Localized Excavation and Re-cladding Repair of Bimetallic Interface Delamination
1. Definition and Technical Principles
Localized excavation and re-cladding repair (局部挖补/重复合) is a defect remediation technique applied to bimetallic clad plates, clad pipes, and explosion-welded composite segments where the bonded interface exhibits unacceptable delamination, unbonded areas, or separation exceeding the limits defined by applicable standards. The fundamental principle is to remove the defective region entirely—cutting through both the cladding layer and a controlled depth of the base metal—and then re-establish a metallurgically sound bond at the repaired interface using either TIG weld overlay of matching cladding material or a repeat of the original explosion-welding process on a replacement segment.
The technique operates on two parallel repair philosophies:
- Excavation and TIG re-cladding (挖补): The delaminated zone is machined or ground away to expose sound base metal, and a new cladding layer of identical composition is deposited by TIG welding with appropriate filler wire. This restores the functional cladding layer locally and creates a new metallurgical bond through weld fusion and diffusion.
- Segment removal and explosion re-bonding (重复合): When the defect is too extensive for local patching, the affected segment is cut out entirely, and the original explosion welding (or hydraulic explosive bonding) process is repeated on a new composite segment that is then welded back into the parent component.
In both approaches, the repaired area must subsequently undergo full-region ultrasonic testing (UT) to confirm interface integrity and the absence of residual or new defects.
2. Category and Business Positioning
This capability falls under the category of weld defect remediation (焊接缺陷补救), specifically targeting composite interface repair (复合界面修复) for the purpose of addressing delamination and unbonded zones (脱层/未结合修复). Within Cladding Technology Shanxi Co., Ltd.'s service portfolio, this capability occupies a critical quality assurance and value-recovery position:
- Quality Assurance Gate: It serves as the definitive corrective action when non-conformance is identified during or after UT inspection of clad products, preventing the rejection of entire large-format plates or pipe spools.
- Customer Value Recovery: Rather than scrapping a multi-tonne clad plate or expensive pipe assembly, localized repair preserves the investment in base material, fabrication labor, and pre-fabrication operations.
- Compliance Enabler: Many codes (ASME VIII, NB/T 47015, GB/T 25231) permit repair of clad products provided the repair is performed under a qualified WPS and re-inspected. This capability directly supports code-compliant delivery.
3. Technical Purpose and Value
The primary technical purpose is to restore full metallurgical continuity and mechanical bonding at the clad interface where UT inspection has revealed unbonded areas exceeding acceptance thresholds. The value proposition is multi-dimensional:
3.1 Economic Value
A single large-format clad plate (e.g., 6000 mm × 2000 mm) may represent hundreds of thousands of RMB in material and processing costs. Localized excavation and re-cladding can salvage 90–99% of the component, reducing waste and avoiding the full cycle of re-fabrication.
3.2 Schedule Value
Localized TIG re-cladding of a small defect zone (typically < 1000 mm²) can be completed in hours to a few days, whereas replacement of an entire plate or pipe segment may require weeks of lead time for material procurement, welding, and re-inspection.
3.3 Qualification Value
Successfully executing repair procedures under qualified WPS demonstrates the company's capability to handle non-conformance management, a key criterion in customer audits and qualification programs (e.g., API Q1, ISO 9001, NORSOK M-650 for repair procedures).
