Zhuojin Pipe Fitting Co., LtdFlat Plate Electrode Overlay Welding for Large Area Horizontal Surfaces
Literature Overview
This paper by Wang Keqi, published in Petrochemical Equipment Technology (1997, Vol. 18, No. 5, pp. 57-59), describes a specialized overlay welding technique using a flat plate electrode (躺板极, literally "lying plate electrode"). The author is affiliated with the Equipment Research Institute of China Petrochemical Fushun Petrochemical Company, an organization with deep expertise in heat exchanger manufacturing and maintenance. The technique is particularly suited for large-area horizontal surface overlay welding, such as the tube sheets and channel covers of heat exchangers.
Core Technical Content
The flat plate electrode overlay welding technique represents a practical innovation for applying corrosion-resistant overlay layers on large flat or slightly curved horizontal surfaces. Unlike conventional overlay methods that use wire or strip electrodes fed through a torch, this method uses a pre-formed flat plate of the overlay material as the electrode itself. The plate is laid on the workpiece surface and melted in situ using an arc striking technique, creating a uniform overlay layer.
Principle of Operation
The technique works as follows:
- A flat plate of the overlay material (e.g., austenitic stainless steel, nickel-based alloy, or duplex steel) is prepared with dimensions matching the desired overlay area.
- The plate is placed on the cleaned and preheated base surface.
- An electric arc is struck between the plate and the base metal using a consumable electrode or a non-consumable tungsten electrode.
- The arc is moved across the plate surface, melting it progressively from one edge to the other.
- The melted plate material flows and wets the base surface, forming a metallurgical bond.
- After solidification, the overlay layer is inspected and machined as needed.
Advantages Over Conventional Methods
| Feature | Flat Plate Electrode | Conventional Wire/Strip Overlay |
|---|---|---|
| Equipment complexity | Low - standard SMAW or GTAW setup | Medium to high - requires feed mechanism |
| Operator skill requirement | Low - straightforward arc striking | Medium - requires feed rate control |
| Production rate | High - large area coverage per pass | Lower - limited by wire feed speed |
| Cost | Low - plate material is economical | Higher - wire/strip is more expensive |
| Applicable surface | Large flat or slightly curved horizontal | Any orientation with proper equipment |
| Uniformity | Good for flat surfaces | Excellent with automated systems |
| Dilution control | Moderate - plate thickness provides buffer | Good - thin layers allow better control |
Application to Heat Exchanger Manufacturing
The primary application described is the overlay welding of heat exchanger tube sheets and channel covers, where the tube-side fluid may be corrosive while the shell-side fluid is benign. The flat plate electrode method allows rapid application of a uniform corrosion-resistant layer on the large flat surface of the tube sheet, covering the tube hole areas and the surrounding plate.
Typical Process Parameters
| Parameter | Value | Notes |
|---|---|---|
| Base material | Carbon steel or low-alloy steel | e.g., SA-266 Gr. B/C, 16MnR |
| Overlay plate material | 304L, 316L, 321, or Alloy 6 | Selected based on corrosion environment |
| Plate thickness | 3-6 mm | Thicker plates provide more dilution buffer |
| Arc voltage | 18-25 V | Depends on electrode type used |
| Current | 200-350 A | SMAW with E309L or similar |
| Travel speed | 150-300 mm/min | Slower for thicker plates |
| Preheat | 100-150 °C | Prevents hydrogen cracking in base metal |
| Interpass temperature | < 200 °C | Controls dilution |
Quality Control Considerations
The flat plate electrode method, while simple, presents specific quality challenges:
- Porosity: Incomplete melting of the plate material or contamination from the plate surface can cause porosity. Surface preparation of the plate (grinding or cleaning) is essential.
- Lack of fusion: Inadequate arc energy or excessive travel speed can result in incomplete bonding between the overlay and base metal. Visual inspection and MT (magnetic particle testing) are used to detect lack of fusion.
- Cracking: High dilution or improper cooling can cause hot cracking or cold cracking. The plate thickness acts as a buffer, but the dilution ratio must still be monitored.
- Uneven thickness: Manual arc movement may produce uneven overlay thickness. Post-weld machining is typically required to achieve uniform thickness.
Engineering Practice Reflections
This technique exemplifies the engineering principle of achieving practical solutions with minimal complexity. In the context of 1990s Chinese petrochemical equipment manufacturing, where access to sophisticated automated overlay welding equipment was limited, the flat plate electrode method provided an accessible and effective solution for corrosion protection of large flat surfaces.
From a metallurgical standpoint, the technique benefits from the self-buffering nature of the plate material. Unlike thin wire overlay, where the first pass is heavily diluted by the base metal, the plate provides sufficient material volume that even with moderate dilution, the resulting overlay composition retains adequate corrosion resistance. This makes the technique more forgiving of parameter variations, which is particularly advantageous in field repair situations.
The technique has found continued relevance in maintenance and repair operations where large flat surfaces need overlay protection but where bringing in sophisticated automated equipment is impractical. The simplicity of the method also makes it suitable for training operators and for use in remote or resource-limited locations.