Causes of Segregation Defects at the End of Pipe and Its Influence on the Microstructure and Properties of Pipe Body
Gas shield solid wire welding process(GMAW)was used for on-site girth welding in a pipeline project.The PAUT(with TOFD)nondestructive testing results of the girth weld after welding failed,and suspected delamination defects were found in the heat-affected zone(HAZ)of the girth weld.The out-of-tolerance defects were further identified as segregation defects by magnetic particle,penetration and ultrasonic detection.The properties and characteristics of defects are analyzed by means of metallography,and analysis of SEM energy spec-trum,and the causes of defects are analyzed in combination with steel making and rolling processes.It is found that the continuous large and long-term decelerations of subsequent plates are the main factors leading to obvious segregation.MnS,Nb and Ti carbon nitride segre-gates and precipitates,and the Vickers hardness of the segregation zone does not meet the requirements of DEC-NGP-S-PL-003-2020-1《Specification of Steel Pipe for Gas Pipeline Project》.However,the tensile strength,impact toughness and DWTT of the pipe body contai-ning the defect meet the requirements of DEC-NGP-S-PL-003-2020-1.
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