Low carbon martensitic stainless steels, called supermartensitic stainless steels (SMSS), have
widely been applied for flowlines transporting corrosive oil and gas. However, intergranular stress
corrosion cracking (IGSCC) in girth welds of the steel has become a major concern recently. A
number of research works have been conducted to clarify the mechanism and to establish the
measures. The most likely mechanism of IGSCC is chromium depletion on grain boundazy because it
is observed in lean grade SMSS, which contains no molybdenum. However, neither depletion nor
carbide of chromium has been detected so far in 2-3% Mo-added high grade SMSS, although the steel
suffers IGSCC under some conditions. In order to clarify the mechanism in high grade steel, effects
of chemical components on IGSCC resistance were investigated. Simulated HAZ specimens with
various conditions of thermal cycles were tested in hot acid environments by U-bend SCC test. The
test results showed that reduction of carbon and addition of titanium are effective to improve the
resistance to IGSCC. Both would inhibit to form chromium carbide that causes to chromium
depletion. Therefore, it suggests that the mechanism of IGSCC in high grade SMSS is chromium
depletion, as in lean grade.