CYCLIC CRACK-GROWTH RATES OF X-60 PIPELINE STEEL IN A NEUTRAL DILUTE-SOLUTION

An experimental investigation was carried out to measure transgranular stress corrosion crack growth rates (CGR) in precracked X-60 steel sp ecimens exposed to neutral (pH 7) dilute simulated ground water (desig nated NS4) white being cyclically loaded in cantilever bending. Testin g was carried out over short-term (generally < 40 days) and long-term periods (similar to 1 y). Loading and environmental conditions in the long-term tests were similar to those for buried natural gas pipelines , with the R ratio (minimum/maximum load) varying from 0.82 to 0.98 an d frequencies ranging from 1 to 400 cycles/day. Cyclic loading conditi ons for specimens in the short-term tests were more severe, with R rat ios ranging from 0.5 to 0.9 and frequencies from 40 cycles/day to 5,00 0 cycles/day. Post-mortem metallographic examination by optical (OM) a nd scanning electron microscopy (SEM) was used to evaluate crack growt h. Average CGR from 4.50 x 10(-8) mm/cycle to 1.25 x 10(-4) mm/cycle ( equivalent to 1.4 x 10(-9) mm/s to 7 x 10(-7) mm/s) were measured, dep ending upon applied load range. For the more severe loading conditions in the short-term tests, the observed growth was attributed to corros ion fatigue. Under the less severe loading conditions of a high R rati o and low frequency, growth was attributed to transgranular stress cor rosion cracking.