ExamFinch.com My Study

Study guide

4.2 Weld Metal and HAZ Discontinuities

Practice this exam

ASNT NDT Level III Basic Examination Exam

Track your progress as you read

Finished sections get a check mark.

1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Level III Role and Certification Path 1.3 Eligibility, Fees, Scheduling, and Retakes 1.4 Study Strategy and Exam-Day Approach

2 Chapter 2: Materials Fundamentals for NDT

2.1 Ferrous Metals and Iron-Carbon Phases 2.2 Non-Ferrous Metals and Alloys 2.3 Mechanical Properties and Crystal Structure

3 Chapter 3: Manufacturing Processes

3.1 Casting Processes and Cast Discontinuities 3.2 Forging, Rolling, and Forming 3.3 Extrusion, Drawing, and Machining 3.4 Heat Treatment Effects

4 Chapter 4: Welding and Joining Processes

4.1 Fusion Welding Processes 4.2 Weld Metal and HAZ Discontinuities 4.3 Brazing, Soldering, and Mechanical Joining

5 Chapter 5: Discontinuity Classification

5.1 Inherent Discontinuities 5.2 Processing Discontinuities 5.3 Service Discontinuities

6 Chapter 6: Specific Discontinuity Types

6.1 Cracks and Fracture Modes 6.2 Porosity, Inclusions, and Lack of Fusion 6.3 Laminations, Seams, Laps, and Undercut 6.4 Relevant vs Non-Relevant Indications

7 Chapter 7: NDT Method Selection and Inspection Planning

7.1 Method Selection Principles 7.2 Surface vs Volumetric Methods 7.3 Inspection Planning and Flaw Expectations

8 Chapter 8: Ultrasonic and Radiographic Testing

8.1 Ultrasonic Testing Principles and Applications 8.2 UT Limitations, Calibration, and Evaluation 8.3 Radiographic Testing Principles and Applications 8.4 RT Limitations, Image Quality, and Safety Basics

9 Chapter 9: Magnetic Particle, Liquid Penetrant, and Visual Testing

9.1 Magnetic Particle Testing 9.2 Liquid Penetrant Testing 9.3 Visual Testing

10 Chapter 10: Electromagnetic, Leak, Acoustic Emission, and Infrared Testing

10.1 Electromagnetic / Eddy Current Testing 10.2 Leak Testing 10.3 Acoustic Emission Testing 10.4 Thermal / Infrared Testing

11 Chapter 11: Specialty Methods and Method Comparison

11.1 Neutron Radiography 11.2 Magnetic Flux Leakage 11.3 Method Comparison and Supplemental Examinations

12 Chapter 12: SNT-TC-1A Recommended Practice

12.1 SNT-TC-1A Overview and Written Practice 12.2 SNT-TC-1A Training and Experience 12.3 SNT-TC-1A Examinations and Recertification

13 Chapter 13: CP-189 Standard Practice

13.1 CP-189 Overview and Scope 13.2 CP-189 Mandatory Requirements 13.3 SNT-TC-1A vs CP-189 Comparison

14 Chapter 14: CP-105 and NDT Training Programs

14.1 CP-105 Purpose and Topical Outlines 14.2 Developing and Approving NDT Training 14.3 OJT, Mentoring, and Instructor Oversight

15 Chapter 15: Building and Managing Certification Programs

15.1 Building Qualification and Certification Programs 15.2 Administering Examinations and Evaluating Competency 15.3 Program Audits and Managing Certifications

16 Chapter 16: Codes, Standards, Procedures, and Quality

16.1 Interpreting Codes, Standards, and Specifications 16.2 Developing and Approving NDT Procedures 16.3 Acceptance Criteria and Quality Control 16.4 External Audits and Inspection Documentation

4.2 Weld Metal and HAZ Discontinuities

Key Takeaways Explain This Topic AI

Lack of fusion and incomplete penetration are planar bonding failures—often better detected by UT than by RT when tight and unfavorably oriented

Porosity is volumetric and gas-related; slag inclusions track flux processes and incomplete interpass cleaning

Undercut and underfill are geometric profile defects; VT is primary, with codes limiting depth and length

Hot (solidification) cracks differ from cold/hydrogen/HAZ cracks in timing, location, and morphology—critical for method timing and technique

Process→flaw mapping (e.g., SMAW→slag, GTAW→tungsten, damp electrodes→hydrogen cracks) is a core Basic exam skill

Last updated: July 2026

4.2 Weld Metal and HAZ Discontinuities

Quick Answer: Weld discontinuities are process-linked flaws in weld metal, fusion line, or HAZ—lack of fusion, incomplete penetration, porosity, slag, undercut, underfill, cracks (hot, cold/hydrogen, HAZ), incomplete sidewall fusion, and tungsten inclusions. Detection depends on morphology: volumetric voids favor RT; planar cracks and LOF often favor UT; surface openings favor VT, PT, and MT.

