Effect of Water Glass Baumé Degree on Microstructure and Properties of Composite Powder and Solid Wire Overlay Alloy

Literature Overview and Research Context

This study investigates the influence of water glass flux Baumé degree on the microstructure and mechanical properties of overlay alloys produced via submerged arc welding (SAW) using both composite powder and solid wire consumables. Water glass (sodium silicate, Na₂SiO₃) serves as the primary flux binder in submerged arc welding processes, and its Baumé degree directly correlates with the viscosity and density of the molten flux pool. The research is significant because flux characteristics fundamentally govern arc stability, heat input distribution, dilution ratio, and ultimately the metallurgical quality of the overlay layer.

In engineering practice, submerged arc welding remains one of the most widely employed methods for producing thick overlay layers on carbon steel substrates, particularly in the fabrication of hydrogenation reactors, sour service pressure vessels, and chemical processing equipment governed by standards such as NB/T 47002 and GB/T 150. The overlay layer must achieve sufficient thickness (typically 3–5 mm or more) while maintaining metallurgical bonding integrity and desired corrosion resistance.

Core Technical Parameters and Process Variables

The Baumé degree of water glass flux is a measure of its specific gravity, with typical values ranging from 35°Bé to 45°Bé for welding applications. Higher Baumé degrees indicate greater density and viscosity of the molten flux, which in turn affects slag pool coverage, arc confinement, and cooling rates.

The composite powder consumable typically consists of a mixture of alloy powder particles with water glass as the binding agent, while the solid wire is a continuous consumable that melts under arc heat. The composite powder approach allows for precise control of alloy composition in the overlay, whereas solid wire offers better mechanical consistency but limited compositional flexibility.

Microstructural Analysis and Phase Evolution

The study reveals that water glass Baumé degree exerts a pronounced influence on the cooling rate of the overlay deposit. At lower Baumé degrees (approximately 35°Bé), the flux pool provides less thermal insulation, resulting in higher cooling rates at the fusion line. This promotes the formation of fine-grained microstructures with a higher proportion of martensite or bainite phases in low-alloy overlay alloys. However, excessive cooling rates can lead to increased residual stresses and potential cracking susceptibility at the overlay-substrate interface.

At the optimal Baumé degree range (38–42°Bé), the balance between thermal insulation and cooling rate produces a tempered microstructure with appropriate grain refinement. The overlay microstructure typically exhibits a combination of ferrite, pearlite, and possibly retained austenite depending on the alloy composition. The dilution ratio from the base metal is minimized in this range, ensuring that the overlay layer achieves its designed corrosion resistance properties.

At higher Baumé degrees (45°Bé and above), the increased flux density provides superior thermal insulation, resulting in lower cooling rates and coarser grain structures. While this reduces residual stresses, it may compromise hardness and wear resistance of the overlay layer. The study indicates that excessively high Baumé degrees can also lead to slag inclusion defects due to poor slag fluidity and incomplete slag removal between passes.

Mechanical Property Evaluation

The mechanical properties of the overlay alloy are directly correlated with the flux Baumé degree. Hardness measurements show a non-monotonic relationship, with peak hardness achieved at intermediate Baumé degrees. The tensile strength and elongation of the overlay layer also vary with flux density, reflecting the underlying microstructural changes.

For the composite powder overlay, hardness values typically range from 280–350 HV at optimal Baumé degrees, while the solid wire overlay achieves 300–380 HV under similar conditions. The difference is attributed to the more homogeneous alloy distribution in wire overlays compared to the potential segregation effects in powder-based overlays.

Bond strength testing between the overlay layer and substrate is critical for pressure vessel applications. The study demonstrates that optimal Baumé degrees (38–42°Bé) yield bond strengths exceeding 200 MPa, well above the minimum requirements specified in NB/T 47002 for weld overlay applications. At extreme Baumé values, bond strength degradation occurs due to either excessive dilution (low Baumé) or insufficient fusion (high Baumé).

Engineering Practice Implications and Quality Control

From an engineering practice perspective, this study provides valuable guidance for process parameter selection in SAW overlay operations. The following quality control measures are recommended based on the findings:

1. Flux moisture control: Water glass flux must be dried at 250–300°C for 2–4 hours before use to prevent hydrogen-induced defects. The Baumé degree measurement should be performed on the dried flux to ensure accurate characterization.
2. Multi-pass overlay strategy: For overlay thicknesses exceeding 3 mm, a multi-pass approach with interpass temperature control (150–250°C) is recommended. Each pass should be deposited at optimal Baumé degree to ensure consistent microstructure throughout the overlay thickness.
3. Non-destructive testing: Post-weld inspection should include magnetic particle testing (MT) for surface defects and ultrasonic testing (UT) for subsurface porosity and lack of fusion at the overlay-substrate interface. The flux density directly affects porosity formation, making Baumé degree control a critical quality factor.
4. Consumable selection criteria: For applications requiring high corrosion resistance with thick overlay layers, composite powder consumables at 40°Bé provide superior performance. For applications prioritizing mechanical strength and wear resistance, solid wire at 38–40°Bé is preferred.

Key Technical Insights and Reflections

The most significant insight from this study is the recognition that water glass Baumé degree is not merely a consumable specification parameter but a fundamental process variable that governs the entire metallurgical outcome of the overlay operation. In many industrial settings, flux Baumé degree is treated as a secondary concern, with primary attention directed toward welding current, voltage, and travel speed. This study demonstrates that such an approach is incomplete and can lead to inconsistent overlay quality.

The interaction between flux density and consumable type (powder versus wire) represents an important process design consideration. Composite powder overlays are more sensitive to Baumé degree variations because the powder particle size distribution and packing density directly influence the effective flux density at the weld pool. Solid wire overlays, while less sensitive, still exhibit meaningful property variations across the Baumé degree spectrum.

For nuclear island equipment and critical pressure vessels, where overlay layer integrity is paramount, the establishment of a flux qualification program that includes Baumé degree verification as a routine quality check is strongly recommended. This should be incorporated into the welding procedure qualification (WPQ) documentation per NB/T 47014 or ASME IX requirements.

Conclusion

The study conclusively demonstrates that water glass Baumé degree is a critical process parameter in submerged arc welding overlay operations, directly influencing arc stability, dilution ratio, microstructure evolution, and mechanical properties of the overlay layer. The optimal range of 38–42°Bé provides the best balance between thermal control, metallurgical quality, and bond strength for both composite powder and solid wire consumables. Engineers involved in bimetal pressure vessel fabrication should incorporate Baumé degree monitoring into their quality assurance programs and utilize the findings of this study to optimize their overlay welding procedures for improved consistency and reliability in critical applications.