Study On The Welding Process Of X90 Submerged-Arc Welded Pipe And Properties Of Welded Joint

X90 pipeline steel has a broad application prospect due to its high strength,toughness,weld ability,and good corrosion resistance.Welding is the key segment in pipe production,in which the weld thermal cycle will affect the micro-structure,which would affect the service life.So it is necessary to investigate the welding technology of X90 pipeline steel.The chemical composition and micro-structure of X90 pipeline steel provided by a domestic steel mill was analysis.The carbon equivalent of X90 pipeline steel was calculated by the formula CEPcm,i.e,0.18%,which indicated low susceptibility to weld cold cracking and good welding ability.The main micro-structure of X90 pipeline steel was granular bainite(GB),a few indistinct bainitic ferrite(FB)in some area,some quasi-polygonal ferrite(QF),dispersively distributed insular or punctuate martensite and retained austenite(M/A).These micro-structure represent high strength and toughness of the base metal.X90 is suitable for pipeline steel for oil and gas pipeline.Then welding material was selected,the welding groove was designed,and tack-final welding procedure was drafted.Six procedures were designed to weld 6 pipe ends 2 m long(?1219mm×16.3mm).The micro-structure,tensile strength,impact toughness and hardness were studied.The results show that Performance meets the standard requirements.The welded joint has the best tensile strength and toughness with BHM-10+YS-SJ105G welding material.The CVN of HAZ represent some ultralow value prove the the local brittle phenomenon;The tensile test specimen fractured at the heat affected zone and the bend test dent at the welded seam,which represent soften phenomenon.So the OM and SEM were employed to analysis Bainite features in CGHAZ and ICCGHAZ,and LePera was used to represent M/A constituent.It is concluded that coarse grain brittle and the necklace distribution of M/A constituent at the prior austenitic grain boundary was essential reason of local brittle in HAZ.Then the soften phenomenon was discussed by SEM and hardness test.The result show that FGHAZ comprise with fine GB and QF,which have more ferrite content and lower hardness than CGHAZ.Meanwhile FGHAZ lack of hard lath micro-structure.As a result,FGHAZ represent soften phenomenon.Finally,a modified welding procedure with low heat input high deposition efficiency was designed,which was employed to product 402.4 t SAWH pipes.Distinctly,the brittle and soften phenomenon was ameliorated.