The vessel was in service at a temperature of
125 °C (255 °F) in the presence of carbonate
ions and CO2-CO-H2O. In this environment,
carbon steel is very susceptible to SCC.
All the cracks were progressing through the
HAZ of the weld. It appears that the cracks originated
in this zone and subsequently propagated
into the base metal and weld. The weld joint
under consideration was locally stress relieved
on site, while other joints were stress relieved
inside a furnace. The vessel was also repaired to
a depth of 10 to 12 mm (0.4 to 0.5 in.) once and
to an additional 2 to 3 mm (0.08 to 0.12 in.) the
following year. After these repairs, no stress
relieving was done. This further enhanced the
stress conditions in the HAZ and aggravated the
SCC.