Simultaneous extrusion process of primary pipe with two asymmetrical branches

The primary pipe is a critical equipment that ensures the safe operation in a nuclear island, therefore; the primary pipe must have extremely high service performance in complex environments characterized by high pressure, temperature, and/or radiation. In addition, generation Ⅲ AP1000 nuclear power plants require a service life of 60 years, which pose great challenges to traditional manufacturing processes, such as casting and section-forging methods with partial welding. The currently popular free-forging method can enhance the resulting properties, but the repeated heating during multiple passes induce coarse grains, and these coarse grains are difficult to refine at key positions. With the rapid development of extrusion devices and optimized extrusion processes, the hot extrusion approach promises to produce primary pipes using a near-net shaping method. However, the huge size and complex shape of the two asymmetrical branches of the primary pipe brings enormous difficulties to the ordinary extrusion process. In this study, a novel simultaneous extrusion process was proposed, wherein a primary pipe with two asymmetrical branches is produced on a uniaxial extrusion press platform with the additional effect of a moving elevating ram. In this study, the principle underlying the simultaneous formation process was first analyzed with respect to the material flow during the extrusion process. The relations between the top-mandrel speed, lift cylinder speed, and branch size were derived to ensure the conditions necessary for the simultaneous formation of the two branches. Next, a finite element model of the proposed primary pipe extrusion process was constructed and the results verified its feasibility. The superiority of this process in preventing shear fracture at the branch root was evaluated by comparing its formation quality with that of traditional unidirectional extrusion. Finally, the influences of billet temperature, extrusion speed, and friction condition on the formation quality were studied to minimize the deformation load, refine the grain, and improve the homogeneity of the microstructure. The results of this research provide a method for reference and an analytical foundation for further development of practical approaches to the formation of primary pipes.