The stress corrosion cracking of a reactor pressure vessel steel in high temperature water at high flow rates

Three types of SA 508 Cl 2 RPV steels with sulfur contents varying between 0.005 and 0.022 wt% were tested for stress corrosion cracking in simulated hot (100-2200C), flowing (35,000 < Reynolds number < 70,000) feedwater (<5 ppb ≤ [O2] ≤ 400 ppb) by means of Constant Extension Rate Tests. Threshold electrode potential values for transgranular stress corrosion cracking (SCC) were determined as a function of sulfur content and temperatures of 100 and 1600C. At 2200C an effect of sulfur content could not be measured. The lowest oxygen concentration to generate SCC at 2200C is estimated to be about 10 ppm. SCC occurs only after a plastic strain of about 95% of the ultimate tensile strength, thereby reducing the constant extension rate technique essentially to constant load testing. Real crack growth rates can be determined after correction for an initiation time. These rate values are independent of the strain rate, but depend on material and temperature. Values of the apparent activation energy of crack propagation were determined as a function of the sulfur content. Apparently, crack initiation occurs only at sulfide inclusions, and crack propagation follows the stress fields of the inclusions. (author)