Numerical Simulation And Experimental Investigation On The Hot Pushed Bending Of Thick-walled Tubes For Nuclear Industry

Considering the wide applications of thick-walled pipes in nuclear power industry, we used P11 alloy steel and 316 L stainless steel thick-walled tubes as the substrate materials to explore the manufacturing method and forming law of the thick-walled elbow and thick-walled tubes with straight sections in this work. The constitutive relation of metal tubes at high temperatures was obtained, and the local induction-heating finite element model was also established by means of the uniaxial hot compression test, DDFORM-3D simulation software, and hot push-bending device. Furthermore, the effects of geometry and process parameters on the forming quality in hot push-bending process were investigated, and the Φ114 mm × 6 mm elbow and Φ323.9 mm × 35 mm × 5D bending tubes with straight sections meeting the requirements in the nuclear power industry were successfully fabricated.Firstly, true stress- strain curves at hot deformation of P11 alloy steel and 316 L stainless steel tubes were obtained by uniaxial hot compression test. The flow stress behaviors of the materials at high temperature were investigated to obtain the optimum formation parameters and establish the practical constitutive model of finite element simulation.Secondly, the hot push-bending process of thick-walled tubes and the effects of some important parameters on forming quality were investigated by the constitutive relation and numerical simulation software. And the established varied-temperature differential model of hot push-bending simulation possessed a higher precision with the actual temperature field. The results showed that the thick-walled pipes could be formed via the combination of push-bending process and local induction heating. A good quality thick-walled elbow of P11 alloy steel with a double curvature axis horn mandrel was successfully built with pushing speed of 5 mm/s, heating frequency of 1000 HZ, and current density of 10 A/mm2. A high quality thick-walled bending tube of 316 L stainless steel with straight sections was formed with the pushing speed of about 30 mm/s, the heating frequency of 1500 HZ, and the current density of 300 A/mm2 via a swing arm system, which had a elbow making the rotation center deviated from the fixture and aligned with the coil.Finally, the actual thick-walled pipe fittings were manufactured by means of hot push-bending machine. The results indicated that the forming thick-walled pipe fittings could meet the required specifications under the guidance of simulation optimization parameters. There was an almost same variation trend between the simulation results and the experimental results of wall thickness and cross-section distortion, verifying the influence laws of geometry and process parameters on forming quality. It was also proved that the constitutive relation at high temperature was accurate and practical, and the finite element model was reliable and close to the actuality.