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newSchoolWorks.com › VoTechWorks › Building & Construction › Welding › Metallurgy for Structural and Pipe Welding › Lesson 6
BLD-WELD01-08 · Lesson 6 · 12 clock hours · lab

Weld Discontinuities: Metallurgical Root Causes and Code Acceptance Criteria

Objective. The student identifies at least six types of weld discontinuities in sectioned or macro-etched coupons, states the metallurgical or procedural cause of each, and applies a simplified AWS D1.1 visual acceptance criterion to determine accept or reject.

Weld Discontinuities: Metallurgical Root Causes and Code Acceptance Criteria

A discontinuity is any interruption in the normal physical structure of a weld or base metal. A defect is a discontinuity that exceeds the acceptance limits of the governing code. Not every discontinuity is a defect—your job as a welder and future credential holder is to know the difference.

The Eight Discontinuities You Must Know

Porosity forms when gas—most often carbon monoxide, hydrogen, or nitrogen—is trapped in the solidifying weld pool. Causes include contaminated base metal (oil, rust, moisture), a damaged or wet electrode coating, insufficient shielding gas coverage, or excessive travel speed that freezes the pool before gas can escape. Porosity appears as rounded voids in a macro-etched section.

Hydrogen cracking (also called cold cracking or delayed cracking) is the most dangerous structural weld defect. Atomic hydrogen diffuses into the heat-affected zone (HAZ) while austenite is present, then becomes trapped as the steel transforms to martensite. Cracking can be delayed hours or days after welding. Causes: high-hydrogen electrodes (not properly dried or stored), high carbon equivalent base metal, rapid cooling, and high restraint. Prevention: use low-hydrogen electrodes (E7018 stored in a rod oven), preheat, and maintain minimum interpass temperature.

Hot cracking (solidification cracking) occurs in the weld metal while it is still partially liquid, along the final solidifying grain boundaries. High sulfur or phosphorus content in the base metal, a deep narrow bead with a high depth-to-width ratio, and excessive heat input all promote hot cracking. Prevention: maintain a favorable bead width-to-depth ratio, select base metal with low sulfur and phosphorus, and control heat input.

Lack of fusion (LOF) is an area where weld metal and base metal—or adjacent weld passes—did not bond metallurgically. Causes include insufficient heat input, improper travel angle, dirty groove faces, or moving too fast. LOF is planar and therefore more severe than porosity.

Undercut is a groove melted into the base metal at the weld toe that is not filled by weld metal. It creates a stress concentration. Causes: excessive current, incorrect electrode angle, or excessive travel speed.

Overlap is the protrusion of weld metal beyond the weld toe onto the base metal surface without fusion. It results from too slow a travel speed or too low a current.

Underfill occurs when the weld face is below the surrounding base metal surface, leaving insufficient throat. Causes include insufficient weld passes or excessive travel speed.

Lamellar tearing is a subsurface cracking in the base metal HAZ caused by through-thickness tensile stress acting on planes of non-metallic inclusions (sulfide stringers) in rolled plate. It is most common in T-joints and corner joints with high restraint and plate rolled in the wrong orientation.

Simplified AWS D1.1 Visual Acceptance Criteria

For structural steel welds inspected visually under AWS D1.1 (statically loaded structures, as a general guide):

Always verify acceptance criteria against the edition of the code that governs your project.

Visual Inspection Tools and Technique

A Certified Welding Inspector (CWI) uses simple, calibrated hand tools:

Inspect in adequate light (minimum 1 foot-candle at the weld surface is a common field guideline). View the weld from multiple angles; surface-breaking cracks are easiest to see when light grazes the surface at a low angle.

Linking Procedure Variables to Metallurgical Cause

When writing a root-cause analysis, trace each variable through the metallurgy:

Safety in the Macro-Etch Lab

Macro-etching uses strong acids (commonly hydrochloric acid solution). Always wear chemical splash goggles, acid-resistant gloves, and a lab apron. Work under a fume hood or in a well-ventilated area. Have an eyewash station within 10 seconds of your work area. Never add water to concentrated acid—add acid to water. Dispose of spent etchant according to your school's hazardous waste procedure.

