9.3 Acceptance Criteria in Fabrication Codes: ASME VIII, AWS D1.1, and API 1104

9.3 Acceptance Criteria in Fabrication Codes: ASME VIII, AWS D1.1, and API 1104

Terminology and Evaluation Foundations

Accurate flaw evaluation requires absolute adherence to standardized metrological definitions. In magnetic particle examination, surface particle accumulations can stem from non-relevant magnetic leakage, cosmetic surface blemishes, or critical structural flaws. An NDT Level III must enforce clear distinctions among four legal terms:

1. Discontinuity: Any interruption in the normal physical structure or configuration of a part, such as cracks, laps, seams, inclusions, porosity, or sharp geometric changes. A discontinuity is completely neutral; its presence does not imply rejection.
2. Flaw: An intentional or unintentional discontinuity that may or may not violate engineering acceptance standards.
3. Defect: A flaw or discontinuity whose size, shape, orientation, location, or severity violates the governing code acceptance criteria. All defects are rejectable and mandate repair, re-work, or scrapping of the component.
4. Indication: The visible accumulation of magnetic particles on the surface of a part. Indications require systematic evaluation to determine if they are false, non-relevant, or relevant.

Indication Classification Taxonomy

• False Indication: Particle accumulations caused by purely mechanical or non-magnetic phenomena, such as gravity settling in depressions, liquid carrier puddling, lint, or particulate entrapment in rough surface scale. False indications disappear upon light wiping and solvent cleaning.
• Non-Relevant Indication: True magnetic leakage fields caused by intentional metallurgical or design features that have no bearing on the structural integrity of the part. Examples include the heat-affected zone (HAZ) fusion boundary between materials of different magnetic permeability, localized cold working, grain boundaries, press fits, and geometric keyways.
• Relevant Indication: An indication caused by a mechanical discontinuity or crack-like flaw located on or immediately beneath the surface, possessing a major dimension greater than 1/16 inch (1.5 mm).

Aspect Ratio Geometry: Linear vs. Rounded Indications

All major fabrication codes—including ASME Section VIII, AWS D1.1, and API 1104—categorize relevant indications into two fundamental geometric classes based on aspect ratio ($L/W$):

+-------------------------------------------------------------------------+
|                  GEOMETRIC INDICATION CLASSIFICATION                    |
+-------------------------------------------------------------------------+
| Classification     | Mathematical Criterion | Morphological Shape       |
|--------------------+------------------------+---------------------------|
| Linear Indication  |   Length > 3 * Width   | Cracks, lack of fusion,   |
|                    |       (L > 3W)         | incomplete penetration,   |
|                    |                        | laps, seams, gouges       |
|--------------------+------------------------+---------------------------|
| Rounded Indication |  Length <= 3 * Width   | Gas porosity, pinholes,   |
|                    |       (L <= 3W)        | slag inclusions, crater   |
|                    |                        | pits, micro-shrinkage     |
+-------------------------------------------------------------------------+

Evaluation Mandate: Indications measuring 1/16 in (1.5 mm) or less in major dimension are deemed non-relevant and are discounted during evaluation, unless an accumulation of minor indications suggests an underlying structural defect.

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ASME Section VIII, Division 1, Mandatory Appendix 6

ASME Section VIII, Division 1 governs unfired pressure vessels. Mandatory Appendix 6 (Methods for Magnetic Particle Examination) details the evaluation rules for all welds, vessel shells, nozzle attachments, and pressure-retaining components.

Appendix 6-4 Acceptance Standards

Under Appendix 6-4, all surfaces examined by magnetic particle testing shall be free from:

1. Any Relevant Linear Indications: Zero tolerance. Any crack, lack of penetration, lack of fusion, or relevant linear indication ($L > 3W$ and $L > 1/16\text{ in} / 1.5\text{ mm}$) is completely rejectable, regardless of length, orientation, or vessel thickness.
2. Relevant Rounded Indications ($L \le 3W$ and $L > 1/16\text{ in}$): Rejectable if the major dimension exceeds 3/16 in (5 mm). This is a single flat limit — Appendix 6-4 does not scale the rounded-indication allowance with material thickness. Candidates who have memorized thickness-banded charts are usually recalling radiographic rounded-indication charts or a customer specification, not Appendix 6.
3. Indications in a Line: Four or more relevant rounded indications separated by 1/16 in (1.5 mm) or less, edge-to-edge, in a line are rejectable.

