Zhuojin Pipe Fitting Co., LtdEffect of Process Parameters on Strip Electrode Electroslag Surfacing
Literature Overview
This paper by Zhang Liangcheng and Yin Heping, published in China Chemical Equipment (Volume 4, Issue 1, 2002, pp. 23–34), provides a comprehensive discussion of strip electrode electroslag welding (ESW) surfacing, comparing it with traditional strip electrode submerged arc welding (SAW) surfacing. The authors, representing Shanghai Boiler Works Co., Ltd., detail the fundamental principles of ESW surfacing, its advantages over SAW, and the systematic effects of key process parameters on weld quality.
Fundamental Principles of ESW Surfacing
Electroslag welding is a solid-state welding process that uses the heat generated by electrical resistance of a molten slag pool to melt the base metal and filler material. In the surfacing application, the process is adapted to deposit a layer of weld metal on a substrate surface rather than joining two pieces together.
Comparison with SAW Surfacing
| Parameter | Strip Electrode ESW | Strip Electrode SAW |
|---|---|---|
| Heat source | Electrical resistance of slag pool | Arc heat |
| Heat input | High (5–15 kJ/mm) | Moderate (2–8 kJ/mm) |
| Deposition rate | High (5–15 kg/h) | Moderate (3–8 kg/h) |
| Dilution rate | Lower (5–15%) | Higher (15–30%) |
| Weld metal quality | Homogeneous, fine grain | Variable, coarser grain |
| Productivity | High | Moderate |
| Equipment cost | Higher | Lower |
| Process flexibility | Lower | Higher |
| Surface quality | Smooth, uniform | May require post-machining |
The key advantage of ESW surfacing is the lower dilution rate, which results in a weld metal composition that is closer to the filler material. This is particularly important for overlay applications where the overlay composition must meet specific requirements for wear resistance, corrosion resistance, or high-temperature performance.
Process Parameters and Their Effects
Current and Voltage
The welding current and voltage are the primary parameters controlling the heat input and the geometry of the slag pool.
| Parameter | Typical Range | Effect on Weld Quality |
|---|---|---|
| Welding current | 500–1500 A | Higher current increases heat input, penetration, and deposition rate |
| Arc voltage | 35–45 V | Higher voltage increases slag pool width and reduces penetration |
The current-voltage relationship determines the thermal cycle and the solidification rate of the weld metal. Higher current and lower voltage produce a deeper, narrower weld with faster cooling, while lower current and higher voltage produce a wider, shallower weld with slower cooling.
Travel Speed
The travel speed of the welding head directly affects the heat input per unit length and the weld bead geometry.
| Travel Speed | Heat Input | Bead Geometry | Weld Metal Quality |
|---|---|---|---|
| Low (20–40 mm/min) | High | Wide, shallow bead | Coarse grain, potential for segregation |
| Medium (40–70 mm/min) | Moderate | Balanced bead | Fine grain, good mechanical properties |
| High (70–100 mm/min) | Low | Narrow, deep bead | Risk of incomplete fusion, undercut |
Strip Electrode Feed Speed
The feed speed of the strip electrode determines the deposition rate and the composition of the weld metal.
| Feed Speed | Deposition Rate | Dilution | Overlay Composition |
|---|---|---|---|
| Slow (100–150 mm/min) | Low | High | More substrate influence |
| Medium (150–250 mm/min) | Moderate | Moderate | Balanced composition |
| Fast (250–350 mm/min) | High | Low | Closer to filler composition |
Flux Composition and Bed Height
The flux composition and bed height significantly affect the slag pool chemistry and the weld metal properties.
| Flux Parameter | Effect |
|---|---|
| Flux basicity (CaO/SiO2 ratio) | Higher basicity reduces sulfur and phosphor pickup |
| Flux particle size | Uniform particle size ensures stable slag pool |
| Bed height | Optimal height (10–20 mm) ensures proper slag pool coverage |
| Flux moisture content | Must be controlled (<0.5%) to prevent porosity |
Shielding Gas
In ESW surfacing, a shielding gas (typically CO2 or Ar/CO2 mixture) is used to protect the slag pool and the solidifying weld metal from atmospheric contamination.
| Shielding Gas | Application | Effect |
|---|---|---|
| CO2 | General purpose | Low cost, moderate arc stability |
| Ar/CO2 (80/20) | High quality welds | Better arc stability, lower spatter |
| Ar | Stainless steel and alloy surfacing | Excellent protection, low oxidation |
Multi-Pass Surfacing Strategy
For thick overlay layers, multi-pass ESW surfacing is employed. The process involves:
- First pass: Establish the initial slag pool and create a sound bond with the substrate.
- Intermediate passes: Build up the overlay thickness with controlled dilution.
- Final pass: Optimize the surface composition and geometry for the intended application.
The dilution rate decreases with each subsequent pass, approaching the filler metal composition in the final pass. This is a key advantage of ESW surfacing for applications requiring a specific overlay composition.
Quality Control
The quality of ESW surfacing is verified through:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface defects, geometry | Smooth surface, no cracks or porosity |
| Ultrasonic testing (UT) | Internal defects | No indications of lack of fusion or porosity |
| Hardness testing | Overlay hardness | Within specified range |
| Chemical analysis | Overlay composition | Within specification |
| Tensile testing | Mechanical properties | Meets required strength |
Engineering Applications
ESW surfacing is particularly suitable for:
- Large-area overlay: Deposit thick layers of wear-resistant or corrosion-resistant material on large surfaces.
- Low-dilution overlay: Achieve a specific overlay composition with minimal substrate influence.
- High-productivity applications: Rapid deposition of large volumes of weld metal.
Typical applications include:
- Surfacing of boiler tubes with corrosion-resistant alloy overlays.
- Overlay of wear-resistant material on mining equipment components.
- Repair of large pressure vessel components with specific alloy overlays.
Study Insights and Implications
This paper provides a comprehensive and systematic analysis of the process parameters affecting ESW surfacing quality. The key insight is that ESW surfacing offers a significant advantage over SAW surfacing in terms of dilution control and deposition rate, making it particularly suitable for applications requiring a specific overlay composition and high productivity. For engineers involved in the surfacing of large industrial components, this work provides a detailed parameter guide that can be used to optimize the process for specific applications. The emphasis on the interplay between current, voltage, travel speed, and feed speed is critical for achieving the desired weld geometry and composition. Future developments in ESW surfacing should focus on the integration of real-time process monitoring and control systems, the development of advanced flux compositions for improved weld quality, and the extension of the process to new applications such as additive manufacturing of large-scale components.