CWI Weld Examination Study Guide: Discontinuities and Causes

What this domain covers

Weld Examination covers what an inspector checks by eye and with gauges before, during and after welding. It also covers how to name discontinuities and their causes. It has one of the larger minimum shares of CWI Part A questions, and it supports the hands-on Part B as well.

Visual examination is the most widely used inspection method and the cheapest. Done at the right stage, it catches problems while they are still easy to fix. The other methods are compared in the NDE methods guide.

Inspection before, during and after welding

Before welding, check the paperwork and the setup:

• The welding procedure specification (WPS) and the welder's qualification.
• Base metal and filler metal identity and condition.
• Joint preparation, fit-up, root opening, alignment and cleanliness.

During welding, check that the WPS is being followed:

• Preheat and interpass temperature.
• Filler metal, technique and root quality.
• Cleaning between passes and back gouging.

Some problems, such as slag left between passes, cannot be seen once the joint is finished, so this stage matters.

After welding, check size, length, profile and surface discontinuities against the acceptance criteria, and confirm any required nondestructive testing (NDT).

Surface discontinuities and their causes

• Undercut: a groove melted at the toe and left unfilled. Typical causes are high current, long arc, fast travel and an electrode angle that aims the arc at one member.
• Overlap: weld metal rolling past the toe without fusing. Typical causes are slow travel, low current and poor angle. It leaves a notch and can hide incomplete fusion underneath.
• Porosity: trapped gas. In gas metal arc welding (GMAW), look for wind or drafts, clogged nozzles, wrong gas flow and contamination. Elongated piping or wormhole porosity often comes from rust, oil, paint or moisture on the joint faces. With self-shielded flux cored arc welding (FCAW), wrong electrode extension can also cause it.
• Arc strikes: on hardenable steel, the small spot heats and quenches quickly, which can form hard, brittle metal and small cracks. Codes require repair by grinding and checking.

Internal discontinuities and their causes

• Slag inclusions come from flux-producing processes: SMAW, FCAW, SAW and ESW. They are rare in GTAW, which uses no flux. Each process is described in the welding processes guide. Causes include poor cleaning between passes and bead shapes that leave notches the next pass cannot melt out.
• Incomplete joint penetration is mostly a geometry problem. Typical causes are a thick root face, a tight root opening, a narrow groove, low current or an oversized electrode.
• Incomplete fusion on a sidewall comes from low heat input, an arc not aimed at the groove face or fast travel. Short-circuiting GMAW and mill scale or oxide on the faces can also cause it.

Cracks

Codes such as AWS D1.1 reject cracks of any size. They are planar with sharp tips, so they concentrate stress and grow under cyclic loading. Rounded porosity, by contrast, is volumetric and less damaging.

Know the main crack types:

• Centerline (solidification) cracks are hot cracks that form as the weld freezes. Deep, narrow beads with a high depth-to-width ratio are a common cause, especially in SAW. Sulfur and phosphorus collect at the center.
• Hydrogen-induced (cold or delayed) cracks need hydrogen, a hard microstructure and stress. They often form in the heat-affected zone (HAZ) and can appear hours after welding. The metallurgy and heat control guide explains the four conditions they need. Underbead cracks are a type of hydrogen crack. Low-hydrogen consumables and preheat are the main remedies.
• Crater cracks form in unfilled craters, often star-shaped. Fill the crater.
• Lamellar tearing is a stepped, terrace-like separation in rolled base metal under a large weld in a T- or corner joint. It is caused by through-thickness shrinkage acting on flat inclusions.

Laminations are different from lamellar tearing. They are flat mill discontinuities that exist before welding. Straight-beam ultrasonic testing (UT) finds them well; they may also show on cut edges.

Linear or rounded?

Many codes call an indication linear when its length is more than three times its width, and rounded otherwise. Always check the governing code's own definition.

Gauges and measurements

Fillet weld size depends on the weld's shape:

• On a convex fillet, size is set by the shorter leg. Extra convexity does not count.
• On a concave fillet, size is limited by the throat, so the gauge checks the throat against that of the specified size.

Other common tools:

• Hi-lo gauge: internal misalignment of pipe ends, plus root opening and wall thickness.
• Weld gauges for undercut depth, reinforcement height and groove angle.
• Rules, calipers, mirrors, magnifiers and flashlights.

Temperature checks

A temperature-indicating crayon melts when the surface is at or above its rating. So:

• To confirm a minimum (such as 150 degrees F preheat), mark with the 150 degree crayon. It should melt.
• To confirm a maximum (such as a maximum interpass), mark with the crayon at the maximum. It should not melt.

Preheat is measured on the base metal at a distance from the joint set by the code. When possible, measure on the face opposite the heating torch to confirm the heat has soaked through.

Viewing conditions

For direct visual examination, ASME Section V, Article 9 sets widely used minimums. The eye must be within about 24 in. (600 mm) of the surface, at an angle of at least 30 degrees. The light must be at least 100 fc (about 1,000 lx). Mirrors and other aids can help reach difficult areas.

Distortion

Weld metal shrinks as it cools, which pulls parts out of shape. To reduce angular distortion in a double-V butt joint, alternate passes between the two sides. Smaller groove volumes, presetting the parts and restraint also help. Large weaves add heat and make distortion worse.

How to study this domain

For each discontinuity, learn three things: what it looks like, where it occurs and what causes it. Then turn it around: given a cause, name the likely discontinuity. The exam often asks it both ways.

Mark J. Curry's tip: report what you see in standard terms. Save the word "defect" for after you have compared it with the acceptance criteria.

Work through the free weld examination practice questions. To see which Part A topic to study first, take the free CWI diagnostic test.

Sources

The Weld Examination questions cite these documents, by document and, where we can confirm it, chapter or section. We never reproduce them.

1. AWS B1.11 (3 questions)
2. AWS D1.1 (2 questions)
3. ASME BPVC Section V (1 question)
4. AWS A3.0 (1 question)

Found a mistake? Use "Report a problem" under any question, and Mark J. Curry checks it.

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