Zhuojin Pipe Fitting Co., LtdPlasma + TIG Welding Joint Microstructure and Properties of Nickel Pipe
Literature Overview
This paper by Wang Xijing, Wei Wankui, Yuan Liangwen, Chai Tingxi, Wang Jiang, and Zhang Dong, published in Hot Working Technology (2016, Vol. 45, Issue 7, pp. 254-257), investigates the plasma + TIG dual-gun welding of 5 mm thick N6 pure nickel pipe. The work was supported by the Gansu Provincial Science and Technology Major Special Plan Project (145RTSA004). The authors used orthogonal experimental design to optimize welding parameters and analyzed the microstructure and mechanical properties of the welded joint.
Core Technical Findings
Pure nickel welding presents unique challenges due to:
- High ductility but susceptibility to cracking
- Susceptibility to porosity from hydrogen
- Poor wetting and fusion characteristics
- Thermal stress sensitivity
The plasma + TIG dual-gun approach combines:
- Plasma arc for deep, narrow penetration
- TIG arc for wider fusion and improved wetting
- Sequential or simultaneous heating for controlled thermal cycles
Key findings from the optimized process:
| Parameter | Value |
|---|---|
| Tensile strength | 328 MPa (90% of base metal) |
| Fracture mode | Mixed fracture dominated by ductile fracture |
| Base metal microstructure | Single-phase austenite |
| TIG weld zone microstructure | Coarse austenite columnar grains (double heat treatment) |
| Plasma weld zone microstructure | Coarse austenite cellular grains |
Process Optimization and Microstructure Analysis
The orthogonal experimental design approach systematically evaluated multiple welding parameters to identify the optimal combination. The dual-gun configuration allows independent control of penetration depth (plasma) and weld width (TIG), providing greater flexibility than single-process welding.
Microstructural evolution in the welded joint:
- Base metal: Fine, uniform austenite grains
- Plasma weld zone (lower weld region): Coarse cellular austenite grains due to high heat input and rapid solidification
- TIG weld zone (upper weld region): Coarse columnar austenite grains due to secondary heating during TIG pass
- Fusion line: Grain coarsening and potential microstructural discontinuity
The mixed fracture mode indicates good toughness, with ductile fracture dominating over brittle fracture. This is favorable for service in applications requiring good crack resistance.
Quality Control and Standards Compliance
For nickel pipe welding, relevant standards include:
- ASTM B160 for nickel pipe
- ASME B31.3 for process piping
- AWS D10.1 for welding procedures
- NACE MR0175 for sour service (if applicable)
Quality control considerations:
- Hydrogen control to prevent porosity (dew point control, flux selection)
- Heat input management to prevent grain coarsening
- Post-weld heat treatment for stress relief and grain refinement
- Non-destructive testing (RT, UT) for defect detection
- Mechanical testing for qualification (tensile, hardness, impact)
Engineering Practice Implications
Pure nickel and nickel alloys are used in chemical processing, power generation, and nuclear applications where corrosion resistance and high-temperature performance are required. The plasma + TIG welding approach offers advantages for:
- Thick-section welding with good penetration
- Controlled dilution in dissimilar welds
- Improved productivity compared to single-process welding
- Better weld geometry control
However, the process requires specialized equipment and operator training. The dual-gun setup increases equipment complexity and cost, which must be justified by the improved weld quality and productivity gains.
Study Insights and Reflections
This research demonstrates the effectiveness of hybrid welding approaches for challenging materials like pure nickel. The plasma + TIG combination provides complementary heat input characteristics that result in superior weld quality compared to either process alone.
The orthogonal experimental design methodology is appropriate for multi-parameter optimization and provides a systematic approach to process development. However, the study focuses on 5 mm thickness; extrapolation to thicker sections would require re-optimization of parameters.
The microstructural analysis reveals the complex solidification patterns resulting from the dual heat source. The coarse grain structure in the weld zones raises concerns about long-term mechanical properties and creep resistance at elevated temperatures. Post-weld heat treatment may be necessary to refine grain structure and relieve residual stresses.
The research provides valuable process development data for nickel pipe welding applications, offering a practical solution for industrial fabrication while highlighting areas for further optimization and quality improvement.