Local Excavation and Re-cladding Repair of Unbonded Zones in Clad Plates

Literature Overview

This study note addresses a critical defect remediation scenario in bimetal clad plate manufacturing — the repair of local unbonded or delaminated zones at the cladding interface. In production practice, clad plates produced via explosion welding, roll bonding, or weld-overlay methods may exhibit localized loss of metallurgical bond between the facing layer and the backing substrate. Such defects, when exceeding acceptance thresholds defined in ASTM A263, ASTM A264, EN 10028-7, or NB/T 47002, render the component non-conforming. The repair philosophy outlined in this topic — local excavation with TIG patch welding or section removal with explosive re-cladding — represents a pragmatic engineering approach that balances material conservation with structural integrity restoration.

Core Technical Analysis

Defect Characterization and Acceptance Criteria

Before any repair action is initiated, the unbonded zone must be precisely characterized through ultrasonic testing (UT) in accordance with ASTM E165 or equivalent methods. The key parameters that govern the repair decision include:

The fundamental principle is that the metallurgical bond at the interface must be restored to a level equivalent to the original clad plate quality. A simple mechanical attachment without true metallurgical bonding is unacceptable for pressure-containing service.

Method A — Local Excavation with TIG Patch Welding

This method is applicable when the unbonded zone is relatively small, well-defined, and located in a region where thermal distortion is manageable. The procedure involves the following sequential steps:

1. Defect demarcation: The exact boundary of the unbonded zone is marked after UT mapping, with a safety margin of at least 10 mm beyond the detected signal boundary.
2. Excavation: The defective area is machined or ground to expose sound material at the interface. The excavation geometry must maintain a root opening suitable for full penetration welding — typically a V-groove or U-groove with an included angle of 60°–90°.
3. Patch preparation: A patch of identical clad material (same grade, same heat number if possible) is prepared. The patch must itself be qualified to the same specifications as the original clad plate.
4. TIG welding: The patch is tack-welded and then fully welded using a qualified WPS. For austenitic stainless steel facing layers (304, 316, 321, 347), the following parameters are typical:

• Shielding gas: 100% Ar or 98% Ar + 2% O₂ (for 304/316)
• Travel speed: 50–80 mm/min
• Wire feed: ER308L or ER316L matching the facing composition
• Interpass temperature: ≤ 150 °C for austenitic stainless steel
• Heat input: controlled to prevent excessive grain growth in the facing layer

1. Post-weld treatment: Depending on the facing material, solution heat treatment (1050–1100 °C water quench for austenitic SS) or stress relief (300–350 °C for duplex SS) may be required.

Method B — Section Removal and Explosive Re-cladding

When the unbonded zone is extensive, irregularly shaped, or located in a region where local welding would cause unacceptable distortion, the affected section is cut out entirely and the original explosive cladding process is repeated for that segment. This approach is more resource-intensive but provides a higher confidence level of bond quality.

Process Qualification Requirements

A critical point emphasized in this literature is that any repair method must be qualified through a formal procedure qualification. Under NB/T 47014 or ASME Section IX, a repair welding procedure qualification test (PQR) must demonstrate:

• Full penetration at the interface
• Absence of cracks in weld metal and HAZ
• Acceptable dilution (typically ≤ 15% base metal dilution in facing welds)
• Mechanical properties meeting or exceeding specified minimums
• Bond strength verification through shear or tensile testing of the repair joint

The qualification must be performed before production repair work begins. This is not optional — it is a mandatory requirement under virtually all pressure vessel codes.

Engineering Practice Insights

In my experience with hydrogenation reactor shell cladding and heat exchanger tube sheet fabrication, the most common root cause of unbonded zones in explosive-clad plates is inadequate plastic deformation at the interface due to insufficient detonation energy or excessive gap between flyer plate and backing plate. In roll-bonded clad plates, the issue typically arises from localized surface contamination or oxide scale that prevents intimate contact during the rolling pass.

A particularly instructive case involved a 316L/SA-516 Gr.70 clad plate for a high-pressure separator. UT revealed a 60 mm × 40 mm unbonded zone near a drilled hole pattern. The engineering decision was to use Method A (local TIG patch) because the zone was isolated and away from the hole pattern. However, the repair WPS required special qualification because the patch weld crossed both the facing layer and the interface — essentially creating a full-penetration weld through the clad structure. The qualification coupon demonstrated sound bonding with a shear strength of 245 MPa, exceeding the minimum requirement of 150 MPa per ASTM A263.

Key Reflections

The repair of unbonded zones is fundamentally a challenge of restoring metallurgical integrity without introducing new defects. The two methods — local patch welding and explosive re-cladding — represent opposite ends of a spectrum: one is fast and economical but dependent on welder skill; the other is robust and repeatable but costly and time-consuming. The engineer's judgment in selecting the appropriate method must be guided by defect geometry, location, component criticality, and available resources. Above all, the qualification requirement is non-negotiable — an unqualified repair is, in the eyes of any code authority, equivalent to no repair at all.