CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Nickel Tube Longitudinal Seam Plasma Plus TIG Welded Joint
Literature Overview
Published in 2016 by Wang Xijing, Wei Wankui, Yuan Liangwen, Chai Tingxi from the State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals at Lanzhou University of Technology, and Wang Jiang and Zhang Dong from Jinchuan Group Co., Ltd., this study investigates the microstructure and mechanical properties of nickel tube longitudinal seam welded joints produced using a combined plasma arc and TIG welding process. The research was supported by the Gansu Provincial Science and Technology Major Special Project (145RTSA004). This work addresses a critical manufacturing challenge in the production of high-purity nickel tubes used in chemical processing, hydrogen energy systems, and nuclear applications.
Core Technical Content
High-purity nickel tubes (typically 99.95% Ni or higher) are essential components in environments requiring exceptional corrosion resistance, particularly in concentrated sulfuric acid, hydrochloric acid, and high-temperature hydrogen systems. The longitudinal seam welding of nickel tubes is a critical manufacturing step that determines the structural integrity and corrosion performance of the finished product. The combined plasma arc plus TIG welding process leverages the deep, narrow penetration of plasma arc welding with the wider weld pool and better surface quality of TIG welding, creating a synergistic approach to producing high-quality welds in nickel.
Process Configuration and Parameters
The plasma arc component provides the primary heat input for achieving full penetration through the tube wall, while the TIG arc serves as a secondary heat source that widens the weld pool and improves surface wetting. This dual-arc configuration allows for precise control of the weld geometry and reduces the risk of burn-through in thin-walled tubes while maintaining adequate penetration.
| Parameter | Plasma Arc | TIG Arc | Combined Effect |
|---|---|---|---|
| Current | 30-80 A | 50-100 A | Balanced penetration and pool width |
| Gas flow (plasma) | 5-15 L/min | N/A | Arc stability and penetration |
| Shielding gas (TIG) | N/A | 10-20 L/min | Oxidation prevention |
| Travel speed | 100-300 mm/min | 100-300 mm/min | Heat input control |
| Nozzle standoff | 2-5 mm | 3-8 mm | Arc focus and coverage |
| Wire diameter | 1.0-1.6 mm | 1.0-1.6 mm | Deposition control |
Microstructural Analysis
The microstructure of the nickel weld zone exhibits several characteristic features:
- Weld metal: Fine, equiaxed grain structure with low sulfur and oxygen content, indicating good deoxidation and low impurity levels.
- Heat-affected zone: Recrystallized grain structure with controlled grain size, showing minimal grain growth due to the relatively low heat input.
- Fusion boundary: Clean interface with minimal segregation and no intermetallic phase formation.
The combined plasma-TIG process produces a weld metal with excellent purity characteristics, as the plasma arc's high energy density promotes effective deoxidation and the TIG arc's wider pool allows for better gas protection. This is critical for nickel applications where even trace amounts of sulfur, oxygen, or carbon can severely degrade corrosion resistance.
Mechanical Properties
The mechanical properties of the welded joint are evaluated through tensile testing, hardness profiling, and impact testing. The weld metal typically exhibits tensile strength within 95% of the base metal strength, with hardness values closely matching the parent nickel. The heat-affected zone shows minimal softening, indicating that the heat input is well-controlled and does not cause excessive grain growth or phase transformation.
Engineering Practice Integration
High-purity nickel tubes are used in critical applications including:
- Chemical processing equipment handling concentrated acids
- Hydrogen production and storage systems
- Nuclear reactor components
- Semiconductor manufacturing equipment
- Food processing equipment requiring high purity
The quality of the longitudinal seam weld is paramount, as it represents the weakest link in the tube's structural integrity and corrosion resistance. Any defect in the weld can lead to premature failure under cyclic loading or in corrosive environments. The combined plasma-TIG process offers several advantages for nickel tube welding:
- Reduced welding time compared to conventional TIG, improving production efficiency
- Lower total heat input, minimizing distortion and residual stress
- Better weld geometry with reduced undercut and improved fusion
- Enhanced surface quality suitable for critical applications
For qualification purposes, the weld procedure must meet the requirements of applicable standards such as ASME Section IX, ISO 15620, or relevant industry specifications. Non-destructive testing including radiographic testing, ultrasonic testing, and dye penetrant testing is mandatory to ensure weld integrity.
Key Questions and Reflections
The study raises important questions about the consistency and repeatability of the combined plasma-TIG process in production environments. While laboratory studies demonstrate excellent results, maintaining consistent weld quality over extended production runs requires careful process monitoring and control. Variations in tube wall thickness, material chemistry, and environmental conditions can all affect weld quality, necessitating adaptive process control strategies.
Another critical consideration is the long-term corrosion performance of the weld in aggressive environments. While the weld metal may exhibit excellent purity characteristics immediately after welding, exposure to high-temperature corrosive media can lead to grain boundary attack, intergranular stress corrosion cracking, or selective corrosion of the weld zone. Long-term corrosion testing under simulated service conditions is essential for validating the weld's durability.
Study Insights and Implications
This research provides valuable insights into the metallurgical behavior of high-purity nickel during combined plasma-TIG welding. The microstructural analysis demonstrates that the dual-arc approach can produce welds with excellent purity and mechanical properties, comparable to the base metal. For engineers involved in nickel tube manufacturing, the key takeaway is that the combined plasma-TIG process offers a viable path to producing high-quality welds with improved efficiency and controlled metallurgy.
The study also highlights the importance of process optimization in achieving consistent weld quality. The interaction between the plasma and TIG arcs, the relative positioning of the two heat sources, and the timing of wire feeding all influence the final weld geometry and properties. Future research should focus on developing real-time monitoring systems that can detect and correct process variations during welding, ensuring consistent quality in production environments. The findings contribute to the growing body of knowledge on advanced welding processes for high-purity metals and should inform future procedure qualification and standard development efforts.