Cyclic Crack Growth Rates of X-60 Pipeline Steel in a Neutral Dilute Solution

Abstract

An experimental investigation was carried out to measure transgranular stress corrosion crack growth rates (CGR) in precracked X-60 steel specimens exposed to neutral (pH 7) dilute simulated ground water (designated NS4) while being cyclically loaded in cantilever bending. Testing was carried out over short-term (generally < 40 days) and long-term periods (~ 1y). 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 and frequencies ranging from 1 to 400 cycles/day. Cyclic loading conditions for specimens in the short-term tests were more severe, with R ratios ranging from 0.5 to 0.9 and frequencies from 40 cycles/day to 5,000 cycles/day. Post-mortem metallographic examination by optical (OM) and scanning electron microscopy (SEM) was used to evaluate crack growth. 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, depending upon applied load range. For the more severe loading conditions in the short-term tests, the observed growth was attributed to corrosion fatigue. Under the less severe loading conditions of a high R ratio and low frequency, growth was attributed to transgranular stress corrosion cracking.