Fatigue Crack Propagation from Notched Specimens of 304 SS in an Elevated Temperature Aqueous Environment

Fatigue crack propagation (FCP) rates for 304 stainless steel (304SS) were determined in 24°C and 288°C air and 288°C water using double-edged notch (DEN) specimens of 304 stainless steel (304 SS). Tests performed at matched loading conditions in air and water at 288°C with 20–60 cc H₂/kg H₂O provided a direct comparison of the relative crack growth rates in air and water over a wide range of crack growth rates. The DEN crack extension ranged from short cracks (0.03–0.25 mm) to long cracks up to 4.06 mm beyond the notch, which are consistent with conventional deep crack tests. Crack growth rates of 304 SS in water were about 12 times the air rate. This 12X environmental enhancement persisted to crack extensions up to 4.06 mm, far outside the range associated with short crack effects. The large environmental degradation for 304 SS crack growth is consistent with the strong reduction of fatigue life in high hydrogen water. Further, very similar environmental effects were reported in fatigue crack growth tests in hydrogen water chemistry (HWC). Most literature data in high hydrogen water show only a mild environmental effect for 304 SS, of order 2.5 times air or less, but the tests were predominantly performed at high cyclic stress intensity or equivalently, high air rates. The environmental effect in low oxygen environments at low stress intensity depends strongly on both the stress ratio, R, and the load rise time, T_r, as recently reported for austenitic stainless steel in BWR water. Fractography was performed for both tests in air and water. At 288°C in water, the fracture surfaces were crisply faceted with a crystallographic appearance, and showed striations under high magnification. The cleavage-like facets on the fracture surfaces suggest that hydrogen embrittlement is the primary cause of accelerated cracking.