The Welding Process And Metallugical Characteristics Of Narrow Gap MAG Welding Of Thick Q690E Steel At Vertical Position With Arc Swinging And Shifting

With gradual implementation of the National Ocean Strategy,jack-up drilling platform has become the mainstream products for offshore oil exploration and development,the depth of its working area keeps going to the deeper waters,and its work environment becomes more and more challenging.Correspondingly,strength level and thickness of marine engineering steel used for the construction had been increased.Leg rack is the core component for the lifting system of jack-up rig,which carries the full weight and the load of the platform,so its welding quality will directly determine the job security and service results of offshore platforms.At present,joining of large and thick high-strength steel for the rack is mainly by using manual metal arc welding.The welding process is complex,the efficiency is low,the quality is unstable,and working conditions is poor.All these results urgently need a green,high-quality,efficient welding process for joining high-strength steel thick plate.Under such a background,in this paper a narrow gap MAG robotic welding process with arc swinging and shifting(NG-S2MAG)at vertical position is proposed for welding of the leg rack for 400ft(?189m)jack-up drilling platform.Compared to the traditional welding process,the new welding process saves about 80%of welding consumables,and shortens the weld period by nearly 50%.On this basis,the process characteristics,thermal characteristics,microstructures and mechanical properties of NG-S2MAG weld of 177.8mm thick Q690E steel,an advanced quenched and tempered high-strength steel,were investigated in this paper.The forming and thermogenic characteristics of the "narrow gap-thin wire-gas shielded-one pass one layer-arc swinging and shifting-vertical welded joint" with 18?22mm gap width were studied.Essentially,during NG-S2MAG welding at vertical position,the arc moves back and forth by swinging and shifting between two side-walls of the groove without vertical movement,and while the arc reaches the dwell point near the side-wall,it moves straight upward and penetrates the side-wall.A three-dimensional dynamic mathematical heat source model of NG-S2MAG welding at vertical position was established by adopting coordinate conversion method.The results of temperature field simulations show that the fusion of two side-walls is asynchronous,which is different from the conventional through-arc or slightly swinging GMAW processes.The pulsed heat input at one side of the base metal,approximate 12.5kJ/cm,is about 1/2 of the total arc energy.It is also the part of the energy resulting in a total heat affected zone(HAZ)width of approximate 3.0mm,while coarsen grained zone(CGHAZ)is only about 0.8mm.The characteristics of the welding process are that during the swinging period of the arc,it moves faster to cross the gap with shorter time;while during the dwell period of the swinging cycle,the arc moves slower upward along the side-wall with a longer period time.The formation of the welded joint seems like a "pileup blocks" with a thinner layer of 2.5?4.0mm in thickness.The characteristics of multi-peak thermal cycle curve for the "narrow gap-thin wire-gas shielded-one pass one layer-arc swinging and shifting-vertical welding process" were revealed.In addition,the sub-regions classification in the original CGHAZ zone of multi-layer NG-S2MAG welded joint were clarified.A new concealed thermocouple method was proposed and adopted to acquire the thermal cycle curves of CGHAZ during multi-layer NG-S MAG welding.It was found that the multi-peak thermal cycle is caused by arc swinging and asynchronous fusion of two side-walls of the groove.This feature determines that the time above 1200? in CGHAZ zone will be less than 1/3 of the conventional through-arc GMAW welding.However,the cooling rate of the new process with less heat input can be compared with the conventional through-arc GMAW welding.Besides,during multi-layer welding the heat of the following layers causes the original CGHAZ to separate into seven sub-regions,including FG3-CG2-CG1HAZ?IC3-CG2-CG1HAZ?SC3-CG2-CG1HAZ?SC3-FG2?CG1HAZ?FG2-CG1HAZ?IC2-CG1HAZ and SC2-CG1HAZ.The area ratio of these sub-regions are about 4%,8%,28%,16%,23%,12%and 9%respectively of the area of the original CGHAZ.In the sub-regions,no CGHAZ and/or CG2-CG1HAZ exist.Within these sub-regions,high temperature tempering and inter-critical heat treatment regions occupies about 70%of the area of the original CGHAZ.This shows that NG-S2MAG welding at vertical position has a unique function of self-optimization.Embitterment mechanism of NG-S MAG welding of Q690E steel at vertical position was revealed.The results of welding thermal simulations show that Q690E steel is sensitive to austenitic grain growth as the thermal cycle temperature above 1200?.Microstructures of CGHAZ and CG2-CG1HAZ are mainly coarse lath martensite,which is hard and brittle,resulting in low impact toughness mainly of transgranular-cleavage fracture.When the peak temperature was lower than 1200?,it contributes to a reheat process to improve low temperature toughness of CGHAZ.However,the toughness increase ratio of martensitic CGHAZ was about 40?50%after high temperature tempering and/or inter-critical heat treatment,that is 20?40%higher than that by normalization.This feature of martensitic CGHAZ is different from that of the traditional pearlitic or bainitic steels.For Q690E steel,it also shows the lath martensite of original CGHAZ tempered into tempered martensite or tempered sorbite,has a better ability to inhibit crack propagation than normalized into fresh martensite.Some process measures to optimize the joint properties of NG-S2MAG welding of Q690E steel at vertical position had been found.In order to control the deterioration of CGHAZ toughness,it is necessary to shorten the time above 1200? and control the thickness of the weld layer within 3.5mm,through adjusting welding current,welding speed and dwell time on both sides of the groove,etc.As for improving the properties of the near-surface weld and its HAZ,tempering bead technique has to be used.On one hand,the above research results will enrich the engineering data and related welding theory about welding of advanced high-strength steel plate of grade 800MPa,by means of the narrow gap-thin wire-gas shielded-one pass one layer-arc swinging and shifting-vertical welding process.On the other hand,the results will provide a new process method and theoretical basis for large thick-walled high-strength steel structure in the field of ocean engineering equipment.