Scrap Criteria for Cracks in Pressure-Bearing Welds and Cladding Layers

The Zero-Tolerance Philosophy

In the fabrication of pressure vessels, heat exchangers, columns, and storage tanks, cracks in pressure-bearing welds, corrosion-resistant overlay layers, or composite interfaces represent an absolute safety bottom line. The principle is unequivocal: after two compliant repair attempts, if a crack persists, or if the crack is located in a high-stress region or critical sealing surface where safe repair is not feasible, the component must be scrapped. This is not a matter of economic preference but of engineering integrity and public safety.

Technical Basis for Crack Rejection

Cracks in welds and overlays are fundamentally different from other defect types such as porosity, slag inclusion, or undercut. While porosity or slag inclusions may be acceptable within certain size and location limits, cracks represent a discontinuity in the material that acts as a stress concentrator and a pathway for corrosion. In a pressure vessel operating under cyclic loading or in a corrosive environment, a crack can propagate rapidly, leading to catastrophic failure.

Defect Type	Acceptance Criteria	Repair Limitation	Final Disposition
Crack in pressure-bearing weld	Zero tolerance after repair	Max 2 repairs per location	Scrap after 2 failed repairs
Crack in overlay/cladding layer	Zero tolerance if in critical area	Max 2 repairs per location	Scrap if unrepairable
Crack at composite interface	Zero tolerance	Max 2 repairs per location	Scrap after 2 failed repairs
Crack in high-stress region	Zero tolerance regardless of repair	May be unrepairable	Scrap immediately
Crack on sealing surface	Zero tolerance	May be unrepairable	Scrap if repair compromises seal

Crack Location and Repairability Assessment

Not all crack locations are equal in terms of repairability. The following assessment framework should be applied:

High-stress regions: Areas near geometric discontinuities such as nozzles, manholes, flanges, and tube-to-tubesheet joints are subject to elevated stress concentrations. A crack in these areas, even if small, poses a significant risk. After two failed repairs, the microstructure in the area may be so compromised that further welding would be counterproductive.
Sealing surfaces: For components where the overlay layer serves as a sealing surface — such as gasketed flange faces, tube sheet sealing areas, or pressure boundary interfaces — a crack that cannot be completely removed and repaired without altering the geometry or surface finish is grounds for immediate scrapping.
Critical overlay layers: In hydrogenation reactors or acid service equipment, the overlay layer is the primary corrosion barrier. If the overlay layer has been cracked and repaired twice, the risk of future cracking is elevated. The metallurgical condition of the overlay after two repair cycles may include excessive dilution, intermetallic precipitation, or reduced toughness.

Metallurgical Considerations

Each repair cycle involves thermal cycling that affects the microstructure of the weld, the heat-affected zone, and the overlay layer. For austenitic stainless steel overlays, repeated thermal cycling can cause:

Grain coarsening in the overlay layer, reducing fatigue resistance.
Carbide precipitation at grain boundaries, increasing susceptibility to intergranular corrosion.
Residual stress accumulation that may exceed the material's yield strength.
Interface degradation at the base metal-overlay junction due to repeated thermal shock.

For nickel-based alloy overlays such as Inconel 625, Hastelloy C276, or Monel 400, the concerns include:

Sigma phase formation in the heat-affected zone after repeated heating above certain temperature thresholds.
Intermetallic compound precipitation that reduces ductility and corrosion resistance.
Cracking susceptibility due to the cumulative effect of thermal gradients.

Engineering Practice and Decision Framework

In practice, the decision to scrap a component after two failed repairs should be supported by a formal engineering assessment. This assessment should include:

Non-destructive examination of the area after the second repair to confirm the presence and extent of the crack.
Metallographic examination of a test coupon or the component itself to evaluate the microstructural condition.
Root cause analysis of the original defect and the failed repairs.
Evaluation of alternative repair methods or processes.
Risk assessment considering the component's operating conditions, service life, and safety classification.

The technical director or authorized responsible engineer must sign off on any decision to proceed beyond two repairs, and this approval must be documented in the fabrication records.

Key Reflections

The zero-tolerance principle for cracks after two failed repairs is a cornerstone of safe pressure vessel fabrication. It reflects the understanding that repeated welding cycles degrade the material's ability to resist future cracking. Engineers must resist the temptation to attempt a third repair "just to see if it works" — the cost of a failed third repair in terms of component integrity, safety, and regulatory compliance far exceeds the cost of scrapping. This principle should be deeply embedded in the quality culture of every fabrication facility.