20 steel thick-wall high-pressure pipes are widely used in chemical industry in China, but brittle fracture accidents of them happen frequently in recent years. The strain aging steel pipes in this research were artificially made in the laboratory and naturally occurred in fabrication or service. The microstructure of strain aging in samples taken from the unused pipes and the accident pipes caused directly or mainly by strain aging were observed with optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that: (1)uniform ferrite and pearlite with uniform lamellar structure can be achieved with normalizing; lamellar pearlite fractures and distorts while a large number of dislocation cells, micro-holes or cracks appear in material after strain aging; and broken distorted pearlite changes to uniform spherical structure after stress relieving; (2) both the samples taken from unused and accident pipes which strain aging artificially in the laboratory and naturally in fabrication or service have the same characteristics as following: the basic changes of microstructure after strain aging cannot be observed with optical microscopy, but can be observed with scanning electron microscopy and transmission electron microscopy.

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