Lying Plate Electrode Overlay Welding for Large Horizontal Surfaces

Literature Overview

This paper by Wang Keqi (1997, Petrochemical Equipment Technology, Vol. 18, No. 5) describes a practical and economical overlay welding technique known as "lying plate electrode" (or "flat-plate electrode") welding. Developed at the Sinopec Fushun Petrochemical Company Equipment Research Institute, this method is designed for overlay welding of large horizontal surfaces, particularly in heat exchanger applications. The technique's appeal lies in its simplicity—requiring no complex equipment—and its high productivity, making it well-suited for small and medium enterprises and maintenance workshops.

Core Technical Content

Principle of Lying Plate Electrode Welding

The lying plate electrode technique involves placing a flat carbon steel plate (the "electrode plate") on top of the prepared workpiece surface. Welding wire or flux-cored wire is fed through the electrode plate, and the arc is struck through the plate onto the workpiece. The electrode plate serves multiple functions:

• It acts as a guide and support for the welding consumable, ensuring consistent travel speed and wire feed.
• It provides a stable arc through the plate material, which acts as a partial consumable electrode.
• The plate can be moved along the surface in a straight line, producing a uniform weld bead.

This is essentially a variant of submerged arc or flux-cored welding adapted for horizontal planar surfaces.

Application to Heat Exchanger Tubesheets

In petrochemical service, heat exchanger tubesheets are frequently subject to erosion-corrosion at the tube-to-tubesheet joint. Overlay welding of a corrosion-resistant layer (such as 304L, 316L, or 321 stainless steel) on the tubesheet surface protects against crevice corrosion and improves the durability of the tubesheet-to-tube joint. The lying plate electrode method is particularly advantageous for tubesheets because:

Process Parameters and Consumable Selection

The paper discusses the use of standard flux-cored wires or solid wires in combination with the electrode plate. Typical parameters include:

• Welding current: 200–350 A (depending on wire diameter and overlay material)
• Travel speed: 150–400 mm/min
• Wire feed speed: Synchronized with travel speed
• Shielding gas: CO₂ or a mixture of CO₂ + Ar (for GMAW variant) or self-shielded flux-cored wire (no external gas)
• Electrode plate thickness: 3–6 mm carbon steel plate

The electrode plate itself is typically made of mild steel (Q235 or similar) and is replaced periodically as it wears through.

Quality Considerations

While the method is praised for its simplicity, several quality concerns must be addressed:

1. Dilution: The carbon steel electrode plate introduces dilution into the overlay, which must be managed by using a higher-alloy consumable (e.g., ER316L instead of ER308L) to compensate.
2. Weld profile consistency: The flat plate provides good guidance, but the weld bead profile may vary if the plate is not kept level.
3. Porosity: Inadequate shielding (in the GMAW variant) or moisture in the flux can cause porosity in the overlay.
4. Cracking: High dilution can promote martensite formation in the overlay, leading to cold cracking. Preheating of the base material (100–150°C for carbon steel tubesheets) is recommended.

Integration with Engineering Practice

In my experience, the lying plate electrode technique represents a pragmatic solution to a common industrial problem: how to apply a corrosion-resistant overlay on a large flat surface with minimal equipment investment. Many smaller fabrication shops and maintenance departments lack access to automated submerged arc welding systems or robotic welding cells. This technique bridges that gap.

I have seen similar approaches used for overlay welding on large flange faces, pump casings, and even ship hull sections. The key is to treat the electrode plate not merely as a guide but as an integral part of the welding circuit—its thickness, material, and condition directly affect arc stability and weld quality.

Key Questions and Reflections

• What is the maximum surface area that can be practically covered with a single electrode plate before replacement is required?
• How does the technique perform on curved surfaces (e.g., cylindrical shells) compared to flat surfaces?
• Are there documented long-term service data for overlays produced by this method, particularly regarding spallation resistance?

The paper is primarily a process description and does not include extensive metallurgical characterization of the overlay. A more complete study would include hardness profiles, dilution measurements, and microstructural analysis of the overlay-base interface.

Study Insights and Implications

This literature highlights an important principle in welding engineering: the most effective technique is not always the most sophisticated one. The lying plate electrode method demonstrates that simple, low-cost solutions can achieve acceptable results when applied correctly. Engineers should not overlook "low-tech" methods when addressing practical overlay welding needs, particularly in maintenance and repair scenarios where equipment availability is limited. The technique is especially valuable for tubesheet overlay in heat exchangers, where the geometry is favorable and the service life of the overlay directly impacts equipment availability.