+-----------------------------------------------------------------------------+
|      ASME SECTION VIII, DIV 1, MANDATORY APPENDIX 6 ACCEPTANCE SUMMARY      |
+-----------------------------------------------------------------------------+
| Indication Class    | Dimensional Threshold       | Acceptance Disposition  |
|---------------------+-----------------------------+-------------------------|
| Relevant Threshold  | Major dimension > 1/16 in   | Subject to evaluation   |
| Linear Indications  | L > 3W                      | REJECT ALL (Zero tol.)  |
| Rounded (any t)     | L <= 3W and > 3/16 in       | REJECT                  |
| Collinear Rounded   | >= 4 separated by <= 1/16"  | REJECT                  |
| Thickness banding   | NOT USED in Appendix 6-4    | Flat 3/16 in limit      |
+-----------------------------------------------------------------------------+

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AWS D1.1 / D1.1M Structural Welding Code - Steel

American Welding Society (AWS) D1.1 governs the fabrication of welded structural steel buildings, bridges, and offshore platforms. In the 2020 edition the inspection requirements moved from Clause 6 to Clause 8, and magnetic particle testing is addressed at 8.10; the old "Table 6.1" became Table 8.1.

AWS D1.1 mandates that MT indications be evaluated against the visual inspection acceptance criteria of Table 8.1 (Table 10.14 governs tubular connections). Table 8.1 is a single table with separate columns for statically loaded nontubular and cyclically loaded nontubular connections — there is no separate "Table 8.2" of MT criteria, and citing one is a reliable way to lose an audit argument. Table 8.2 in D1.1:2020 carries ultrasonic acceptance-rejection criteria, not MT criteria. The code creates a fundamental engineering distinction between two loading regimes:

1. Statically Loaded Nontubular Connections (Table 8.1, statically loaded column)

• Cracks: Zero tolerance. The weld shall have no cracks, regardless of length or location.
• Weld/Base Metal Fusion: Thorough fusion must exist between adjacent layers of weld metal and between weld metal and base metal.
• Piping Porosity: For groove welds in tension, the sum of visible piping porosity diameters shall not exceed $3/8\text{ in}$ ($10\text{ mm}$) in any $12\text{ in}$ ($300\text{ mm}$) length of weld. In fillet welds, the sum of visible piping porosity of diameter $1/32\text{ in}$ ($0.8\text{ mm}$) or greater shall not exceed $3/8\text{ in}$ ($10\text{ mm}$) in any linear inch of weld, and shall not exceed $3/4\text{ in}$ ($19\text{ mm}$) in any $12\text{ in}$ length of weld. Note that D1.1 does not prohibit piping porosity outright for statically loaded groove welds — that is the cyclically loaded rule.
• Undercut: For welds in primary members transverse to the applied tensile stress, undercut shall not exceed $1/32\text{ in}$ ($0.8\text{ mm}$). In all other cases, undercut shall not exceed $1/16\text{ in}$ ($1.6\text{ mm}$).

2. Cyclically Loaded Nontubular Connections (Table 8.1, cyclically loaded column / Fatigue Critical)

In cyclically loaded structures (such as highway bridges, crane runways, and wind turbine towers), fluctuating dynamic stresses cause microscopic stress concentrations to propagate rapidly into catastrophic fatigue cracks. Consequently, acceptance criteria are far more stringent:

• Cracks: Zero tolerance. No cracks are permitted under any circumstances.
• Linear Discontinuities & Porosity in Tension Zones: In welds subject to cyclic tensile stress, no piping porosity or linear indications are permitted to remain on the surface.
• Undercut at Weld Toes: In primary members subject to transverse cyclic tensile stress, undercut shall not exceed 0.01 in (0.25 mm) in depth. In all other cases, undercut shall not exceed $1/32\text{ in}$ ($0.8\text{ mm}$).

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API 1104 Welding of Pipelines and Related Facilities

American Petroleum Institute (API) standard 1104 governs cross-country transmission pipelines carrying crude petroleum, natural gas, and hazardous liquids. Section 9 carries the acceptance standards for imperfections located by radiographic, magnetic particle, liquid penetrant, and ultrasonic testing, with a dedicated magnetic particle subsection.

Evaluation Rules (Section 9, magnetic particle acceptance standards)

1. Linear Indications:
   • Cracks: All cracks are rejectable EXCEPT shallow crater cracks or star cracks located at weld bead stop-starts that do not exceed 5/32 in (3.96 mm) in length.
   • Linear Indications Other Than Cracks: Indications such as incomplete fusion, cold lap, or inadequate joint penetration are evaluated against pipe diameter and weld length:
     • Linear indications evaluated as incomplete fusion (IF) are unacceptable when they exceed 1 in (25 mm) in total length in any continuous 12 in (300 mm) length of weld, or 8% of the weld length. Note that the code expresses this as an aggregate rule; there is no separate "individual indication shall not exceed 1/2 in." limit for linear MT indications, and inventing one is a common error.
2. Rounded Indications:
   • Rounded indications are evaluated against the corresponding porosity standards in Section 9 (the radiographic acceptance rules for individual pores and cluster porosity).
   • Cluster porosity occurring in the finish pass is unacceptable when the diameter of the cluster exceeds 1/2 in (13 mm), when the aggregate length of cluster porosity in any continuous 12 in (300 mm) of weld exceeds 1/2 in (13 mm), or when an individual pore within a cluster exceeds 1/16 in (1.6 mm) in size.

