Fatigue Crack Propagation Rates for Notched 304 Stainless Steel Specimens In Elevated Temperature Water

Abstract

Fatigue crack propagation (FCP) rates for 304 stainless steel (304 SS) were determined in 24°C and 288°C air and 288°C water with 20–60 cc H2/kg H2O using double-edged notch (DEN) specimens. Tests performed at matched loading conditions in air and water provided a direct comparison of the relative crack growth rates over a wide range of test conditions. Crack growth rates of 304 SS in water were about 12 times the air rate for both short cracks (0.03–0.25 mm) and long cracks up to 4.06 mm beyond the notch, which are consistent with conventional deep crack tests. 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). Prior to the recent tests reported by Wire and Mills [1] and Evans and Wire [2], most literature data in high hydrogen water showed only a mild environmental effect for 304 SS, of order 2.5 times air or less. However, the tests were predominantly performed at high cyclic stress intensities or high frequencies where environmental effects are small. The environmental effect in low oxygen environments at low stress intensity depends strongly on both the stress ratio, R, and the load rise time, Tr. Fractographic examinations were performed on specimens tested in both air and water to understand the operative cracking mechanisms associated with environmental effects. In 288°C water, the fracture surfaces were crisply faceted with a crystallographic appearance, and showed striations under high magnification. The cleavage-like facets suggest that hydrogen embrittlement is the primary cause of accelerated cracking.