Lying Plate Electrode Weld Overlay Technique

Literature Overview

The lying plate electrode (躺板极) weld overlay technique is a specialized method for applying overlay layers to large, flat, or gently curved surfaces, particularly in the fabrication of pressure vessels, heat exchangers, and structural components. This technique uses a flat plate electrode that is placed on the workpiece and moved along the welding direction, providing a continuous and uniform deposition of overlay material. This study note examines the principles, advantages, limitations, and applications of the lying plate electrode technique, drawing from both theoretical understanding and practical experience.

Principle of Lying Plate Electrode Welding

The lying plate electrode technique operates on the principle of electroslag welding (ESW) or submerged arc welding (SAW) with a modified electrode configuration. Instead of a conventional wire electrode, a flat plate of the overlay material is used as the electrode. The plate is fed into the welding zone at a controlled rate, and the arc is struck between the plate and the workpiece. The molten metal from the plate and the workpiece pool together, and the overlay is deposited as the plate advances.

Key Components

Component	Function	Specification
Plate electrode	Source of overlay material	5–20 mm thick, 50–200 mm wide
Electrode holder	Supports and feeds the plate	Adjustable angle, cooling system
Flux cover	Shields the weld, controls chemistry	Rutile or basic flux
Power source	Provides welding current	DC or AC, 200–800 A
Travel mechanism	Controls plate movement	Mechanical or hydraulic

Advantages of the Technique

The lying plate electrode technique offers several advantages over conventional wire electrode methods:

High deposition rate: The plate provides a large volume of molten metal per unit time, enabling rapid build-up of thick overlay layers
Uniform composition: The plate material has a consistent composition, reducing dilution variation
Low dilution: The large volume of overlay metal reduces the proportion of base metal in the weld, typically achieving dilution rates of 5–15%
Good penetration: The electroslag or submerged arc process provides deep penetration into the base material, ensuring good fusion
Cost-effective: The plate material is less expensive per unit volume than wire or powder, and the high deposition rate reduces labor costs

Applications and Limitations

Suitable Applications

Application	Overlay Thickness	Base Material	Overlay Material
Pressure vessel heads	5–15 mm	Carbon steel	Stainless steel
Heat exchanger tubesheets	3–10 mm	Low-alloy steel	Nickel alloy
Structural beams	2–8 mm	Carbon steel	Wear-resistant alloy
Ship hull plates	3–10 mm	Weathering steel	Corrosion-resistant alloy
Storage tank bottoms	2–5 mm	Carbon steel	Stainless steel

Limitations

Geometry restriction: The technique is limited to flat or gently curved surfaces with a radius of curvature greater than 500 mm
Equipment requirements: Specialized electrode holders and travel mechanisms are required
Minimum thickness: The overlay thickness must be at least 3 mm for the technique to be effective
Surface preparation: The base surface must be flat and clean, requiring machining or grinding
Inspection access: The flux cover must be removed for inspection, which can be labor-intensive

Process Parameters

The welding parameters for the lying plate electrode technique are optimized based on the plate thickness, base material, and required overlay properties:

Parameter	Range	Effect
Current	300–700 A	Higher current increases deposition rate and penetration
Voltage	25–35 V	Higher voltage increases arc length and dilution
Travel speed	50–150 mm/min	Higher speed reduces deposition rate and penetration
Plate thickness	5–20 mm	Thicker plate allows higher current and speed
Plate angle	0–15°	Angle affects molten pool shape and dilution
Flux type	Rutile, basic	Basic flux provides lower hydrogen and better mechanical properties

Parameter Optimization

The optimization of welding parameters involves balancing deposition rate, dilution, and mechanical properties. For a typical application with 10 mm thick 309L plate on 12Cr1MoV base:

Initial trial: Current 500 A, voltage 30 V, speed 100 mm/min, plate angle 5°
Dilution measurement: Metallographic analysis of cross-section
Adjustment: If dilution > 20%, increase current or decrease speed
Property verification: Hardness, tensile strength, and corrosion testing

Quality Control

The quality of the overlay is verified through a combination of non-destructive and destructive testing:

Visual inspection: After flux removal, checking for surface defects, porosity, and undercut
Ultrasonic testing: For subsurface defects and bond strength verification
Magnetic particle testing: For surface cracks in the overlay and heat-affected zone
Hardness testing: Grid pattern testing to verify uniform hardness across the overlay
Metallographic examination: Cross-section to measure dilution and examine microstructure
Tensile testing: Transverse tensile tests to verify bond strength (minimum 350 MPa for most applications)

Engineering Practice Case

A case study involved the overlay of a 12Cr1MoV pressure vessel head with 8 mm of 309L stainless steel using the lying plate electrode technique. The vessel head had a diameter of 3000 mm and a thickness of 60 mm. The process involved:

Machining of the head surface to remove 3 mm and achieve Ra 12.5 μm
Preheat to 200 °C using electric blankets
Lying plate electrode welding with 10 mm 309L plate, current 550 A, voltage 32 V, speed 120 mm/min
Two passes to achieve 8 mm total thickness
Post-weld heat treatment at 640 °C for 4 hours
Full UT and MT inspection

The resulting overlay had a dilution of 10%, hardness of 200–220 HV, and excellent bond strength (420 MPa). The vessel passed all inspection requirements and was commissioned successfully.

Study Insights

The lying plate electrode technique is a valuable tool in the cladding engineer's arsenal, particularly for large, flat surfaces where high deposition rates and low dilution are required. The technique is most effective when the overlay thickness is 5 mm or greater, as the high deposition rate provides significant time savings. However, engineers must be aware of the geometry limitations and the specialized equipment requirements. The key to successful application is careful parameter optimization and thorough quality control, ensuring that the dilution is controlled and the bond strength is adequate. The literature reinforces the importance of process qualification and the need for detailed documentation of welding parameters and inspection results.