Investigation on welding parameters and bonding characteristics of underwater wet friction taper plug welding for pipeline steel

The Q235 and X65 steel tapered plugs were used to perform friction taper plug welding (FTPW) on X65 pipeline steel in underwater wet condition. The work concentrates on defect formation, torque variation, energy input during FTPW, and also the weld microstructures and hardness. Energy input is proportional to torque-time integration under constant rotating speed and is critical to realize metallurgical bonding between the hole and plug. Increasing burn-off can prolong welding time, thus enhancing energy input, so that defect can be eliminated. Using X65 plug can produce defect-free welds since the energy input is adequate. However, when Q235 plug is used, lack of bonding is commonly found as a kind of root defect in friction taper plug (FTP) weld owing to the insufficient energy input. The weld zone microstructure mainly consists of upper bainite in X65 plug weld, but Widmanstatten structure in Q235 weld. In heat-affected zone, the microstructure is found as upper bainite in both X65 and Q235 plug welds.