CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of Sintered Flux for Strip Electrode Electroslag Welding Overlay of Austenitic Stainless Steel
Literature Overview
Strip electrode electroslag welding (ESW) is a highly productive process for depositing thick cladding layers on large structural components, particularly for austenitic stainless steel overlays on carbon steel substrates in pressure vessel and heat exchanger fabrication. The performance of strip ESW is critically dependent on the flux formulation, which governs slag viscosity, electrical resistivity, chemical reactivity, and inclusion behavior. This study focuses on the development of a sintered flux specifically designed for strip ESW overlay of austenitic stainless steel (304L and 316L) on carbon steel substrates. The study addresses the flux chemistry, sintering process, slag properties, and the resulting clad layer quality, providing a comprehensive framework for flux development and qualification.
Core Technical Content
Flux Chemistry and Design Principles
The sintered flux is designed based on the following principles: (1) the slag must have sufficient viscosity to retain the molten pool and prevent metal splash; (2) the slag must have appropriate electrical resistivity to maintain a stable arc; (3) the slag must be chemically compatible with the austenitic stainless steel to avoid detrimental reactions such as excessive oxidation or chromium depletion; (4) the slag must effectively deoxidize the molten pool and absorb inclusions.
The flux composition is based on a calcium fluorite (CaF₂) and calcium silicate (CaSiO₃) system with additions of aluminum oxide (Al₂O₃), magnesium oxide (MgO), and iron oxide (FeO). The following table presents the chemical composition of the developed flux:
| Component | Content (wt%) | Function |
|---|---|---|
| CaF₂ | 35-40 | Slag viscosity control, arc stability |
| CaSiO₃ | 25-30 | Slag fluidity, deoxidation |
| Al₂O₃ | 10-15 | Slag viscosity increase, inclusion absorption |
| MgO | 8-12 | Slag basicity control, refractory protection |
| FeO | 5-8 | Slag reducibility, heat input control |
| MnO | 3-5 | Deoxidation, manganese pickup |
| SiO₂ | 5-7 | Slag fluidity adjustment |
The flux is manufactured by a sintering process in which the raw materials are blended, pressed into pellets, and sintered at 1150-1250°C for 2-4 hours. The sintering temperature and time are critical process parameters that affect the flux density, grain size, and chemical homogeneity. The target flux density is 1.8-2.0 g/cm³, which ensures good slag coverage and minimal flux consumption.
Slag Properties and Their Effect on Weld Quality
The slag properties are characterized by measuring the slag viscosity, electrical resistivity, and surface tension at temperatures ranging from 1200°C to 1600°C. The viscosity is measured using a rotational viscometer, the electrical resistivity is measured using a four-probe method, and the surface tension is measured using a pendant drop method.
The following table presents the measured slag properties and their effect on weld quality:
| Slag Property | Measured Value | Optimal Range | Effect on Weld Quality |
|---|---|---|---|
| Viscosity at 1400°C (Pa·s) | 0.85 | 0.5-1.5 | Controls metal pool retention and splash |
| Electrical Resistivity at 1400°C (Ω·m) | 0.62 | 0.4-0.8 | Controls arc stability and heat input |
| Surface Tension at 1400°C (N/m) | 0.38 | 0.3-0.45 | Controls slag-metal interface stability |
| Basicity (CaO/SiO₂) | 2.1 | 1.8-2.5 | Controls deoxidation and inclusion behavior |
The measured slag properties fall within the optimal ranges, indicating that the flux formulation is well-suited for strip ESW overlay of austenitic stainless steel. The basicity of 2.1 ensures effective deoxidation and manganese pickup, while the viscosity of 0.85 Pa·s provides good metal pool retention without causing excessive slag entrapment.
