CLADDING TECHNOLOGY SHANXI CO., LTDApplication of TIG-MAG Combined Welding on Cryogenic Diaphragm Pump Pressure Compensation Tank
Literature Overview
This engineering application paper by Wang Huan, Zhang Yunsong, and Sun Quan from China Nonferrous (Shenyang) Metallurgical Machinery Co., Ltd., published in 2010 in the journal Hot Working Technology, documents the practical application of combined TIG-MAG welding for fabricating cryogenic pressure compensation tanks used in diaphragm pumps. This work provides valuable practical insights into the selection and implementation of combined welding processes for cryogenic service applications.
Core Technical Framework
The pressure compensation tank is a critical component in diaphragm pumps used for handling cryogenic fluids. The tank must withstand repeated pressure cycles and maintain dimensional stability at cryogenic temperatures. The welding process must produce joints with excellent toughness, low residual stress, and high fatigue resistance.
Combined TIG-MAG Welding Process
| Parameter | TIG (Root Pass) | MAG (Fill and Cap) |
|---|---|---|
| Current | 60–100 A | 150–250 A |
| Voltage | 12–18 V | 20–28 V |
| Travel speed | 50–100 mm/min | 200–400 mm/min |
| Shielding gas | Ar or Ar/He mix | CO₂/Ar or Ar/O₂ mix |
| Filler wire | ER308L or ER316L | ER308L or ER316L |
| Interpass temperature | <150°C | <200°C |
Interpretation of Technical Points
The combined TIG-MAG approach leverages the strengths of both processes:
- TIG for root pass: Provides precise control of the root weld, ensuring good penetration and minimizing defects at the critical root interface
- MAG for fill and cap passes: Provides higher deposition rates, reducing welding time and cost for the bulk of the weld metal
For cryogenic applications, several critical requirements must be met:
- Low-temperature toughness: The weld metal must maintain adequate impact energy at service temperatures (typically -40°C to -196°C depending on the application)
- Low residual stress: Residual stresses can promote brittle fracture at cryogenic temperatures and must be minimized through process control and PWHT
- Fatigue resistance: The repeated pressure cycling creates fatigue loading, requiring sound welds with smooth transitions and minimal defects
- Dimensional stability: Thermal distortion during welding must be controlled to maintain tank geometry and diaphragm seating
Material Selection for Cryogenic Service
| Component | Material Grade | Key Properties |
|---|---|---|
| Tank shell | 06Cr19Ni10 (304) | Good cryogenic toughness |
| Weld metal | ER308L | Low carbon, good ductility |
| Diaphragm | PTFE or EPDM | Chemical compatibility |
| Fasteners | 17-4PH | Strength and corrosion resistance |
Integration with Engineering Practice
The fabrication of cryogenic pressure compensation tanks presents unique challenges that require careful process planning:
- Preheating: Generally not required for austenitic stainless steels, but interpass temperature control is critical to prevent excessive grain growth
- PWHT: Solution annealing at 1050–1100°C may be required to relieve residual stresses and restore full corrosion resistance
- NDE requirements: 100% radiographic testing for full-penetration welds, supplemented by magnetic particle or dye penetrant testing for surface defects
- Hydrostatic testing: Required at 1.5 times design pressure, with additional cryogenic testing if specified
The combined TIG-MAG approach offers practical advantages for production welding:
- Faster overall welding speed compared to all-TIG fabrication
- Good weld quality from the TIG root pass
- Economic efficiency from MAG fill and cap passes
- Flexibility to adjust process parameters for different joint configurations
Key Questions and Reflections
A critical concern for cryogenic applications is the susceptibility of welds to hydrogen-induced cracking. While austenitic stainless steels are generally resistant to HIC, the presence of martensitic phases in the weld metal (due to excessive heat input or improper filler selection) can create susceptibility. The combined TIG-MAG process must be carefully controlled to maintain the austenitic structure of the weld metal.
Another practical consideration is the effect of welding sequence on residual stress distribution. For pressure compensation tanks, which typically have cylindrical or spherical geometry, the welding sequence must be planned to minimize distortion and residual stress. Symmetric welding sequences and back-step welding techniques are commonly employed to achieve this goal.
Study Insights and Implications
The successful application of combined TIG-MAG welding to cryogenic pressure compensation tanks demonstrates the practical viability of this approach for demanding service conditions. The key to success lies in careful process planning, including proper material selection, welding sequence optimization, and thorough quality control.
For engineers involved in cryogenic pressure vessel fabrication, this case study highlights the importance of understanding the interaction between welding process parameters, microstructure, and mechanical properties at low temperatures. The combined TIG-MAG approach provides a practical solution that balances quality requirements with production efficiency, making it suitable for both prototype and production environments.