Stripped-Electrode Electroslag Weld Overlay on Heat Exchanger Flanges
1. Definition and Technical Principles
Stripped-electrode electroslag weld overlay (带极电渣堆焊) is a specialized solid-state joining and surface engineering process applied to the sealing faces and raised faces of heat exchanger equipment flanges. Unlike conventional arc welding overlay methods such as TIG or MIG, this technique exploits the unique thermal characteristics of electroslag welding (ESW) to deposit one or more layers of corrosion-resistant or wear-resistant alloy onto a base carbon steel or low-alloy steel flange substrate.
The process operates on the principle of electroslag welding with a continuously fed pigtail (stripped) electrode. A molten slag pool is maintained at the weld interface, which serves as the heat source and flux medium. As the stripped electrode is fed downward at a controlled rate, the metal arc burns through the slag pool, melting the electrode tip and the base metal simultaneously. The resulting weld pool is confined by a water-cooled copper mold, which ensures rapid solidification, controlled dilution, and consistent geometry of the overlay layer. The slag pool provides excellent deoxidation, inclusion removal, and thermal regulation, resulting in overlay deposits with low hydrogen content, minimal porosity, and superior metallurgical integrity compared to arc-welded overlays on the same base material.
The key distinguishing feature of this process for flange applications is the use of a pigtail electrode (a continuously fed, stripped rod electrode rather than a consumable stick or wire electrode). This electrode geometry enables uninterrupted deposition, high deposition rates (typically 8–25 kg/h), and uniform layer thickness across the large flat surfaces of flange sealing faces. The process is performed in a vertical or near-vertical orientation, with the flange positioned on a rotating fixture or the electrode assembly traversing the flange face.
2. Category and Business Positioning
Within the cladding and weld overlay industry, stripped-electrode electroslag weld overlay occupies a distinct niche that bridges conventional welding overlay and advanced cladding technologies. It is categorized as follows:
- Process Category: Weld overlay / surfacing (distinct from explosion welding or hydraulic explosive bonding)
- Equipment Category: Heat exchanger flanges, shell-and-tube exchanger flanges, heat exchanger cover flanges, and associated piping flanges
- Industry Vertical: Petrochemical, refining, natural gas processing, power generation, and chemical process industries
- Competitive Positioning: This technique is positioned as a cost-effective, high-deposition-rate alternative to multi-pass TIG or MIG overlay for thick overlay requirements on large flange diameters, while offering superior metallurgical quality compared to manual surfacing methods
For Cladding Technology Shanxi Co., Ltd., this capability represents a specialized qualification in the weld overlay route (as opposed to hydraulic explosive bonding or explosion welding routes). It addresses a specific market segment where heat exchanger manufacturers and EPC contractors require reliable, code-compliant overlay solutions for flanges operating in corrosive or erosive service conditions. The technique is particularly valued for its ability to produce thick, homogeneous overlay layers (typically 3–8 mm per pass) with controlled dilution of base metal into the overlay, which is critical for maintaining corrosion resistance in the final service environment.
3. Technical Purpose and Value
The primary technical purposes of applying stripped-electrode electroslag weld overlay to heat exchanger flanges are:
- Corrosion Resistance Enhancement: Deposit austenitic stainless steel (e.g., 304, 304L, 316, 316L), duplex stainless steel (e.g., 2205), or nickel-based alloys (e.g., Hastelloy, Inconel) onto carbon steel or low-alloy steel flanges exposed to corrosive process media such as sour gas, hydrogen sulfide, chlorides, and high-temperature oxidizing environments.
- Wear Resistance Improvement: Apply overlay layers resistant to erosion-corrosion in high-velocity fluid applications or abrasive media service.
- Material Cost Optimization: Avoid the use of expensive fully alloyed flanges by overlaying only the sealing face and raised face areas of carbon steel flanges, reducing material costs by 40–70% while maintaining equivalent service performance.
- Repair and Retrofit: Restore worn or corroded flange faces on existing heat exchangers during turnaround or maintenance activities, extending equipment life without full replacement.
- Intermetallic Compatibility: Provide a transition layer between dissimilar base and overlay materials to mitigate cracking risks during welding, cooling, and service.
The business value of this capability is significant. Heat exchanger flanges are high-volume components in petrochemical and power generation projects, and the overlay requirement is often specified in project engineering documents. Possessing qualified capability in electroslag weld overlay for flanges enables the company to participate in competitive bids for large-scale EPC projects, offers a differentiated service compared to competitors limited to TIG/MIG overlay, and provides customers with a technically superior solution for thick overlay applications where arc welding would require excessive passes and carry higher risk of defects.
