Weld flaw acceptance limits for elongated indications

Workmanship acceptance criteria are not fitness-for-service criteria. They exist to police the consistency of a qualified welding process, and the limits they set are what a competent welder working to a qualified procedure should achieve — not what the joint can structurally tolerate. A weld rejected on workmanship grounds is very often still structurally sound; that is precisely why an engineering critical assessment to BS 7910 or API 579 exists as a separate, and much more demanding, route.

Almost every code splits flaws into two classes. Planar flaws — cracks, lack of fusion, incomplete penetration — are rejected outright at essentially any size, because they are sharp, they concentrate stress, and they grow under fatigue or environmental loading. Volumetric flaws — slag inclusions, tungsten inclusions, porosity — are blunt, do not concentrate stress in the same way, and are therefore permitted up to a size limit that scales with the section thickness.

The limits themselves follow a common pattern: a single-indication length limit, a width limit, and an aggregate limit over a reference length so that a chain of individually acceptable indications cannot add up to an unacceptable run of weakened section. ASME VIII-1 UW-51(b)(2) allows an elongated indication of 6 mm for thickness up to 19 mm, t/3 between 19 and 57 mm, and 19 mm above 57 mm, with the aggregate length limited to t in any 12t of weld. ASME B31.3 Table 341.3.2 for Normal Fluid Service limits an individual slag or tungsten inclusion to t/3 in length and the lesser of 2.5 mm or t/3 in width, with the same aggregate rule. API 1104 §9.3.5 works to fixed dimensions rather than thickness fractions, reflecting the narrow wall range of transmission pipeline: 50 mm length, 1.6 mm width, and 50 mm aggregate in any 300 mm of weld.

This calculation covers elongated slag and tungsten inclusions only. Porosity, undercut, incomplete penetration, incomplete fusion, burn-through, cracks and geometric imperfections each have their own clauses and, in several codes, their own charts. AWS D1.1 ultrasonic acceptance works differently again — it is amplitude based, with a discontinuity severity rating d = a − b − c in decibels compared against a class table that varies with thickness and probe angle, so a length limit does not apply to it at all. Always work from the edition of the code named in the contract; clause numbers and limits move between editions.

ASME VIII-1 UW-51(b)(2):  L_allow = 6 mm (t ≤ 19) · t/3 (19 < t ≤ 57) · 19 mm (t > 57)
                          aggregate ≤ t in any 12t of weld
ASME B31.3 Normal Fluid Service:  L_allow = t/3 ,  W_allow = lesser of 2.5 mm and t/3
                          aggregate ≤ t in any 12t of weld
API 1104 elongated slag inclusion:  L_allow = 50 mm , W_allow = 1.6 mm
                          aggregate ≤ 50 mm in any 300 mm of weld