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newSchoolWorks.com › VoTechWorks › Building & Construction › Welding › Gas Tungsten Arc Welding (GTAW) › Lesson 8
BLD-WELD01-06 · Lesson 8 · 10 clock hours · demo

Weld Discontinuities, Visual Inspection, and NDE Introduction

Objective. Students will identify at least eight GTAW-related weld discontinuities from sample coupons and photographs, apply AWS D1.1 visual acceptance criteria to accept or reject each, and describe when liquid penetrant testing (PT) is used on GTAW welds.

Why Discontinuities Matter in GTAW

Gas Tungsten Arc Welding (GTAW) produces some of the cleanest welds in the trade, yet the process has its own set of defects. Because GTAW is used on critical joints — stainless steel pipe, pressure vessels, aluminum aerospace components, and structural steel root passes — a defect that might be tolerable elsewhere can be cause for rejection here. Your job as the welder is to recognize discontinuities before they become service failures.

A discontinuity is any interruption in the normal physical structure of a weld. A defect is a discontinuity that exceeds the acceptance limit of the applicable code. Not every discontinuity is a defect; the code decides.

Eight Common GTAW Discontinuities

Tungsten inclusions are fragments of the non-consumable tungsten electrode trapped in the weld metal. They appear on X-ray as bright (dense) spots. Cause: touching the tungsten to the puddle or filler rod, or using too-high amperage that melts the tip.

Porosity is gas pockets trapped in solidifying weld metal. In GTAW, causes include contaminated base metal or filler rod, moisture, insufficient shielding gas coverage, or a gas lens clog. Porosity may be scattered, clustered, or linear.

Lack of fusion (LOF) occurs when the weld metal does not bond to the base metal or to a previous pass. In GTAW, it often results from too-low heat input, incorrect torch angle, or moving too fast.

Undercut is a groove melted into the base metal at the weld toe that is not filled by weld metal. It reduces the load-carrying cross-section and creates a stress concentration.

Cracks are the most serious discontinuity. Hot cracks form while the metal is still near solidification temperature; cold cracks (hydrogen-assisted) appear hours or days later. Cracks are always defects under every structural code.

Overlap (cold lap) is weld metal that rolls over the base metal at the toe without fusing. It is caused by excessive filler addition or insufficient heat.

Arc strikes are localized areas where the arc accidentally contacted the base metal outside the weld zone, causing a small hardened spot. They are a potential crack initiation site and are rejectable under AWS D1.1.

Discoloration on stainless steel and titanium indicates oxidation from inadequate shielding gas coverage on the back side (purging) or face. Color is an indirect indicator: bright silver is ideal; straw and light gold are generally acceptable on stainless; blue, grey, and black indicate damaging oxidation and chromium carbide precipitation that reduce corrosion resistance.

AWS D1.1 Visual Acceptance Criteria

AWS D1.1 is the Structural Welding Code — Steel. Table 6.1 lists visual acceptance criteria for statically and cyclically loaded structures. Key limits you will apply today include:

When you use a weld gauge, measure the actual fillet leg size, convexity, concavity, and throat. Compare your measurement to the drawing callout and the code limit. Record every measurement — a written record is your professional documentation.

Visual Inspection Procedure

1. Allow the weld to cool enough to handle safely; wear heat-resistant gloves.

2. Remove slag (GTAW produces very little) and clean the surface.

3. Use a bright LED light at a low angle (raking light) to reveal surface profile.

4. Use a 10× magnifying glass for fine cracks and undercut depth.

5. Use the weld gauge to measure undercut depth, fillet leg size, and reinforcement height.

6. Record accept or reject with the measured value and the code limit on your inspection log.

Introduction to Liquid Penetrant Testing (PT)

Some discontinuities are too fine to see with the naked eye. Liquid penetrant testing (PT) is a nondestructive examination (NDE) method that finds surface-breaking discontinuities on non-porous materials. It is widely used on GTAW stainless steel and aluminum welds where radiography is not practical.

