IGSCC crack initiation and growth in mill annealed alloy 600 tubing in high temperature caustic

TitleIGSCC crack initiation and growth in mill annealed alloy 600 tubing in high temperature caustic
Publication TypeBook
Year of Publication1997
AuthorsBrisson, BW, Ballinger, R, McIlree, AR
Abstract

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.