CLADDING TECHNOLOGY SHANXI CO., LTDTIG Repair Welding for Small Surface Defects in Cladding Layers
Overview and Technical Context
In the fabrication of bimetal pressure vessels and cladding products, even the most rigorously controlled welding processes inevitably produce minor surface defects such as shallow pores, small slag inclusions, and localized lack of fusion. These defects, if left unrepaired, can serve as initiation sites for corrosion, stress corrosion cracking, or fatigue failure under service conditions. The TIG (Gas Tungsten Arc Welding) repair welding technique stands out as the preferred method for addressing such small-scale defects due to its exceptional arc stability, precise heat input control, and superior weld bead profile management. This study note examines the technical fundamentals, process parameters, and practical considerations associated with TIG repair welding in the context of clad plate and weld-overlay pressure vessel fabrication.
Core Technical Principles
TIG repair welding operates on the principle of localized, controlled melting with minimal thermal disturbance to the surrounding base metal and cladding layer. The tungsten electrode provides a concentrated arc with a stable, non-consumable heat source, while the shielding gas (typically argon or argon-helium mixtures) prevents atmospheric contamination. The key advantage lies in the ability to precisely control the weld bead geometry and penetration depth, ensuring that the repair blends smoothly with the surrounding intact cladding surface without excessive dilution of the overlay alloy composition.
For cladding applications, the repair must maintain the metallurgical integrity of the overlay layer. This means that the repair weld metal must match or exceed the chemical composition and mechanical properties of the original cladding alloy. The repair procedure must be qualified under NB/T 47014 or ASME IX, with specific attention to the repair procedure qualification requirements for overlay welding.
Typical Process Parameters
| Parameter | Recommended Range | Notes |
|---|---|---|
| Welding current | 80–200 A | Dependent on repair size and base thickness |
| Arc voltage | 12–20 V | Maintain stable arc length |
| Travel speed | 50–150 mm/min | Slower for deeper repairs |
| Shielding gas flow | 12–20 L/min | Argon or Ar/He mix |
| Preheat temperature | 50–150°C | Based on base material and thickness |
| Interpass temperature | ≤250°C | Prevent excessive grain growth |
| Heat input | 0.5–2.0 kJ/mm | Low to moderate |
| Electrode diameter | 1.6–3.2 mm | Match repair geometry |
Defect-Specific Repair Strategies
Shallow Surface Pores
Surface pores in cladding layers typically result from inadequate shielding gas coverage, contamination of the weld zone, or excessive travel speed. The repair approach involves grinding the pore to a smooth, concave profile that blends with the surrounding surface, followed by TIG filling using matching filler wire. The key is to avoid overfilling, which would create a raised bead that alters the surface finish and potentially creates stress concentration points. For corrosion-resistant overlay layers such as 304L, 316L, or Inconel 625, the filler wire must match the overlay composition exactly to prevent galvanic coupling issues.
Small Slag Inclusions
Slag inclusions in TIG-clad layers are less common than in arc welding processes but can occur when multi-pass overlay welding is employed. The repair requires complete removal of the slag inclusion by grinding or machining, creating a smooth transition zone. The repair weld must be deposited in a single pass if possible to minimize the number of thermal cycles in the already-sensitive overlay region. Post-repair, the area should undergo dye penetrant testing (PT) to confirm complete inclusion removal and sound weld fill.
Localized Lack of Fusion
Lack of fusion defects in cladding layers are more critical than surface pores because they compromise the bond strength between the overlay and base metal. TIG repair requires careful preparation of the repair groove to ensure adequate fusion with the surrounding intact weld metal. The groove geometry should provide sufficient access for the torch and filler wire, with a root opening of 1.0–2.0 mm and groove angles of 60°–90°. The repair must be followed by ultrasonic testing (UT) to verify complete fusion, particularly at the overlay-base metal interface.
Quality Control and Inspection
The repair area must undergo a comprehensive inspection regime following the repair welding operation. Visual inspection (VT) confirms the surface profile and absence of surface discontinuities. Dye penetrant testing (PT) detects any surface-breaking cracks or lack of fusion that may have developed during the repair process. For repairs at the overlay-base metal interface, ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) is essential to verify bond integrity. The repair area should be included in the hydrostatic test scope, and any repairs on sealing surfaces must undergo a dedicated leak test.
A critical consideration is the number of repair cycles permitted. Most standards limit the number of repairs at any single location to two attempts. Exceeding this limit indicates a systemic process issue that requires root cause analysis rather than continued repair attempts. Each repair attempt must be documented in the quality records, including the defect description, repair procedure, filler material used, and inspection results.
Engineering Practice Insights
In my experience with hydrogenation reactor fabrication, TIG repair welding has been the most frequently used repair method for minor defects in Inconel 625 overlay layers. The challenge lies in maintaining the low carbon content of the 625 alloy during repair welding, as excessive heat input can promote carbide precipitation and sensitization. Using a low-carbon filler wire (ERNiCr-3 or equivalent) and maintaining interpass temperatures below 150°C has proven effective in preserving the overlay's corrosion resistance.
For thin-wall clad pipe repair, the heat input must be further reduced to prevent distortion and base metal sensitization. In such cases, pulsed TIG welding provides an additional degree of control, allowing the arc to be modulated between peak current (for penetration) and background current (for heat control). This technique is particularly valuable for repairs on stainless steel clad pipes where the base metal is susceptible to intergranular corrosion if overheated.
Key Reflections
The TIG repair welding technique, while seemingly straightforward, demands a high degree of welder skill and procedural discipline. The success of a repair depends not only on the welding parameters but also on the preparation quality, filler material selection, and inspection thoroughness. A poorly executed repair can create a defect more severe than the original, particularly in critical cladding applications where the overlay serves as the primary corrosion barrier. Engineers must approach each repair with the understanding that it represents an additional thermal cycle on already-stressed material, and the repair procedure must be conservative in its thermal management to avoid introducing new defects or degrading existing properties.