Residual stresses may exist in components that have been cold formed or machined and in assemblies after welding. The common method for reducing these stresses is by:
- heating to a suitable temperature for the metal in question,
- holding it there long enough to reduce the residual stresses and
- then cooling slowly so as to minimise the further development of residual stresses.
In the case of duplex stainless steels, their metallurgy is such that they are prone to the formation of detrimental and unwanted phases at the temperatures that may normally be considered for stress relieving. Thus, as one Handbook states “There is no satisfactory temperature below the solution annealing temperature at which stress relief can be employed without the danger of the formation of intermetallic phases which will lower corrosion resistance and reduce toughness.”
For Alloy 2205 the minimum solution temperature referred to is 1040 deg C. More critically however, the cooling from this temperature needs to be a quench and this will introduce new considerations such as distortion and further residual stresses. Similarly for other duplex stainless steels.
Practical considerations dictate that it may not be possible to stress relieve products and equipment in duplex stainless steels but on a case by case basis it may be possible to find a solution by considering at the outset the motivation for this heat treatment operation.