When using physical plate coupons, edges may be sharp. Handle with leather gloves. Apply lockout/tagout procedures on any power sources before handling coupons from the weld area.

Key terms10

DiscontinuityAny interruption in the normal physical structure of a weld or base metal; may or may not be a rejectable defect depending on code limits.
DefectA discontinuity whose size, shape, or location exceeds the acceptance limits of the governing code, making the weld rejectable.
Hydrogen cracking (cold cracking)Cracking caused by diffusible hydrogen trapped in the heat-affected zone, often occurring hours or days after welding in high-restraint or high-carbon-equivalent joints.
Hot cracking (solidification cracking)Cracking that occurs in partially solidified weld metal along grain boundaries, promoted by high sulfur or phosphorus content and a deep, narrow bead geometry.
Lack of fusion (LOF)A planar discontinuity where weld metal failed to bond metallurgically to the base metal or to an adjacent weld pass.
UndercutA groove melted into the base metal at the weld toe that is not refilled with weld metal, creating a stress concentration.
Lamellar tearingSubsurface cracking in rolled base metal caused by through-thickness tensile stress acting on planes of sulfide or other non-metallic inclusions.
Heat inputThe amount of energy transferred per unit length of weld, calculated from voltage, amperage, and travel speed; controls cooling rate and HAZ size.
Carbon equivalent (CE)A formula-based index that estimates how hardenable—and therefore how susceptible to hydrogen cracking—a steel is, based on its alloy content.
Macro-etchA cross-sectional metallurgical examination technique in which a polished weld coupon is exposed to an acid solution to reveal the weld bead profile, fusion zone, and discontinuities.

Worked examples4

1. A fillet weld has a specified leg size of 5/16 in. You measure the actual leg with a fillet weld gauge and read 1/4 in. Is this weld acceptable under a general AWS D1.1 visual criterion requiring the weld to meet specified size?
  1. Convert both dimensions to the same unit: specified = 5/16 in. = 0.3125 in.; measured = 1/4 in. = 0.250 in.
  2. Calculate the shortfall: 0.3125 − 0.250 = 0.0625 in. (1/16 in.).
  3. Apply the criterion: AWS D1.1 requires the fillet weld leg to meet the specified size; any weld that is undersized is rejectable.
  4. Decision: 0.250 in. < 0.3125 in. required — REJECT. The weld must be supplemented with an additional pass to achieve at minimum 5/16 in.
Answer: REJECT — the measured leg of 1/4 in. is 1/16 in. short of the specified 5/16 in.; the weld must be built up to the required size.
2. You measure undercut depth at the weld toe of a primary structural member using an undercut gauge. The gauge reads 0.040 in. The simplified AWS D1.1 limit for undercut on primary members is 1/32 in. Is this acceptable?
  1. Convert the code limit to decimal inches: 1/32 in. = 0.03125 in.
  2. Compare: measured undercut = 0.040 in.; allowable limit = 0.03125 in.
  3. 0.040 in. > 0.03125 in., so the undercut exceeds the allowable depth.
  4. Decision: REJECT. The undercut must be repaired by grinding the toe to a smooth transition and, if needed, depositing additional weld metal.
Answer: REJECT — 0.040 in. exceeds the 1/32 in. (0.031 in.) limit for undercut on primary members.
3. Calculate the heat input for a SMAW pass made at 150 A and 24 V with a travel speed of 6 in./min. Use the standard formula: Heat Input (kJ/in.) = (Amps × Volts × 60) ÷ (Travel Speed in./min × 1000).
  1. Identify values: A = 150, V = 24, travel speed = 6 in./min.
  2. Multiply amps × volts × 60: 150 × 24 × 60 = 216,000.
  3. Multiply travel speed × 1000: 6 × 1000 = 6,000.
  4. Divide: 216,000 ÷ 6,000 = 36 kJ/in.
Answer: Heat input = 36 kJ/in. This is a moderate-to-high heat input level; increasing travel speed or reducing current would lower it and reduce the risk of excessive grain growth in the HAZ.
4. A macro-etched section shows a crack running along the centerline of the weld bead from the root to partway through the weld. The weld was made on steel with elevated sulfur content using a deep, narrow bead. Identify the discontinuity type and state whether any size is acceptable under AWS D1.1.
  1. Analyze location and geometry: centerline crack along the last-solidifying grain boundary in the weld metal — this matches the definition of hot cracking (solidification cracking).
  2. Identify contributing causes: high sulfur content promotes low-melting-point grain boundary films; deep narrow bead geometry concentrates solidification shrinkage stress along the centerline.
  3. Apply code criterion: AWS D1.1 prohibits cracks of any size in structural welds — there is no minimum accept threshold.
  4. Decision: REJECT regardless of crack length or depth. The weld must be removed and replaced, and the WPS must be revised to correct bead geometry and specify low-sulfur base metal.
Answer: Hot cracking (solidification cracking) — REJECT. No crack of any size is acceptable under AWS D1.1. The procedure must be corrected before rewelding.

