Zhuojin Pipe Fitting Co., LtdPAW Plus TIG Welding Technology for Non-Alloy Zirconium R60702
Literature Overview
This technical paper, published in China Chemical Equipment in 2019 by Kong Maichuan et al. from Lanzhou Lanshi Heavy Equipment Co., Ltd., reports on the welding of non-alloy zirconium alloy R60702 using a combined Plasma Arc Welding (PAW) plus Tungsten Inert Gas (TIG) welding approach. The study addresses a practical fabrication challenge in the chemical processing and nuclear industries, where zirconium equipment must withstand aggressive chemical environments and radiation. The work was conducted at the Gansu Provincial Key Laboratory for Special Material Welding in Pressure Vessels, reflecting the specialized nature of zirconium welding expertise.
Material Characteristics and Weldability Analysis
R60702 is a commercial non-alloy zirconium alloy characterized by high purity and excellent corrosion resistance in various chemical environments. Its key material properties relevant to welding include:
| Property | Characteristic | Welding Implication |
|---|---|---|
| High oxygen affinity | Rapid oxidation above 400°C | Requires excellent inert gas shielding |
| Low thermal conductivity | Heat concentrates at weld zone | Higher risk of overheating and grain coarsening |
| Elastic modulus | Lower than steel | Greater susceptibility to distortion |
| Hydrogen embrittlement | Absorbs hydrogen from moisture | Post-weld hydrogen control critical |
| Contamination sensitivity | Reacts with many materials | Cleanliness of equipment and environment essential |
The fundamental weldability challenge for zirconium is its extremely high reactivity with atmospheric gases. Above approximately 400°C, zirconium rapidly absorbs oxygen, nitrogen, and hydrogen from the atmosphere, forming brittle intermetallic compounds that severely degrade mechanical properties. Even trace contamination can render the weld unacceptable. This necessitates rigorous gas shielding protocols that go far beyond conventional welding practice.
PAW+TIG Combined Welding Process
The PAW+TIG approach combines the deep, narrow penetration of plasma arc welding with the wider, shallower bead of conventional TIG welding. This combination is particularly advantageous for zirconium welding of plates thicker than 8 mm, as demonstrated in this study.
Process Configuration and Parameters
The combined process operates as follows:
- PAW pass: The plasma arc provides deep, narrow penetration for the root and hot passes, minimizing the total heat input and reducing the volume of material exposed to high temperatures.
- TIG pass: The conventional TIG arc is used for cap passes and final surface finishing, producing a smooth, well-formed surface profile.
- Gas shielding: Multi-stage shielding is employed, including primary arc shielding, secondary trailing shield, and back-side shielding to protect the hot root surface.
The critical process parameters identified in the study include:
| Parameter | PAW Setting | TIG Setting |
|---|---|---|
| Welding current | Higher (plasma arc) | Lower (conventional TIG) |
| Travel speed | Controlled for penetration | Adjusted for bead width |
| Shielding gas flow | Multi-stage, high flow rate | Primary and trailing shield |
| Back-side gas | Essential for root protection | Continuous flow |
Weld Quality Assessment
The authors performed comprehensive weld quality evaluation including:
- Mechanical properties testing: Tensile tests confirmed that weld joint strength meets relevant standard requirements. The weld metal strength is comparable to or slightly lower than the base metal, which is typical and acceptable for zirconium welds.
- Metallographic examination: Microstructural analysis revealed uniform weld metal microstructure without excessive grain coarsening or contamination-induced phases. The absence of oxide inclusions or nitrogen-bearing phases confirms effective gas shielding.
- Visual inspection: Weld bead profiles showed consistent geometry with proper fusion and no visible defects.
The key finding is that with appropriate process parameters and protection measures, high-quality zirconium welds can be achieved using the PAW+TIG combination. The method is specifically recommended for plate thicknesses exceeding 8 mm, where single-process approaches may struggle to balance penetration depth with bead quality.
Engineering Practice Considerations
For industrial zirconium welding applications, several practical considerations emerge from this work:
- Equipment cleanliness: All welding equipment contacting zirconium must be dedicated or thoroughly cleaned to prevent contamination. Tungsten electrodes, gas nozzles, and even the operator's clothing must be free of foreign materials.
- Gas purity: Shielding gas must be of high purity (typically 99.999% argon or helium), with dew point below -70°C to minimize hydrogen absorption.
- Temperature monitoring: The workpiece should be cooled below 400°C before exposure to air, either through active cooling or by maintaining continuous gas shielding until cooling is complete.
- Joint design: For plates over 8 mm, the PAW+TIG combination reduces the number of passes compared to TIG-only welding, which is beneficial for maintaining overall weld quality in thick sections.
Study Insights and Reflections
The PAW+TIG combination represents a pragmatic engineering solution to the challenge of welding thick zirconium sections. Plasma arc welding alone can produce excessive heat input in the cap passes, leading to surface oxidation and poor bead profile. Conversely, TIG welding alone requires excessive passes for thick plates, increasing both the risk of contamination and the overall heat input. The hybrid approach leverages the strengths of each process: PAW for efficient deep penetration and TIG for controlled surface quality.
The emphasis on gas shielding protocol is appropriate. In zirconium welding, the single most common cause of weld failure is contamination from inadequate shielding. The multi-stage shielding approach described in this work—combining primary, secondary, and back-side protection—represents best practice for critical zirconium applications.
This work provides a validated process route for zirconium fabrication in the chemical processing industry, where equipment integrity is paramount. The demonstrated capability to achieve standard-compliant welds in sections over 8 mm thick expands the range of zirconium fabrication that can be economically achieved, reducing the need for alternative joining methods such as brazing or mechanical fastening.