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Previous fatigue crack propagation (FCP) tests on a single heat of 304 stainless steel (304 SS) specimens showed a strong acceleration of rates in high temperature water with 40–60 cc H2/kg H2O at 288°C, with rates up to 20X the air rates. The accelerated rates were observed under fully reversed conditions (R = −1) (Wire and Mills, 2001) and high stress ratios (R = 0.7 and 0.83) (Evans and Wire, 2001). In this study, a second heat of 304 SS has been tested at 243°C and 288°C and lower positive stress ratios (R = 0.3, 0.5). The second heat showed the large acceleration of rates at 288°C observed previously. Rates were up to two times lower at 243°C, but were still 7–8X the air rates. A time-based correlation successfully correlates the accelerated rates observed, and is nearly identical to fits of literature data in hydrogen water chemistry (HWC), which has hydrogen added at a lower level of about 1 cc/kg H2O. The accelerated rates on the second heat were not stable under two different test conditions. In contrast to the first heat, the second heat showed a reduction in environmental enhancement at long rise times, accompanied by a change in fracture mode. Addition of a constant load hold time of 1200 s between cycles also caused a marked reduction in crack propagation rates in both heats, with reduction to nearly air rates in the second heat. The differing rise time effects between the two heats could be rationalized by time-dependent deformation. More hold time testing is required to define the material and loading conditions which lead to reduced rates.

Fatigue crack propagation (FCP) testing of 304 stainless steel (304 SS) specimens showed a strong acceleration of rates in high temperature water with 40–60 cc H2/kg H2O at 243°C and 288°C, with rates up to 20X the air rates [1]. However, FCP rates were markedly reduced for a second heat at long rise times and for both heats with addition of a constant load hold time of 1200 s at a high stress ratio [2]. Such behavior had not been previously reported in the literature and merited further investigation. Tests have been extended to include two additional heats and a wider set of loading conditions. FCP rates were accelerated at long rise times in the two additional heats, consistent with a large series of tests on wrought, weld, and cast austenitic stainless steel materials recently reported by Nomura, et al. [3]. Hold time tests at a lower stress ratio showed that small increases or decreases in rate occur with holds at minimum or maximum load, but the changes were within normal data scatter. The rate reductions are not a generic result of less frequent cycling, but are limited to specific loading parameters or heats. A time-based correlation successfully describes the accelerated rates observed on all four heats, and is nearly identical to fits of literature data in PWR water and hydrogen water chemistry (HWC). A power law fit with separate terms for rise time, ΔK, and stress ratio provides an equivalent correlation.

Fatigue crack growth rate tests were performed on a 304 stainless steel compact tension (CT) specimen in water with 40–60 cc/kg H2. Data in the literature for CT tests show minor environmental effects in hydrogenated water, but higher effects in oxygenated water. However, the PWR data presented by Bernard, et al (1979) were taken at low stress ratios (R = 0.05) and high stress intensity levels (ΔK = 16–41 MPa√m). The purpose of these tests is to explore the crack growth rate characteristics of 304 SS in hydrogenated water at higher R values (0.7 and 0.83) and lower ΔK values (11.0 and 7.7 MPa√m) Each set of R, ΔK conditions were tested at frequencies of 0.1, 0.01 and 0.001 Hz. The results show a pronounced effect on crack growth rates when compared to available literature data on air rates.