CLADDING TECHNOLOGY SHANXI CO., LTDInfluence of Process Parameters on Strip Electrode Electroslag Welding Overlay
Literature Overview
This study note investigates the influence of process parameters on strip electrode electroslag welding (ESW) overlay, a process widely used for producing thick overlay layers on large structural components such as pressure vessel heads, heat exchanger tubesheets, and heavy machinery bases. Strip electrode ESW overlay offers deposition rates of 10–20 kg/h, significantly exceeding those of conventional arc welding processes, making it the process of choice for overlay thicknesses exceeding 10 mm. The literature under review systematically examines the effects of welding current, voltage, travel speed, strip width, and slag composition on overlay quality, including dilution rate, layer uniformity, defect formation, and mechanical properties. The findings provide a comprehensive process window for engineers to optimize ESW overlay for specific applications.
Fundamental Principles of Strip Electrode ESW Overlay
Strip electrode ESW overlay operates on the principle of electroslag welding, where the heat source is the resistance heating of a molten slag pool rather than the electric arc itself. The strip electrode melts from its trailing edge into the slag pool, and the molten metal solidifies beneath the slag, forming a dense, uniform weld deposit. The process is inherently stable and produces minimal spatter, making it suitable for automated production. The key process parameters and their effects are summarized in the following table:
| Parameter | Typical Range | Primary Effect |
|---|---|---|
| Welding current (A) | 400–800 | Controls heat input and penetration |
| Voltage (V) | 35–50 | Controls slag pool depth and fluidity |
| Travel speed (mm/min) | 200–600 | Controls deposit thickness and dilution |
| Strip width (mm) | 12–30 | Controls deposit width and bead profile |
| Strip thickness (mm) | 2.0–5.0 | Controls deposit thickness per pass |
| Slag composition | CaO-SiO₂-MnO-Al₂O₃ | Controls slag viscosity and fluidity |
| Slag preheat (°C) | 800–1200 | Controls slag melting and stability |
The interaction between these parameters is complex and non-linear. For example, increasing current raises the temperature of the slag pool, which increases slag fluidity and can lead to excessive penetration and dilution if not compensated by increasing travel speed. Similarly, increasing strip width without adjusting current can lead to incomplete melting of the strip edges, resulting in lack of fusion defects.
Effect of Current and Voltage on Overlay Quality
Welding current is the primary parameter controlling heat input in ESW overlay. The literature demonstrates that current has a direct influence on the following quality indicators:
- Dilution rate: Increasing current from 400 A to 700 A increases dilution from approximately 15% to 35%, as the deeper penetration melts more base material into the overlay.
- Layer hardness: Higher dilution introduces more base material carbon and alloying elements into the overlay, which can increase hardness but also increase cracking susceptibility.
- Defect formation: Excessive current (> 750 A for typical strip widths) can cause slag inclusions, as the increased heat input destabilizes the slag pool and entrains unmelted slag particles.
Voltage controls the depth and fluidity of the slag pool. A voltage that is too low results in a shallow, viscous slag pool with poor wetting of the strip electrode, leading to incomplete melting and cold lap defects. A voltage that is too high creates a deep, fluid slag pool that increases dilution and can cause undercut at the overlay edge. The literature identifies an optimal voltage range of 38–45 V for most strip electrode ESW overlay applications, corresponding to a slag pool depth of 8–15 mm.
The following table presents the optimal parameter combinations identified in the literature for different overlay thickness requirements:
| Target Overlay Thickness (mm) | Current (A) | Voltage (V) | Speed (mm/min) | Strip Width (mm) | Passes |
|---|---|---|---|---|---|
| 5–8 | 450–550 | 38–42 | 350–450 | 15–20 | 2–3 |
| 8–15 | 550–650 | 40–45 | 300–400 | 20–25 | 3–5 |
| 15–25 | 600–750 | 42–48 | 250–350 | 25–30 | 5–8 |
Effect of Travel Speed and Strip Dimensions
Travel speed is the primary parameter controlling deposit thickness per pass and the linear heat input. The relationship between travel speed and deposit thickness is inversely proportional: slower speeds produce thicker deposits but increase heat input and dilution. The literature recommends a deposit thickness per pass of 2.0–3.5 mm for optimal quality, corresponding to travel speeds of 250–450 mm/min for typical strip widths.
