核电主管道非对称双管嘴同时挤压成形工艺

Simultaneous extrusion process of primary pipe with two asymmetrical branches

针对AP1000核电主管道侧向双管嘴非对称分布的特点,本文在单轴单向压力机平台上增加提升油缸的运动作用,提出双管嘴同时挤压成形的新工艺.首先,分析了双管嘴同时挤压成形的工艺原理并建立了可实现同时成形的上顶杆及提升油缸的速度与管嘴尺寸之间的解析关系.其次,建立双管嘴同时挤压成形的有限元模型,分析了同时挤压成形方案的可行性及在避免管嘴处材料撕裂缺陷方面的优势.最后,从降低成形载荷和关键部位晶粒尺寸以及提高组织均匀性的角度,分析了坯料温度、挤压速度和摩擦条件三个重要因素的影响规律,为实施主管道挤压成形提供工艺参考.

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,generationⅢAP1000 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 ofprimary pipes.