Cessation of environmentally-assisted cracking in a low-alloy steel: experimental results

Abstract

The presence of dissolved metallurgical sulfides in pressure vessel and piping steels has been linked to environmentally-assisted cracking (EAC), a phenomenon observed in laboratory tests that results in fatigue crack growth rates as high as 100 times that in air. Previous experimental and analytical work based on diffusion as the mass transport process has shown that surface cracks that are initially clean of sulfides will not initiate EAC in most applications. This is because the average crack tip velocity would not be sufficiently high to expose enough metallurgical sulfides per unit time and produce the sulfide concentration required for EAC. However, there is a potential concern for the case of a relatively large embedded crack breaking through to the wetted surface. Such a crack would not be initially clean of sulfides, and EAC could initiate. Previous experiments have suggested that under some conditions, EAC could be persistent. This paper presents the results of a series of experiments conducted on two heats of an EAC susceptible, high-sulfur, low-alloy steel in 243°C low-oxygen water to further study the phenomenon of EAC persistence at low crack tip velocities. A load cycle profile that incorporated a significant load dwell period at minimum load was used. Experiments using compact tension specimens with various initial precrack depths were employed to simulate the breakthrough of embedded cracks. The results showed that EAC ceased after several hundred hours of cycling. This indicates that significant dwell periods can allow sufficient time for sulfur diffusion to turn off EAC provided that the initial crack tip velocities are not unusually high.