The use of alternatives to an alloy that is not readily available must be approached very cautiously. There needs to be an analysis of the requirements of the application and properties of the alloys.

Titanium (Ti) and Niobium (Nb) may be used in stainless steel as they combine more readily with the carbon in the metal than does chromium and thereby the formation of grain boundary carbides may be avoided and as a consequence of this, inter granular corrosion may be prevented. The 2 elements, Ti and Nb are referred to as stabilisers and the grades 321 and 316Ti are the stabilised versions of 304 and 316 respectively. These elements also improve the mechanical properties of the grade at elevated temperatures.

Minimising the problem of chromium carbide formation and hence intergranular corrosion is also achieved by ensuring the carbon content is kept low. 0.03% is allowed in terms of a 316L specification but often actual levels are event lower.

HOWEVER, stabilised grades also offer higher strengths at elevated temperatures whilst lowering the carbon content (as in 316L) has the opposite effect. To illustrate:

At 400 deg C the maximum allowable stresses in terms of ASME 8 are

• For 316 : 111MPa
• For 316Ti: 111Mpa
• For 316L: 91Mpa

Stabilised grades of stainless steel are very useful because they offer a combination of properties. This needs to be recognised when their availability is a problem and thus use of an alternative may be considered.

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.