4. Key Process and Implementation Points
4.1 Pre-Weld Preparation
Proper pre-weld preparation is critical to achieving defect-free overlay deposits. The following steps are mandatory:
- Base Material Inspection: Verify the flange material grade (e.g., ASTM A105, ASTM A182 F316, 16Mn, 20# steel) through material certification and visual/NDT inspection. Confirm hardness, microstructure, and absence of cracks, inclusions, or segregation.
- Surface Cleaning: Remove all mill scale, rust, oil, paint, and contaminants from the overlay area using grinding, wire brushing, or solvent cleaning. The surface must be clean and free of oxide layers to ensure proper slag wetting and fusion.
- Edge Preparation: Prepare the overlay area geometry, including chamfering of flange edges and machining of the sealing face to the required flatness and roughness (typically Ra 6.3–12.5 μm before overlay).
- Heat Treatment (if required): For low-alloy steel flanges with carbon equivalent (CE) above 0.45% (e.g., 15CrMo, 12Cr1MoV), preheat to 150–300°C to reduce cracking susceptibility. Post-weld heat treatment may be required per the applicable code.
4.2 Process Parameters
The following table summarizes typical process parameters for stripped-electrode electroslag weld overlay on heat exchanger flanges:
| Parameter | Typical Range | Notes |
|---|---|---|
| Electrode Type | Pigtail (stripped) electrode, Φ8–Φ12 mm | Material matched to overlay alloy (e.g., 309, 316L, 2205) |
| Electrode Feed Rate | 2.5–6.0 m/h | Adjusted for layer thickness and electrode diameter |
| Traverse Speed | 150–400 mm/min | Depends on flange diameter and overlay width |
| Electrode Current | 350–650 A | DC or AC, depending on electrode material |
| Electrode Voltage | 28–42 V | Slag pool voltage |
| Slag Flux | ES-12, ES-14, or equivalent | Flux composition matched to electrode and base material |
| Slag Pool Depth | 10–20 mm | Critical for heat input control and dilution |
| Overlay Layer Thickness | 3–8 mm per pass | Multiple passes for thicker requirements |
| Preheat Temperature | 100–300°C (base material dependent) | Higher for high CE materials |
| Interpass Temperature | ≤300°C (stainless overlay), ≤400°C (carbon steel) | Controlled to prevent sensitization and cracking |
4.3 Overlay Sequence and Layer Design
The overlay layer design is a critical engineering decision that directly impacts the metallurgical quality and service performance of the flange. A typical layer design for a carbon steel flange with a 316L overlay requirement consists of three layers:
- Transition Layer (Layer 1): Deposit an austenitic stainless steel with higher nickel content (e.g., 309 or 309L) to reduce dilution from the carbon steel base and prevent cracking. This layer acts as a metallurgical buffer between the ferritic base and the austenitic overlay.
- Transition Layer (Layer 2): Deposit a second transition layer of 309L or 316L with reduced nickel content, further reducing the carbon and iron content in the weld metal.
- Final Overlay Layer (Layer 3): Deposit the final corrosion-resistant layer of 316L, 2205, or the specified service alloy. This layer must meet the required chemical composition and mechanical properties for the intended service environment.
The dilution rate from the base metal into each overlay layer must be calculated and controlled. For the transition layers, dilution of up to 25–30% is typically acceptable, while the final overlay layer must have dilution below 10% to ensure the required corrosion resistance. The electroslag welding process offers superior dilution control compared to TIG or MIG due to the slag pool's ability to regulate heat input and metal transfer.
4.4 Post-Weld Operations
- Cooling Control: Allow the flange to cool slowly to room temperature, or apply post-weld heat treatment if required by the applicable code (e.g., PWHT per NB/T 47015 or ASME Section IX).
- Machining: Machine the overlay surface to the required flatness (typically ≤0.05 mm/TIR for raised face flanges) and surface finish. Use carbide or ceramic tooling to avoid embedding tool material in the overlay.
- Post-Weld Heat Treatment: For stainless steel overlays on low-alloy steel base, PWHT may be required to relieve residual stresses. The temperature and duration must be specified per the applicable code and material specification.
