The corrosion resistant characteristics of stainless steels may be
adversely affected by the sensitization process occurring, while
Welding-stainless, in a certain temperature interval from about 600 to 900'C
(from about 1100 to 1650'F) which promotes the precipitation (gathering) of
chromium carbides at grain boundaries and the parallel loss of anticorrosive
chromium from the base metal.
Another is to use a different type of base
metal welding rod including an
amount of titanium (type 321) or columbium (also known as niobium) (type 347)
which tend to form readily titanium carbides (or columbium carbides) (and by
this action the carbon becomes unavailable for chromium) at sensitization
temperatures leaving the chromium free to perform its anticorrosive
task.
Note that the filler metal for this material, if required, should be
always of type 347. Why? Because titanium (in type 321) being reactive, is not
readily recovered during deposition, so that it would not be available when it
is needed most. Columbium however is not reactive, it will stay put through
melting, and, when the material is heated to the "sensitization" temperature,
will do its job of producing columbium carbides in preference to chromium
carbide, and so it will save the day.
The third strategy for safely
Welding-stainless is to perform a solution heat treatment at elevated
temperature (about 1050 oC or 1900 oF), for repairing a condition of corrosion
susceptibility. This puts again in solution (called solid solution) the chromium
carbides originated during Welding-stainless sensitization of regular 18/8
stainless (like types 302 or 304). This process however must contend with
problems of heavy oxide formation if not done in vacuum or protective
atmosphere, and of distortions.
Stainless steel welding electrode type 309 and 310,
used for elevated temperature applications, and type 316 or better type 316L
used for enhanced corrosion resistance, are generally not prone to sensitization
and are used with filler wires of similar composition.
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