Corrosion Fatigue Behavior of S135 Drill Pipe Steel in H₂S Enviornment

Fatigue tests under different stress amplitude were conducted on S135 drill pipe steel in air and H₂S environments, and the fatigue lives under different conditions were obtained. The fatigue life formulas of the drill pipe steel in different environments were obtained by a data regression analysis method, and the fatigue fracture mechanism was analyzed. The results show that the tested steel had an obvious characteristic of fatigue limit in the air environment. The fatigue lives of tested steel in the H₂S environment were lower than those in the air under the same equivalent stress amplitude, the fracture happened under relatively low equivalent stress amplitude and no fatigue limit existed. The fatigue cracks in the tested steel were all initiated at or near the surface in different environments and under different equivalent stress amplitude, and the crack initiation region exhibited mainly cleavage fracture. The crack propagation region showed a main characteristic of fatigue striations in the air while of cleavage fracture and cleavage steps in the H₂S environment. On the cleavage fracture surface existed lots of second cracks, indicating a characteristic of hydrogen embrittlement fracture.