- Final Inspection: Perform visual inspection (VT), magnetic particle inspection (MT) or liquid penetrant inspection (PT) of the overlay surface, and dimensional verification of the machined face.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Qualification (WPQ)
The welding procedure must be qualified per the following standards, depending on the project specification and jurisdiction:
- NB/T 47014-2011 — Qualification of welding procedure for pressure vessels and pressure components (Chinese national standard for pressure equipment)
- ASME Section IX — Qualification rules for welding, brazing, and bonding procedures (for ASME code-stamped equipment)
- GB/T 19866-2005 — Welding procedure qualification test method (general welding)
- GB/T 24231-2009 — Electroslag welding of carbon steel and low alloy steel (process-specific standard)
The WPQ must demonstrate that the specified process parameters, electrode materials, flux compositions, preheat temperatures, interpass temperatures, and post-weld treatments produce weld metal meeting the required mechanical and metallurgical properties. Test specimens must be subjected to tensile testing, bend testing (face, root, and side bends per ASME Section IX or NB/T 47014), and hardness testing. For corrosion-resistant overlays, additional corrosion testing (e.g., pitting resistance per ASTM G48, intergranular corrosion per ASTM A262) may be required.
5.2 Welder Qualification
Welders operating the stripped-electrode electroslag weld overlay process must be qualified per:
- NB/T 47015-2011 — Qualification of welders for pressure vessels and pressure components
- ASME Section IX — Qualification of welders (for ASME code equipment)
- TSG Z6002-2010 — Supervision regulations for safety technology of special equipment (welder qualification)
Welder qualification requires the successful completion of a qualification test using the qualified welding procedure, with acceptance criteria including visual inspection, NDT (typically MT or PT), and mechanical testing of test coupons.
5.3 Acceptance Criteria for Finished Overlay
| Acceptance Item | Criteria | Reference Standard |
|---|---|---|
| Visual Inspection (VT) | No cracks, undercut, porosity, or surface defects | NB/T 47013, ASME V |
| Magnetic Particle Inspection (MT) | No linear indications ≥1.5 mm; no clusters of indications | NB/T 47013, ASME V Article 7 |
| Penetrant Inspection (PT) | No surface-breaking cracks or linear defects | ASME V Article 6, GB/T 18851 |
| Hardness | Overlay hardness within ±50 HV of specified value | Per material specification |
| Chemical Composition | Overlay layer composition within specified limits (e.g., Cr ≥16%, Ni ≥10% for 316L) | ASTM A240, GB/T 4237 |
| Flatness | Raised face flatness ≤0.05 mm/TIR (per class) | ASME B16.5, GB/T 9119 |
| Overlay Thickness | Minimum thickness per design specification (typically ≥3 mm for corrosion service) | Per project specification |
| Intergranular Corrosion (if required) | No intergranular corrosion per ASTM A262 Practice E or Practice A | ASTM A262, NACE TM0172 |
| Pitting Corrosion (if required) | Pitting resistance equivalent number (PREN) ≥19 for 316L, ≥24 for 2205 | ASTM G48, NACE TM0169 |
5.4 Applicable Flange Standards
- ASME B16.5 — Pipe flanges and flanged fittings (metallic)
- ASME B16.47 — Large-diameter steel flanges
- GB/T 9119 — Steel flanges (Chinese national standard)
- GB/T 150 — Pressure vessels (flange design and selection)
- NB/T 47011 — Pressure vessel flanges (Chinese standard)
- API 605 — Flanges for heat exchangers and similar equipment
6. Common Risks and Controls
6.1 Cracking Risks
Cracking is the primary metallurgical risk in electroslag weld overlay of dissimilar materials. The following controls are implemented:
- Hydrogen-Induced Cracking (HIC): Control by using low-hydrogen flux (ES-12, ES-14), ensuring proper flux drying (250°C for 2 hours minimum), and controlling preheat and interpass temperatures. Hydrogen content in weld metal must be below 5 mL/100g for carbon steel base materials.
- Hot Cracking: Mitigated by selecting appropriate electrode composition (higher Mn and S content for austenitic electrodes), controlling sulfur and phosphorus content in base material, and maintaining proper slag composition to promote fluidity and inclusion removal.
- Cold Cracking: Controlled by preheating the base material (150–300°C for low-alloy steels), limiting carbon equivalent (CE ≤ 0.45% preferred), and using post-weld heat treatment when required.
