Abstract :
Environmentally assisted cracking (EAC) can cause increases in fatigue crack growth rates of 40–100 times the rate in air for low alloy steels. EAC is activated by a critical level of dissolved sulfides at the crack tip. Sulfide inclusions (MnS) in the steel produce corrosive sulfides in solution following exposure by a growing crack. In stagnant, low oxygen water conditions considered here, diffusion is the dominant mass transport mechanism acting to change the sulfide concentration within the crack. Previous diffusion analysis and experimental work (Wire, G.L., Li, Y.Y., 1996. Press. Vessel. Pip. 323, 269–289) has shown that surface cracks will not initiate EAC for most loading conditions. Diffusion analysis is extended herein to cover breakthrough of embedded defects with large sulfide inventories. The mass transport via diffusion is limited by the sulfide solubility. As a result, deep cracks in high sulfur steels are predicted to retain undissolved sulfides for extended but finite periods of time, tdiss, which increase with the crack length and the metallurgical sulfide content in the steel. The predicted crack extension due to the limited duration of EAC is a small fraction of the initial embedded defect size and would not greatly change calculated crack depths. The calculations are in agreement with recent experiments by Li (Li, Y.Y., 1997. ASME Pressure Vessel and Piping Conference, 27–31 July, 1997, Orlando, FL).
