Microstructure And Mechanical Properties Of Underwater Friction Taper Plug Welding For Dissimilar Steel

In this study,X52 pipeline steel is chosen as the base material and Q345 C low alloy structural steel with Q235 C carbon structural steel are chosen as the consumable rod to perform underwater FTPW(Friction taper plug welding)experiment.The interactions between the process parameters and weld forming,weld defect formation,microstructure evolution and mechanical properties of two types(blind-hole or through-hole)underwater FTPW are studied.The experimental results provide an important basis for the application of underwater FTPW in local repairing of the subsea metal structures.In the range of 6500~7500rpm rotating speed and 40~45kN axial force,defect free welds are obtained by underwater blind-hole FTPW with Q345 C consumable rod.With the same rotating speed,the width of forged zone and HAZ at the corner near bottom hole would gradually expand with the increase of the axial force.With the same axial force,however,the rotating speed has no obvious effect on it.The microstructure of the weld zone mainly consists of lath martensite and lath bainite,and its hardness is 450HV1.The bonding line near to the hole side is a band of fine-grained ferrite.The heat affected zone of the base material near the bonding line mainly consists of different sizes lath martensite and lath bainite,and its hardness is up to 430HV1.The hardening tendency of FTPW joint is obvious.The tensile performance of the upper part of the underwater blind-hole FTPW joint is better than that of the lower part of the joint.The upper part of the joint tensile specimens are basically broken at the base material.The tensile strength is 550.7~609.1MPa,and the elongation is 13.82~19.84%.The tensile specimens in the lower part of the joint are basically broken at the bonding line.Most of their tensile strength is below 550 MPa,and the elongation is generally less than 10%.The V-notch impact absorbed energy at 0? in the center of the weld and bonding line would reach 64 J and 65 J as the best,which is much lower than that of the base material(267J).The CTOD value of base material is the highest with the average value of 0.993 mm.The weld CTOD value(0.129~0.163mm)is much lower,and the heat affected zone CTOD value is the lowest(0.020~0.071mm).The highest CTOD value of weld and heat affected zone are 16.41% and 7.15% of the base material respectively.Underwater through-hole FTPW experiments are performed using Q235 C steel as plug.The results indicate that lack of bonding and incomplete filling defects would form under the condition of 7000 rpm rotating speed with 30~35kN axial force.With the axial force increasing to 35 kN,the weld defect can be avoided,and the width of forged zone would gradually expand.The first eutectoid ferrite,side plate ferrite,polygonal ferrite,acicular ferrite and Widmanst?tten ferrite microstructure could be observed in underwater through-hole FTPW weld,and its hardness is between 158~245HV1.The microstructure of bonding line and heat affected zone of the base material are similar to the underwater blind-hole FTPW joint.Most of tensile specimens of underwater through-hole FTPW joint are broken at the bonding line.The tensile strength is 344.3~448.2MPa with the welded joint coefficient 53.8~70.0%,and the elongation is only 3.18~5.56%.The others fracture in the base material,indicating their tensile performance could reach the same level to the base material.The tensile strength is 576.9~588.3MPa with the welded joint coefficient 90.1~91.9%,and the elongation is 18.28~19.94%.The impact specimens with notch at the bonding line are broken along the bonding line with a maximum impact absorbed energy of 28 J,which is much lower than that of the base material(267J).