Sigma Precipitation in HAZs.When the
higher molybdenum alloys such as 904L, AL-
6XN, and 254SMO were first developed, one of
the anticipated corrosion problems was attack
of single-phase precipitates in weld HAZs. This
form of attack has subsequently proved to be
either superficial or nonexistent in most applications,
probably because the compositions of the
alloys have been skillfully formulated to minimize
phase-related hot-rolling problems.
More recently, nitrogen has been added to
molybdenum-bearing austenitic stainless steels
to retard the precipitation of chromium- and
molybdenum-rich intermetallic compounds (σ
or χ phases). The incubation time for intermetallic
precipitation reactions in Fe-Cr-Ni-Mo
stainless alloys is significantly increased by
raising the alloy nitrogen content. This has
allowed the commercial production of thick
plate sections that can be fabricated by multipass
welding operations. In addition to suppressing
the formation of deleterious phases,
nitrogen, in cooperation with chromium and
molybdenum, has a beneficial effect on localized
corrosion resistance in oxidising acid-chloride
solutions.