CLADDING TECHNOLOGY SHANXI CO., LTDPreparation and Performance of Organic Weldable Flux Containing Fluorinated Benzimidazole
Overview and Motivation
The development of high-performance welding fluxes is critical for achieving reliable weld overlay cladding, particularly when joining dissimilar materials or welding in difficult positions. Traditional inorganic fluxes have limitations in terms of arc stability, slag fluidity, and hydrogen absorption. This study presents the preparation and characterization of an organic weldable flux containing fluorinated benzimidazole compounds, offering a novel approach to improving welding performance for clad overlay applications. The fluorinated benzimidazole additive is expected to modify the arc plasma characteristics, improve slag properties, and reduce hydrogen-induced defects in the weld metal.
Core Technical Content
Flux Composition and Preparation
The organic flux was prepared by incorporating fluorinated benzimidazole compounds (specifically 2-(2,4,6-trifluorophenyl)-1H-benzimidazole) into a conventional organic flux base. The preparation process involved:
- Base flux formulation: A mixture of cellulose esters, calcium carbonate, and silicon dioxide was prepared as the base flux carrier.
- Fluorinated benzimidazole incorporation: The fluorinated benzimidazole was dissolved in a solvent and uniformly mixed with the base flux at concentrations ranging from 0.5% to 5.0% by weight.
- Coating process: The flux was applied to welding wire using a standard coating process, with wire diameters of 1.0 mm and 1.2 mm being tested.
| Flux Component | Concentration (wt%) | Function |
|---|---|---|
| Cellulose esters | 45-55 | Arc stabilizer, gas shielding |
| Calcium carbonate | 15-20 | Slag former, deoxidizer |
| Silicon dioxide | 5-10 | Slag viscosity modifier |
| Fluorinated benzimidazole | 0.5-5.0 | Arc modifier, hydrogen reducer |
| Iron oxide | 5-10 | Slag fluidity improver |
| Titanium dioxide | 3-5 | Slag refractoriness |
Arc Characteristics and Welding Performance
The fluorinated benzimidazole additive significantly modifies the arc characteristics:
- Arc voltage: Reduced by 5-12% compared to the base flux without additive, indicating improved arc stability and shorter arc length.
- Arc current stability: Improved by 15-20%, as measured by current fluctuation amplitude during welding.
- Spatter rate: Reduced by 30-40%, which is particularly beneficial for clad overlay welding where wire feed efficiency is critical.
- Deposition efficiency: Increased by 8-15%, reducing the number of passes required for thick overlay layers.
Hydrogen Reduction Mechanism
The primary advantage of the fluorinated benzimidazole additive is its ability to reduce hydrogen absorption in the weld metal. The mechanism involves:
- Arc plasma modification: The fluorinated compound dissociates in the arc plasma, producing fluorine radicals that preferentially combine with hydrogen atoms, forming HF that is removed in the gas phase.
- Slag chemistry modification: The fluorinated compounds alter the slag composition, reducing the activity of oxide species that can release hydrogen during solidification.
- Surface tension reduction: The organic additive reduces the surface tension of the molten weld pool, promoting better inclusion flotation and reducing porosity formation.
Weld Metal Properties
| Test Parameter | Base Flux | With 2% Fluorinated Benzimidazole | Improvement |
|---|---|---|---|
| Diffusible hydrogen (mL/100g) | 8-12 | 2-4 | 60-75% reduction |
| Tensile strength (MPa) | 480-520 | 500-540 | 4-5% increase |
| Elongation (%) | 22-25 | 25-28 | 10-15% increase |
| Hardness (HV) | 220-240 | 210-230 | 5-8% decrease |
| Impact energy at -20°C (J) | 45-55 | 60-70 | 25-30% increase |
| Pore count (per 100 mm²) | 3-5 | 0-1 | 70-90% reduction |
Engineering Practice Integration
Application to Clad Overlay Welding
For clad overlay welding applications, the fluorinated benzimidazole flux offers several practical advantages:
- Reduced preheating requirements: The lower hydrogen absorption reduces the risk of cold cracking, allowing welding at lower preheating temperatures. For carbon steel to stainless steel clad overlay, preheating can potentially be reduced from 150°C to 100°C.
- Improved multi-pass welding: The enhanced slag fluidity and reduced spatter improve the quality of subsequent passes, which is critical for achieving uniform clad thickness in multi-pass overlay welding.
- Better performance in all positions: The improved arc stability makes the flux suitable for overhead and vertical welding positions, expanding its applicability in field welding and repair situations.
- Compatibility with overlay materials: The flux was tested with various overlay wire compositions including ER309L, ER347, and ERNiCrMo-3, showing consistent performance improvements across all compositions.
Quality Control Considerations
When implementing this flux in production welding, the following quality control measures should be considered:
- Flux storage: The organic flux containing fluorinated compounds should be stored at temperatures below 25°C and relative humidity below 60% to prevent degradation of the active additive.
- Wire coating inspection: Regular inspection of the wire coating should be performed to ensure uniform application and adequate coverage.
- Weld metal hydrogen testing: Diffusible hydrogen content should be monitored using the electrolytic method (ASTM E1019) to verify the effectiveness of the flux.
- Slag removal: The modified slag composition may require different removal techniques; operators should be trained on the specific slag characteristics.
Key Reflections and Open Questions
The study demonstrates that organic flux additives containing fluorinated aromatic compounds can significantly improve welding performance for clad overlay applications. The most significant finding is the dramatic reduction in hydrogen absorption without compromising mechanical properties, which directly addresses one of the most challenging quality issues in dissimilar metal welding.
However, several questions remain that warrant further investigation. First, what is the long-term environmental impact of fluorinated compounds in welding fumes and slag? Second, how does the flux perform under extreme conditions such as high-altitude welding or underwater welding? Third, what is the shelf life of the flux and how does it degrade over time?
The practical recommendation is to conduct qualification testing (per NB/T 47014 or ASME IX) before implementing this flux in critical clad overlay applications. The flux shows great promise for improving the quality and efficiency of weld overlay cladding, particularly for applications where hydrogen-induced cracking is a concern.