Failure Analysis of Butt Weld of Bimetal Composite Pipes

摘要

The study aims to summarize the main failure morphology of butt weld of bimetal composite pipes which are used in a gathering pipeline of one condensate gas field with high temperature and high pressure, and then analyze the failed pipes with the methods such as SEM, energy spectroscopy, and metallographic examination. The experiments show that there is no Mo which is the key element of metal material to resist the pitting corrosion and stress corrosion in the organization of the welds. Through the microscopic morphology analysis it shows that the process of the gradual shedding of corrosion product film gives rise to the failure of pipelines. The gradual shedding of corrosion product will expose the new matrix to the corrosive environment which will accelerate further erosion. The cycle of the erosion continues and eventually leads to pipelines' leakage. The weld residual stress plays an essential role in the weld failure, besides, the existence of the residual stress will accelerate the collapse of the passivation film. The factors which lead to the weld failure are the special structure, organization of the weld, corrosive media, and the residual stress of the weld. In view of the failure situation, we put forward the corresponding suggestions as follows.