PWSCC Performance of Alloy 600 Tubesheet Expansions: Effect of Microstructure and Expansion Technique

Abstract

Primary water stress corrosion cracking (PWSCC) of Alloy 600 steam generator tubing of pressurized water reactor plants has occurred at regions of residual stress in tubesheet roll transitions of mechanically expanded tubes. Hydraulic expansion (HE) in later designs is considered to develop lower residual stresses. The susceptibility of alloy 600 tubing to PWSCC is also dependent upon the microstructure. This paper presents results obtained under accelerated PWSCC test conditions of the effect of both the expansion technique and the microstructure on the PWSCC performance of expanded specimens. A mechanical stress re1 ief or "kiss roll" (KR) applied to roll transitions, consisting of intermediate diametral expansion of one roll step, was used for one set of specimens. This configuration has exhibited PWSCC sensitivity in service. A second set of specimens was hydraulically expanded. Each KR and HE set contained Alloy 600 tubing in subsets of (1) a mill annealed (MA), PWSCC-susceptible heat and (2) a thermally treated (TT) heat. The TT tubing possessed extensive grain boundary carbide precipitates produced by prolonged heating at 725 degrees C. All tubing was 22.2 mm OD x 1.27 mm wall, expanded into 22.7 mm-diameter holes in steel collars. Specimens were exposed to 400 degrees C - 20.8 MPa steam with 76 kPa hydrogen and an ID-to-OD pressure differential of 10.4 MPa. The aggressiveness of the steam was enhanced by traces of chloride, fluoride, and sulfate obtained by dosing the make-up water with 30 ppm of each anion as the sodium salt. This environment can initiate PWSCC- type cracks in reverse U- bend tubing specimens approximately 20 times more rapidly than typical lithiated, borated, hydrogen-containing PWR reactor cool ant at 330%. The expanded specimens were exposed until throughwall cracking developed or for 1500 hours. The results confirmed the superior PWSCC resistance of TT tubing over MA tubing in either expansion configuration. The results also indicated an enhanced PWSCC resistance of HE over KR for either microstructure. Throughwall cracking developed by 92 hours in all MA-KR specimens. All MA-HE specimens developed throughwall cracks by 286 hours. The TT-KR specimens required 1148 hours for 100% cracking. No throughwall cracks occurred in the TT-HE specimens in 1500 hours. The cracks occurred at prototypical locations with respect to expansion transition regions.