Intergranular Stress Corrosion Cracking of 13%Cr and 18%Cr Ferritic Stainless Steels in High Temperature High Purity Water

IGSCC of 13%Cr and 18%Cr Ferritic stainless steels with various carbon, nitrogen, and titanium contents, has been studied in high temperature aqueous environments. Conventional ferritic stainless steels became susceptible to IGSCC in high temperature water irrespective of the existence of chloride ions, when they were cooled from temperatures above 1050 C as is typically done during welding thermal cycles. The IGSCC was not dependent so much upon the test environment as upon the heat treatment and chemical composition of the steel, confirming the assumption that the solution at the crack tip might be acidic even in high purity water. The IGSCC was also found to be caused by chromium depletion along the grain boundaries resulting from precipitation of chromium carbides and nitrides. This can be prevented by adding sufficient amounts of titanium to the steel (13%Cr–Ti: Ti ≥ 15 [C+ N]%, 18%Cr–Ti: Ti ≥ 15 [C + N]% and Ti ≥ 0.25%). The reason why much more titanium than stoichiometrically required to stabilize carbon and nitrogen in steel is necessary, was discussed in connection with the cooling rate in the temperature range from 1000 to 900 C, after heating above 1050 C. And the difference between 13%Cr steel and 18%Cr steel was explained by considering the effect of chromium in retarding the diffusion of carbon and nitrogen.