Effect of Degree of Ordering by Varying Cooling Rate below 550<sup>o</sup>C on the Initiation of Primary Water Stress Corrosion Cracking(PWSCC) in Alloy 600 Tube

This study investigated the effect of degree of ordering in Alloy 600 steam generator (SG) tube material on the initiation of primary water stress corrosion cracking (PWSCC), using a slow strain rate test (SSRT). The material was cooled in air from 1,100<sup>o</sup>C to 550<sup>o</sup>C, and below 550<sup>o</sup>C the cooling rate was varied by water quench (WQ), air cooling (AC), and furnace cooling (FC). The change in cooling rate was intended to control the degree of ordering. The PWSCC initiation experiment was conducted using a double tube compact tension (DTCT) specimen at a constant load in the primary water environment of a nuclear reactor. The results of the PWSCC initiation tests showed that the materials cooled at a fast rate were more susceptible. This experiment showed that cracking probability was higher when the degree of order was low due to a fast cooling rate. This can be interpreted to mean that a higher residual entropy is formed by fast cooling, since the higher entropy provides a greater driving force for crack initiation. The nature of the PWSCC activation process can be well explained by the ordering reaction of Alloy 600 (Q= 190 kJ/mol). For the first time, crack formation during the ordering reaction in Alloy 600 was detected using an acoustic emission sensing method. To explain the PWSCC resistance of Alloy 690, the kinetics of the ordering reaction of Alloy 600 and Alloy 690 were compared based on their activation energy.