4.1 Fusion Welding Processes

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

4.1 Fusion Welding Processes

Key Takeaways Explain This Topic AI

Fusion welding melts base metal (and usually filler) to form a continuous metallurgical bond; process choice controls heat input, slag, and atmosphere exposure

SMAW, GMAW, GTAW, FCAW, and SAW differ in electrode/ consumable form, shielding method, and typical discontinuity risk profiles

Heat input (voltage × current / travel speed, with process efficiency) governs HAZ width, cooling rate, hardness, and hydrogen/crack risk

Weld metal solidifies directionally from the fusion line toward the centerline; last-to-freeze zones concentrate segregates, porosity, and solidification cracks

Level III inspection plans start from joint type and process, then select NDT methods that match expected flaw morphology and access

Last updated: July 2026

4.1 Fusion Welding Processes

Quick Answer: Fusion welding joins parts by melting base metal (often with filler) so solidification creates a continuous bond. Arc processes (SMAW, GMAW, GTAW, FCAW, SAW) dominate structural work; resistance, electron-beam, and laser welding serve specialized applications. Heat input and solidification control weld metal and HAZ quality—and therefore which NDT methods a Level III should prescribe.

The ASNT NDT Level III Basic exam treats welding as a primary manufacturing process: candidates must map process → expected discontinuities → appropriate NDT methods. This section builds that process foundation before Section 4.2 catalogs specific weld and HAZ flaws.

Why Fusion Welding Matters for NDT

A welded joint is not a homogeneous continuation of base metal. It contains:

Weld metal (fusion zone) — solidified melt of base metal plus any filler.

Heat-affected zone (HAZ) — base metal heated enough to change microstructure but not melted.

Unaffected base metal — outside the thermal cycle.

Discontinuities form in the weld, at the fusion line, and in the HAZ. Process variables (current, voltage, travel speed, polarity, shielding, preheat, interpass temperature) change heat input and cooling rate, which change hardness, residual stress, hydrogen retention, and crack susceptibility. Level III procedures must reflect the actual process—not a generic “weld inspection” template.

Arc Welding Process Families

Process Electrode / arc Shielding Typical uses NDT-relevant traits

SMAW (stick) Consumable covered electrode Flux coating → gas + slag Field construction, repair, carbon steel Slag inclusions; hydrogen from coatings if wet; versatile positions

GMAW (MIG/MAG) Continuous solid wire External gas (inert or active) Production fab, thinner to medium plate Spatter, LOF at toes if parameters wrong; little slag

GTAW (TIG) Nonconsumable tungsten + optional filler Inert gas (Ar, He) Root passes, stainless, aluminum, critical joints Tungsten inclusions if electrode dips; high quality, low H

FCAW Continuous tubular wire with flux core Self-shielded or gas-assisted Structural steel, high deposition Slag; more fumes; good outdoors (self-shielded)

SAW Continuous wire under granular flux blanket Flux covers arc Long seams, heavy plate, vessels Deep penetration; slag; limited to flat/horizontal usually

SMAW — Shielded Metal Arc Welding

SMAW uses a covered stick electrode. The covering generates shielding gas and forms slag that protects the solidifying puddle. Strengths: simple equipment, outdoor use, all-position capability with proper electrodes. Risks for NDT: slag inclusions if slag is not cleaned between passes; porosity from moisture or contamination; hydrogen-assisted cracking if cellulosic or damp coatings introduce diffusible hydrogen into hardenable steels without adequate preheat/interpass control.

GMAW — Gas Metal Arc Welding

GMAW feeds solid wire continuously under external gas. Modes include short-circuit, globular, spray, and pulsed spray—each with different heat and penetration patterns. Short-circuit transfer is cooler and can leave lack of fusion or cold lap on heavy sections if travel or technique is poor. Spray transfer gives deeper penetration but is mostly flat/horizontal. Because there is little slag, slag inclusions are uncommon, but porosity from gas interruption, wind, or contaminated wire/base metal remains a concern.

GTAW — Gas Tungsten Arc Welding

GTAW produces clean, controlled welds with a nonconsumable tungsten electrode. Filler is added separately when needed. Preferred for roots, stainless steels, nickel alloys, aluminum, and thin sections. Low hydrogen and excellent visibility of the puddle reduce many arc defects—but tungsten inclusions occur if the electrode contacts the weld pool or filler. Radiography shows tungsten as bright (high density) spots; UT may scatter on metallic inclusions.

FCAW — Flux-Cored Arc Welding

FCAW uses tubular wire filled with flux. Self-shielded FCAW needs no external gas (good outdoors); gas-shielded FCAW uses CO₂ or mixed gas for better toughness in many designs. Deposition rates are high. Like SMAW, slag must be removed between passes. Incomplete slag removal and high deposition can hide lack of interpass fusion and slag lines that RT and UT must catch on multipass joints.

SAW — Submerged Arc Welding

SAW buries the arc under a blanket of granular flux. High current, deep penetration, and high deposition suit long straight seams on plate and vessels. The flux forms slag and can contribute to slag inclusions if not managed. Geometry often limits position (flat or horizontal fillet). Thick, narrow HAZs and deep weld metal demand volumetric methods (RT or UT) for critical service.

