Weld Inspection Frequency and Sampling Plan
A shop is fabricating 200 identical steel beams using submerged arc welding (SAW). The welding coordinator has proposed a visual inspection of 10% of the welds and no other non-destructive testing (NDT) based on the argument that the process is automated and consistent. The contract specifies that welds must meet ISO 5817 quality level B. The client requires a inspection and testing plan (ITP) that complies with ISO 3834-2. The coordinator's plan includes only visual inspection and dimensional checks. The shop has previously experienced porosity issues with SAW when the flux was not properly baked. The client has not specified NDT requirements beyond the standard.
1. Evaluate the adequacy of the proposed inspection plan against ISO 3834-2 and ISO 5817 requirements.
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ISO 3834-2 requires that the manufacturer plan inspections and tests to ensure that the product meets specified requirements. The plan must consider the criticality of the welds and the potential for defects. For ISO 5817 quality level B, which is stringent, visual inspection alone is often insufficient, especially for submerged arc welds where internal defects like porosity or lack of fusion may occur. The previous porosity issues indicate a risk that must be addressed. ISO 3834-2 does not prescribe specific NDT percentages but requires that the extent of inspection be based on the welding process, material, and service conditions. For pressure-retaining or structural welds, NDT such as ultrasonic testing (UT) or radiographic testing (RT) is typically required. The proposed 10% visual inspection is inadequate because: (1) Visual inspection cannot detect internal defects. (2) The sample size of 10% may not be statistically sufficient to detect sporadic defects. (3) The plan does not address the known risk of porosity from flux handling. Therefore, the plan does not meet ISO 3834-2 requirements for planning and may not ensure compliance with ISO 5817 level B.
2. Recommend a revised inspection and testing plan that addresses the risks and complies with the standards.
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The revised ITP should include: (1) Visual inspection of 100% of welds to ISO 5817 level B. (2) NDT: at least 10% volumetric NDT (UT or RT) of the welds, with sampling distributed across the production. If any defect is found, increase sampling to 100% until the process is under control. (3) Given the porosity risk, include a check of flux baking and handling procedures, and consider adding a pre-weld check of flux condition. (4) For the first 5 beams, perform 100% NDT to establish process capability. (5) Include destructive testing of a representative sample (e.g., macro section) to verify weld quality. (6) Specify acceptance criteria per ISO 5817 level B. (7) Define responsibilities and qualifications for inspection personnel (e.g., ISO 9712). (8) Include a feedback loop for non-conformances. This plan aligns with ISO 3834-2 by demonstrating a risk-based approach and ensuring that the inspection extent is sufficient to detect potential defects. It also addresses the specific porosity issue by adding process controls.