7.2 Defect Removal, Grinding & Re-Inspection

7.2 Defect Removal, Grinding & Re-Inspection

Once a non-conforming weld defect is identified via radiographic testing (RT), ultrasonic testing (UT), or visual examination (VT), the repair process transitions to field execution under Sections 10.3-10.6 of API Standard 1104 (22nd Edition). The quality and structural integrity of a finished repair weld depend heavily on the precision of defect excavation, meticulous cavity preparation, strict preheating enforcement, low-hydrogen filler metal control, and rigorous non-destructive re-examination. Certified Welding Inspectors (CWIs) must monitor every stage of defect removal and re-inspection to ensure strict adherence to qualified Repair Welding Procedure Specifications (Repair WPS).

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1. Excavation Methods and Cavity Preparation

Defect removal requires excavating the non-conforming weld volume while preserving sufficient base metal and adjacent sound weld metal to ensure proper joint geometry and re-welding access.

Excavation Techniques

• Mechanical Grinding: The most common and controlled method for shallow and partial-thickness defect removal. Utilizes high-speed pneumatic or electric grinders equipped with aluminum oxide or zirconia grinding wheels, pencil grinders, and carbide burrs. Mechanical grinding generates minimal heat, eliminating the risk of thermal shock or metallurgical alteration in the surrounding metal.
• Air Carbon Arc Gouging (CAC-A): Utilizes a carbon-graphite electrode and a high-velocity compressed air jet to melt and blow away defective weld metal rapidly. CAC-A is highly effective for deep, full-thickness excavations on heavy-wall pipe (t > 0.500 in.).
• Mandatory Post-Gouging Grinding: When air carbon arc gouging or thermal cutting is used, API 1104 mandates that the gouged cavity surface MUST be mechanically ground down to clean, bright metal. This removes carbon pick-up/carburized layers, oxide scale, slag inclusions, and thermally affected microstructures that would otherwise cause severe embrittlement and cracking during repair re-welding.

Excavation Cavity Geometry

The excavated groove must be carefully shaped to enable complete fusion and sound bead deposition:

• Side Slope / Included Angle: Cavity walls must be smoothly sloped at a minimum included angle of 45° to 60° (or side slopes of 10° to 20° from vertical) to prevent steep sidewall drop-offs.
• Radius and Transition: The bottom of the cavity must feature a smooth, rounded root radius (minimum 1/8 in. / 3.2 mm radius) rather than sharp V-notches, eliminating stress concentrations and lack-of-sidewall-fusion traps.
• Tapered Ends: The ends of the repair trench must taper gradually into the sound existing weld metal at a slope no steeper than 3:1 (horizontal to depth).

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2. Pre-Repair Cavity Inspection & Verification

Before striking an arc to deposit repair weld metal, the excavated cavity must undergo mandatory inspection to confirm complete removal of the original defect:

• Visual Examination (VT): Verification of smooth contour, absence of burrs, and cleanliness (free of oil, grease, rust, moisture, and grinding swarf).
• Surface Non-Destructive Examination (MT/PT): API 1104 mandates or strongly recommends surface NDE of the ground cavity prior to welding. Magnetic Particle Testing (MT)—using wet fluorescent or dry powder technique—or Liquid Penetrant Testing (PT) must be performed across the cavity.
• Verification of Crack Tips: When repairing authorized cracks, MT or PT is mandatory to verify that the crack tips have been completely removed and that no secondary micro-fissures propagate into adjacent base metal.

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3. Preheat and Thermal Control During Repair Welding

Repair welding takes place under significantly higher mechanical constraint than original production welding because the surrounding pipe joint is fully welded and rigid. This severe restraint drastically increases residual tensile stresses and hydrogen cracking susceptibility.

Preheat Temperature Requirements

• Per the repair WPS: the repair procedure specifies preheat and interpass minimums/maximums, method, and location (10.3.3 g). Because the repair is made in a fully restrained, completed joint, repair preheat is typically set at or above the production preheat — the qualified values control.
• Preheat zone: preheat is applied uniformly around the repair area, extending beyond the excavation edges as the procedure specifies.
• Interpass maintenance: temperature is monitored with calibrated crayons or pyrometers immediately before each pass; dropping below the specified minimum before a pass violates the WPS.

Post-Weld Thermal Management

To allow dissolved atomic hydrogen to diffuse safely out of the repair weld metal and heat-affected zone before cooling to ambient temperature, post-weld heating (soak) at 400°F to 600°F (204°C to 316°C) or application of insulated slow-cooling blankets is commonly mandated by company specifications.

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4. Filler Metal Selection and Handling

The choice of welding consumables for repair operations is critical to achieving sound mechanical properties:

• Low-Hydrogen Consumables: While original mainline pipe welding may utilize cellulosic SMAW electrodes (e.g., AWS E8010-P1), repair welding frequently mandates low-hydrogen electrodes (e.g., AWS E7018-H4R, E8018-G, E9018-M) or semi-automatic FCAW/GMAW processes to minimize diffusible hydrogen content (≤ 4 mL/100g of deposited weld metal).
• Consumable Storage and Handling: Low-hydrogen SMAW electrodes must be stored in temperature-controlled holding ovens (250°F - 300°F) after removal from hermetically sealed containers. electrodes exposed to ambient atmospheric moisture beyond permitted time limits (typically 4 hours) must be re-baked per manufacturer specifications or discarded.

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5. NDT Re-Examination Protocol & Acceptance (Sections 10.5-10.6)

After the repair weld has cooled to ambient temperature (and after any mandated post-weld delay period, such as 24 hours for hydrogen crack detection on high-strength pipe), the repair area must undergo rigorous non-destructive examination.

Coverage Requirements

• Re-Inspection Extent: The re-inspection must cover the entire length of the repaired area, plus an overlapping inspection zone extending a minimum of 2 inches (50 mm) beyond each end of the repair into the adjacent un-repaired weld metal.
• Dual NDT Method Protocol: The repaired weld must be evaluated using the original NDT volumetric inspection method (Radiographic Testing RT or Automated Ultrasonic Testing AUT) PLUS surface examination (Magnetic Particle MT or Liquid Penetrant PT).

Acceptance Standards

The repaired weld zone must meet the exact same acceptance criteria specified in Section 9 (or Annex A ECA) as the original production weld. If the re-inspected repair meets Section 9 limits, it is accepted; if it contains non-conforming defects, it is subject to the double repair prohibition and cut-out rules.