Porosity Cluster Found by RT on a Pipe Butt Weld
A 150 mm nominal bore, 8 mm wall carbon steel pipe butt weld is examined by radiography. The film shows a cluster of rounded indications in the root region over a 12 mm length. The largest single indication is about 1.2 mm. The applicable code acceptance criterion for this service is that rounded indications are unacceptable if they exceed a specified fraction of wall thickness in a given length, and clustered porosity above a density limit is rejectable. The welder used a cellulose electrode with a cellulosic root and reports the root gap was 'a bit wide.' The RT film quality was verified as sensitivity 2-2T. The contractor proposes to accept the weld because each pore is small.
1. How should the inspector evaluate the cluster, and what is the flaw in the contractor's argument?
Read the worked answer
Acceptance is not judged pore by pore. Codes evaluate rounded indications by size, by the number in a defined length, and by clustering. A 12 mm long cluster with a largest indication near 1.2 mm must be compared against the code's chart or table for the wall thickness and length of weld evaluated. The contractor's argument that each pore is small ignores the density and cluster rules, which exist because a group of pores reduces effective section and can link under load. The inspector should measure the cluster length, count and size the indications, compare to the acceptance table, and record the result. If the cluster exceeds the density or size limits, the weld is rejectable and repair is required. The inspector should also confirm the RT technique was appropriate for the root region, since root porosity can be missed or exaggerated by technique.
2. What likely cause should be investigated, and what verification would you require after repair?
Read the worked answer
Root porosity in a cellulosic-electrode root is commonly caused by an excessive root gap, poor fit-up, damp electrodes, contamination, or arc instability at the root. The reported wide root gap is a plausible contributor, since a wide gap can cause the root to bridge poorly and trap gas. The inspector should review the fit-up records, electrode storage and issue, and the welder's technique. After repair, the weld should be re-examined by RT to the same technique and acceptance criteria, and the repair should be made to a qualified repair procedure. The inspector should also check that the repair did not introduce a new flaw such as lack of fusion at the repair boundary. Root cause should be corrected before the next joints, or the same defect will recur.