The PT sequence:

1. Clean and dry the surface — remove oil, scale, and coatings.

2. Apply penetrant — the visible red dye soaks into any surface opening by capillary action.

3. Dwell time — the penetrant must remain on the surface long enough to enter discontinuities; follow the manufacturer's minimum time.

4. Remove excess penetrant — wipe and lightly rinse; do not flush the discontinuity.

5. Apply developer — white powder draws the dye out of the discontinuity, producing a red indication against the white background.

6. Interpret — size, shape, and location of indications are sketched on a report form.

7. Clean — remove all chemicals after inspection.

PT safety: penetrant chemicals are flammable and irritating to skin and eyes. Work in a ventilated area. Wear nitrile gloves and safety glasses. Follow SDS requirements for the specific kit.

Safety Points for This Lesson

Key terms9

DiscontinuityAny interruption in the normal physical structure of a weld or base metal; becomes a defect only when it exceeds a code acceptance limit.
Tungsten inclusionA fragment of the non-consumable tungsten electrode that becomes trapped in GTAW weld metal, typically caused by contact between the electrode and the puddle or filler rod.
UndercutA groove melted into the base metal at the weld toe that is not filled by weld metal, reducing cross-section and creating a stress concentration.
Lack of fusion (LOF)Failure of the weld metal to fuse completely with the base metal or an adjacent weld pass, often caused by insufficient heat input or incorrect technique.
PorosityGas pockets trapped in solidifying weld metal; in GTAW commonly caused by contamination, moisture, or inadequate shielding gas coverage.
Liquid penetrant testing (PT)A nondestructive examination method that uses a dye drawn into surface-breaking discontinuities by capillary action, then revealed with a contrasting developer.
Dwell timeThe minimum time a penetrant must remain on a surface before removal, allowing it to fully enter any open discontinuity.
Arc strikeA localized area outside the weld zone where the arc accidentally contacted the base metal, creating a hard, potentially cracked spot; rejectable under AWS D1.1.
Weld gaugeA precision hand tool used to measure fillet weld leg size, throat, reinforcement height, undercut depth, and other weld profile dimensions.

Worked examples4

1. You measure a fillet weld leg with a weld gauge. The drawing calls for a 5/16 in. fillet weld. Your gauge reads 1/4 in. on one leg. AWS D1.1 allows a fillet weld to be undersized by no more than 1/16 in. for welds 5/16 in. and larger (single pass). Is this weld accept or reject?
  1. Identify the specified size: 5/16 in.
  2. Identify the measured size: 1/4 in. = 4/16 in.
  3. Calculate the shortage: 5/16 − 4/16 = 1/16 in.
  4. Compare to the allowable undersize tolerance: the shortage equals 1/16 in., which is at the limit — not greater than the limit.
  5. Conclusion: the weld is at the borderline of the tolerance. Unless the code provision for that specific condition says 'less than 1/16 in.' the weld is at the maximum allowable undersize. Document the measurement and flag for re-inspection or engineering review depending on your site quality plan.
Answer: The weld is exactly at the allowable undersize limit of 1/16 in.; it does not exceed the limit, so it is technically accept — but the measurement must be documented and the weld should not be further undersized anywhere along its length.
2. You are measuring undercut depth with a weld gauge on a statically loaded structural joint. You measure 0.040 in. (approximately 1 mm). The AWS D1.1 limit for undercut on a statically loaded member is 1/32 in. Is this accept or reject?
  1. Convert the AWS D1.1 limit to decimals: 1/32 in. = 0.03125 in.
  2. Compare to the measured value: 0.040 in. > 0.03125 in.
  3. The measured undercut exceeds the code limit.
Answer: Reject. The undercut depth of 0.040 in. exceeds the AWS D1.1 limit of 1/32 in. (0.031 in.) for statically loaded members. The weld must be repaired — typically by grinding smooth and re-welding the toe.
3. During PT, the dwell time on a penetrant kit label states a minimum of 10 minutes and a maximum of 60 minutes at temperatures between 50 °F and 125 °F. The shop is at 68 °F. You apply penetrant at 9:00 AM. At what earliest time may you remove the excess and proceed to developer?
  1. Identify start time: 9:00 AM.
  2. Identify minimum dwell time: 10 minutes.
  3. Add minimum dwell time to start time: 9:00 AM + 10 min = 9:10 AM.
  4. Confirm the temperature is within the stated range: 68 °F is between 50 °F and 125 °F — yes.
Answer: You may begin removing excess penetrant no earlier than 9:10 AM. Do not exceed 60 minutes or the penetrant may dry in the discontinuity, masking the indication.
4. A coupon has three separate pores visible on the weld surface. Their diameters measured under a 10× magnifier are approximately 0.030 in., 0.045 in., and 0.020 in. The applicable code limits individual pore diameter to 3/32 in. (0.094 in.). Are any of these individual pores rejectable on size alone?
  1. Convert the code limit: 3/32 in. = 0.09375 in.
  2. Compare each pore: 0.030 in. < 0.094 in. — within limit; 0.045 in. < 0.094 in. — within limit; 0.020 in. < 0.094 in. — within limit.
  3. None of the three pores exceeds the individual size limit.
Answer: No individual pore is rejectable on diameter alone. However, you must also check frequency and clustering rules in the applicable code table, because pores that individually pass may still produce a rejectable cluster pattern.

