The Stress concentration is the main reason of damage to ferromagnetic metal pipelines. Through effective detection,the use status of metal pipelines can be predicted and the safety of equipment can be effectively guaranteed. The ferromagnetic effect of the material under stress is analyzed by the energy balance theory of ferromagnetic materials. The 3D simulation model of ferromagnetic materials is built by simulation software based on the metal band theory, and the magnetic effect characteristics of the system under tension stress are calculated. The weak magnetic field detecting sensor based on differential symmetrical structure is designed and manufactured. The stress concentration system of different types of pipelines is detected by the stress detection system based on the weak magnetic sensor. The stress concentration is verified by other stress detection methods. The results show that the weak magnetic sensor can effectively detect the weak magnetic field signal on the pipeline surface, and the weak magnetic field signal shows violent fluctuation characteristics in the stress concentration region. Various detection methods verify the effectiveness of the weak magnetic stress detection. Weak magnetic stress detection technology can effectively detect pipeline stress concentration caused by defects or deformation.

The 2.25Cr1Mo steel that was frequently used in petrochemical equipment was welded by hot welding process at 750 ℃ and 850 ℃. The welding residual stress of every welded specimen was measured by the small blindhole method. The electrochemical workstation was used to measure the stress corrosion property of every welded specimen in alkaline, acidic and neutral medium. The microstructure of welded joint was analyzed by metallographic technique. The results showed the hot welding process could obviously reduce the welding residual stress of 2.25Cr1Mo steel welded plate and raise the stress corrosion property of it in every medium. In addition, the higher the process temperature was, the bigger the reduced degree of welding residual stress was, the more the stress corrosion property was improved. At 850 ℃, the welding residual stress was reduced by the degree of above 60% and the stress anticorrosion property was raised by the degree of above 30%. The microstructure of welded joint was improved after high temperature welding. The mechanical property was maintained.