Strip width and thickness also significantly influence overlay quality. A wider strip produces a wider deposit but requires higher current to ensure complete melting. A thicker strip increases deposit thickness per pass but also increases the risk of incomplete melting at the strip core. The literature recommends matching strip dimensions to the target deposit thickness, with a general guideline of strip thickness being approximately 70–80% of the target deposit thickness per pass.
Slag Composition and Its Influence
The slag composition is a critical but often overlooked parameter in ESW overlay. The slag must possess sufficient fluidity to ensure complete wetting of the strip electrode and the weld pool, while maintaining enough viscosity to prevent excessive metal entrainment. The literature identifies the following slag composition ranges as optimal:
| Slag Component | Optimal Range (wt%) | Function |
|---|---|---|
| CaO | 35–45 | Increases basicity, stabilizes slag |
| SiO₂ | 15–25 | Controls viscosity |
| MnO | 10–20 | Deoxidizes, controls fluidity |
| Al₂O₃ | 5–10 | Increases viscosity, prevents slag splatter |
| Na₂O / K₂O | 3–8 | Fluxing agent, reduces melting temperature |
| CaF₂ | 5–10 | Increases fluidity, reduces surface tension |
A slag with excessive basicity (high CaO/SiO₂ ratio) becomes too viscous, leading to poor wetting and slag inclusions. A slag with insufficient basicity becomes too fluid, leading to excessive dilution and slag entrainment. The literature recommends a CaO/SiO₂ ratio of 1.5–2.5 for most overlay applications, with adjustments based on the specific alloy system.
Defect Analysis and Process Optimization
The literature identifies four primary defect types in strip electrode ESW overlay and their relationship to process parameters:
| Defect | Primary Cause | Optimal Parameter Range |
|---|---|---|
| Slag inclusions | Low voltage, high viscosity slag | Voltage 38–45 V, CaO/SiO₂ = 1.5–2.5 |
| Lack of fusion | Low current, excessive speed | Current > 500 A, speed < 450 mm/min |
| Excessive dilution | High current, low speed | Current < 650 A, speed > 300 mm/min |
| Undercut | Excessive voltage, wide strip | Voltage < 45 V, strip width < 25 mm |
A systematic approach to process optimization involves conducting a parameter sweep, varying one parameter at a time while holding others constant, and evaluating the resulting overlay quality through metallographic examination, hardness profiling, and NDT. The literature recommends using a Taguchi L9 orthogonal array for initial parameter screening, followed by focused optimization of the most influential parameters.
Study Insights and Engineering Implications
The study of process parameters in strip electrode ESW overlay reveals that this process, while highly productive, demands precise parameter control to achieve acceptable quality. The interdependence of current, voltage, travel speed, and slag composition creates a multi-dimensional process space where small deviations can lead to significant quality variations. Engineers should adopt a data-driven approach to process development, using statistical experimental design methods to identify optimal parameter combinations and establish robust process windows. The literature also emphasizes the importance of slag composition control, as the slag is the medium through which all process parameters are translated into weld quality. A well-designed slag composition can compensate for minor parameter fluctuations, providing process robustness that is critical for production environments. Furthermore, the literature advocates for real-time process monitoring, including current and voltage feedback control, slag temperature measurement, and travel speed verification, to ensure consistent quality throughout production runs. In conclusion, strip electrode ESW overlay is a powerful tool for thick overlay production, but its effectiveness depends entirely on a deep understanding of the parameter interactions that govern process stability, metallurgical quality, and defect prevention.