Intergranular stress corrosion cracking behavior of austenitic stainless steels in hydrogenated high-temperature water

The influence of material factors on the intergranular stress corrosion cracking (IGSCC) susceptibility of austenitic stainless steels under hydrogenated high-temperature water were studied using the constant extension rate technique (CERT) using specimens with a cold deformed hump. A remarkable and beneficial effectiveness of sensitization on IGSCC susceptibility was recognized in spite of its significant chromium depletion at the grain boundary. To make clear the cause of the reverse effect of sensitization on IGSCC in high-temperature water with different potentials, the effects of grain boundary composition such as chromium and molybdenum and carbide precipitation on IGSCC were examined in hydrogenated water. The results show that the IGSCC susceptibility in hydrogenated high-temperature water is mainly influenced by the distribution of carbide precipitation rather than by the chemical composition at the grain boundary. Careful examination of the initiation sites for the IGSCC shows that grain boundary sliding occurs as the first step of the IGSCC process. Based on this observation, grain boundary sliding (GB Sliding) behavior was studied by creep testing in air. Significant intergranular fracture was recognized after low-temperature creep test less than 550 degrees C. The area fraction of the intergranular fracture surface of solution-treated Type 316 (UNS S31600) was slightly more than that of sensitized material with carbide precipitation at the grain boundary. This similar dependence of carbide precipitation on IGSCC and creep behavior suggested that GB Sliding might play an important role in IGSCC by itself or in conjunction with other reactions such as crack tip dissolution. Also, the observed activation energy of creep behavior within the temperature 380 degrees C to 500 degrees C was 110 KJ/mol. This value is close to that of IGSCC (107 KJ/mol) under hydrogenated high-temperature water. Therefore, it seems reasonable to conclude that GB Sliding might play a significant role in the IGSCC initiation and propagation process under hydrogenated high-temperature water.