CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Process Parameters on Strip Electrode Electroslag Weld Overlay
Literature Overview
This study note examines a 2002 publication by Zhang Liangcheng and Yin Heping from Shanghai Boiler Works Co., Ltd., published in the journal China Chemical Equipment. The paper investigates the influence of process parameters on strip electrode electroslag welding (ESW) overlay, a process widely used for the manufacture of large-diameter bimetallic pressure vessels, heat exchanger shells, and hydrogenation reactor liners. Strip electrode ESW overlay is particularly valued for its high deposition rate, low dilution, and excellent metallurgical quality, making it the preferred process for thick overlay layers on large components.
Principles of Strip Electrode ESW Overlay
Strip electrode ESW overlay utilizes a continuous strip of overlay material (typically stainless steel or nickel-based alloy) as the electrode, which is melted in a slag pool formed between the strip electrode and the base material surface. The process is characterized by a stable arc, high heat input, and a large molten pool, which results in low dilution of the base material into the overlay (typically 5-15% compared to 30-50% for conventional arc welding). This low dilution is critical for ensuring the corrosion and high-temperature resistance properties of the overlay layer.
Key Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Overlay Quality |
|---|---|---|
| Welding current | 400-1200 A | Higher current increases deposition rate but may increase dilution |
| Welding voltage | 35-50 V | Higher voltage increases arc length and slag pool stability |
| Travel speed | 150-400 mm/min | Higher speed reduces heat input per unit length |
| Strip electrode feed rate | 100-300 mm/min | Must be synchronized with travel speed for uniform deposition |
| Slag composition | Flux-specific | Affects slag fluidity, deoxidation, and desulfurization |
| Base material preheat | 100-250°C | Reduces residual stress and prevents cracking |
The interplay between welding current, travel speed, and strip electrode feed rate is the most critical process window to control. An imbalance between the strip electrode feed rate and the travel speed can lead to either incomplete melting of the strip electrode (resulting in cold laps and unmelted inclusions) or excessive melting (resulting in undercut and excessive dilution). The optimal balance is typically achieved when the deposition rate matches the travel speed to produce a uniform overlay layer with consistent thickness.
Process Parameter Optimization
Current and Voltage Relationship
The welding current and voltage are interdependent parameters that determine the arc power and heat input. For strip electrode ESW overlay, the typical heat input ranges from 3000 to 8000 J/mm. Higher heat inputs result in deeper penetration into the base material, which increases dilution. For overlay applications where dilution must be minimized (e.g., 316L overlay on carbon steel), the heat input should be kept at the lower end of the range.
| Heat Input (J/mm) | Dilution (%) | Penetration Depth (mm) | Deposition Rate (kg/h) |
|---|---|---|---|
| 3000 | 5-8 | 0.5-1.0 | 4-6 |
| 5000 | 8-12 | 1.0-2.0 | 7-10 |
| 7000 | 12-18 | 2.0-3.0 | 10-14 |
Travel Speed and Deposition Uniformity
Travel speed directly affects the overlay layer thickness and uniformity. A slower travel speed results in a thicker overlay layer but increases the risk of excessive dilution and distortion. A faster travel speed produces a thinner layer with lower dilution but may lead to incomplete melting of the strip electrode. The optimal travel speed must be determined through qualification welding trials that verify both the dilution ratio and the overlay layer thickness uniformity.
For multi-pass overlay, the travel speed and current should be adjusted for each pass to maintain consistent dilution. The first pass typically uses a slightly higher current to ensure adequate bonding to the base material, while subsequent passes use reduced current to minimize dilution and maintain the overlay composition.
Quality Control and Defect Prevention
Common Defects in Strip Electrode ESW Overlay
| Defect | Appearance | Root Cause | Prevention |
|---|---|---|---|
| Unmelted strip electrode | Cold lap at overlay surface | Feed rate too high relative to current | Reduce feed rate or increase current |
| Excessive dilution | Overlay hardness lower than specification | Heat input too high | Reduce current, increase travel speed |
| Slag inclusion | Non-metallic inclusions in overlay | Slag not fully removed between passes | Thorough slag removal, proper slag composition |
| Surface undercut | Groove at strip electrode edge | Travel speed too fast, current too low | Increase current, reduce travel speed |
| Cracking | Longitudinal or transverse cracks | High residual stress, hydrogen embrittlement | Increase preheat, reduce heat input |
Non-Destructive Testing Requirements
The quality of strip electrode ESW overlay is verified through a combination of non-destructive testing methods. Ultrasonic testing (UT) is the primary method for detecting internal defects such as lack of fusion, slag inclusions, and porosity. The ASME Section V Code requires UT to be performed in accordance with the applicable technical requirements. Penetrant testing (PT) is used to detect surface cracks and other surface-breaking defects. For critical applications, radiographic testing (RT) may be required to verify the soundness of the overlay layer.
The dilution ratio is verified through chemical analysis of the overlay layer, typically performed at multiple locations across the overlay surface. The dilution should be within the specified range (typically 5-15% for stainless steel overlay on carbon steel) to ensure the overlay meets the required corrosion resistance and mechanical properties.
Engineering Practice Considerations
Shanghai Boiler Works Co., Ltd. is a leading manufacturer of large-scale pressure vessels and heat exchangers in China, and their experience with strip electrode ESW overlay is directly applicable to the fabrication of hydrogenation reactors, high-pressure heat exchangers, and other critical petrochemical equipment. The process parameters developed in this study have been validated through extensive engineering practice and are applicable to overlay thicknesses ranging from 6 mm to 50 mm.
For large-diameter vessels (typically above 2000 mm), the strip electrode ESW overlay process is performed in a horizontal position with the vessel rotated to ensure uniform overlay around the circumference. The overlay is applied in multiple passes, with each pass building up 3-5 mm of overlay material. The interpass temperature must be controlled to prevent excessive softening of the base material and to minimize residual stresses.
A critical practical consideration is the management of distortion. The high heat input of ESW overlay can cause significant longitudinal and transverse distortion of the base material. This is particularly problematic for thin-walled vessels or components with complex geometries. Pre-compensation techniques, such as welding sequence optimization and the use of backing bars, are essential to control distortion within acceptable limits.
Study Insights and Implications
This 2002 publication from Shanghai Boiler Works provides valuable engineering data on strip electrode ESW overlay process parameters that remain relevant in current practice. The systematic investigation of process parameter effects on overlay quality demonstrates the importance of qualification welding trials in establishing reliable process windows. The emphasis on dilution control and slag quality management highlights the metallurgical considerations that are critical to achieving high-quality overlay layers.
The process parameters reported in this study should be used as a starting point for qualification welding trials, with adjustments made based on the specific base material, overlay material, and component geometry. The fundamental principles of process control—balancing heat input, travel speed, and electrode feed rate—remain the cornerstone of successful strip electrode ESW overlay.
Engineers working on bimetallic pressure vessel fabrication should pay close attention to the slag composition and its effect on overlay quality. The slag acts as a protective atmosphere, a heat source, and a deoxidizer, and its composition must be carefully controlled to ensure consistent overlay quality. The use of appropriate fluxes and the thorough removal of slag between passes are critical quality control measures that cannot be overlooked.