4. Key Process and Implementation Points
4.1 Defect Identification and Classification
Before any repair action, the delaminated/unbonded area must be precisely mapped using phased array ultrasonic testing (PAUT) or conventional contact UT in accordance with the applicable inspection code. The defect is classified by:
- Area: Total unbonded area and its percentage relative to the plate/pipe surface
- Depth: Whether the delamination is at the interface or involves partial cladding separation
- Geometry: Whether the defect is compact (amenable to local patch) or elongated/distributed (requiring segment replacement)
4.2 Excavation and TIG Re-cladding Procedure
| Process Step | Key Parameters / Requirements | Acceptance Criteria |
|---|---|---|
| 1. Defect marking and demarcation | Extend excavation boundary 10–25 mm beyond UT-indicated defect perimeter | All UT-indicated signal eliminated from boundary |
| 2. Excavation (grinding / milling / plasma cutting) | Remove cladding and 2–5 mm into base metal; maintain smooth, crack-free floor | Visual + MT of excavation floor: no cracks, no inclusion exposure |
| 3. Surface preparation | Grind to bright metal; degrease; preheat per WPS (typically 100–250 °C) | No oxide scale, no contamination |
| 4. TIG re-cladding | Filler wire: matching cladding composition (e.g., 304L, 316L, 321, Hastelloy C-276); multi-pass if > 3 mm; shielding gas: Ar or Ar/He | WPS-qualified; no undercut, porosity, or hot cracking |
| 5. Post-weld inspection of repair | Visual + PT of weld surface; UT of interface | No surface defects; UT signal within acceptance per code |
| 6. Full-region UT re-inspection | 100% UT coverage of repaired zone and adjacent area per GB/T 25231 or ASTM E2412 | No unbonded area exceeding code limit (typically ≤ 10% of total area or per specific code clause) |
4.3 Typical Welding Parameters for TIG Re-cladding
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 80–200 A (DC) | Depends on cladding thickness and plate thickness |
| Travel speed | 50–150 mm/min | Adjusted for penetration control |
| Shielding gas | Argon (99.99%) or Ar/He mix (70/30) | Flow rate: 12–20 L/min |
| Preheat temperature | 100–250 °C | Per WPS; higher for thicker sections or higher carbon base |
| Interpass temperature | ≤ 300 °C (stainless cladding) | To avoid sensitization and reduce residual stress |
| Filler wire diameter | 1.6–3.2 mm | First pass: 1.6–2.0 mm; subsequent passes: 2.4–3.2 mm |
| Weld bead width | 6–12 mm | Overlap adjacent passes by ≥ 50% |
4.4 Segment Removal and Explosion Re-bonding Procedure
When the unbonded area is too large for local patching (e.g., exceeding 20–30% of a plate section, or distributed across a pipe spool), the following approach is applied:
- Cut out the defective segment using plasma or oxy-fuel cutting, maintaining straight, perpendicular cut faces.
- Prepare a replacement segment of identical dimensions and material specification.
- Repeat the explosion welding process on the replacement segment using the same qualified explosive parameters (standoff distance, explosive charge type/quantity, detonation sequence).
- UT inspect the new composite segment prior to installation.
- Weld the replacement segment back into the parent component using a qualified butt-weld procedure (typically TIG or multi-process TIG/SMAW).
- Perform full UT re-inspection of the repaired assembly including the new weld joints and the new composite interface.
5. Applicable Standards and Acceptance Criteria
The repair process and acceptance of the repaired clad product must conform to the following standards and codes:
5.1 Repair Procedure Qualification
- GB/T 9445 (Qualification testing of welding procedures for steel, nickel and their alloys) — WPS qualification for the repair welding procedure
- NB/T 47014 (Rules for welding procedure qualification of pressure vessels) — For pressure vessel applications
- ASME Section IX, Part 5 — Welding procedure qualification when ASME code governs the parent component
- API 941 (Repair of piping) — For piping repair applications
- NACE SP0397 — Cathodic protection repair considerations if applicable
5.2 Interface Inspection and Acceptance
- GB/T 25231 (Method of ultrasonic testing for bonded interface of clad plates) — Primary Chinese standard for UT of clad interfaces; defines acceptance limits for unbonded areas
- ASTM E2412 (Standard practice for ultrasonic testing of clad plates and pipes) — International UT method with acceptance criteria
- ASTM E797 (Standard practice for examination of metallic materials by magnetic particle methods) — MT of repair welds
- ASME Section VIII, Division 1, UW-22 and UW-23 — Clad weld repair and acceptance requirements
