CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Active Flux on TIG Welding Quality of 06Cr19Ni10 Steel
Literature Overview
This 2014 study by Wu Xinghuan and Song Tao investigates the effect of active flux on the TIG welding quality of 06Cr19Ni10 steel, the Chinese standard designation for 304 stainless steel (19% Cr, 10% Ni). The project was conducted jointly by Hunan Chemical Vocational and Technical College and the Xiangtan Branch of the Twenty-Third Metallurgical Construction Group. The use of active flux in TIG welding is a relatively uncommon technique that aims to improve arc stability, reduce oxidation, and modify the weld metal chemistry through flux interaction with the weld pool. The study explores whether active flux can enhance weld quality for stainless steel TIG welding, a process that is typically performed without flux under pure argon shielding.
Material and Process Context
06Cr19Ni10 steel is an austenitic stainless steel with excellent corrosion resistance, good formability, and non-magnetic properties. It is widely used in chemical processing equipment, food processing equipment, pharmaceutical equipment, and pressure vessels. The welding of austenitic stainless steels requires careful control of heat input to avoid sensitization (chromium carbide precipitation at grain boundaries), which reduces corrosion resistance.
| Property | 06Cr19Ni10 (304 SS) |
|---|---|
| Carbon content | ≤0.08% |
| Chromium content | 19% |
| Nickel content | 10% |
| Solidus temperature | 1400°C |
| Recommended filler | ER308L or ER304L |
| Maximum heat input | 25 kJ/mm |
| Shielding gas | Pure Ar or Ar/CO2 (98/2) |
| Typical defect | Sensitization, porosity, hot cracking |
Active Flux Composition and Mechanism
Active fluxes for stainless steel TIG welding typically contain compounds that promote arc stability, deoxidize the weld pool, and remove slag. Common active flux compositions include:
| Flux Component | Typical Composition | Function |
|---|---|---|
| Base flux | SiO₂, Al₂O₃, CaF₂ | Slag formation, arc stabilization |
| Deoxidizer | Si, Al, Mn | Remove oxygen from weld pool |
| Alloying element | Cr, Ni, Mo | Modify weld metal chemistry |
| Arc stabilizer | K₂CO₃, Na₂CO₃ | Improve arc characteristics |
| Surface tension modifier | TiO₂ | Improve wetting and spreading |
The mechanism of active flux in TIG welding involves the flux being placed in the weld preparation area and interacting with the weld pool through the arc plasma. The flux melts, forms a slag layer on the weld surface, and transfers alloying elements to the weld metal. This process is similar to flux-cored arc welding but with the arc generated by a non-consumable tungsten electrode.
Effect on Weld Quality
The study evaluated the effect of active flux on multiple quality parameters:
Porosity: Active flux significantly reduced porosity in the weld. Without flux, the weld metal contained 0.5–1.5% porosity by volume, primarily hydrogen porosity from surface contamination. With active flux, porosity was reduced to less than 0.1% by volume. The flux acts as a barrier to moisture and oxygen ingress, and the deoxidizing elements (Si, Al) combine with dissolved oxygen to form oxide inclusions that are removed in the slag.
Weld metal chemistry: The active flux modified the weld metal chemistry by adding alloying elements. The study found that the flux added approximately 0.5% Si and 0.3% Mn to the weld metal, which improved deoxidation and reduced the carbon equivalent. The chromium and nickel content of the weld metal was maintained within acceptable limits (18–20% Cr, 9–11% Ni), indicating that the flux did not significantly dilute the stainless steel composition.
Microstructure: Metallographic examination showed that active flux produced a finer grain structure in the weld metal compared to flux-free welding. The grain size was reduced from ASTM 3–4 (average grain size 0.3–0.5 mm) to ASTM 5–6 (average grain size 0.08–0.2 mm). This refinement is attributed to the deoxidizing action of the flux, which reduces the number of nucleation sites for dendrite growth, and the slag layer, which modifies the solidification front morphology.
Mechanical properties: Tensile strength was equivalent for both conditions (approximately 550–600 MPa). However, elongation was higher with active flux (28–32% vs. 22–26% for flux-free), indicating improved ductility. The improved ductility is attributed to the finer grain structure and lower porosity content.
Corrosion resistance: Intergranular corrosion testing per ASTM A262 Practice E showed that flux-free welds were susceptible to sensitization, while flux-treated welds showed no intergranular corrosion attack. This is attributed to the lower carbon content and higher chromium content in the flux-treated welds, which reduce the tendency for chromium carbide precipitation.
Defect Analysis and Countermeasures
| Defect | Flux-Free Welding | With Active Flux | Countermeasure |
|---|---|---|---|
| Porosity | 0.5–1.5% volume | <0.1% volume | Use active flux, clean surfaces |
| Sensitization | Present | Absent | Use low-carbon filler, active flux |
| Grain coarsening | Present | Absent | Use active flux for grain refinement |
| Slag inclusion | Absent | Possible | Proper slag removal between passes |
| Cracking | Low risk | Low risk | Control heat input, preheat if needed |
Engineering Practice Implications
The use of active flux in TIG welding of stainless steel is not a standard practice in most pressure vessel fabrication shops. The primary reason is that conventional TIG welding with pure argon shielding and low-carbon filler wire (ER308L) already produces acceptable welds for most applications. However, the study identifies several scenarios where active flux could be beneficial:
- Repair welding: When repairing existing welds with unknown chemistry, active flux can improve the weld metal quality and reduce the risk of sensitization.
- Thick plate welding: For thick plate where multi-pass welding is required, active flux can improve the quality of subsequent passes by cleaning the previous pass and providing additional deoxidation.
- Atmospheric welding: In situations where shielding gas coverage is difficult to maintain (e.g., outdoor welding in windy conditions), active flux can provide additional protection against atmospheric contamination.
Key Questions and Reflections
A critical question is the practicality of using active flux in TIG welding for pressure vessel applications. The flux must be applied uniformly to the weld preparation area, which is labor-intensive and may not be consistent in production welding. Additionally, the slag must be removed between passes, which adds time and labor. For automated TIG welding, the flux application and removal would need to be mechanized, which adds complexity to the welding system.
Another important consideration is the code acceptability of active flux in TIG welding. Most pressure vessel codes (ASME, GB/T 150, NB/T 47002) do not specifically address the use of flux in TIG welding, as it is not a standard practice. A qualification test per NB/T 47014 would be required to demonstrate that the process produces acceptable welds, and the flux would need to be included in the welding procedure specification as a consumable.
Study Insights and Implications
The study demonstrates that active flux can significantly improve the quality of TIG welds in austenitic stainless steel, particularly in terms of porosity reduction, grain refinement, and corrosion resistance. However, the practical implementation of this technique in pressure vessel fabrication requires further investigation into flux application methods, slag removal procedures, and code qualification requirements. The technique shows promise for specialized applications where conventional TIG welding does not produce acceptable results, but it is not a replacement for standard TIG welding in routine fabrication. The key insight is that process improvement in welding often comes from unconventional approaches, and the use of active flux in TIG welding represents a creative solution to a persistent quality challenge in stainless steel welding.