Nuclear Power Large-Diameter Thick-Wall Long Pipe Fitting Vertical Extrusion Process and Quality Control Technology

Project Overview and Award Recognition

This news item, published in Forging & Stamping Technology in 2014 (Vol. 39, No. 5, p. 50), reports that the project titled "Nuclear Power Large-Diameter Thick-Wall Long Pipe Fitting Vertical Extrusion Process and Quality Control Technology" received the Third Prize of the 2013 Beijing Municipal Science and Technology Award. The project was led by the Advanced Manufacturing Technology and Equipment Research Center of the Beijing Institute of Mechanical Engineering, with participation from Inner Mongolia Northern Heavy Industry Group Co., Ltd., Tianjin University of Technology, and Inner Mongolia University of Technology. The project was funded as part of the National Science and Technology Major Project and was successfully accepted in Beijing in May 2011.

Technical Significance of Nuclear Power Pipe Fittings

Nuclear power plant pipe fittings represent some of the most demanding engineering components in terms of material requirements, manufacturing precision, and quality assurance. The combination of large diameter, thick wall, and long length presents unique manufacturing challenges that require specialized processes and rigorous quality control. Key technical requirements include:

The vertical extrusion process is particularly suited for producing large-diameter thick-wall pipe fittings because it provides superior control over the deformation pattern, reduces the risk of surface defects, and allows for more uniform material flow compared to horizontal extrusion.

Vertical Extrusion Process Analysis

The vertical extrusion process involves placing the heated billet vertically in a container and applying downward force through a punch to force the material through a die, producing a hollow section. For nuclear power pipe fittings, the process parameters are carefully optimized to achieve the required dimensional accuracy and material quality:

• Billet preparation: The starting billet must be free of surface and internal defects, with controlled chemistry and microstructure. For alloy steels such as P91 or P92, the billet is typically produced through vacuum induction melting or electroslag remelting followed by controlled reheating.
• Heating: The billet is heated to the appropriate forging temperature (typically 1150-1200°C for P91) using induction heating or gas heating, with careful control to avoid excessive grain growth or decarburization.
• Extrusion parameters: The extrusion temperature, speed, and reduction ratio are optimized to achieve uniform deformation without excessive work hardening or material flow instability.
• Homogenization: After extrusion, the pipe fitting is subjected to homogenization heat treatment to relieve residual stresses and achieve a uniform microstructure.
• Normalizing and tempering: The final heat treatment cycle establishes the required mechanical properties, including tensile strength, yield strength, impact toughness, and creep resistance.

Quality Control Technology

The quality control system for nuclear power pipe fittings must be comprehensive and traceable, covering every stage of the manufacturing process:

1. Incoming material inspection: Chemical analysis, mechanical property verification, and non-destructive examination of the starting material.
2. Process monitoring: Continuous monitoring of heating temperatures, extrusion forces, and cooling rates during the forming process.
3. In-process inspection: Dimensional checks at critical process stages, metallographic examination of microstructure, and hardness testing.
4. Final non-destructive testing: 100% ultrasonic testing for volumetric defects, magnetic particle testing or penetrant testing for surface defects, and radiographic testing where applicable.
5. Final mechanical testing: Tensile, impact, and hardness testing on test coupons processed alongside the production fittings.
6. Hydrostatic testing: Pressure testing to verify the integrity of the fitting and detect any leaks or weak areas.

The PDCA (Plan-Do-Check-Act) cycle is inherently embedded in this quality control system, with continuous monitoring and feedback loops ensuring that process parameters remain within specified limits and that any deviations are promptly corrected.

Engineering Practice and Industry Impact

The successful development of this vertical extrusion process and quality control technology represents a significant advancement in the domestic manufacturing capability for nuclear power pipe fittings. Prior to this project, China relied heavily on imports for large-diameter thick-wall nuclear pipe fittings, which posed supply chain risks and quality control challenges. The project's success has enabled domestic manufacturers to produce these critical components to the required quality standards, reducing dependence on foreign suppliers and supporting the growth of the domestic nuclear power industry.

The collaboration between research institutions, manufacturing companies, and universities exemplifies the effective integration of academic research and industrial application. The Beijing Institute of Mechanical Engineering provided the process development and quality control expertise, while Inner Mongolia Northern Heavy Industry Group contributed manufacturing capabilities and production experience.

Study Insights and Reflections

This project achievement is a testament to the importance of sustained investment in advanced manufacturing technologies for critical infrastructure components. The development of vertical extrusion technology for nuclear power pipe fittings required not only technical innovation but also rigorous quality management, extensive process validation, and regulatory compliance with nuclear safety standards.

For engineers involved in nuclear component manufacturing, the key lessons from this project include the importance of process control in achieving consistent material quality, the value of comprehensive non-destructive testing in detecting potential defects, and the necessity of maintaining full traceability throughout the manufacturing chain. The vertical extrusion process offers particular advantages for large-diameter thick-wall sections, providing superior control over material flow and dimensional accuracy compared to alternative forming methods.

In conclusion, the recognition of this project with the Beijing Municipal Science and Technology Award reflects the significant technical achievement in developing domestic capabilities for manufacturing nuclear power large-diameter thick-wall long pipe fittings through vertical extrusion, contributing to both national energy security and the advancement of advanced manufacturing technology in China.