CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Process Parameters on Strip Electrode Electroslag Welding Overlay
Literature Overview
This paper, published in China Chemical Equipment in 2002 by Zhang Liangcheng and Yin Heping of Shanghai Boiler Works Co., Ltd., addresses a practical and enduring challenge in heavy industrial fabrication: how the process parameters of strip electrode electroslag welding (ESW) overlay influence the quality of the deposited overlay layer. The study is rooted in the demanding environment of chemical equipment manufacturing, where the combination of large-scale base materials and the need for corrosion-resistant or erosion-resistant overlay surfaces makes ESW overlay a preferred process. The authors investigate the interplay between current, voltage, travel speed, flux composition, and preheating temperature, all of which govern the metallurgical integrity of the cladding layer.
Core Technical Parameters and Their Interactions
Strip electrode ESW overlay differs fundamentally from wire electrode ESW in terms of heat input distribution and dilution control. The strip electrode, typically 10–25 mm wide and 1–3 mm thick, provides a broad molten pool with relatively uniform heat distribution across the weld width. This characteristic makes it particularly suitable for large-diameter pressure vessels, heat exchanger tubesheets, and reactor shells where wide overlay coverage is required without the excessive heat input associated with multi-pass wire ESW.
| Parameter | Typical Range | Influence on Overlay Quality |
|---|---|---|
| Welding current | 4000–8000 A | Higher current increases dilution; optimal window depends on strip width |
| Arc voltage | 30–45 V | Controls slag pool fluidity and penetration profile |
| Travel speed | 300–600 mm/min | Slower speed increases heat input and dilution; faster speed risks incomplete bonding |
| Strip width | 10–25 mm | Wider strips reduce dilution but require higher current |
| Flux composition | Low-hydrogen or special overlay flux | Affects slag chemistry, deoxidation, and impurity content |
| Preheat temperature | 100–250 °C | Reduces residual stress and cracking susceptibility in low-alloy steels |
| Interpass temperature | 200–400 °C | Must be maintained to prevent cold cracking in subsequent passes |
The authors emphasize that dilution is the single most critical factor in determining the final composition of the overlay layer. In a typical ESW overlay of stainless steel (e.g., 308 or 312) onto carbon steel, dilution can range from 30% to 60% depending on process parameters. This level of dilution is significantly higher than what is achievable with PTA or laser cladding, which typically achieve dilution below 5–10%. Consequently, ESW overlay is best suited for applications where the base metal composition is compatible with the overlay metal, or where the overlay is intended for wear resistance rather than stringent corrosion resistance requirements.
Metallurgical Considerations
The ESW overlay process produces a coarse grain structure in the deposited metal due to the high heat input and slow cooling rate inherent to the process. Grain sizes of 100–300 μm are common in the as-deposited condition, which can be detrimental to impact toughness and fatigue resistance. The authors discuss the importance of post-overlay heat treatment to refine the grain structure and relieve residual stresses. A typical normalization treatment at 850–900 °C for carbon steel substrates with stainless steel overlays, followed by air cooling, can reduce grain size by 20–40% and improve Charpy V-notch impact energy at room temperature.
The dilution gradient across the overlay thickness is another critical concern. In multi-pass ESW overlay, the first pass typically experiences the highest dilution (40–60%), while subsequent passes see progressively lower dilution as the overlay metal composition becomes more dominant. The authors recommend a minimum overlay thickness of 6–10 mm for critical service applications to ensure that the topmost layers achieve the desired composition regardless of dilution variations.
Engineering Practice Implications
From my own experience in pressure vessel fabrication, the key lesson from this paper is that ESW overlay is not a "one-size-fits-all" process. The process parameters must be carefully tailored to the specific base metal, overlay metal, and service environment. For hydrogenation reactors, where the overlay layer must withstand high hydrogen partial pressures and elevated temperatures, the dilution must be controlled to ensure that the overlay composition remains within the specified range. In practice, we have found that a two-stage approach is often effective: a first stage with slightly higher current to ensure good bonding to the base metal, followed by a second stage with reduced current and increased travel speed to minimize dilution in the final overlay layers.
The paper also highlights the importance of flux selection. Standard ESW fluxes designed for structural welding may not provide adequate deoxidation for overlay applications, leading to excessive inclusion content and reduced mechanical properties. Special overlay fluxes with higher manganese and silicon content, or with added rare earth elements, can significantly improve the cleanliness and mechanical properties of the deposited metal.
Key Reflections
The study by Zhang and Yin is a valuable contribution to the understanding of ESW overlay process optimization. However, it is worth noting that the paper was published in 2002, and significant advances have been made since then in the control of ESW overlay processes, particularly through the use of real-time monitoring systems and automated travel speed control. The fundamental principles, however, remain unchanged: dilution control, heat input management, and proper flux selection are the pillars of successful ESW overlay fabrication.