Zhuojin Pipe Fitting Co., LtdStrip Electrode Electroslag Surfacing on Heat Exchanger Equipment Flanges
Literature Overview
The paper by Ye Yufen, published in Welding Technology (2006, Vol. 35, No. 2, pp. 73-74), describes the application of strip electrode electroslag double-layer surfacing on flanges for heat exchanger equipment. The author, from Zhejiang Zhenhai Refining and Chemical Inspection and Safety Company, presents a practical engineering solution for restoring or enhancing the surface properties of critical heat exchanger flanges. This work addresses a common maintenance challenge in the refining and chemical industries where flange surfaces are subject to severe wear and corrosion.
Technical Background
Heat exchanger flanges operate under demanding conditions involving high temperatures, corrosive process fluids, and frequent mechanical engagement during maintenance activities. The flange sealing surface must maintain dimensional accuracy and surface integrity to ensure reliable leak-tight joints. When the original surface is damaged, surfacing welding provides an economical and effective means of restoration.
Electroslag surfacing, and particularly strip electrode electroslag surfacing, offers several advantages for this application:
- Deep penetration with minimal dilution from the base metal
- High deposition rates suitable for large-area surfacing
- Excellent slag protection resulting in clean weld metal
- Good control over surfacing layer composition and thickness
- Capability for double-layer surfacing with different compositions
Process Configuration
The double-layer strip electrode electroslag surfacing process involves two distinct layers:
| Layer | Purpose | Typical Composition | Typical Thickness |
|---|---|---|---|
| First layer (transition) | Bonding and dilution control | Semi-stainless or low-alloy steel | 2-3 mm |
| Second layer (functional) | Surface performance | Stainless steel or nickel alloy | 3-5 mm |
The transition layer serves to buffer the chemical and thermal mismatch between the base metal and the functional surfacing layer. Without this intermediate layer, excessive dilution from the base metal into the functional layer would compromise its performance characteristics.
Welding Parameter Analysis
The author conducted systematic welding tests and procedure qualification to determine the optimal surfacing parameters. The following table summarizes the key parameters and their effects:
| Parameter | Range Tested | Optimal Value | Effect on Quality |
|---|---|---|---|
| Current (I) | 400-700 A | 550-650 A | Higher current increases penetration and deposition rate |
| Voltage (U) | 30-45 V | 35-40 V | Affects arc stability and slag fluidity |
| Travel speed (V) | 200-500 mm/min | 300-400 mm/min | Controls heat input and bead profile |
| Strip electrode width | 20-40 mm | 30 mm | Matches to flange surface geometry |
| Preheat temperature | 100-300 °C | 150-200 °C | Prevents cracking in high-carbon base metals |
| Interpass temperature | ≤ 250 °C | 150-200 °C | Controls microstructure and residual stress |
| Slag composition | Various formulations | Optimized for fluidity | Affects slag removal and surface quality |
Parameter Interaction Analysis
The interplay between current, voltage, and travel speed is critical to achieving consistent surfacing quality. The following relationships were identified:
- Current and penetration: Increasing current increases penetration depth and dilution rate. For the transition layer, higher current is beneficial to ensure adequate bonding with the base metal. For the functional layer, lower current is preferred to minimize dilution.
- Voltage and slag fluidity: Higher voltage increases arc length and slag temperature, improving slag fluidity but potentially causing excessive spatter and oxidation.
- Travel speed and bead profile: Higher travel speeds produce narrower, flatter beads with lower heat input. Lower speeds produce broader, more convex beads with higher heat input and greater dilution.
The optimal parameter combination must be selected based on the specific requirements of each layer. The transition layer benefits from higher heat input to ensure thorough melting and bonding with the base metal, while the functional layer requires lower heat input to preserve its alloy composition.
Quality Control and Defect Analysis
Common Defects and Countermeasures
The following table presents common defects encountered in strip electrode electroslag surfacing of flanges, along with their causes and countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Porosity | Moist slag, contaminated base metal | Dry slag, clean and prepare base metal |
| Cracking | High carbon equivalent, rapid cooling | Increase preheat, control interpass temperature |
| Undercut | Excessive current, incorrect travel speed | Reduce current, adjust travel speed |
| Poor bonding | Insufficient penetration, surface contamination | Increase current, improve surface preparation |
| Slag inclusion | Poor slag fluidity, rapid cooling | Optimize slag composition, maintain temperature |
| Excessive dilution | High current, low travel speed | Reduce current, increase travel speed |
Inspection and Verification
Quality verification of the surfacing welds involves multiple inspection methods:
- Visual inspection: Examination of bead profile, surface finish, and absence of obvious defects.
- Magnetic particle testing (MT): Detection of surface and near-surface cracks and lack of fusion.
- Ultrasonic testing (UT): Verification of bonding between surfacing layers and base metal.
- Hardness testing: Confirmation of hardness profile through the surfacing layers.
- Metallographic examination: Assessment of microstructure, dilution, and bonding quality on test coupons.
- Chemical analysis: Verification of surfacing layer composition, particularly for the functional layer.
Engineering Practice Considerations
Surface Preparation
Proper surface preparation is essential for successful electroslag surfacing of flanges. The surface must be free from rust, paint, scale, and other contaminants. Mechanical grinding or shot blasting to a white metal finish is recommended. Any existing welds or repairs on the flange surface must be ground flush to ensure uniform surfacing.
The geometric accuracy of the flange surface must also be verified before surfacing. Any significant out-of-roundness or face runout should be corrected by machining before surfacing, as the surfacing process cannot compensate for major geometric deviations.
Post-Surfacing Machining
After surfacing, the flange surface must be machined to the required dimensions and surface finish. The machining allowance should be sufficient to remove any surface irregularities and defects while maintaining adequate surfacing layer thickness. A minimum surfacing layer thickness of 1-2 mm should be maintained after machining to ensure the functional properties of the surfacing layer are preserved.
The machining operation should be performed with appropriate coolants to avoid overheating and potential cracking of the surfacing layer. Carbide or ceramic-tipped cutting tools are recommended for machining hard surfacing materials.
Study Insights and Reflections
This paper exemplifies the practical engineering approach to solving real-world maintenance challenges. The author does not merely describe the process but provides detailed parameter information and quality control procedures that can be directly applied in industrial settings.
The double-layer approach is particularly noteworthy. By using a transition layer between the base metal and the functional surfacing layer, the process achieves both strong bonding and excellent surface performance. This approach is applicable to many other surfacing applications where dissimilar materials must be joined with high reliability.
The emphasis on procedure qualification and systematic parameter optimization reflects a mature engineering mindset. Rather than relying on experience alone, the author employed controlled testing to establish the process window and identify the optimal parameter combinations. This approach provides a repeatable and reliable process that can be transferred to other similar applications.
One area that could benefit from further investigation is the long-term performance of the surfacing layers under actual service conditions. While laboratory testing provides valuable information, the true performance of the surfacing welds is ultimately determined by their behavior in service. Long-term monitoring and post-service failure analysis can provide valuable feedback for process improvement.
In conclusion, Ye Yufen's paper presents a well-developed engineering solution for the surfacing of heat exchanger flanges using strip electrode electroslag welding. The systematic approach to process development, the detailed parameter information, and the comprehensive quality control procedures make this work highly valuable for engineers working in maintenance welding applications. The double-layer surfacing strategy and the emphasis on procedure qualification represent best practices that should be adopted in similar applications across the process industries.