Plasma TIG Hybrid Welding of Pure Nickel Pipe Joint Microstructure and Properties A Study Note

Literature Overview

This paper, published in Hot Working Technology (2016, Vol. 45, No. 7, pp. 254–257) by Wang Xijing, Wei Wankui, Yuan Liangwen, Chai Tingxi, Wang Jiang, and Zhang Dong from Lanzhou University of Technology and Jinchuan Group, investigates the microstructure and mechanical properties of pure nickel (N6) pipe longitudinal weld joints produced using a plasma+TIG double torch welding method. Funded by the Gansu Provincial Science and Technology Major Project (145RTSA004), this research addresses the practical challenges of welding pure nickel components used in hydrogen production, ammonia synthesis, and other high-temperature chemical processing applications.

Core Technical Content

Pure nickel welding is notoriously difficult due to several metallurgical challenges: high thermal conductivity leading to wide, shallow welds; susceptibility to hot cracking from sulfur and phosphorus impurities; tendency to form porosity from hydrogen absorption; and difficulty achieving complete fusion. The plasma+TIG hybrid approach combines the deep penetration of plasma arc with the wide wetting and shielding of TIG arc to overcome these challenges.

The experimental configuration used 5 mm thick N6 pure nickel pipe with longitudinal butt joints on flat plate simulation specimens. Orthogonal experimental design was employed to optimize welding parameters, and the optimal parameters produced weld joints with tensile strength of 328 MPa, reaching 90% of the base metal strength.

Microstructural Characterization

The microstructural analysis reveals distinct zones within the weld joint, each with characteristic features:

The plasma welding zone, occupying the lower and middle portions of the weld, exhibits cellular dendritic austenite with coarse grain structure. This morphology results from the high energy density and rapid solidification rates characteristic of plasma arc welding. The TIG welding zone in the upper portion shows columnar crystal growth, reflecting the thermal gradient direction during solidification and the influence of the secondary heat input from the TIG arc.

The dual heat source creates a complex thermal cycle in the weld metal. The TIG arc provides a secondary heating effect on the plasma-deposited layers, resulting in two thermal cycles for the upper weld regions. This re-heating promotes grain coarsening and can affect mechanical properties and corrosion resistance.

Mechanical Properties and Fracture Analysis

The fracture analysis indicates a mixed fracture mode dominated by ductile fracture characteristics. This is a positive indicator of weld quality, as it suggests adequate toughness and resistance to crack propagation. The 90% tensile strength ratio to base metal meets typical qualification requirements for pressure vessel and piping applications per ASME B31.3 and related codes.

The achievement of 90% base metal tensile strength is particularly significant for pure nickel welding, where many conventional welding processes produce joints with significantly lower strength ratios due to incomplete fusion, porosity, or cracking. The plasma+TIG hybrid approach demonstrates that near-base-metal strength can be achieved with proper parameter optimization.

Process Optimization and Engineering Considerations

The orthogonal experimental design methodology used in this study provides a systematic approach to parameter optimization. Key parameters likely investigated include:

1. Plasma arc current and transfer mode (DCSP, DCRP, or pulsed).
2. TIG arc current and polarity.
3. Travel speed and its relationship to both arc currents.
4. Shielding gas flow rates for both torches.
5. Torch configuration (leading/trailing arrangement of plasma and TIG).
6. Nozzle diameters and stand-off distances.

The plasma+TIG configuration offers several practical advantages for nickel pipe welding:

• Deep penetration: The plasma arc provides concentrated energy for deep penetration in thicker pipe walls.
• Wide surface wetting: The TIG arc ensures good surface fusion and bead appearance.
• Reduced porosity: The combined shielding from both arcs provides superior protection against atmospheric contamination.
• Improved root fusion: The deep plasma penetration ensures complete root fusion, critical for leak-tight joints.

Engineering Practice Implications

Pure nickel components are widely used in hydrogen production facilities, ammonia plants, and other chemical processing applications where high-temperature corrosion resistance and strength are required. The successful welding of N6 nickel pipe joints using the plasma+TIG method has direct implications for:

1. Pipeline integrity: Longitudinal welds in nickel-lined or nickel-clad pipes require reliable joining to maintain corrosion barrier integrity.
2. Pressure vessel fabrication: Nickel alloy pressure vessels and heat exchangers require high-quality welds meeting code requirements.
3. Repair operations: Field repair of nickel components in chemical processing plants benefits from proven welding procedures.

However, several practical challenges remain for production implementation:

• Equipment complexity: Dual torch systems require synchronized control and increased capital investment.
• Procedure qualification: Each pipe diameter, thickness, and joint configuration requires individual WPS qualification.
• Post-weld requirements: Stress relief heat treatment may be necessary to reduce residual stresses, but must be carefully controlled to avoid sensitization or grain growth.
• Non-destructive testing: Special NDT techniques may be required for nickel welds due to their low radiographic contrast and acoustic impedance characteristics.

Key Questions and Reflections

The study does not address the corrosion performance of the weld joint in actual service environments. Pure nickel welds may exhibit different corrosion behavior from the base metal due to microstructural differences, impurity segregation, and residual stress effects. Long-term corrosion testing in representative service environments would be essential for complete qualification.

Additionally, the study uses flat plate specimens, which may not fully represent the geometric constraints of actual pipe welding, including circumferential weld geometry, pipe diameter effects on heat dissipation, and positional welding challenges. The transition from flat plate qualification to actual pipe welding requires additional testing and possibly parameter adjustment.

The coarse grain structure observed in both the plasma and TIG weld zones raises concerns about long-term mechanical performance, particularly under creep or fatigue conditions. Grain refinement through post-weld treatment or modified welding parameters could improve the overall joint quality.

Study Insights and Implications

This research demonstrates that the plasma+TIG hybrid welding approach is a viable and effective method for joining pure nickel pipe components with near-base-metal mechanical properties. The achievement of 90% tensile strength ratio and predominantly ductile fracture behavior validates the process for engineering applications. For engineers involved in nickel alloy welding for chemical processing, hydrogen production, and other demanding applications, this study provides valuable process data and metallurgical insights. The key takeaway is that hybrid welding processes can overcome the inherent challenges of pure nickel welding, but require systematic parameter optimization, comprehensive qualification testing, and careful attention to post-weld treatment to ensure long-term joint reliability in critical service environments.