Mechanical properties and microstructure of X70 butt joint welded by self-shielded flux cord wire

The present paper deals with the research work of self shielded flux cored wire (FCAW-S) for X70 large diameter pipe.Welding procedure qualifications tests and mechanical properties tests were carried out by Pipeline Research Institute of CNPC to assess field weldability and should be in accordance with API 1104 and the additional standard requirements.The results of welding procedure qualifications tests show that the self-made FCAW-S has good all-positions weldability, detachability, arc stability and less spatter loss coefficient and total amount of weld fumes.The tensile strength of theweld is about 730-760 MPa and the average impact absorbing energy at -20℃ is 125 J.Tests results show that this self-made FCAW-S can provide a qualified weld for X70 pipeline steel.The microstructure of X70 butt joint weld exhibits acicular ferrite, polygonal ferrite, proeutectoid ferrite, bainite morphology but shows no weidmanstaten structure in HAZ (heat-affected zone) coarse grain zone.The main microstructure in columnar crystal zone and HAZ coarse grain zone is bainite.Bainitic microstructure in the butt joint indicated this self-made FCAW-S have the capability to meet all mechanical properties requirements of this welding material.