Numerical Simulation Of Expansion Drawing For Large Diameter BFe10-1-1 Cupronickel Pipe And Study On Its Annealed Microstructure And Properties

BFe10-1-1 cupronickel pipe is a key material in the fields of marine engineering,ships,thermal power and nuclear power.At present,domestic large diameter cupronickel pipe has problems such as small diameter,low yield and poor corrosion resistance,which cannot meet the needs of high-end pipes.In this paper,combined with the Jiangsu Science&Technology Program（SBE2017030563）,the numerical simulation of expansion drawing and annealing heat treatment process for the engineering trial production of large diameter BFe10-1-1 cupronickel pipe are carried out.The conclusions of the research are as follows:The relationship between the wall-reduction amount and the diameter-expansion amount of the cupronickel pipe in the expansion drawing without external die is established.The simulation analysis shows that by expanding the diameter of the mandrel and reducing the wall of the external die at the same time,a large diameter cupronickel pipe with better shape and size can be obtained.After the eccentric pipe blank is expansion drawing with external die,the eccentricity of pipe is reduced.When the mandrel is deflected,the eccentricity of the pipe increases after pipe blank is expansion drawing with external die.Based on the optimization principles of minimum expansion force and better pipe size,the optimized die parameters and expanding process are obtained as follows:mandrel die angleα=7°～11°,length of mandrel sizing belt L_M=20～25 mm,external die angleβ=13°～17°,length of external die sizing belt L_ED=27.5～32.5 mm,expansion coefficient per pass r/r₀≤1.05,wall reduction per pass t/t₀=0.83～1.0.The best semi-hard finished annealing process for the cold drawn super large diameter thin walled BFe10-1-1 cupronickel pipe with a size ofΦ457×5.0 mm is 640℃*10 min.The tensile strength and elongation after fracture of the pipe in this state are 352 MPa and 35.7%,respectively.The immersion and erosion corrosion rate are 0.2935μm/a and 0.7338μm/a,respectively.The best process for complete softening annealing is720℃*10 min.The tensile strength and elongation after fracture of the tube are 317 MPa and 52.5%,respectively,and the immersion and erosion corrosion rates are 0.2473μm/a and 0.5965μm/a,respectively.After the BFe10-1-1 cupronickel pipe is corroded by seawater,a protective Cu₂O product film is mainly formed on the surface.The samples annealed at 720℃*10 min has the highest proportion of lowΣCSL grain boundaries,the largest average size of grain clusters,and fewer grains close to the<111>orientation.Meanwhile,the corroded surface is prone to form a thicker and denser oxide film,thus showing more excellent corrosion resistance.The immersion corrosion experiment shows that the initial oxide film could accelerate the corrosion of large diameter BFe10-1-1 cupronickel pipe.The twin grain boundary（that isΣ3 CSL grain boundary）in the pipe has lower corrosion sensitivity than random grain boundary.The grains close to the<101>orientation in the pipe have stronger corrosion resistance.64 figures,18 tables and 93 references are contained.