CLADDING TECHNOLOGY SHANXI CO., LTDTIG and MAG Combination Welding on Low-Temperature Diaphragm Pump Pressure Compensation Tank - Literature Study Note
Application Context and Technical Challenges
This 2010 study by Wang Huan, Zhang Yunsong, and Sun Quan from the Pressure Vessel Workshop of China Nonferrous (Shenyang) Metallurgical Machinery Co., Ltd., documents the application of TIG and MAG combination welding on a low-temperature diaphragm pump pressure compensation tank. This case study represents a practical engineering solution to a specific manufacturing challenge, and its value lies in demonstrating how hybrid welding approaches can address complex fabrication requirements that single-process methods cannot adequately meet.
Equipment and Service Conditions
Low-temperature diaphragm pumps are used in chemical processing, oil and gas, and cryogenic applications where precise flow control and reliable sealing are required. The pressure compensation tank associated with such pumps must withstand:
- Low-temperature service (potentially down to -40°C or lower)
- Cyclic pressure loading
- Corrosive process media
- Fatigue from repeated pressurization and depressurization
The material selection for such tanks typically involves carbon steel for the pressure shell and stainless steel (304 or 316L) for the internal wetted surface, creating a bimetallic construction that requires careful welding strategy.
Process Selection Rationale
The decision to employ a TIG and MAG combination approach reflects a practical engineering judgment based on the specific requirements of the joint:
| Process | Application Zone | Rationale |
|---|---|---|
| TIG (GTAW) | Root pass and critical welds | Precise heat control, no filler dilution, excellent penetration |
| MAG (GMAW) | Fill and cap passes | High deposition rate, productivity |
The root pass, which forms the foundation of joint integrity, is performed by TIG welding to ensure full penetration and proper fusion without excessive dilution. The subsequent fill and cap passes are executed by MAG welding to achieve the required weld volume efficiently.
Process Parameters and Welding Procedure
A typical combination welding procedure for this application would involve the following parameter settings:
| Parameter | TIG (Root Pass) | MAG (Fill/Cap) |
|---|---|---|
| Current (A) | 80-150 | 180-250 |
| Voltage (V) | 16-20 | 22-28 |
| Travel speed (cm/min) | 5-10 | 15-25 |
| Shielding gas | Ar 100% | Ar 80% + CO2 20% |
| Wire diameter (mm) | 2.4 (filler rod) | 1.2-1.6 |
The procedure qualification under NB/T 47014 or ASME IX would require demonstration of mechanical properties, including tensile strength, bend test results, and hardness profiles across the weld cross-section.
Quality Assurance Considerations
For low-temperature pressure vessels, the quality assurance program is more demanding than for ambient temperature service:
- Impact testing (Charpy V-notch) at the minimum design temperature is mandatory
- Heat-affected zone toughness must be verified to prevent brittle fracture
- Hydrogen-induced cracking susceptibility must be assessed, particularly for the carbon steel base material
- Intergranular corrosion testing of stainless steel welds may be required if the service environment is aggressive
Non-destructive examination typically includes:
- Radiographic testing (RT) of all complete penetration welds
- Magnetic particle testing (MT) of the carbon steel surfaces
- Penetrant testing (PT) of the stainless steel surfaces
- Hydrostatic pressure test at 1.5 times the design pressure
Engineering Practice Lessons
This case study illustrates several important principles of pressure vessel fabrication:
- Process combination is not a compromise but an optimization - using each process where it performs best yields superior results compared to using a single process throughout.
- The root pass quality determines the overall joint integrity - investing in TIG for the root pass pays dividends in reduced defect rates and improved service life.
- Low-temperature service demands additional attention to toughness - the same welding procedure that might be acceptable for ambient temperature service may be inadequate for cryogenic applications.
Study Reflections
The practical nature of this study makes it particularly valuable for fabrication engineers who must make real-time decisions about process selection, parameter setting, and quality assurance. The experience of combining TIG and MAG welding on a specific component provides transferable knowledge for similar applications, while the low-temperature context adds an additional layer of complexity that distinguishes this work from routine pressure vessel welding.
Summary
The application of TIG and MAG combination welding on a low-temperature diaphragm pump pressure compensation tank demonstrates the effectiveness of hybrid process approaches in addressing complex fabrication requirements. By leveraging the precision of TIG for critical root passes and the productivity of MAG for fill and cap passes, the fabrication achieves both quality and efficiency. The low-temperature service conditions add important constraints that must be addressed through careful procedure qualification and comprehensive quality assurance, making this case study a valuable reference for engineers working on cryogenic pressure equipment.