Relationship between coercivity and stress of X80 pipeline steel

Abstract

The operation environment along the long-distance oil and gas pipelines is complex, and the abnormal external load during pipeline operation may lead to local stress concentration, which will increase the operation risk of pipeline.The coercivity-based stress measurement method is a new technology to evaluate the safety margin and prevent the safety accidents of pipeline. Herein, the magnetic properties of ferromagnetic materials, such as atomic magnetic moment, domain structure and hysteresis loop, were explored in depth. The mechanism of mechanical and magnetic coupling in coercivitybased stress measurement was clarified. The existing expressions of relationship between coercivity and stress in uniaxial stress state were introduced. The model of relationship between coercivity and stress in biaxial stress state was deduced.Besides, experiments were performed to verify the relationship between coercivity and stress of X80 pipeline steel in the uniaxial and biaxial stress states. The experimental results show that there is a good linear relationship between the coercivity and stress of X80 pipeline steel in uniaxial stress state, the coercivity is related to the biaxial stress in the biaxial stress state, and the laws are consistent with the models derived herein. The research result provides a theoretical and experimental basis for the field application of coercivity-based stress measurement method, which is proved to be capable of accurately measuring the stress of X80 pipeline steel, and it is a reliable method to evaluate the safety margin of in-service pipeline.

Keywords

• x80 pipeline steel
• coercivity
• mechanical and magnetic coupling
• stress measurement
• biaxial stress state