The effects of heat treatment and environment on corrosion fatigue

The effects of aging treatment on the fatigue behavior of alloy 600 in air saturated and deaerated pure water were studied. Crack growth rates ranging from 10-7 mm/cycle to 10-4 mm/cycle were measured for stress intensity factors beginning at 6 MPa√m and ranging up to 30 MPa√m at a load ratio (Pmin/Pmax) of 0.1. Two thermal treatments were studied: solution annealed material aged at 700 degrees C for either 2 or 120 hours. The 2-hour age resulted in a highly sensitized microstructure while the 120-hour age resulted in a open-quotes desensitizedclose quotes microstructure. The results of the fatigue testing indicate that there is no degradation in the fatigue resistance of alloy 600 in deaerated water. For the thermal treatments studied, the environment had no effect on the measured crack growth behavior above a ΔK of 15 MPa√m. For lower values of the stress intensity factor, the 120-hour aging treatment exhibited higher growth rates and appeared to have a lower threshold stress intensity factor. A frequency effect was detected in air saturated water for material aged for 2 hours with the lower frequency resulting in a slightly higher crack growth rate. In addition to the fatigue tests, alloy 600 steam generator tubing was procured from a PWR vendor in order to characterize tubing that is in-service in older steam generators. The results of this work indicate that, for the mill annealed tubing studied, alloy 600 can undergo grain boundary chromium depletion at the operating temperatures which exist in steam generators