When susceptible stainless steels are heated in or
cooled through the temperature range
425-900°C, chromium tends to combine with carbon
to form chromium carbides that precipitate preferentially
at grain boundaries. The adjacent areas being depleted
in chromium become sensitized to Intergranular Corrosion
(IGC).
Sensitization may occur following service above 400°C
or if rapid cooling is not achieved after welding, solution
heat treatment or full stress relief. In aggressive
environments, welded fabrications may exhibit IGC as
a narrow band in the heat-affected zone (HAZ). It is
important to note that sensitization is not of significant
conseqence unless the steel is exposed to a corrosive
environment.
A loss of corrosion resistance may also be expected,
particularly in higher Cr and/or Mo steels containing
ferrite if sigma is formed during similar thermal treatments.
Methods of Minimizing Intergranular Corrosion
- IGC
Solution anneal above 1040°C followed by a rapid
quench.
Use UNS S34700,
a Nb stabilized grade, or S32100,
a Ti stabilized grade.
Use a low carbon, 0.03% max. carbon grade such as UNS
S30403, S31603,
S31703 or
N08904.
In today's technology with AOD melting furnaces, carbon
is easily and routinely reduced to very low residuals.
AOD melting is making the low carbon grades the standard
for welded fabrication.ASTM A262 practice A to E are
standard tests to determine susceptibility to IGC. Practice
E, the Huey test, is widely used.