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Comparative Acceptance Standards Across Major Fabrication Codes

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Role and Legal Responsibilities of the NDT Level III

The NDT Level III serves as the highest technical authority in the interpretation, implementation, and arbitration of code criteria.

1. Procedure Authoring and Acceptance Criteria Synthesis

The Level III must translate complex code clauses into clear, unambiguous shop-floor inspection procedures and Technique Sheets. An operational technique sheet must explicitly list:

• Exact mathematical cutoff dimensions for relevant linear and rounded indications.
• The applicable rounded-indication limit for the governing code (a flat 3/16 in. under ASME Section VIII Appendix 6; thickness or product-form banding only where a customer specification actually imposes it).
• Rejectable criteria for collinear indications.
• Surface preparation and post-cleaning mandates.

2. Arbitration of Border-Line and Non-Relevant Indications

When Level II technicians encounter ambiguous surface indications, the Level III arbitrates:

• Distinguishing Non-Relevant Permeability Indications: At the heat-affected zone (HAZ) of high-strength structural welds, localized phase transformations (martensite or bainite formation) create sharp permeability gradients that produce fuzzy, diffuse magnetic particle accumulations resembling linear indications. The Level III instructs the technician to demagnetize the joint, reduce magnetizing current by $25%$, or examine the area with a light hand-held AC yoke. If the indication diffuses or vanishes, it is non-relevant. If it remains sharp and distinct, it is a planar crack.
• Mechanical Exploration (Feather Grinding): Where an indication appears to be a superficial scratch or die mark, the Level III may authorize light blend grinding (not to exceed the minimum wall thickness allowance of the design code). The area is re-examined. If the indication is completely removed, the part is acceptable; if the indication continues into the metal, it is confirmed as a propagating crack and rejected.

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Worked Flaw Evaluation Scenarios & Level III Exam Traps

Scenario 1: ASME Section VIII Vessel Shell Weld Evaluation

Discontinuity Data: An MT technician examines a longitudinal seam weld on a $0.50\text{ in}$ ($12.7\text{ mm}$) thick pressure vessel shell fabricated to ASME Section VIII, Division 1. The technician discovers two distinct indications:

• Indication A: A sharp indication measuring $0.180\text{ in}$ ($4.6\text{ mm}$) long by $0.030\text{ in}$ ($0.76\text{ mm}$) wide.
• Indication B: An elliptical indication measuring $0.150\text{ in}$ ($3.8\text{ mm}$) long by $0.080\text{ in}$ ($2.0\text{ mm}$) wide.

Level III Technical Evaluation:

1. Relevance Check: Both Indication A ($0.180\text{ in}$) and Indication B ($0.150\text{ in}$) have major dimensions $> 1/16\text{ in}$ ($0.0625\text{ in}$). Both are relevant.
2. Aspect Ratio Evaluation for Indication A: $L/W = \frac{0.180\text{ in}}{0.030\text{ in}} = 6.0$ Because $L/W > 3.0$, Indication A is a Linear Indication. Under Appendix 6-4, all relevant linear indications are strictly REJECTABLE (zero tolerance).
3. Aspect Ratio Evaluation for Indication B: $L/W = \frac{0.150\text{ in}}{0.080\text{ in}} = 1.875$ Because $L/W \le 3.0$, Indication B is a Rounded Indication.
4. Dimensional Limit for Indication B: Appendix 6-4 rejects relevant rounded indications with a major dimension greater than $3/16\text{ in}$ ($0.1875\text{ in}$) at any thickness. Indication B has length $0.150\text{ in} \le 0.1875\text{ in}$, so Indication B is individually ACCEPTABLE.
5. Final Disposition: The weld is REJECTED due to Indication A.

Scenario 2: Group of Four Collinear Rounded Indications

Discontinuity Data: On the same vessel weld, an inspector identifies four pinhole indications arranged along a straight line in the weld cap. Each indication measures $0.090\text{ in}$ in diameter. The edge-to-edge separation distances between adjacent indications are $0.040\text{ in}$, $0.050\text{ in}$, and $0.055\text{ in}$ respectively.

Level III Evaluation:

1. Each indication is rounded ($L/W = 1.0$) and relevant ($0.090\text{ in} > 1/16\text{ in}$). Individually, each is $\le 3/16\text{ in}$.
2. There are four indications in a line.
3. The edge-to-edge spacing between each adjacent indication is less than $1/16\text{ in}$ ($0.0625\text{ in}$).
4. Disposition: Under ASME Section VIII, Division 1, Appendix 6-4, four or more relevant rounded indications in a line separated by $1/16\text{ in}$ (1.5 mm) or less edge-to-edge are REJECTABLE. The weld must be excavated and repaired.