CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Electroslag Welding Strip Cladding Flux and Process Parameters
Literature Overview and Significance
Electroslag welding (ESW) with strip electrodes is a widely used method for depositing thick overlay layers on large structural components, including pressure vessel shells, heat exchanger tubesheets, and large-diameter pipes. The flux in ESW serves multiple critical functions: it maintains the molten slag pool, controls the welding atmosphere, modifies the weld metal chemistry through alloying, and influences the solidification microstructure. The research under review provides a comprehensive investigation of flux composition design and process parameter optimization specifically for strip cladding applications, addressing the unique requirements of overlay welding such as controlled dilution, desired overlay composition, and adequate bond strength with the base metal.
Flux Composition Design and Functionality
The flux composition in strip ESW cladding is fundamentally different from that used in structural ESW, as it must serve the dual purpose of creating a stable slag pool and delivering specific alloying elements to the overlay. The study examines four flux systems based on different fluxing agents and alloying additions.
| Flux Type | Primary Fluxing Agent | Alloying Additions | Melting Point (°C) | Application |
|---|---|---|---|---|
| Type A | CaF2 + SiO2 | Cr, Mo | 1150–1250 | Stainless steel overlay |
| Type B | CaF2 + Al2O3 | Ni, Cr | 1200–1300 | Nickel alloy overlay |
| Type C | Na2O + B2O3 | Mo, W | 1050–1150 | High-temperature alloy overlay |
| Type D | CaF2 + TiO2 | Cr, Nb | 1100–1200 | General purpose overlay |
Type A flux, based on calcium fluoride and silica with chromium and molybdenum additions, is identified as the most versatile for stainless steel overlay applications. The CaF2 provides a low melting point and good fluidity, while SiO2 acts as a deoxidizer and contributes to slag viscosity control. Chromium additions in the flux serve as a secondary alloying source, compensating for dilution from the base metal and ensuring the overlay meets the required chromium content for corrosion resistance. The study demonstrates that a flux composition of 60% CaF2, 20% SiO2, 10% Cr2O3, 5% MoO3, and 5% TiO2 produces an overlay with 18–22% Cr and 2–3% Mo when used with a 309L strip electrode on carbon steel base metal.
The slag chemistry is characterized by its basicity (CaO/SiO2 ratio) and its influence on weld metal inclusion content. A basicity of 2.5–3.5 is recommended for stainless steel overlay, as it produces a clean slag with minimal oxide inclusions while maintaining adequate fluidity. The study warns that basicity below 2.0 leads to excessive slag viscosity and poor slag removal, while basicity above 4.0 increases the risk of slag inclusions in the weld metal and promotes hot cracking.
Process Parameter Optimization
The process parameters for strip ESW cladding include welding current, voltage, travel speed, strip width, and electrode stick-out. These parameters are interdependent and must be optimized as a system to achieve the desired overlay thickness, dilution, and microstructure.
| Parameter | Typical Range | Influence on Overlay | Optimization Target |
|---|---|---|---|
| Current | 8000–12000 A | Deposition rate, dilution | Maximize rate while controlling dilution |
| Voltage | 38–45 V | Slag pool stability, strip melting | Maintain stable slag pool |
| Travel speed | 25–45 cm/min | Layer thickness, cooling rate | Achieve target thickness per pass |
| Strip width | 40–60 mm | Bead width, deposition rate | Match to component geometry |
| Stick-out | 50–80 mm | Heat input, arc stability | Minimize strip oxidation |
| Interpass temperature | 200–400 °C | HAZ properties, cracking risk | Balance weldability and productivity |
The study identifies the current-to-travel-speed ratio as the primary determinant of overlay thickness per pass. A ratio of approximately 200–250 A per cm/min produces a layer thickness of 3–5 mm, which is the optimal range for strip ESW cladding. At higher ratios, the layer becomes too thick, leading to excessive cooling rate gradients and potential cracking. At lower ratios, the layer is too thin, reducing productivity without significant quality benefit. The voltage must be maintained within a narrow window (38–42 V for standard strip widths) to ensure stable slag pool formation; deviations of more than 2 V cause slag pool instability and strip feeding irregularities.
The electrode stick-out length is a frequently overlooked parameter that significantly affects strip oxidation and weld metal composition. The study recommends a stick-out of 50–80 mm, as shorter stick-outs cause excessive arc heat concentration at the tip and increased strip oxidation, while longer stick-outs lead to poor electrical contact and unstable arc. A preheated strip with a controlled oxide layer is recommended for the first pass to ensure reliable arc initiation.
Microstructure and Dilution Control
The dilution rate in strip ESW cladding is inherently higher than in other welding methods due to the large heat input and the direct contact of the molten slag pool with the base metal. The study quantifies dilution rates of 20–35% for the first pass and 10–20% for subsequent passes, depending on the base metal composition and flux alloying content. This dilution must be compensated by adjusting the strip electrode composition or the flux alloying additions.
| Pass Number | Dilution Rate (%) | Effective Cr Content | Effective Mo Content |
|---|---|---|---|
| 1st pass | 25–35 | 12–16% | 1.0–1.8% |
| 2nd pass | 15–22 | 16–20% | 1.8–2.5% |
| 3rd pass | 10–15 | 18–22% | 2.2–3.0% |
The microstructure of the overlay evolves from a mixed ferrite-austenite structure in the first pass (due to high dilution and lower effective alloy content) to a predominantly austenitic structure in subsequent passes. This microstructural evolution is beneficial, as the final surface layers possess the best corrosion resistance. The study recommends a minimum of three passes for critical applications to ensure the top two passes achieve a fully austenitic microstructure with adequate chromium and molybdenum content.
Defect Analysis and Quality Assurance
Common defects in strip ESW cladding include slag inclusions, gas porosity, hot cracking, and insufficient bond strength. The study conducts a systematic defect analysis using metallographic examination and non-destructive testing. Slag inclusions are the most frequent defect, typically located at the interface between the overlay and the base metal or between successive passes. They are caused by insufficient slag fluidity, excessive interpass temperature, or inadequate slag removal between passes. The recommended countermeasure is to maintain interpass temperature below 300 °C and to ensure complete slag removal by chipping and grinding between passes.
Hot cracking is associated with high sulfur and phosphorus content in the base metal or consumables and is particularly prevalent in the first pass where the dilution rate is highest. The study recommends using consumables with sulfur content below 0.015% and phosphorus content below 0.025%, and preheating the base metal to 200–300 °C to reduce the cooling rate and allow sulfur to diffuse away from grain boundaries. Bond strength testing reveals that the first pass interface is the weakest link, with shear bond strength typically 80–90 MPa compared to 120–150 MPa for subsequent interfaces. This is attributed to the higher dilution and the formation of a thin, brittle intermetallic layer at the interface. The study recommends a minimum first pass thickness of 3 mm to provide adequate material for grinding away any brittle interfacial layer during post-weld finishing.
Engineering Practice Implications
The research provides a robust framework for strip ESW cladding process development that can be directly applied to pressure vessel fabrication. The emphasis on flux composition as a primary lever for controlling overlay chemistry is particularly valuable, as it allows process engineers to achieve the desired overlay composition even with varying base metal compositions. The systematic approach to dilution compensation through multi-pass design and flux alloying additions provides a practical methodology for process qualification. The defect analysis and countermeasures are directly applicable to quality control procedures in fabrication shops, and the recommended NDT acceptance criteria align with NB/T 47014 and ASME IX requirements. The overall contribution of this research is a comprehensive process guide that enables reliable, high-quality strip ESW cladding in industrial production.