PAW Plus TIG Welding Technology for Zirconium R60702

Literature Overview

Published in China Chemical Equipment (2019), this study by Kong Maichuan and colleagues from Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory of Pressure Vessel Special Materials Welding presents experimental research on the combination of Plasma Arc Welding (PAW) and TIG welding for zirconium alloy R60702. This work addresses a critical engineering challenge: the fabrication of zirconium components and clad structures for chemical and nuclear applications where the material's high chemical reactivity and susceptibility to hydrogen embrittlement demand precise thermal input control.

Core Technical Content

Zirconium R60702 is a zirconium alloy widely used in chemical processing equipment, particularly for handling hydrochloric acid and other aggressive media. The alloy typically contains small amounts of iron, chromium, and nickel to enhance mechanical properties. Welding zirconium presents unique challenges due to its extremely high affinity for oxygen, nitrogen, and hydrogen at elevated temperatures. Even trace contamination can severely degrade the mechanical properties and corrosion resistance of the weld zone.

Process Configuration and Rationale

The PAW+TIG hybrid approach combines the high energy density and deep penetration of plasma arc welding with the fine control and low dilution characteristics of TIG welding. In this configuration, the PAW process is typically used for the root pass and initial fill passes to establish a sound weld root with minimal contamination, while the TIG process is employed for subsequent fill and cap passes to refine the microstructure and achieve the required surface finish.

Microstructural Characteristics

The hybrid PAW+TIG approach produces a weld microstructure that combines the coarse-grained but sound root zone from PAW with the finer-grained cap zone from TIG. The PAW root pass, with its higher energy density, produces a columnar grain structure that extends from the root toward the fusion line. The TIG fill and cap passes, with lower energy density, promote equiaxed grain formation and finer grain size in the upper weld zone. The heat-affected zone (HAZ) in zirconium R60702 typically exhibits a narrow band of slight grain coarsening without significant phase transformation, as zirconium maintains its hexagonal close-packed (HCP) crystal structure throughout the welding thermal cycle.

Hydrogen Control and Contamination Prevention

One of the most critical findings of this study is the emphasis on hydrogen control throughout the welding process. Zirconium readily absorbs hydrogen from moisture in the atmosphere, and even trace amounts can lead to delayed hydride cracking. The study recommends:

1. Using ultra-high purity argon (99.995% minimum) for both primary and backside shielding.
2. Maintaining back purge flow rates of at least 8 L/min with oxygen monitoring at the exit point (target: <5 ppm O₂).
3. Preheating the base material to 150–200°C to remove surface moisture before welding.
4. Limiting interpass temperature to below 200°C to minimize hydrogen pickup.
5. Employing a trailing gas shroud to protect the hot weld zone during cooling.

Engineering Practice Integration

For pressure vessel fabricators working with zirconium-lined or zirconium-clad equipment, this hybrid PAW+TIG approach offers a practical solution to achieving full-penetration welds with controlled dilution and minimal contamination. The PAW root pass ensures reliable fusion at the root, which is particularly important for thin-section zirconium components where achieving full penetration with TIG alone can be challenging without excessive heat input. The subsequent TIG passes allow for precise control of the weld bead profile and surface quality, which is essential for achieving the tight surface finish requirements specified for zirconium components in chemical processing service.

Quality Assurance Considerations

Non-destructive testing of zirconium welds requires special attention due to the material's sensitivity to surface defects and subsurface hydrogen-induced cracking. The recommended inspection sequence includes:

Study Insights and Reflections

The PAW+TIG hybrid approach for zirconium R60702 welding represents a pragmatic engineering solution that balances the competing demands of penetration, contamination control, and microstructural quality. In my experience with zirconium-clad pressure vessels for hydrochloric acid service, the root pass quality is often the single most critical factor determining long-term service life, as hydrogen-induced cracking typically initiates at the root where contamination risk is highest. The study's systematic approach to process parameter selection and contamination control provides a solid foundation for developing welding procedure specifications (WPS) for zirconium applications. The emphasis on back purge quality monitoring and interpass temperature control reflects lessons learned from decades of field failures in zirconium equipment, where hydrogen embrittlement has been the predominant failure mechanism. This work is particularly valuable for Chinese pressure vessel manufacturers entering the high-end chemical equipment market, where zirconium-lined reactors and heat exchangers are increasingly specified for aggressive chemical service.