Crack initiation and early propagation of X70 steel in simulated near-neutral pH groundwater

The near-neutral pH environmentally assisted cracking (EAC) initiation and propagation behavior of X70 pipeline steel was investigated in NS4 solution deaerated with N-2/5% carbon dioxide (CO2) under the low/frequency cyclic load. The test results show that an increase in the applied stress level promotes the crack initiation. Based on the crack initiation sites, the cracks are divided into two types, pit cracks and non-pit cracks. Non-pit crack initiation is found to be the dominative cracking mechanism on the original external pipe surface, but as the tests progress, the contribution of pit cracks becomes significantly pronounced. The pits acted as the crack initiation sites are related to the localized dissolution at/around the nonmetallic inclusions. Therefore, it is expected that the EAC initiation resistance of steel can be improved by reducing the inclusion content in the steel. The crack propagation rate is found to be essentially independent of the applied stress level. By comparing the crack initiation lifetime of the X70 steel to the test data of X65 steel reported in the literature, it is found that X70 steel is more resistant to the EAC than the X65, owing to the effect of microstructure.