This section is high-yield for the Basic exam: questions frequently present a process or morphology and ask for the discontinuity name, cause, or best method. Pair this material with Section 4.1 process knowledge.

  • Lack of Fusion and Incomplete Penetration
  • Lack of Fusion (LOF)

Lack of fusion is failure of weld metal to fuse completely with base metal or with prior weld beads. Forms include:

Sidewall lack of fusion — bead does not wet/fuse the groove face.

Interpass lack of fusion — successive beads do not fuse to each other.

Lack of fusion at root or toe — incomplete bonding at critical stress raisers.

Causes: low heat input, wrong travel angle, excessive travel speed, improper joint prep (rust, paint, mill scale), wrong torch/gun angle, or short-circuit modes on heavy sections. LOF is typically planar and may be tightly closed— RT contrast can be poor if the plane is not aligned with the radiation path. Angle-beam UT oriented to reflect from the unfused plane is often more reliable for critical planar LOF.

Incomplete (Lack of) Penetration

Incomplete penetration means the weld did not reach the required depth or root fusion for the joint design (e.g., root faces not consumed in a single-V groove intended as CJP). It leaves an unfused root plane that acts like a crack under load. Distinguish design-intent PJP (partial penetration allowed by drawing) from rejectable incomplete penetration on a CJP joint. VT of the root (when accessible), RT, and UT root scans are common detection approaches.

Porosity

Porosity is trapped gas voids in solidifying weld metal—typically rounded or elongated (“wormhole” / piping porosity). Sources:

Contaminants (oil, rust, moisture, paint) on joint or wire.

Moisture in electrode coatings or flux.

Inadequate or turbulent shielding (wind on GMAW/GTAW).

Excessive arc length or wrong gas.

Base metal chemistry (e.g., high sulfur can contribute to gas issues in some systems).

  • Porosity form Appearance Common context
  • Scattered Random round voids General contamination or gas issues
  • Clustered Local group of voids Local contamination or stop/start
  • Linear / aligned Chain along axis Solidification path gas
  • Piping / wormhole Elongated tunnels Gas evolution during freeze
  • Surface-breaking Open pores on face VT/PT accessible

RT is excellent for volumetric porosity (dark spots on film/digital). UT may lose back reflection or show discrete signals. Surface-open pores: VT/PT . Acceptance codes often limit size, spacing, and accumulated length.

Slag Inclusions

Slag inclusions are nonmetallic solid material trapped in the weld—almost always associated with flux-bearing processes (SMAW, FCAW, SAW) when slag is not fully removed between passes or is poured over by the next bead. Morphology is often elongated or irregular , following bead boundaries or interpass lines.

Detection: RT (slag often less dense than steel → dark indications; shape more irregular than gas porosity); UT (reflectors along fusion or interpass planes). Prevention is process control and thorough slag removal—NDT finds what cleaning missed.

Undercut and Underfill

Undercut is a groove melted into the base metal at the toe (or root) of the weld and left unfilled. It is a stress raiser and fatigue initiator. Causes: excessive current, long arc, wrong travel angle, or too high travel speed. VT is primary; depth/length limits appear in AWS and fabrication codes. MT/PT may also show undercut edges if they trap particles/penetrant, but measurement is visual/mechanical.

Underfill (or insufficient throat/face reinforcement below flush when flush is required) means the weld face or root is below the adjacent surface or below required reinforcement. It reduces section thickness. VT and weld gauges assess profile. Underfill is geometric, not a crack, but can be rejectable under workmanship standards.

Cracks — Hot, Cold, HAZ, and Hydrogen

Cracks are the most severe class of weld discontinuity. Subdivide by mechanism and location.

Crack type When formed Typical location Drivers Notes for NDT

Solidification (hot) crack During solidification (liquid films present) Weld centerline, crater, interdendritic High S/P, deep narrow beads, high restraint, concave beads Open or tight; RT/UT/MT/PT depending on surface

Liquation crack HAZ partial melting of segregates HAZ near fusion line Susceptible alloys, high heat input Often fine; surface methods + UT

Hydrogen-assisted (cold) crack Hours after cool-down (delayed) HAZ (often toe/root), sometimes weld Diffusible H + hard microstructure + stress Delayed MT/PT critical; UT for subsurface

Reheat / stress-relief crack During PWHT or high-temp service Coarse HAZ, notches Creep-strengthened alloys, restraint Process history matters

Lamellar tearing During welding of restrained T/corner joints Base metal under fusion line, rolling plane Through-thickness strain + poor Z-direction ductility UT from accessible face; plate quality

Hot vs Cold — Exam Distinctions

Hot cracks: form while metal is hot; often centerline , may show oxidized fracture surfaces; linked to solidification chemistry and bead shape (deep/narrow vulnerable).

