Study On Longitudinal Seam Of Nickel Pipe In Double Guns Of Plasma & TIG Welding

Nickel and nickel alloys have a lot of advantages, such as high melting point, good mechanical properties and corrosion resistance, which are widely used in petrochemical,marine, medical and other fields. Compared to nickel seamless pipes, the manufacturing process of nickel and nickel alloys’ welding pipes is simple, with high pass rate, saving material and low cost. Nickel and nickel alloys’ welding pipes increase the diameter size of nickel seamless pipes and expand domestic and foreign markets of nickel pipes.There are often defects like hot cracking, porosity and biting edge during pure nickel welding, in addition, there are high requirements in the concentricity of pipe interface and assembly gap of pipes when longitudinal seams of nickel pipes are welded by plasma welding. In this paper,the double guns of plasma & TIG welding are tested in welding the longitudinal seams of 3 to5 mm thickness nickel pipes without filler wire, which can make full use of the advantages of both welding methods and improve production efficiency and finished product rate of nickel welding pipes.In the pre-experiment, the double guns of plasma & TIG welding are imitately tested in the flatbed of 5mm thickness pure nickel N6 without filler wire for welding longitudinal seams of nickel pipes. The welding parameters, used the orthogonal experimental design to optimize, of nickel pipe’s longitudinal seams are determined: welding speed is 180mm/min,plasma welding current is 180 A, the current of TIG is 240 A, the distance between plasma welding gun and TIG welding gun is 120 mm. the welds on the front and back are good and the tensile strength of welded joints is 90% of base material, whose fracture is mixed fracture based on ductile fracture. Microstructure analysis indicates that base material N6 is a single phase of austenite, the TIG zone, middle and upper welds, is mainly composed of coarse columnar crystals of austenite because of the twice heat. the plasma welding zone of welds is composed of coarse cellular crystal of austenite. The reasonable choice of back protectors and air flow is crucial when the double guns of plasma & TIG welding weld in the flatbed of pure nickel. When the back shielding gas of the argon is nothing or low flow,oxygen and water in the air go into welds and form pores,which will reduce joints strength. the protection effect,with the back shielding gas flow is 10 L / min,is best.To explore the corrosion resistance of plasma & TIG welds, the experient measures and compares the corrosion resistance of base material N6, plasma & TIG welds and plasma welds, which are tested in seven different concentrations of acid and salt corrosion solution by immersion corrosion test and electrochemical test. The results are consistent with microstructure analysis. The corrosion resistance of plasma & TIG welds is better than plasma welds, but less than base material of pure nickel.According to the welding process of double guns of plasma & TIG welding in the purenickel plates, the longitudinal seams welding process parameters of 3.5mm thickness nickel pipes without filler wire are determined. And further experiments detect and analyze the longitudinal seams properties of plasma & TIG welded joints and plasma welded joints without TIG. The tensile strength of plasma welded joints and plasma & TIG welded joints both have reached more than 80% of the base material, but the surface of plasma welds is recessed, the stability of continuous welding is poor, the defects of biting edge and porosity are prone. Nickel pipes, welded by plasma & TIG welding, are significantly eliminated surface defects of welds and the finished product rate is improved. The X-ray flaw detection results of welding pipes showed that plasma welding pipes are prone to welding defects and plasma & TIG welding pipes have a high finished product rate. The Hydrostatic test results show that the plasma and plasma & TIG welds are no leakage and stable operation at the operating pressure of the medium. To sum up, the longitudinal seams of nickel pipes in double guns of plasma & TIG welding without filler wire is an ideal method that has a high finished product rate and reduces production costs. Welded joints’ properties can meet the production requirements and whose corrosion resistance is better.