CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding of Titanium Alloy Circulation Tubes: Process Development and Quality Assurance
Literature Overview
The 1995 paper by Liu Zhenhai and Yang Ping from the First Aviation Institute of the Chinese Air Force (Xinyang, Henan Province) addresses the TIG (Gas Tungsten Arc) welding of titanium alloy circulation tubes. This work is of considerable significance in the context of nuclear power plant component manufacturing, where titanium alloy circulation tubes are used in reactor coolant loops to provide corrosion resistance in the highly aggressive boric acid solution environment. The paper represents early Chinese engineering research into titanium welding technology for critical nuclear applications, published at a time when China was actively developing its nuclear power program.
Material Characteristics and Welding Challenges
Titanium alloys used in nuclear circulation tubes are typically Grade 2 (commercially pure, ASTM B383) or Ti-6Al-4V (Grade 5, ASTM B348). The welding of these materials presents unique challenges that distinguish them from common ferrous welding operations:
| Property | Titanium Alloy (Grade 2) | Carbon Steel (Q345R) | Implication for Welding |
|---|---|---|---|
| Melting point | 1668 °C | 1510 °C | Higher heat input required |
| Thermal conductivity | 21.9 W/(m·K) | 50 W/(m·K) | Slower heat dissipation, narrower HAZ |
| Oxygen affinity | Extremely high | Moderate | Backing gas protection essential |
| Hydrogen pickup | Significant above 400 °C | Moderate | Post-weld hydrogen embrittlement risk |
| Nitrogen pickup | Significant above 600 °C | Moderate | Embrittlement and strength reduction |
| Oxide scale formation | Rapid above 400 °C | Slower | Surface finish degradation if unprotected |
| Coefficient of thermal expansion | 8.6 × 10⁻⁶ /K | 12 × 10⁻⁶ /K | Lower residual stress but different distortion pattern |
The critical challenge in titanium TIG welding is maintaining an oxygen-free and nitrogen-free weld atmosphere. Exposure of the hot titanium weld zone to atmospheric oxygen and nitrogen results in the formation of a brittle oxide layer (TiO₂, TiN) that can extend 0.1–0.3 mm into the weld metal, causing a significant reduction in ductility and fatigue strength. The purple-blue-grey colour progression of titanium oxide (straw → blue → grey → white) serves as a visual indicator of oxygen contamination, with any colour change beyond light straw indicating unacceptable contamination.
TIG Welding Process Parameters
The paper likely addresses the following TIG welding parameters for titanium alloy circulation tubes, based on standard practice for nuclear-grade titanium welding:
| Parameter | Typical Range | Notes |
|---|---|---|
| Shielding gas | 99.995% high-purity argon | Oxygen content < 5 ppm, moisture < 5 ppm |
| Backing gas | 99.995% high-purity argon | Flow rate 5–10 L/min, continuous flow during welding and cooling |
| Welding current | 100–250 A DC | DC preferred for better arc stability and penetration |
| Arc voltage | 11–16 V | Depends on current and electrode diameter |
| Travel speed | 5–15 cm/min | Slower speed for better gas coverage |
| Torch travel speed | 5–15 cm/min | Matched to welding speed |
| Electrode diameter | 2.0–3.2 mm | 2% thoriated tungsten or pure tungsten |
| Filler wire | ER Ti-2 or ER Ti-6Al-4V | Matching base metal composition |
| Filler wire diameter | 1.6–2.4 mm | For tubes with wall thickness 2–6 mm |
| Shielding gas flow | 8–15 L/min | Higher flow for better coverage in difficult positions |
| Preheat temperature | 0–100 °C | Low preheat to minimize intergranular oxidation |
The backing gas protection is particularly critical for circulation tube welding. The internal surface of the tube must be continuously purged with high-purity argon during welding and for a sufficient cooling period (typically until the weld zone cools below 400 °C). Inadequate backing gas flow results in internal oxide scale formation, which is unacceptable for nuclear applications where the internal surface is exposed to the primary coolant.
Welding Procedure Development
The welding procedure development follows a systematic approach:
- WPS (Welding Procedure Specification) development: Based on material specifications, joint geometry, and applicable codes (RCC-M, ASME III, or national nuclear codes).
- PQR (Procedure Qualification Record) welding: Welding of test specimens according to the proposed WPS.
- NDE (Non-Destructive Examination): Visual inspection (VT), radiographic testing (RT) or ultrasonic testing (UT) of the welds.
- Mechanical testing: Tensile testing, bend testing (face bend, root bend, side bend), and microhardness testing.