6.2 Dilution and Composition Control
Excessive dilution of base metal into the overlay layer can degrade corrosion resistance. Controls include:
- Using a multi-layer overlay design with transition layers to progressively reduce dilution
- Optimizing slag pool depth and electrode feed rate to control heat input
- Performing chemical analysis of the overlay layer after each pass to verify composition
- Limiting the number of passes and interpass temperature to prevent excessive base metal melting
6.3 Slag Inclusion Defects
Electroslag welding is susceptible to slag inclusions if the slag pool is not properly maintained. Controls include:
- Maintaining proper slag pool depth (10–20 mm) throughout the welding operation
- Using high-quality, properly dried flux with consistent composition
- Controlling traverse speed and electrode feed rate to prevent slag entrapment
- Performing 100% MT or PT inspection of the overlay surface and cross-section testing for slag inclusion verification
6.4 Distortion and Residual Stress
Thermal distortion of the flange during overlay can affect dimensional accuracy and sealing performance. Controls include:
- Using a clamping fixture to restrain the flange during welding
- Employing symmetric welding sequences to balance thermal input
- Applying controlled preheat and post-weld heat treatment to relieve residual stresses
- Performing dimensional verification after machining and, if required, after PWHT
6.5 Sensitization of Stainless Steel Overlay
For austenitic stainless steel overlays (304, 316, etc.), sensitization (chromium carbide precipitation at grain boundaries) can occur if the interpass temperature exceeds 425°C. Controls include:
- Limiting interpass temperature to ≤300°C for stainless steel overlays
- Using low-carbon electrode materials (304L, 316L, 309L) where possible
- Performing solution heat treatment (1050–1100°C followed by rapid quench) if sensitization is detected, though this is rarely practical for large flanges
- Verifying intergranular corrosion resistance per ASTM A262 if sensitization is a concern
7. Application Scenarios Across Company Technology Routes
Cladding Technology Shanxi Co., Ltd. operates three primary technology routes: TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. The stripped-electrode electroslag weld overlay capability fits within the TIG/MIG weld overlay route but occupies a distinct process niche. The following analysis positions this capability within the company's overall technology portfolio:
7.1 Relationship to TIG/MIG Weld Overlay Route
While TIG and MIG overlay are the company's primary weld overlay processes, they have limitations for thick overlay applications on large flange surfaces:
- TIG Overlay: Excellent for thin, high-quality overlay layers (1–3 mm) with precise dilution control. However, deposition rate is low (0.5–2 kg/h), making it impractical for thick overlays on large flange diameters. TIG is preferred for small flanges, repair applications, and high-purity overlay requirements.
- MIG Overlay: Higher deposition rate than TIG (2–5 kg/h), suitable for medium-thickness overlays. However, dilution control is less precise than TIG, and the process is more susceptible to porosity and spatter. MIG is preferred for general-purpose overlays where deposition rate is a priority.
- Stripped-Electrode Electroslag Overlay: Highest deposition rate (8–25 kg/h), suitable for thick overlays (3–8 mm per pass) on large flange surfaces. Superior dilution control compared to MIG, and better metallurgical quality than manual surfacing. This process fills the gap between TIG/MIG overlay and explosion welding for thick overlay requirements on flat surfaces.
The three processes are complementary: TIG for thin, high-quality overlays; MIG for medium-thickness general-purpose overlays; and electroslag for thick, high-volume overlays on large flange surfaces. The company can select the optimal process based on the specific requirements of each project.
7.2 Relationship to Hydraulic Explosive Bonding and Explosion Welding Routes
Hydraulic explosive bonding and explosion welding are solid-state bonding processes that produce metallurgical bonds without melting. They are ideal for producing clad plates and pipes with precise thickness ratios and no dilution. However, they are not suitable for flange overlay applications for the following reasons:
- Geometry Limitation: Explosive bonding processes are designed for flat plates and cylindrical tubes, not for the complex geometry of flange sealing faces and raised faces.
- Thickness Limitation: Explosive bonding produces clad layers with typical thickness ratios of 10–50% of the total thickness, which may not meet the thick overlay requirements for flange sealing faces.
- Process Complexity: Explosive bonding requires specialized facilities, safety zones, and regulatory approvals that are not practical for flange overlay operations.
Therefore, stripped-electrode electroslag weld overlay serves as the primary process for flange overlay applications within the company's portfolio, while hydraulic explosive bonding and explosion welding are reserved for clad plate and pipe fabrication. The company's three technology routes are complementary, each addressing different product geometries and application requirements.
