rule of thumb, carbon steel 2.5 minutes per mm thickness (Lloyd’s Register Energy – Stoomwezen). With workpieces thicker than 1 inch, 15 minutes per inch to a maximum of 4 hours (Asme VIII).