|Title||IGSCC crack initiation and growth in mill annealed alloy 600 tubing in high temperature caustic|
|Year of Publication||1997|
|Authors||Brisson, BW, Ballinger, R, McIlree, AR|
SCC crack initiation and crack growth rates have been measured in mill annealed alloy 600 tubing as a function of stress intensity factor in 10% caustic at 315 degrees C. Tests were conducted using internally pressurized smooth and pre-cracked tubing. Samples were polarized to either +150 my (pre-cracked tube test) or +225 mV (initiation test) with respect to a nickel electrode. Crack initiation and growth from the external tube surface were monitored using a multi-frequency AC potential drop system. The AC potential drop system allowed detection of initiation from a smooth surface as well as the monitoring of crack extension in real time. Crack growth rates were obtained over the stress intensity factor, K, range between 4 and is MPa root m. Values for K were estimated based on fractographic analysis of samples after testing and an estimate of the K-solution for a thin walled tube. Average crack growth rates ranged from 2 to 14 mm/yr. Estimates of the uncertainty in both crack growth rates and K were made. Growth rates obtained fall within the overall range of the existing data base for da/dt in alloy 600. The data and analysis suggest a threshold value of K for K-driven crack growth of approximately 4 MPa root m. However, since the scatter in the existing data is very large. More importantly, from the standpoint of life prediction it was observed that da/dt responded to and was a function of stress intensity factor for cracks as small as 0.2 mm in depth and probably smaller. Additionally, initiation from a smooth surface occurred at multiple sites followed by link up to form a main crack front.