CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Sintered Flux for Austenitic Stainless Steel Strip Electrode Electroslag Cladding
Literature Overview
The research by Xie Xiang, Bao Yefeng, Yang Ke, Jiang Yongfeng, and Li Li from the School of Mechanical and Electrical Engineering, Hohai University, published in Electric Welding Machine (2011), focuses on the development of sintered flux specifically designed for strip electrode electroslag welding (ESW) cladding of austenitic stainless steel. Strip electrode ESW is a high-deposition-rate process widely used for cladding thick-section pressure vessels, storage tanks, and heat exchanger shells where large overlay thicknesses are required.
Core Technical Content and Flux Development
The sintered flux in strip electrode ESW serves multiple critical functions: it maintains the slag pool composition and viscosity, provides thermal insulation, absorbs alloying elements for dilution control, and protects the molten metal from atmospheric contamination. The flux composition directly influences the dilution rate, microstructure, mechanical properties, and corrosion resistance of the resulting overlay layer.
The development of flux for austenitic stainless steel ESW cladding requires careful balance of several competing requirements. The flux must be sufficiently basic to promote clean welds and adequate deoxidation, but not so basic as to cause excessive slag inclusion. The alloying additions to the flux must compensate for dilution from the base metal while maintaining the target composition of the overlay layer. The melting range and viscosity of the flux must be compatible with the ESW process parameters to ensure stable operation.
| Flux Component | Function | Typical Content |
|---|---|---|
| CaF2 | Fluxing, slag fluidity control | 20–35% |
| SiO2 | Viscosity control | 5–15% |
| Al2O3 | Viscosity, refractoriness | 5–15% |
| CaO / MgO | Basicity, deoxidation | 10–20% |
| MnO / FeO | Alloying, oxygen activity control | 5–15% |
| Cr2O3 / TiO2 | Alloying for stainless steel overlay | 3–10% |
| Binds (sodium silicate, etc.) | Sintering, pelletization | 5–10% |
Process-Material Interaction and Key Technical Points
In strip electrode ESW cladding, the strip electrode (typically austenitic stainless steel such as 308L, 309L, or 316L) is fed through the slag pool and melted to deposit the overlay layer. The slag pool composition, determined by the flux, controls the melting rate of the strip electrode, the dilution from the base metal, and the thermal profile of the weld.
A critical aspect of flux design for austenitic stainless steel ESW is the control of dilution. The base metal (typically carbon steel or low-alloy steel) dilutes into the overlay layer, reducing the chromium and nickel content below the levels required for adequate corrosion resistance. The flux must be alloyed with chromium and nickel-bearing compounds to compensate for this dilution. The target dilution rate for stainless steel ESW overlays is typically 20–30%, which requires careful flux alloying design.
The sintering process for the flux involves mixing the raw materials, adding a binder (typically sodium silicate or starch-based), granulating, and sintering at controlled temperatures (typically 900–1100 °C). The sintering temperature and time affect the density, strength, and melting characteristics of the flux pellets. Properly sintered flux exhibits uniform particle size, adequate compressive strength, and consistent melting behavior—all of which contribute to process stability during ESW operation.
The microstructure of the ESW overlay layer is characterized by a fine-grained equiaxed structure, significantly different from the columnar structure typical of other welding processes. This is attributed to the high heat input and slow cooling rate in ESW, which promotes heterogeneous nucleation at the solid-liquid interface. The fine grain structure contributes to good mechanical properties and corrosion resistance in the overlay layer.
Engineering Practice and Standards Compliance
Strip electrode ESW cladding is extensively used in the fabrication of large-diameter pressure vessels, storage tanks, and heat exchanger shells per NB/T 47002, ASME VIII Div.1, and API 934. The process offers deposition rates of 5–15 kg/h, significantly higher than manual or semi-automatic methods, making it economically attractive for thick overlay requirements.
The flux must be qualified as part of the welding procedure qualification per NB/T 47014 or ASME IX. Qualification testing includes dilution analysis, mechanical property testing (hardness, tensile strength, impact toughness), and corrosion testing (intergranular corrosion per ASTM A263/A264/A265, where applicable). The flux composition and sintering parameters must be documented and controlled to ensure batch-to-batch consistency.
Common defects in ESW overlays include slag inclusion, porosity, lack of fusion at the interface, and excessive dilution. Each defect has specific causes related to flux properties: slag inclusions result from excessive slag viscosity or insufficient slag removal between passes; porosity is associated with flux moisture content; lack of fusion indicates inadequate heat input or improper flux melting behavior; and excessive dilution points to insufficient flux alloying.
Key Questions and Reflections
The development of flux for ESW cladding is an iterative process that requires close coordination between materials science and welding engineering. A key question is the optimal balance between flux basicity and alloying content. Higher basicity improves slag fluidity and inclusion removal but may reduce the effectiveness of alloying additions. Conversely, high alloying content can alter the melting behavior of the flux and affect process stability.
Another important consideration is the storage and handling of sintered flux. Flux absorbs moisture from the atmosphere, and elevated moisture content leads to hydrogen-induced porosity and cracking in the overlay. Proper flux storage (sealed containers, controlled humidity) and pre-welding baking (typically 250–400 °C for 1–2 hours) are essential quality control measures.
Study Insights and Conclusion
This work demonstrates the importance of flux development in achieving high-quality ESW overlays for austenitic stainless steel cladding applications. The systematic approach to flux composition design—balancing fluxing, alloying, and melting characteristics—provides a framework that can be adapted for other overlay applications. For engineers working on large-scale cladding projects, the key takeaway is that flux quality is a critical determinant of overlay quality, and that flux development should be considered an integral part of the welding procedure qualification process. The work reinforces the principle that in ESW cladding, the flux is not merely a passive medium but an active participant in the metallurgical transformation of the overlay layer.