Research On Resistance Welding For The End Of Bimetal Mechanical Composite Pipe

Mechanical composite pipes are increasingly used in oil field today,but welding cracks easily occur in sealing zone for those excessing?300mm.Due to cracks in sealing and higher costs in end resurfacing welding,the research,different from traditional process,firstly makes metallurgical composite zone at pipe's end by resistance welding.Then butt welding is conducted.Since?355×?10+2?mm of L415/316L bimetal mechanical composite pipe is researched,three big problems are obvious in resistance welding:too thick specimens,too large thickness ratio,dissimilar steel welding,which make resistance seam welding difficult to implement.The research mainly focus on resistance spot welding for the pipe's end,following work in three aspects:pressure calculation and simulation,analysis for microstructure and performance of welding spots,analysis for microstructure and performance of butt welding after spot welding.By means of the calculation on residue contact pressure of bimetal mechanical composite pipe and the model analysis on welding heat loading with cracks,it is revealed that residue contact pressure is changing in production and use process,due to the differences in thermal conductivity and coefficient of thermal expansion between inner and outer pipe.When the diameter of pipe is larger and hits reverse yielding temperature,the binding force is more likely reducing,which causes the moving between inner and outer layers and the tend to crack.Exert Y axis stress?radial force?on zone 30mm from pipe's end,which means constrain force of welding spot;comparing with condition in no radial force,the limit equivalent residual pressure decreases about 20MPa.Hence,resistance welding on the end of pipe partly reduces residual stress in welding joint and decline the cracking possibility in sealing zone.After resistance welding for the end of pipe,the microstructure in welding joint maintains as-cast structure in nugget,which means less equiaxed cells in the center with more columnar cells around.Austenite and ferrite are main tissues.The performance is possibly affected because of the differences in strength,toughness and corrosion resistance between base metal and more directional columnar cells.From welding joints shear test and observation of fracture morphology,it is obvious that the inner and outer layer make higher bonding;there are many dimples in fractures,illustrating the fracture properties of ductile fracture.Some impurities exist at the dimple aggregation segregation,which might lead to cracking.Alloying element content including Cr,Ni sharply reduces in dissimilar steel mixing zone combining composition analysis and microhardness,resulting dramastical changing in hardness,strength and corrosion resistance.Electrochemical experiments show that corrosion potential of the spot on the 316L stainless steel surface is 0.15⁰.25V lower than that of base metal,while the corrosion current density is higher about 0.05⁰.4mA·cm^-2,which probably decreases the corrosion resistance to some extent.The butt welding joints'microstructure is austenite and ferrite distributed among austenite cells.Fusion zone base metal side is basically organized with uniform size pearlite and ferrite.On the whole,the strength and hardness of joints are higher than that of base metal and the broken zone is located on base metal,which means welding point plays a fixing role and stress concentration is away from the weld.Bending tests show that the spot welding does not reduce the overall-joint toughness.The intergranular corrosion tests show that the spot welding has little influence on the intergranular corrosion tendency of the whole joint.Therefore,the process is practical to some extent for its ensuring both in mechanical properties and corrosion resistance,apart from the further studies in stress concentration resulting from spot welding...