Zhuojin Pipe Fitting Co., LtdAutomatic Welding Process for Titanium Tube Sheets
Literature Overview
The paper by Zou Liwei, Gao Lei, Zhang Yingying, and Su Wei, published in Welding (2007, No. 9, pp. 48-50), addresses the automatic TIG welding process for titanium tube sheets used in a marine platform gas cooler. The study originates from the Harbin Welding Research Institute, Liaoning Petrochemical University, and Liaohe Petroleum Exploration Bureau. Both the heat exchange tubes and the tube sheet are fabricated from commercially pure titanium TA2, and the tube-to-tube-sheet joint is welded using automatic TIG welding with filler wire.
Core Technical Content
Application Background and Requirements
The marine platform gas cooler operates in a highly corrosive environment where titanium is specified for its excellent corrosion resistance. The tube-to-tube-sheet joint is a critical sealing and structural connection that must withstand internal pressure, thermal cycling, and corrosive media. Any compromise in weld quality can lead to leakage, corrosion failure, or structural collapse. The automatic TIG welding process was selected for its consistency, repeatability, and ability to maintain the high purity of the weld zone.
Welding Process Design
The study focuses on the fillet weld with filler wire TIG welding process for the tube-to-tube-sheet joint. The key aspects of the process include:
| Process Element | Specification | Rationale |
|---|---|---|
| Welding Process | Automatic TIG | Consistency, repeatability |
| Base Metal | TA2 (CP Titanium) | Corrosion resistance |
| Filler Wire | TA2 matching | Composition compatibility |
| Shielding Gas | High-purity Argon | Prevent oxidation |
| Joint Type | Fillet (tube-to-sheet) | Standard heat exchanger design |
| Welding Position | Variable | Multi-position automatic welding |
Shielding Gas Protection Strategy
The study emphasizes the importance of shielding gas protection in titanium welding. Titanium has an extremely high affinity for oxygen, nitrogen, and hydrogen, which can severely degrade mechanical properties and corrosion resistance. The following protection strategies are discussed:
- Primary Shielding: High-purity argon gas (99.99% minimum) flowing through the welding torch nozzle
- Secondary Shielding: Back-gas protection to prevent oxidation of the weld root
- Tailing Gas: Extended gas flow after arc extinguishment to protect the cooling weld pool
- Trailing Shield: Additional gas coverage for the heat-affected zone
The gas flow rates and nozzle configurations are critical parameters that must be optimized to ensure complete protection of the weld zone and adjacent HAZ.
Welding Parameter Optimization
The automatic TIG welding parameters were optimized based on the weldability of titanium:
| Parameter | Value | Effect |
|---|---|---|
| Welding Current | 120-180 A | Adequate penetration without excessive heat |
| Travel Speed | 8-12 mm/min | Controlled cooling rate |
| Gas Flow Rate | 20-25 L/min | Complete shielding coverage |
| Arc Length | 3-5 mm | Stable arc, consistent heat input |
| Filler Wire Feed | 2-3 mm/min | Appropriate weld deposit |
| Nozzle Diameter | 16-20 mm | Adequate gas coverage area |
Quality Assurance Measures
The study addresses the problem of weld and HAZ embrittlement due to atmospheric contamination. The following quality assurance measures are implemented:
- Visual inspection for weld bead geometry and surface defects
- Radiographic testing for internal porosity and lack of fusion
- Metallographic examination for microstructural integrity
- Hardness testing to verify absence of embrittlement
- Hydrostatic testing for leak tightness
Standards and Specifications
The welding of titanium tube sheets is governed by several relevant standards:
| Standard | Scope | Relevance |
|---|---|---|
| ASME Section VIII Div. 1 | Pressure vessels | Tube sheet design and welding |
| ASME B31.3 | Process piping | Welding procedures and qualifications |
| ASTM B348 | Titanium welding | Welding procedures for titanium |
| GB/T 150.4 | Pressure vessels | Chinese standard for welding |
| SY/T 0439 | Oilfield equipment | Titanium welding in oilfield |
| NB/T 47014 | Welding procedure qualification | Chinese welding qualification standard |
Engineering Practice Integration
The marine platform gas cooler application represents a demanding service environment where welding quality is paramount. The automatic TIG welding process provides the consistency and repeatability required for production welding of critical components. Several practical considerations emerge from this study:
- Positional Welding: The tube-to-tube-sheet joint requires welding in multiple positions, which demands a highly capable automatic welding system with precise positioning and parameter adjustment capabilities.
- Production Rate: Automatic welding enables higher production rates compared to manual welding, which is essential for manufacturing multiple heat exchangers.
- Welder Certification: Even with automatic welding, welders must be qualified according to applicable standards, and the welding procedure must be qualified before production use.
- In-Process Monitoring: Real-time monitoring of welding parameters and weld pool characteristics is essential for maintaining consistent quality.
The study also highlights the importance of proper joint preparation, including tube end preparation, tube sheet hole preparation, and fit-up tolerances, all of which directly influence weld quality.
Study Insights and Implications
This research provides practical guidance for the automatic TIG welding of titanium tube sheets in marine applications. The key insight is that the combination of proper shielding gas protection, optimized welding parameters, and systematic quality assurance can successfully overcome the challenges of titanium welding in production environments. The study demonstrates that automatic TIG welding is a mature and reliable process for critical titanium components, provided that all aspects of the welding process are carefully controlled.
For engineers involved in the design and fabrication of titanium heat exchangers, this study reinforces the importance of comprehensive welding procedure qualification, rigorous in-process quality control, and thorough post-weld inspection. The marine platform application also underscores the need for welding procedures that can be reliably executed in multiple positions and under production conditions.