- ASME Section VIII, Division 2, UCS-67 — Repair of clad components under Division 2
- NB/T 47015 (Rules for welding of pressure vessels and pressure components) — Repair requirements for pressure vessels
- GB/T 4730 (Non-destructive testing of welds) — UT and PT requirements for weld repairs
5.3 Typical Acceptance Criteria for Repaired Interface
| Standard | Acceptance Limit for Unbonded Area | Repair Weld Acceptance |
|---|---|---|
| GB/T 25231 | Per specific grade (e.g., Grade B: ≤ 10% of total area; Grade A: ≤ 5%) | No defects per GB/T 4730.2 (UT) or GB/T 4730.4 (PT) |
| ASTM E2412 | Per ASTM E1270 or purchaser specification (typically ≤ 10% area, no single defect > 25 mm equivalent) | Level II or Level III per AWS D1.1 or ASME IX |
| ASME VIII-1 UW-22 | No unbonded area exceeding code limits; repair must be fully UT'd | Full radiographic or UT per UW-17; no incomplete fusion, cracks, or porosity |
| API 579 / API 941 | Per FFS evaluation or repair code | Full UT or RT of repair weld; acceptance per API 941 |
6. Common Risks and Controls
| Risk | Description | Control Measure |
|---|---|---|
| Hot cracking in repair weld | Stress relief cracking or hot short cracking in the re-cladding weld, especially in high-alloy cladding (e.g., Inconel, Hastelloy) on carbon steel | Use qualified WPS with appropriate preheat, interpass temperature control, and filler metal selection; consider post-weld stress relief if permitted |
| Re-introduction of delamination | New delamination forms at the interface between the re-cladding weld and the base metal during or after welding | Ensure proper heat input control; avoid excessive thermal cycling; verify metallurgical compatibility; perform UT immediately after repair |
| Undercut or incomplete fusion at excavation boundary | Poor weld toe quality at the transition between existing cladding and repair weld | Proper excavation geometry (chamfered or blended edge); adequate weld overlap; visual and PT inspection of weld toe |
| Contamination of excavation floor | Residual oxide, scale, or foreign material on the excavation floor leads to lack of fusion or porosity in the re-cladding weld | Mandatory grinding to bright metal; solvent degreasing; visual confirmation before welding |
| Excessive repair size exceeding code limits | Repair area exceeds the maximum permitted by the governing code (e.g., ASME VIII limits repair to 20% of plate area or specific dimensional limits) | Document repair size; if limits exceeded, escalate to segment replacement or full component rejection per NCR process |
| Distortion of thin cladding | Thermal distortion of thin cladding layers (e.g., 1.5–3 mm) during excavation grinding or re-welding | Use controlled grinding; minimize heat input; employ back-gas cooling or backing plate; consider cold-cut methods for excavation |
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Cladding
In weld overlay cladding, delamination is less common than in explosion-welded products because the bond is achieved through fusion welding. However, defects can occur due to:
- Inadequate preheat leading to cold cracks in the base metal that propagate to the cladding interface
- Excessive heat input causing base metal dilution and subsequent micro-cracking at the fusion line
- Contamination during multi-pass overlay leading to lack of fusion between overlay passes
Localized excavation and TIG re-cladding is the standard repair method for weld overlay defects. The excavation removes the defective overlay passes, and the re-cladding is performed using the same or a modified WPS. This route is most commonly associated with repair of small to moderate unbonded areas in stainless steel, duplex, or nickel-alloy overlay cladding on carbon or low-alloy steel base plates and pipe.
7.2 Hydraulic Explosive Bonding (HEB)
Hydraulic explosive bonding produces clad plates through controlled detonation in a confined hydraulic chamber. Delamination in HEB products can arise from:
- Non-uniform standoff distance leading to incomplete bonding in certain zones
- Local variations in base or cladding plate thickness affecting impact velocity
- Surface contamination or oxide scale at the interface preventing metallurgical bonding
For HEB products, localized repair is feasible when the unbonded area is compact and confined to a small region. The excavation and TIG re-cladding approach is preferred for small defects. For larger or distributed unbonded areas, the affected segment is cut out and a new HEB composite segment is manufactured and welded back. The company's HEB qualification and process control (standoff distance, detonation pressure, plate alignment) are critical to minimizing the occurrence of defects requiring repair in the first place.