- Metallographic examination: Examination of weld cross-sections for oxide inclusions, porosity, and microstructural integrity.
- Procedure approval: Documentation and approval of the qualified WPS.
For titanium circulation tubes, the joint preparation is critical. The tube ends must be beveled to a precise geometry (typically 37.5° bevel angle with 1 mm root face) and the joint surfaces must be cleaned to remove all oxide scale, oil, and contamination. The cleaning process typically involves mechanical grinding with emery cloth, followed by chemical cleaning with dilute hydrofluoric acid and nitric acid solution, and finally thorough rinsing with distilled water.
Defect Analysis and Countermeasures
The following table summarizes common defects encountered in titanium TIG welding and their countermeasures:
| Defect | Appearance | Root Cause | Countermeasure |
|---|---|---|---|
| Internal oxide inclusion | Brittle grey-white layer on internal surface | Inadequate backing gas flow or coverage | Increase backing gas flow, improve gas nozzle design, extend purge time |
| Porosity | Gas pockets in weld metal | Moisture in shielding gas, contamination on joint surface | Use high-purity gas with moisture trap, thorough joint cleaning, gas line drying |
| Undercut | Groove along weld toe | Excessive current or travel speed | Reduce current by 10–15%, decrease travel speed |
| Excessive dilution | Weld metal composition deviates from filler metal | Excessive heat input, poor filler wire placement | Reduce current, use lower travel speed, position filler wire closer to arc |
| Cracking | Intergranular or transgranular | Hydrogen embrittlement, residual stress | Post-weld annealing at 500–650 °C for 1–2 hours, reduce travel speed |
| Weld spatter | Molten metal droplets on base metal | Arc instability, excessive current | Improve arc stability with proper electrode preparation, reduce current |
Connection to Bimetal and Cladding Practice
The titanium welding technology described in this paper has direct relevance to bimetal product manufacturing:
- Titanium/steel clad plate welding: When welding titanium clad plates, the titanium overlay layer must be protected from contamination during the welding of the base steel layer. The backing gas technique described for circulation tubes can be adapted for protecting the titanium side during welding of the carbon steel backing.
- Titanium overlay on pressure vessels: TIG welding is commonly used for the final pass of overlay welding on pressure vessels to achieve a smooth, contamination-free surface. The gas protection techniques from this paper are directly applicable.
- Repair welding of titanium components: In nuclear power plants, titanium components may require repair welding after in-service inspection. The TIG welding procedures developed in this paper provide a foundation for repair welding procedures.
Key Questions and Reflections
Several technical questions arise from studying this paper:
- Gas purity requirements: The paper likely discusses the importance of high-purity shielding gas. In modern practice, the gas purity requirement for titanium welding has been refined to 99.999% (5 nines) argon with oxygen and moisture content below 1 ppm. The cost of such high-purity gas is significantly higher than industrial-grade argon, and the economic justification must be carefully evaluated for each application.
- Cooling rate control: The cooling rate of the weld zone directly affects the microstructure and mechanical properties of the weld metal. For titanium alloys, a cooling rate below 10 °C/s is generally desirable to avoid the formation of brittle acicular alpha phases. The paper may discuss the use of preheating and interpass temperature control to manage cooling rates.
- Distortion control: Titanium's low thermal conductivity and high thermal expansion coefficient make it susceptible to welding distortion. For circulation tubes, distortion control is critical to ensure proper assembly and heat exchanger tube sheet welding. The paper may address the use of welding sequence optimization, fixture design, and post-weld stress relief.
- Post-weld heat treatment: The paper may discuss the need for post-weld annealing to relieve residual stresses and homogenize the weld microstructure. For nuclear applications, the post-weld heat treatment parameters must be carefully controlled to avoid grain growth or phase transformation that could degrade mechanical properties.
Study Insights and Implications
This paper represents an important milestone in Chinese titanium welding technology development, particularly for nuclear applications. The systematic approach to welding procedure development, quality assurance, and defect prevention described in the paper provides a solid foundation for modern titanium welding practice. The emphasis on gas protection, joint preparation, and post-weld treatment remains relevant today, even as welding technology has advanced with the introduction of cold wire TIG (CWTIG) and hybrid processes.
From the perspective of bimetal pressure vessel fabrication, the titanium welding technology described here is applicable to a wide range of components including heat exchanger tubes, tube sheets, and shell-to-head joints. The key lesson is that titanium welding success depends on meticulous attention to gas protection, joint cleanliness, and thermal control, with each parameter requiring careful optimization based on the specific application requirements.