Corrosion fatigue behavior of S135 high‐strength drill pipe steel in a simulated marine environment

The fatigue and corrosion fatigue lives of S135 drill pipe steel were tested in air and in a simulated marine environment. The empirical equations for fatigue lives in different environments were obtained via regression analysis, and fatigue fracture mechanisms were elucidated. Results showed that an endurance limit existed in air, and fatigue cracks were initiated from or near the surface of specimens. The morphologies in air were cleavage fracture at the crack initiation zone and fatigue striations at the crack propagation zone. In the marine environment, fatigue striations were less at higher stress amplitudes, and fatigue lives were lower than that in air. Fatigue cracks were initiated from the corrosion pits on and near the surface of specimens. The morphologies were characteristic cleavage fracture at the crack initiation zone and cleavage fracture and secondary cracking at the crack propagation zone. Less fatigue striations occurred on the main fracture surface. The fracture mechanism in the marine environment can be summarized as follows: corrosion pit formation, crack initiation, crack propagation, piercing, and fracture.