Identification and Repair Methods of Welding Defects in Pressure Vessel Manufacturing

Welding is a critical process in the fabrication of pressure vessels, directly impacting structural integrity and operational safety. However, welding defects such as cracks, porosity, incomplete fusion, and undercutting can compromise the vessel’s performance. Timely identification and proper repair of these defects are essential to ensure safety, reliability, and compliance with design standards.

1. Common Welding Defects

Cracks: Can occur in the weld metal or heat-affected zone (HAZ) due to excessive residual stress, improper preheating, or rapid cooling. Cracks are highly critical as they can propagate under pressure, leading to catastrophic failure.

Porosity: Gas trapped during welding can form small voids within the weld, weakening its mechanical strength. Causes include moisture, contaminated filler material, or poor shielding gas coverage.

Incomplete Fusion and Penetration: Occurs when the weld metal does not fully bond with the base metal or adjacent weld passes, reducing load-carrying capacity.

Undercutting and Overlap: Undercutting creates grooves at the weld toe, while overlap is excess weld metal protruding over the base metal; both act as stress concentrators.

2. Identification Methods

Visual Inspection: The first line of defense, identifying surface defects such as cracks, undercuts, and overlaps.

Non-Destructive Testing (NDT):

Radiographic Testing (RT): Detects internal porosity, slag inclusions, and incomplete penetration.

Ultrasonic Testing (UT): Measures internal discontinuities and weld thickness, effective for both planar and volumetric defects.

Magnetic Particle Testing (MT): Detects surface and near-surface cracks in ferromagnetic materials.

Dye Penetrant Testing (PT): Reveals surface-breaking defects in non-ferromagnetic or ferromagnetic metals.

3. Repair Methods

Grinding and Rewelding: Surface defects like undercuts and minor cracks can be removed by grinding, followed by rewelding under controlled parameters.

Preheating and Post-Weld Heat Treatment (PWHT): Used to reduce residual stress, prevent crack propagation, and restore toughness in high-strength or alloy steels.

Weld Overlay or Cladding: For severe defects in critical areas, overlay welding can reinforce the region and restore structural integrity.

Acceptance Criteria Compliance: Repairs must meet standards such as ASME BPVC Section VIII or EN 13445, including proper weld procedure qualification and re-inspection.

4. Preventive Measures

Proper selection of welding consumables and procedures based on material type and operating conditions.

Controlled preheating, interpass temperature, and cooling rates to minimize residual stresses.

Skilled and certified welders to ensure consistent quality.

Conclusion
Accurate identification and effective repair of welding defects are essential in pressure vessel manufacturing. Combining visual inspection, advanced NDT methods, and proper repair techniques ensures structural integrity, operational safety, and compliance with regulatory standards. Proactive prevention during welding further reduces defect occurrence, enhancing overall vessel reliability.

References

ASME Boiler and Pressure Vessel Code (BPVC), Section VIII – Rules for Construction of Pressure Vessels.

AWS D1.1 – Structural Welding Code: Steel.

Totten, G.E. (2006). Steel Heat Treatment: Metallurgy and Technologies. CRC Press.

Kou, S. (2003). Welding Metallurgy, 2nd Edition, Wiley.

EN 13445 – Unfired Pressure Vessels, European Standard.