Resistance, Electron Beam, and Laser (Overview)

Process Energy / bond mechanism Typical product Discontinuity / NDT notes

Resistance (spot, seam) Joule heating at faying surfaces under electrode pressure Sheet assemblies, auto bodies Stuck welds, expulsion, undersized nuggets; often peel/tear tests + limited NDT

Electron beam (EBW) Focused electrons in vacuum (or partial vacuum) Aerospace, precision thick joints Deep narrow fusion zone; porosity, incomplete penetration; specialized RT/UT

Laser beam (LBW) Focused laser Automotive, medical, thin precision Keyhole porosity, underfill, LOF; optical access and geometry limit methods

These processes still create fusion zones and HAZs, but joint geometry is often lap or narrow butt with limited access. Level III plans may emphasize process control, destructive sampling, or specialized UT/RT rather than classical multipass arc-weld techniques.

Heat Input and Thermal Cycle

Heat input (approximate arc energy per unit length) is commonly expressed as:

H = (V × I × 60) / S (with units consistent; multiply by process efficiency η when comparing processes)

where V = arc voltage, I = current, S = travel speed. Higher heat input generally means:

Wider, softer HAZ in carbon steels (slower cool → less martensite risk, but more grain growth).

Greater distortion and residual stress patterns that interact with restraint.

Deeper melt and wider fusion zone—but if travel is too slow with wrong parameters, excess melt-through or undercut can appear.

Low heat input (fast travel, cool short-circuit modes) can leave lack of fusion , incomplete penetration, and hard HAZs in hardenable steels—raising hydrogen crack risk if moisture is present. Preheat and interpass temperature control cooling rate independently of arc heat input and are critical on alloy and thick carbon steels.

Level III procedure writers specify preheat, interpass max/min, and heat-input limits when codes or material groups require them—and they align NDT timing (e.g., delayed MT/PT for hydrogen cracking) with that thermal history.

Solidification of Weld Metal

Weld pools freeze from the fusion line (cool base metal) toward the centerline and trailing edge. Grains often grow epitaxially from the base metal into the weld, producing columnar solidification structures. Consequences:

Last-to-freeze centerline can segregate impurities (S, P) and host solidification (hot) cracks under transverse strain.

Gas that does not escape before freeze becomes porosity (rounded).

Impurities and oxides can form inclusions aligned with solidification fronts.

Multipass welds remelt portions of prior beads; incomplete remelting leaves lack of interpass fusion or trapped slag at bead boundaries.

Understanding freeze direction helps explain why solidification cracks run along the weld centerline and why RT/UT scan plans should address centerline and fusion-line regions on critical joints.

Joint Types and Geometry

• Joint type Description Common weld types Inspection access notes
• Butt Edges aligned in same plane Groove welds (V, U, J, square) Best access for RT/UT of full thickness
• T-joint Edge to face Fillet, partial/full penetration groove Fillet root often hard to RT; UT angle beams
• Lap Overlapping plates Fillet, plug, spot Limited volumetric access; surface methods + process control
• Corner Edge to edge at angle Fillet or groove Geometry shadows radiation and UT paths
• Edge Parallel edges joined Edge weld Thin members; often VT/PT primary

Complete joint penetration (CJP) vs partial joint penetration (PJP) and fillet-only designs change the critical unfused root plane. Codes and drawings define whether that plane is acceptable; NDT acceptance criteria must match the design intent—not assume every joint is full penetration.

When Level III Selects NDT by Process

Think in process → risk → method chains:

Multipass SMAW/FCAW carbon steel pressure vessel seam → slag + LOF + porosity risk → RT and/or UT volumetric + MT/PT surface as required by code.

GTAW stainless root + GMAW fill → tungsten risk in root, LOF in fill → RT sensitive to tungsten density; UT for planar LOF if geometry allows.

High-deposition SAW long seam → deep weld, slag, possible centerline issues → automated UT or RT per fabrication code.

Field SMAW repair with cellulosic electrodes, thick hardenable steel, inadequate preheat → hydrogen crack risk in HAZ → delayed MT (and VT) after cooling/hold period, plus volumetric if specified.

Laser lap weld on thin sheet → keyhole porosity, small nugget → specialized UT or process monitoring; conventional RT may be marginal.

Visual testing (VT) is always first: profile, undercut, underfill, spatter, arc strikes, and workmanship. PT/MT address surface-breaking cracks and open discontinuities after cleaning. RT excels at volumetric porosity and many slag indications and shows tungsten brightly. UT is preferred for planar lack of fusion and cracks when orientation and access allow—especially important because tight LOF may show poorly on RT.

Procedure and Qualification Links

Welding procedure specifications (WPS), procedure qualification records (PQR), and welder performance qualifications control process parameters. The Level III does not replace the welding engineer but must understand that essential variables (process, position, thickness, F-number, preheat, etc.) change discontinuity risk. When production switches from SMAW to FCAW, or from manual to SAW, inspection procedures and technique sheets may need revision—same “weld,” different flaw population.

Study Focus

Memorize the five major arc processes by electrode type and shielding; connect heat input and solidification to HAZ hardness and centerline cracking; map joint types to access for RT/UT; practice process → flaw → method reasoning rather than isolated definitions.