Transgranular stress corrosion cracking of X-60 pipeline steel in simulated ground water

An investigation was carried out on API X-60 steel specimens with multiple edge cracks to determine transgranular stress corrosion crack (TGSCC) growth rates in simulated ground water (NS-4 solution). Loading and environmental conditions were similar to those for buried natural gas pipelines in service, Specimens were loaded statically or cyclically in cantilever bending with the maximum crack lip stress intensity factor (K-max) varying from 34 MPa root m to 38 MPa root m. Static tests were carried out for periods extending to 111 days. For the cyclic tests, which lasted up to 378 days, the stress ratio (R = minimum/maximum load) was controlled between 0.5 to 0.9, with frequencies ranging from 40 cycles/day to 5,000 cycles/day. TGSCC with crack features similar to those found in the field was produced successfully in the laboratory. Depending upon the loading conditions and test period, the crack growth varied from similar to 10 mu m to 1,800 mu m, corresponding to maximum average crack growth rates from 6.1 x 10(-10) mm/s to 2.6 x 10(-7) mm/s, For a low R ratio and high frequency, the observed growth was the result of corrosion fatigue. Under the conditions of a static load or high R ratio and low frequency, growth was caused by TGSCC. A superposition model for crack growth was proposed and applied to the experimental data, giving good agreement between the predicted and observed crack growth rates.