The math in this lesson1

Learners compare leg, undercut and pore measurements to code limits and compute penetrant dwell-time windows.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Discontinuity identification station rotation: students examine a set of 12 prepared sample coupons and photographs (tungsten inclusions, porosity, lack of fusion, undercut, cracks, overlap, arc strikes, discoloration) and complete an ID worksheet, then check answers against an answer key. · Go/no-go visual inspection drill: instructor presents six completed GTAW coupons; students use a weld gauge and bright light to measure and classify each as accept or reject per AWS D1.1 Table 6.1, documenting measurements in a log. · PT demonstration (liquid penetrant): instructor applies dye penetrant to a cracked stainless coupon, performs dwell time, removes excess, and applies developer; students observe indication development and sketch the indication pattern on a report form.
Materials: Prepared defect coupon set (12 samples), weld gauge, magnifying glass (10×), AWS D1.1 structural code Table 6.1 handout, liquid penetrant test kit (visible dye), bright LED inspection light, inspection report forms
Check: Discontinuity ID worksheet score 80% or higher (at least 10 of 12 correct); go/no-go decisions match instructor key on at least 5 of 6 coupons — both criteria required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

GTAW process knowledge / AWS SENSE
1. Which discontinuity is unique to GTAW and results from the electrode contacting the weld puddle?
Visual inspection / AWS D1.1 Certified Welder
2. Under AWS D1.1, arc strikes on the base metal outside the weld zone are:
NDE methods / NCCER Welding
3. During liquid penetrant testing, what is the purpose of the developer?
GTAW process quality / AWS SENSE
4. A GTAW bead on stainless steel pipe shows a gray-to-black discoloration on the back side of the weld. This most likely indicates:
Visual inspection / AWS D1.1 Certified Welder
5. You measure undercut depth on a completed GTAW fillet weld and record 0.025 in. The AWS D1.1 limit for statically loaded members is 1/32 in. What is the correct disposition?
NDE methods / NCCER Welding
6. Which sequence correctly describes the liquid penetrant testing process?
GTAW technique / AWS SENSE
7. A welder notices lack of fusion (LOF) on the sidewall of a GTAW groove weld. Which corrective action during welding is most likely to prevent this in the future?
Welder qualification / ASME Sec. IX pipe
8. A GTAW welder qualification test under ASME Section IX requires a visual examination of the test coupon. Which discontinuity would always cause the coupon to be rejected, regardless of size?
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