| In the year of 1995, The Welding Institute of UK has proposed a new solid-state joining technology namely friction hydro pillar processing(FHPP) which is of high expectations for the use of welding and repairing the subsea metal structures in marine engineering. Based on the same principle, friction taper plug welding(FTPW) was then improved when a tapered hole/plug is used. So far, although research works for underwater FHPP/FTPW are still being persisted in developing welding equipment and confirming the feasibility, desired achievement is rare. Hence, to the absent acknowledge, we would like to propose that some basic questions need to be clarified may include the parameter design of the device, the feasibility and process window in fabricating sound welds, the metallurgy bonding mechanism and microstructure evolution in the weld, mechanical properties and failure mechanisms of the joint, etc.In the present study, aiming at the use of FHPP/FTPW in offshore engineering, we have developed a dedicated welding system which has the highest axial force capability and the power, and then carried out a systematic research for joining DH36 and X65 steels in air and underwater conditions. Based on the abundant experiment, not only have a lot of defect free welds been obtained, but also the welding metallurgy characteristics and underwater welding processing issues have been basically resolved. The microstructural evolution for underwater FTPW weld was investigated under scanning electron microscope, electron back scattering diffraction analysis, and transmission electron microscopy. Moreover, the joints’ properties of tensile, bending and impact were also investigated systematically. Main conclusions are as follows:(1) Based on the performance analysis, a FHPP/FTPW equipment has been developed successfully. It involves a highly integrated welding spindle, a hydraulic power system and an automatic control system. Main technical parameters of welding system are the maximum rotational speed of 8000 rpm, the maximum axial force of 60 kN, the maximum torque of 100 Nm, and a total power output of 93 kW. Initial trials indicate that, on the machine, not only can a continuous process including drilling, welding, cutting and milling at the location where interested be performed, but a continuous seam welding process on X65 pipe structures.(2) In the air medium, defect free DH36 welds could be easily fabricated by FTPW in the parameters of 7000~7500rpm rotational speed and 25~40kN axial force. During FTPW process, the welding thermal cycle curve indicated that near the side wall of the hole the peak temperature would reach 1180℃ or higher, but 965℃ at the bottom. The t8/5 in the above two locations were about 26 s and 34.5s respectively. When Q235 B plug was used, Widmanst?tten structures and grain boundary ferrite with over heated characteristics could observed in weld metal. Decreasing the weld energy input could improve the overheating behavior through increasing the cooling velocity. At optimized parameters, the tensile properties of the weld could reach the level same to the base material.(3) In the water medium, to DH36 steel, both of lack of bonding and incomplete filling defects would form frequently in FHPP welds. However, with FTPW, a lot of defect free welds could be obtained when the hole was tapered in angle range of 18~24°。In such conditions, the water medium can hardly influence the welding qualities. When a 24° tapered hole was used, combining with a 21° tapered plug, the suggested defect free process window is of 7000~7500rpm rotational speed and 25~45k N axial force.(4) Based on the OM, SEM and EBSD analysis results, the microstructural evolution near bonding interface and bonding mechanism of the hole and plug during underwater FTPW for such steel was investigated. It is believed that, several material from the burn-offed plug in thermoplastic state can flow into the gap between the hole and plug. Under the couple state of frictional heating and axial forcing, these materials would recrystallize along the initial side wall of the hole during FTPW, so that several new grain boundaries were performed. This is identified as the metallurgical bonding pattern of the plug material and the hole sidewall in underwater FTPW process.(5) OM, SEM, TEM and EBSD were used to investiated the microstructure of underwater DH36 weld with DH36 plug. It is found that the microstructure of weld zone(WZ) is consist of lath martensite and coarsening lath bainite, in HAZ, it is mainly of lath bainite. In Q235 B plug weld, overheated microstructures of grain boundary ferrite and side plate ferrite could observed in the WZ. When Q345 B plug is used, however, a number of fine acicular ferrite with random grain orientations.(6) The DH36 underwater FTPW joints exhibit good tensile and impact properties. At optimized parameters, the tensile strength of the weld could reach the level same to the base material, but the elongation is of 10~25%. At 0℃, the impact energy of weld zone and bonding line would reach 39.5J and 45 J as the best. Moreover, a significant improvement of impact toughness that 10~20J higher than that with DH36 plug could be found by using Q345 B plug owing to the formation of fine acicular ferrite.(7) Defect free X65 welds could be obtained in underwater condition by FTPW with 7000 rpm rotational speed and 25~45kN axial force. The microstructure of WZ includes lath martensite, bainite, and acicular ferrite. In HAZ it is mainly of upper bainite. The X65 underwater FTPW weld exhibits a tensile property same to the base material, R=6T bending 180° without any crack, and 70~100J impact energy which could satisfy the marine use according to AWS D3.6 underwater welding code. However, the too high hardness value of 480HV10 in weld zone could not meet the AWS D3.6 request. |
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