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.