Fillet Gauge
My courseAsk Tack AIFree practice checksCWI Part ACWI Part BCWI Part C D1.1CoursesGuidesCWI Part A practice test: 165 questions, closed book, 10 domainsCWI Part B practice test: replicas, tool kit and a timed planCWI Part C practice test and D1.1 exam guideAll guidesAll toolsAboutContact

Weld porosity: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

Weld porosity is gas trapped in weld metal as it freezes, leaving round or stretched voids. Pores can break the face or sit buried inside. Inspectors sort porosity by pattern: scattered pores spread through the weld, a tight cluster, elongated pores drawn out along the bead, and piping, a long tube-shaped void often called a wormhole.

Two views of weld porosity. A: a weld bead from above with scattered pores, a tight cluster and one elongated pore. B: a groove weld cross-section with three round voids and a long tube-shaped piping void rising toward the face
Surface patterns on top, a cut section below. Original drawing, not to scale.
Use this diagram

How to recognize porosity

  1. Look for smooth, round edges. A gas pore has a rounded outline and a shiny or dull smooth wall. Jagged, angular edges point to slag instead.
  2. Read the pattern. Pores spread at random along the bead are scattered porosity. A tight group in one small area, often at an arc start or stop, is cluster porosity. Pores stretched in the welding direction are elongated. A tube that runs into the weld at an angle, sometimes surfacing as a deep pinhole, is piping.
  3. Check the location. Porosity lives in the weld metal. A hole in the plate beside the weld is not porosity, and neither are raised dots stuck on the plate.
  4. Think about what you can't see. Surface pores are often the tip of more porosity below. A buried pore shows up on a radiograph as a dark, round spot with soft edges.

Try one question

Question 1 of the free 10-question check on CWI Part B. No signup. Then ask Tack, the AI study buddy, why the answer is right and the others are wrong.

Question 1 of 10 · free checkUntimed

Welding Inspection and Discontinuities

A surface pore on a replica is fully covered by the 3/32 in and 1/8 in circles on a V-WAC gauge, but shows outside the edge of the 1/16 in circle. What diameter should be recorded for the pore?

Press A–D or 1–4 to choose an answer while this question is on screen.

Choose an answer to begin

How to measure it on a replica

On a replica or a real weld you record two things: the size of the largest pore and how many pores there are in a given length. A gauge with reference circles, such as a V-WAC, lets you find the smallest circle that fully covers the pore. Then count pores along the length the specification names and add up their diameters if the criteria ask for a sum. A dial caliper's depth rod can confirm how deep an open pore goes. The numbers you compare against always come from the governing document, never from memory.

Ask Tack before you buy AI your Fillet Gauge study buddy

See what's on the CWI Part B exam, test yourself with 5 quick questions, or find the plan that fits you.

The CWI Part B course

Available now · instant access

Learn the tool kit, the specification and the qualification records, then practice.

  • 7modules of lessonsEvery task on the outline
  • 3full-length timed exams46 questions in 120 minutes each
  • 338practice questionsEach one explained
  • 2worked calculationsStep by step
  • 21specification drillsFind it fast in the Book of Specifications
  • 1week-by-week study planSet to your exam date

Turn a practice answer into a study step. In untimed practice, ask Tack AI about any question you answer, follow up, then see your weakest topics and what to drill next.

Part B course$99Get the Part B course · $99

Instant access. Terms and refunds · Secure checkout by Stripe · Billed by Nori Software

Compare plans

Every answer comes with a full explanation.

Common mix-ups

  • Porosity or slag. Round and smooth is gas. Angular and dark, often on a pass boundary, is slag.
  • Porosity or spatter. Porosity is a hole in the weld. Spatter is a droplet stuck on top of the plate.
  • Piping or a crack. Piping has a rounded tube wall and a blunt end. A crack is a sharp line with pointed tips.
  • Elongated or a lack of fusion. Lack of fusion is flat and follows a groove face. An elongated pore is rounded in cross-section.

Why it forms

Gas has to come from somewhere: moisture in flux or on the joint, oil, paint or rust, shielding gas blown away by a draft, a blocked nozzle, or too long an arc. Fast travel freezes the pool before the gas can float out. Cluster porosity at a start often means the shielding hadn't settled yet.

Porosity on the CWI exam

Porosity is the discontinuity most candidates can name, and the one most often measured. On Part B, Welding Inspection and Discontinuities is at least 34% of the practical, and pore size and pore count on replicas belong there. On Part A, which is closed book, it fits Visual Examination and NDE Fundamentals. On Part C you find the governing visual criteria in the code, D1.1 Clause 8 for the D1.1 endorsement, and read them in your own copy.

Practice it

Name porosity against its look-alikes in the weld discontinuities quiz. Then compare it with the two other things trapped inside welds: slag inclusions and tungsten inclusions. The weld discontinuities guide has every type in one table.

Check the rest of Part B

Take the free 10-question check. It's weighted to the Part B outline, scored by domain and needs no signup, so you see whether discontinuities or another domain costs you the most points.

Sources

Show the list

Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

Your next step

Ready to see where you stand on CWI Part B?

Free, untimed and scored by domain. No signup.

Start the free 10-question check

Part B course. Lessons, timed practice exams and a plan around your exam date. Get Part B course · $99 See the plans or read the free sample lesson.Terms and refunds

Want a timed run? Try the free 25-question mini-mock. The first 5 need no email.

Prefer paper? Free printables

From the blog

Requirements, fees and paySlag inclusion: what it is and how to spot itRequirements, fees and payTungsten inclusion: what it is and how to spot itRequirements, fees and payIncomplete fusion: what it is and how to spot itRequirements, fees and payIncomplete joint penetration: what it is and how to spot itRequirements, fees and payWeld undercut: what it is and how to spot itRequirements, fees and payWeld overlap: what it is and how to spot itRequirements, fees and payWeld underfill: what it is and how to spot itRequirements, fees and payLongitudinal weld crack: what it is and how to spot itRequirements, fees and payTransverse weld crack: what it is and how to spot itRequirements, fees and payCrater crack: what it is and how to spot itRequirements, fees and payToe crack: what it is and how to spot itRequirements, fees and payRoot crack: what it is and how to spot itRequirements, fees and payUnderbead crack: what it is and how to spot itRequirements, fees and payLamination in steel plate: what it is and how to spot itRequirements, fees and payLamellar tearing: what it is and how to spot itRequirements, fees and payArc strike: what it is and how to spot itRequirements, fees and payWeld spatter: what it is and how to spot itRequirements, fees and payExcessive weld reinforcement: what it is and how to spot itRequirements, fees and payBurn-through: what it is and how to spot it