Cold/hydrogen cracks: form near ambient after hydrogen diffusion and residual stress act on susceptible (often martensitic) microstructures; delayed hours to days; common at toes and roots of hardenable steels with moisture-bearing consumables and low preheat.

Level III procedures for hardenable steels may require inspection delay (e.g., 24–48 h) before final MT/PT so delayed cracks have time to open. Immediate inspection after welding can miss hydrogen cracks that form overnight.

Incomplete Sidewall Fusion and Related Planar Flaws

Incomplete sidewall fusion is LOF along the groove face—classic in narrow grooves, steep bevels, or when the arc is directed to the center rather than washing the walls. On RT, a straight dark line along the fusion boundary may appear if gapped; tight LOF may be invisible. UT with beams aimed at the sidewall is preferred for critical service.

Related planar issues:

Cold lap — weld metal rolls over base metal without fusion (often toe).

Unfused root face on CJP joints — incomplete penetration family.

Tungsten Inclusions

Tungsten inclusions occur almost exclusively with GTAW (and related tungsten-electrode processes) when the electrode touches the pool or filler, or when current is excessive for electrode diameter. Tungsten particles are high density : on radiographs they appear light/bright (opposite of porosity’s dark spots). They may be discrete spots or clusters. UT can also detect metallic inclusions. Prevention: proper electrode grind, stick-out, and technique—not “more amperage.”

Process → Flaw Mapping (Critical for Basic Exam)

  • Process / condition Higher-risk discontinuities
  • SMAW, poor interpass cleaning Slag inclusions, interpass LOF
  • SMAW/FCAW, wet electrodes, hardenable steel, low preheat Hydrogen cold cracks in HAZ
  • GMAW short-circuit on thick groove Lack of fusion, cold lap
  • GTAW electrode dip Tungsten inclusions
  • Contaminated joint, wind on gas shield Porosity
  • Excess current / long arc Undercut, spatter, burn-through
  • High restraint, high S/P, concave crater Hot cracks, crater cracks
  • SAW multipass, incomplete flux/slag control Slag, LOF at bead boundaries
  • High heat input on susceptible alloy Liquation cracking, wide HAZ issues
  • Restrained T-joint on poor Z-ductility plate Lamellar tearing
  • Matching NDT Methods to Weld Flaws
  • Discontinuity Preferred methods Caveats
  • Porosity RT primary; UT supplemental Surface pores: VT/PT
  • Slag inclusion RT , UT Irregular shape vs round porosity on RT
  • LOF / incomplete sidewall fusion UT angle beam often best; RT if open gap Tight planar LOF may miss on RT
  • Incomplete penetration RT , UT root techniques; VT if root accessible Know CJP vs PJP design
  • Undercut / underfill VT (+ gauges) Profile, not volumetric
  • Surface cracks MT (ferro), PT (nonferro), VT Delayed exam for hydrogen cracks
  • Subsurface cracks UT ; RT if sufficiently open/oriented Orientation vs beam/film critical
  • Tungsten inclusion RT (bright spots); UT GTAW history is the clue
  • Lamellar tearing UT in base metal under weld Plate quality + joint design

Magnetic particle (MT) detects surface and near-surface cracks on ferromagnetic welds—excellent for toe cracks and undercut-related openings after proper magnetization direction (field roughly perpendicular to expected crack).

Liquid penetrant (PT) serves nonferromagnetic alloys (stainless, aluminum, nickel) and any nonporous metal for surface-breaking cracks and open pores; surface must be clean and free of coatings that block penetrant.

Radiographic testing (RT) images volumetric density differences through the joint—porosity, many slag pockets, tungsten, and some incomplete penetration. Sensitivity depends on thickness, energy, geometry, and technique quality. Tight cracks and LOF parallel to the beam can be missed.

Ultrasonic testing (UT) finds planar flaws when the beam intersects them—LOF, cracks, incomplete penetration. Procedure design (angles, skips, calibration, DAC/TCG) is Level III territory. Austenitic welds need specialized techniques due to noise and anisotropy.

Visual testing (VT) remains mandatory baseline: profile, undercut, underfill, arc strikes, spatter, overlap, and obvious surface cracks.

Exam Scenarios

Radiograph shows rounded dark spots scattered in weld metal → porosity (gas), not slag (more irregular) or tungsten (bright).

Bright white spots on RT of GTAW root → tungsten inclusions .

Crack at HAZ toe 36 hours after welding quenched-and-tempered steel with damp cellulosic electrodes → hydrogen-assisted cold crack → delayed MT .

Centerline crack in deep, narrow single-pass weld during cooling from solidification range → hot/solidification crack .

Straight reflector along groove face on angle-beam UT; RT almost clean → likely tight sidewall LOF .

Study Focus

Separate volumetric (porosity, slag) from planar (LOF, cracks); hot vs cold crack timing and location; RT bright vs dark (tungsten vs porosity); process tables until mapping is automatic.