Microstructure And Mechanical Properties Of Underwater Friction Taper-plug Welds For X65 Pipeline Steel

Friction taper-plug welding(FTPW)is an innovative solid-phase joining process with huge potential and promising application in the repair of underwater steel structures such as marine platform and submarine pipeline.Up to now,not too many literatures related to underwater FTPW could be found and researches associated with the weld formation,bonding mechanism,material flow and weldability of different plugs are even less.In this paper,X65 pipeline was selected as the base material and four kinds of plugs were used to perform underwater FTPW experiments.The weld formation and bonding mechanism,material flow behavior during welding process and the influence of plug materials and welding parameters on the defects formation,microstructural evolution and mechanical properties were investigated.Within the experimental condition and process parameters of this paper,main conclusions are summarized as flows:Using X65 and Q345 C plugs can obtain defect-free welds with 7000 rpm rotating speed,14 mm burn-off,30~50kN and 20~40kN axial forces respectively.When Q235 C plug is used,lack of bonding is commonly found as a kind of root defect owing to the insufficient energy input.Intermittent crack were found along the bonding interface of all the 316 L plug welds,which should be caused by the residual tensile stress around the bonding interface.Microcracks can easily emerge at and may propagate along the ?-? phase boundaries to form macrocracks.Metallurgical bonding of hole bottom and sidewall is easy to be realized for X65 plug welds,but lack of bonding defect is very likely to appear at the rounded transient.Increasing plug burn-off can prolong welding time,thus enhancing energy input and reducing cooling rate of rounded transient which is beneficial for metallurgical bonding to be realized between the plug and base material.For X65 plug welds produced under generally low axial force,the shortcoming of material flow and energy input in heat dominated phase(HDP)and heating-shear transition phase(HSTP)is a key factor for the formation of incomplete filling defect at the rounded transient.Material flow and energy input during HDP and HSTP can be improved by increasing the axial force,which would result in better material flow,higher weld power,more energy input in HDP and HSTP as well as a closer distance to hole bottom.These are all in favor of obtaining high quality welds.X65 plug welds is dominated by upper bainite with hardness generally below 300HV10.Microstructure in the heat-affected zone(HAZ)of Q345 C plug welds is mainly upper bainite with hardness lower than 300HV10,but in the weld zone(WZ),quenched structures including lower bainite and lath martensite results in hardness generally ranging from 300~400HV10.Microstructure in the HAZ of Q235 C plug welds is also mainly upper bainite.The maximum hardness in HAZ is 230HV10.Microstructure in the WZ of Q235 C plug welds is mainly coarse Widmanstatten structure with maximum hardness up to 240HV10.Using 316 Lplugs can significant reduce WZ hardness for austenite dominated microstructure after FTPW process.Almost all the tensile specimens of X65 and Q345 C plug welds fractured in the base material far away from the plug area.Charpy impact absorbed energy of weld zone at 0? is above the requirement of AWS D3.6 Underwater Welding Code.Mean value is higher than 40 J at all parameters and 298 J could be reached under higher axial force for X65 plug welds.