The math in this lesson2

Learners compute heat input in kJ/in at 150 A, 24 V, 6 in/min and compare leg and undercut readings to limits.
A-SSE.1N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Examine a set of eight instructor-prepared macro-etched weld cross-sections showing porosity, hydrogen cracking, hot cracking, lack of fusion, undercut, overlap, underfill, and lamellar tearing; sketch each defect and annotate cause and prevention in the lab notebook. · Role-play as a CWI using a visual inspection kit (fillet weld gauge, undercut gauge, 6-inch scale, magnifying glass) on physical plate coupons and record accept/reject decisions using a simplified D1.1 visual inspection checklist—instructor then reveals correct answers and leads discussion. · Write a one-page root-cause analysis for three assigned discontinuities, linking welding procedure variables (travel speed, heat input, moisture, interpass temperature) to the specific metallurgical mechanism that produced each defect.
Materials: Set of 8 macro-etched weld cross-section samples, visual inspection kit (fillet and undercut gauges, steel scale, 10× loupe), AWS D1.1 simplified visual acceptance criteria handout, root-cause analysis worksheet, lab notebooks
Check: Correct identification of discontinuity type and cause on at least 6 of 8 samples; accept/reject decisions on physical coupons match instructor key with no more than one error; root-cause analysis demonstrates correct metallurgical reasoning on all three samples. 80% overall.

Practice check8

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

Metallurgy and Weld Discontinuities
1. Which discontinuity can occur hours or days after welding is completed and is most directly caused by diffusible hydrogen trapped in the heat-affected zone?
AWS D1.1 Code Application / Visual Inspection
2. A visual inspection reveals a groove at the weld toe of a primary structural member measuring 0.025 in. deep. Applying a simplified AWS D1.1 undercut limit of 1/32 in. for primary members, what is the correct decision?
Metallurgy and Weld Discontinuities
3. Lamellar tearing is most likely to occur in which type of joint and base metal condition?
Metallurgy and Weld Discontinuities
4. Which of the following best describes 'lack of fusion' as seen in a macro-etched cross-section?
Welding Procedure and Defect Prevention
5. A welder uses damp E7018 electrodes without drying them in a rod oven before welding high-restraint structural joints. Which defect is the PRIMARY risk?
AWS D1.1 Code Application / Visual Inspection
6. According to AWS D1.1, what is the correct accept/reject decision for a surface crack of any length found during visual inspection of a structural weld?
Metallurgy and Welding Procedure Variables
7. Increasing travel speed while keeping amperage and voltage constant will have which primary effect on weld metallurgy?
Metallurgy and Weld Discontinuities
8. During a lab macro-etch examination, you observe cracks radiating from the centerline of the weld toward the outer surface in a bead with a noticeably greater depth than width. The base metal has elevated phosphorus content. What is the most likely discontinuity?
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