Reasons for Cracking Failure of an Elbow of External Petroleum Pipeline in a Station

An elbow of external oil pipeline in a station in Tahe oilfield cracked. In order to determine the cracking reason of the oil pipeline elbow, the morphology of failure position were observed by metallographic microscope and scanning electron microscope, the pipeline corrosion products were analyzed by X-ray diffraction, and the mechanical properties of the pipe were tested. The results showed that there were abnormal structures such as upper bainite, bainite and martensite in the microstructure of pipeline, so the material had high strength and hardness, and low plasticity and toughness. A small amount of inclusions and high residual stress in the material caused the improvement of cracking sensitivity of the elbow. Stress corrosion cracking occurred in the pipelines under the action of internal stress and corrosive medium. With the continuous action of stress, the pipeline cracked and leaked.