ASME B31.3 Acceptance Criteria Cheat Sheet for Welding Inspectors

If there's one table every piping and welding inspector must know cold, it's ASME B31.3 Table 341.3.2 — the acceptance criteria for welded joints. This single table appears on the API 570 exam, the AWS CWI exam, and on virtually every piping inspection job interview. It defines what's acceptable, what's rejectable, and what requires repair.

This article is a practical cheat sheet: the key limits, how to apply them, and how to use the SkillJet.co code reference drawer during exam practice to look up criteria in real time.

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Understanding the Service Categories

ASME B31.3 classifies fluid services into categories that determine the level of inspection and the stringency of acceptance criteria:

Category	Description	Examination
A	Severe cyclic conditions	100% radiography + 100% visual
B	Normal fluid service	Spot/random radiography per design
C	Harmful human exposure	Enhanced examination per design
D	Low-risk fluid service	Visual examination only (typically)

Most exam questions and field scenarios focus on Category B (normal service), but you must know the differences across all categories.

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Table 341.3.2 — Key Acceptance Limits

Visual Examination Acceptance Criteria

Imperfection	Category A	Category B	Category D
Cracks	None permitted	None permitted	None permitted
Undercut (depth)	None permitted	≤ 1/32 in (0.8 mm) or T/2, max 1/16 in (1.6 mm)	≤ 1/16 in (1.6 mm)
Surface porosity	None permitted	Minimal, scattered	Moderate
Convexity (excess weld metal)	≤ 1/32 in (0.8 mm) + 1 mm per face	≤ 1/16 in (1.6 mm) + 1.5 mm	≤ 1/8 in (3.2 mm)
Concavity	Not permitted if it reduces thickness below minimum	Permitted if thickness maintained	Permitted if thickness maintained
Weld reinforcement	≤ 3 mm (1/8 in)	≤ 3 mm (1/8 in)	≤ 4 mm (5/32 in)

Radiographic / Ultrasonic Examination Criteria

Imperfection	Category A	Category B
Cracks	None	None
Lack of fusion	None	None
Incomplete penetration	None	None (in root)
Slag inclusions	None	≤ T/3 length, max 50 mm
Porosity (rounded)	Minimal (per chart)	Per ASME Section VIII Div. 1 Appendix 4 chart
Tungsten inclusions	None	Minimal, scattered

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The Most Frequently Tested Defects

1. Undercut

Undercut is the most commonly tested weld defect on certification exams. The key rule for Category B: undercut depth shall not exceed the lesser of 1/32 inch (0.8 mm) or half the weld thickness, with an absolute maximum of 1/16 inch (1.6 mm). Additionally, the total accumulated length of undercut in any 12-inch weld length shall not exceed 2 inches.

Exam tip: When a question gives you a weld with 1.5 mm undercut on a 10 mm thick weld, the limit is min(0.8 mm, 5 mm) = 0.8 mm. Since 1.5 mm > 0.8 mm, the weld is rejectable.

2. Porosity

Porosity acceptance depends on the size, distribution, and service category. For radiographic examination, refer to the porosity charts in ASME Section VIII, Division 1, Appendix 4. The key concept: rounded indications are evaluated by size and distribution, not just count.

3. Lack of Fusion and Incomplete Penetration

These are never acceptable in any category. If a radiograph shows lack of fusion or incomplete penetration (in the root), the weld must be repaired regardless of service category. This is one of the few absolute rules in B31.3.

4. Surface Concavity

Surface concavity (often called "suck-back" in root passes) is acceptable only if it does not reduce the wall thickness below the required minimum. If the measured thickness at the concavity is less than the minimum required thickness, the weld is rejectable.

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How to Use This on the Exam

On the API 570 and CWI exams, acceptance criteria questions typically present a scenario: a weld measurement or a radiographic description, and ask whether the weld is acceptable. The approach:

1. Identify the service category — the question will state it or give you clues.
2. Identify the defect type — undercut, porosity, lack of fusion, etc.
3. Look up the limit in Table 341.3.2 (or recall it from memory).
4. Compare the given value to the limit and determine accept/reject.

With practice, you can answer these in under 30 seconds. The SkillJet.co welding and piping simulator includes dozens of acceptance criteria questions with the code reference drawer available during practice, so you can verify your answers against the actual table.

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Beyond the Table: Related B31.3 Requirements

Pressure Testing (Chapter VI)

• Hydrostatic test pressure = 1.5 × design pressure (minimum)
• Pneumatic test pressure = 1.2 × design pressure
• Test temperature must be above the material's ductile-to-brittle transition temperature

Heat Treatment (Table 331.1.1)

• Pre-heat and PWHT requirements depend on material, thickness, and service
• For P-No. 1 materials, PWHT is typically required above 25 mm (1 inch) thickness
• For Cr-Mo alloys (P-No. 4, 5), PWHT thresholds are lower

Welding Documentation (Chapter VII)

• WPS (Welding Procedure Specification) must be qualified by PQR (Procedure Qualification Record)
• Welders must be qualified per WPQ (Welder Performance Qualification)
• Essential variables must be documented and within ranges

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Practice Makes Permanent

Reading acceptance criteria is one thing. Applying them to realistic weld scenarios under time pressure is what separates passing candidates from failing ones. The SkillJet.co QC simulator lets you practice with the code reference drawer open — exactly like the real API 570 exam — so you build the muscle memory of navigating to the right table and the right row quickly.

Start with the free diagnostic to see where you stand, then drill the acceptance criteria questions until you can answer them in your sleep. When you're ready, take the final certification mock exam to confirm you're exam-ready.