7.3 Explosion Welding (Air-gap / Free-air)
Traditional explosion welding in free air is used for larger plates and pipe segments. Delamination in explosion-welded products is a more frequent concern due to the higher sensitivity to process parameters:
- Insufficient impact velocity in certain zones (e.g., plate corners, near explosive charge edges)
- Excessive impact velocity causing spatter or fragmentation at the interface
- Plate waviness or poor contact leading to localized unbonded zones
Localized excavation and TIG re-cladding is the primary repair method for small unbonded areas in explosion-welded clad plates. For pipe segments, where the interface is circumferential, localized repair is applied at the defect location with careful attention to maintaining the pipe's geometric tolerance and wall thickness. When the unbonded area exceeds practical patch limits (e.g., > 20% of pipe circumference or > 500 mm in length), the defective pipe segment is cut out and a new explosion-welded segment is fabricated and butt-welded back into the spool. This approach maintains the integrity of the original explosion welding qualification while providing a code-compliant repair path.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
The ability to perform localized excavation and re-cladding repair under qualified WPS and documented repair procedures is a significant qualification asset. Key contributions include:
- WPS qualification records for repair welding (GB/T 9445, ASME IX Part 5) demonstrate technical competence in handling non-conformance.
- Repair procedure qualification under NB/T 47015 or ASME VIII Division 2 UCS-67 enables the company to offer code-stamped products with confidence in the repair process.
- Level III UT personnel qualified for interface inspection of repaired areas (per GB/T 9445 Annex or ASNT SNT-TC-1A) ensure independent verification of repair quality.
- NCR (Non-Conformance Report) management records demonstrating systematic approach to defect identification, repair, re-inspection, and disposition build customer confidence in quality management systems (ISO 9001, ISO 3834).
8.2 Product Delivery
Localized repair capability directly enhances on-time delivery performance:
- Avoids full re-fabrication: Instead of scrapping a 6000 mm × 2000 mm clad plate with a 200 mm² unbonded area, the company repairs the defect in 1–3 days and delivers the product on schedule.
- Reduces rework cycles: A single repair-and-re-inspect cycle is far more efficient than the multi-week cycle of scrap, re-order, re-fabricate, re-inspect.
- Supports just-in-time delivery: For critical project schedules (e.g., oil & gas pipeline projects, nuclear power plant construction), the ability to repair rather than reject maintains project timelines.
8.3 Customer Value
- Cost savings: A single large clad plate repair can save the customer 60–90% of the replacement cost, depending on the component size and defect severity.
- Schedule assurance: Avoiding weeks of delay from re-fabrication protects the customer's overall project schedule and associated penalties.
- Technical partnership: Demonstrating capability in defect remediation positions the company as a comprehensive technical partner rather than a simple fabricator, strengthening long-term customer relationships.
- Code compliance: Providing code-compliant repair documentation (WPS, WPQ, NCR, UT reports) ensures the customer's regulatory and insurance requirements are met.
9. Implementation Checklist
- Receive NCR from UT inspection identifying unbonded/delaminated area with precise location, dimensions, and UT signal characteristics.
- Evaluate defect size and geometry against repair limits per governing code (ASME VIII, NB/T 47015, GB/T 25231).
- Determine repair method: local excavation + TIG re-cladding, or segment removal + explosion re-bonding.
- Verify that the repair welding procedure is qualified under applicable WPS (GB/T 9445, ASME IX Part 5, NB/T 47014).
- Prepare repair work instruction including excavation method, welding parameters, preheat/interpass temperature, and inspection requirements.
- Execute excavation: remove defective cladding and controlled depth of base metal; inspect floor for cracks and inclusions (visual + MT).
- Perform TIG re-cladding per qualified WPS with appropriate filler wire, shielding gas, and heat input.
- Inspect repair weld: visual + PT of surface; UT of interface; RT if required by code.
- Perform full-region UT re-inspection of repaired area and adjacent zone per GB/T 25231 or ASTM E2412.
- Document all activities in the NCR file: repair procedure reference, welder qualification, UT reports, final disposition.
- Close NCR with approval from quality authority and customer representative (if required).
10. Conclusion
Localized excavation and re-cladding repair is an indispensable capability in the bimetallic cladding manufacturing value chain. It bridges the gap between defect detection and product delivery, transforming potential scrap into salvageable, code-compliant product. For Cladding Technology Shanxi Co., Ltd., this capability strengthens the company's qualification portfolio, enhances on-time delivery performance, and delivers measurable cost and schedule value to customers. The key to successful execution lies in rigorous WPS qualification, disciplined process control during excavation and re-cladding, and thorough UT verification of the repaired interface—ensuring that the final product meets or exceeds the original quality standard of the unbonded-free clad component.