7.3 Cross-Route Integration Opportunities
There are opportunities to integrate the electroslag weld overlay capability with the company's other technology routes:
- Clad Plate + Electroslag Flange Overlay: Produce clad plates via explosion welding for heat exchanger tubesheets, then apply electroslag overlay to the flanges of the same heat exchanger. This provides a unified solution for the entire heat exchanger, with consistent metallurgical quality and code compliance.
- Hydraulic Explosive Bonding + Electroslag Overlay: For large-diameter heat exchanger flanges, use hydraulic explosive bonding to produce the base clad plate, then apply electroslag overlay to the sealing face. This combines the advantages of solid-state bonding (no dilution) with the geometric flexibility of welding overlay.
- Multi-Process Qualification Packages: Develop integrated WPQ packages that cover TIG, MIG, and electroslag overlay for the same material combinations, enabling the company to offer customers a comprehensive weld overlay solution with process flexibility.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The stripped-electrode electroslag weld overlay capability contributes to the company's qualification portfolio in the following ways:
- Welding Procedure Qualifications (WPQ): Each material combination (base material × overlay material) requires a separate WPQ per NB/T 47014 or ASME Section IX. Building a library of qualified procedures for common combinations (e.g., 16Mn base with 309/316L overlay, 15CrMo base with 309/316L overlay) enables rapid response to customer specifications.
- Welder Qualifications: Maintaining a pool of qualified welders for electroslag overlay ensures the company can staff projects with the required personnel. Welder qualifications are typically valid for 6–12 months, requiring periodic requalification.
- Equipment Qualification: The electroslag welding equipment (power source, electrode feed mechanism, traverse system, slag flux handling) must be calibrated and qualified per the applicable code. This ensures consistent process performance across multiple projects.
- NDT Qualification: The company must maintain qualified NDT personnel for MT and PT inspection of overlay welds, per NB/T 47013 or ASME V.
8.2 Product Delivery
The electroslag weld overlay capability directly supports product delivery in the following ways:
- Increased Production Capacity: The high deposition rate of electroslag welding (8–25 kg/h) enables the company to process large volumes of flanges in a short time, supporting tight project schedules.
- Reduced Rework Rates: The superior metallurgical quality of electroslag overlay (low hydrogen, low porosity, low slag inclusions) reduces the need for rework, improving on-time delivery rates.
- Consistent Quality: The automated nature of the process (controlled electrode feed, traverse speed, and slag pool depth) produces consistent overlay quality across multiple flanges, reducing quality variability.
- Large Flange Capability: The process is well-suited for large-diameter flanges (up to DN2000 or larger), which are challenging for TIG or MIG overlay due to the large surface area and thick overlay requirements.
8.3 Customer Value
The stripped-electrode electroslag weld overlay capability delivers the following value to customers:
- Cost Savings: Overlaying carbon steel flanges with corrosion-resistant alloys reduces material costs by 40–70% compared to fully alloyed flanges, while providing equivalent service performance.
- Extended Equipment Life: High-quality overlay layers provide long-term corrosion and wear resistance, extending the service life of heat exchangers and reducing unplanned shutdowns.
- Code Compliance: Qualified procedures, welders, and NDT ensure that all overlay work meets applicable codes (NB/T 47014, ASME Section IX, TSG 21), providing customers with confidence in the integrity of the equipment.
- Technical Expertise: The company's expertise in electroslag weld overlay provides customers with technical support for overlay design, process selection, and quality verification, reducing the customer's technical risk.
- Integrated Solutions: The company's ability to offer TIG/MIG overlay, hydraulic explosive bonding, and explosion welding from a single source simplifies the customer's procurement and qualification process, reducing project complexity.
9. Conclusion
Stripped-electrode electroslag weld overlay on heat exchanger flanges is a specialized, high-value capability within the company's weld overlay portfolio. It addresses a specific market need for thick, high-quality overlay layers on large flange surfaces, filling the gap between TIG/MIG overlay and explosion welding. The process offers superior deposition rates, excellent metallurgical quality, and precise dilution control, making it the optimal choice for thick overlay applications on heat exchanger flanges in the petrochemical, refining, and power generation industries.
By investing in this capability—through WPQ development, welder qualification, equipment maintenance, and NDT capability—the company positions itself as a comprehensive cladding and weld overlay solutions provider, capable of addressing the full spectrum of surface engineering requirements from thin TIG overlays to thick electroslag overlays to solid-state bonded clad plates and pipes. This integrated approach provides customers with a single-source solution for their surface engineering needs, reducing project complexity and technical risk while delivering cost-effective, code-compliant, and high-quality products.