Clad Layer Quality Assessment
The clad layers produced using the developed flux were evaluated for chemical composition, microstructure, mechanical properties, and bond strength. The following table summarizes the results:
| Property | 304L Clad Layer | 316L Clad Layer | Requirement |
|---|---|---|---|
| Cr (wt%) | 18.5-19.2 | 17.8-18.5 | ≥17.0 |
| Ni (wt%) | 8.2-8.8 | 11.5-12.2 | ≥8.0 (304L), ≥10.5 (316L) |
| Mo (wt%) | <0.3 | 1.8-2.2 | ≥1.5 (316L) |
| C (wt%) | 0.025-0.035 | 0.020-0.030 | ≤0.030 (L grades) |
| Hardness (HV) | 180-210 | 190-220 | ≤250 |
| Tensile Strength (MPa) | 520-580 | 540-600 | ≥485 |
| Bond Strength (MPa) | 480-520 | 500-550 | ≥0.8 × base metal TS |
The clad layers meet all specified requirements, with the bond strength exceeding the minimum requirement by more than 20%. The low carbon content in the L-grade clad layers confirms the effectiveness of the flux in controlling carbon pickup during the ESW process.
Interpretation of Technical Points
The study demonstrates that the flux formulation is the most critical factor in the success of strip ESW overlay. A poorly formulated flux can lead to excessive carbon pickup, chromium depletion, slag inclusion, or arc instability, all of which compromise the clad layer quality. The developed flux addresses these issues through a carefully balanced composition that provides the required slag properties while maintaining chemical compatibility with the austenitic stainless steel.
The sintering process is also a critical factor in flux quality. The sintering temperature and time must be optimized to achieve the target flux density and chemical homogeneity without causing excessive grain growth or phase separation. The study found that sintering at 1200°C for 3 hours produced the optimal flux properties, with a density of 1.92 g/cm³ and a grain size of 15-25 μm. Sintering at lower temperatures resulted in insufficient densification, while sintering at higher temperatures caused grain coarsening and phase separation.
Integration with Engineering Practice
The developed flux was applied in the fabrication of a large hydrogenation reactor shell (diameter 4.5 m, wall thickness 120 mm) with a 316L austenitic stainless steel overlay of 15 mm thickness. The overlay was deposited in 6 passes using a 6 mm wide strip electrode and the developed flux. The total welding time was approximately 48 hours, and the clad layer thickness was uniform within ±0.5 mm across the entire shell circumference. The clad layer was verified by chemical analysis, hardness testing, and ultrasonic thickness measurement, and all results met the specified requirements.
A practical challenge encountered during the fabrication was the control of slag entrapment at the pass boundaries. The high productivity of strip ESW means that the slag is continuously replaced, but at pass boundaries, there is a risk of slag carry-over from the previous pass into the next pass. This was mitigated by using a slag removal device between passes and by adjusting the flux composition to produce a slag with lower surface tension, which promotes easier slag removal. The slag removal device was a simple mechanical scraper mounted on the welding carriage, which was adjusted to remove the solidified slag immediately after each pass.
Key Questions and Reflections
The study raises the question of the long-term stability of the flux properties. Sintered fluxes can degrade over time due to moisture absorption, which affects the slag properties and weld quality. The study recommends storing the flux in a sealed container at a temperature above 100°C and limiting the storage time to 6 months. However, in practice, fluxes may be stored for longer periods, and the effect of aging on flux properties should be systematically studied. Additionally, the study does not address the effect of flux composition on the environmental impact of the welding process. The use of CaF₂ in the flux can release fluorine-containing gases during welding, which are harmful to the welder's health and the environment. Future flux development should consider reducing the CaF₂ content or replacing it with alternative flux components that provide similar slag properties with lower environmental impact.
Study Insights and Implications
The development of a sintered flux for strip ESW overlay of austenitic stainless steel is a critical enabler of high-productivity cladding for large structural components. The study provides a systematic approach to flux development, from chemistry design to sintering process optimization to clad layer quality verification. The key implication for engineers is that flux development should be treated as a materials engineering problem, not merely as a consumable selection exercise. The flux formulation must be tailored to the specific application, considering the base metal, the overlay material, the welding parameters, and the quality requirements. Engineers should invest in flux development and qualification, and they should establish flux property monitoring procedures to ensure consistent weld quality throughout the production lifecycle.