Abstract: Instantaneous circumferential fracture occurred in a section of elbow in a high-pressure pipeline of a synthetic ammonia enterprise. The causes of elbow fracture were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope and energy spectrum analysis, mechanical property test, low-magnification test and metallographic examination. The results show that the nitrogen content of the elbow was high, and the correct heat treatment was not carried out after cold bending, which caused strain aging embrittlement and work hardening. At the same time, the existence of ring loose, banded structure and widmanstatten structure in the material further reduced the mechanical properties of the elbow. Under the action of a certain impact force, the elbow underwent brittle fracture.