4.1 Fusion Welding Processes

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:

1. Weld metal (fusion zone) — solidified melt of base metal plus any filler.
2. Heat-affected zone (HAZ) — base metal heated enough to change microstructure but not melted.
3. 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

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)

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

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:

1. Multipass SMAW/FCAW carbon steel pressure vessel seam → slag + LOF + porosity risk → RT and/or UT volumetric + MT/PT surface as required by code.
2. GTAW stainless root + GMAW fill → tungsten risk in root, LOF in fill → RT sensitive to tungsten density; UT for planar LOF if geometry allows.
3. High-deposition SAW long seam → deep weld, slag, possible centerline issues → automated UT or RT per fabrication code.
4. 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.
5. 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.