Stress Corrosion Crack Growth Rates in Type 304 Stainless Steel in Simulated BWR Environments

Stress corrosion cracking of Type 304 stainless steel has been studied with fracture-mechanics-type standard 25.4-mm-thick compact tension specimens in simulated boiling-water reactor environments at 289°C and 8.3 MPa. Tests were performed with either constant or cyclic loading. The latter tests used a positive sawtooth waveform with an unloading time of 1 or 5 s, a load ratio R (minimum load to maximum load) of 0.2 to 0.95, and a frequency f of 8 × 10⁻⁴ to 1 × 10⁻¹ Hz. Crack lengths and crack growth rates were determined by the compliance method; crack mouth opening displacement was measured with in-situ clip gauges. Fractography was used to examine the mode of cracking and to confirm the compliance method for crack length determination. The test environments were high-purity deionized water with 0.2- to 8-ppm dissolved oxygen, and water with 0.2-ppm dissolved oxygen and 0.1-ppm sulfate (as H₂SO₄). Two heats with a carbon content of 0.06 wt percent were investigated in solution-heat-treated and furnace-sensitized conditions. Degree of sensitization varied from ∼0 to 20 C/cm² as measured by the electrochemical potentiokinetic polarization method. The first heat was tested in water with 0.2- and 8-ppm dissolved oxygen and with 0.2-ppm dissolved oxygen and 0.1-ppm sulfate. The loading conditions encompassed the range f=8×10⁻² to 8 × 10⁻⁴ Hz, K_max=28 to 72 MPa•m^1/2, and R = 0.95. Under these conditions, the crack growth rates were ∼0 to 3 × 10⁻⁹ m/s. The effects of water chemistry transients which produced changes in the concentration of dissolved oxygen or sulfate in the environment were also investigated. The second heat was tested in water with 8-ppm dissolved oxygen. The influence of load ratio and frequency was investigated over the range R = 0.5 to 1.0 and f = 1 × 10⁻¹ to 2 × 10⁻³ Hz, at maximum stress intensity K_max = 28 to 38 MPa•m^1/2. Under these conditions, crack growth rates varied from 1 × 10⁻¹⁰ to 3 × 10⁻⁹ m/s. Crack growth rate increased significantly at low R values. However, the growth rate at R = 0.95 was not significantly different from that under constant load. Correlation of the crack growth rate data with crack-tip strain rates is discussed.