8.2 Welding Discontinuities and Weldment Examination

8.2 Welding Discontinuities and Weldment Examination

Metallurgical and Thermal Dynamics of Welded Joints

Fusion welding creates an localized metallurgical casting process within a component. During deposition of the molten weld pool, localized regions of the weld metal and parent material undergo extreme thermal cycles, steep temperature gradients ($>1000^\circ\text{C}/\text{mm}$), and rapid solid-state phase transformations.

• The Fusion Zone (FZ): The solidified weld metal exhibiting a cast dendritic microstructure. As the molten weld metal cools, solidification shrinkage and thermal contraction generate severe residual tensile stresses that frequently reach the yield strength of the material.
• The Heat-Affected Zone (HAZ): The narrow band of parent base metal immediately adjacent to the fusion line that was not melted, but whose microstructure and mechanical properties were substantially altered by the welding thermal spike. In medium- and high-carbon or low-alloy steels, rapid cooling (quenching) by the massive surrounding base metal transforms austenite into brittle, untempered martensite, rendering the HAZ highly susceptible to hydrogen-assisted cold cracking.
• Solidification Shrinkage & Restraint: When thick structural plates or rigid assemblies are joined, mechanical restraint prevents the cooling weld metal from contracting freely. High transverse and longitudinal tensile residual stresses are locked into the joint, driving the formation of hot and cold weld discontinuities.

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Classification and Morphology of Welding Discontinuities

1. Longitudinal Weld Cracks

Longitudinal cracks run parallel to the longitudinal axis of the weld seam. They occur primarily in two distinct metallurgical zones:

• Centerline Solidification Cracks (Hot Cracking): Occur during weld pool solidification along the weld centerline where advancing columnar dendrites growing from opposite bevel faces meet. Low-melting-point eutectic segregates (such as iron sulfides $\text{FeS}$ and phosphides) are rejected into the remaining liquid at the centerline. High transverse contraction stresses tear this liquid film apart, creating an open, longitudinal centerline fissure.
• Fusion Line Cracks: Occur along the boundary separating the weld deposit from the parent base metal, often triggered by severe geometric notch stress concentrations combined with hard, unrefined grain structures.
• MT Indication: Forms a crisp, continuous or intermittent linear indication running directly down the center or along the toe of the weld bead.

2. Transverse Weld Cracks

Transverse cracks run across the weld bead, oriented perpendicular ($90^\circ$) to the longitudinal axis of the weld seam. They frequently initiate in the weld metal and propagate across the fusion boundary into the adjacent Heat-Affected Zone and parent metal.

• Metallurgical Causes: Driven by high longitudinal tensile residual stresses combined with excessively hard microstructures or delayed hydrogen ingress (hydrogen-induced cold cracking).
• High-Alloy and Thick Plate Susceptibility: Common in high-strength quenched and tempered steels (e.g., ASTM A514, HY-80, AISI 4340) welded with inadequate preheat or insufficient post-weld interpass temperature control.
• MT Indication: Appears as sharp, knife-edge linear indications cutting directly across the weld crown rippling, frequently extending past the weld toes into the adjacent base plate.

3. Crater Cracks (Termination Cracks)

Crater cracks are hot shrinkage cracks that form in the shallow depression (weld crater) left at the termination point of a weld bead when the welding arc is extinguished abruptly.

• Solidification Mechanism: When a welder breaks the arc without pausing to backfill the crater, the outer perimeter of the puddle solidifies rapidly while the center remains molten. As the molten central core freezes and contracts without additional filler metal, three-dimensional tensile shrinkage stresses tear the unsolidified grain boundaries.
• Morphology: Crater cracks commonly form a characteristic star-shaped, multi-pointed radiating pattern (often termed "star cracks"), or small single longitudinal/transverse fissures contained entirely within the crater.
• MT Indication: Distinct star-like, cross-shaped, or radiating cluster indications confined within the weld stop crater.

4. Lack of Side-Wall and Inter-Pass Fusion (LOF)

Lack of fusion represents a planar discontinuity resulting from the failure of the deposited weld metal to fuse (coalesce) completely with the adjacent base metal preparation bevel (side-wall lack of fusion) or with previously deposited adjoining weld passes (inter-pass lack of fusion).

• Primary Causes: Insufficient welding heat input, excessive travel speed, improper electrode angle causing the arc to ride on the puddle rather than the bevel face, heavy unground mill scale, or severe magnetic arc blow deflecting the arc.
• Physical Signature: LOF constitutes a sharp, planar separation often accompanied by thin oxide films. When exposed to the surface, it represents an intense stress concentrator.
• MT Indication: Forms an exceptionally straight, razor-sharp linear indication running parallel to the weld seam along the weld toe or fusion line.

5. Incomplete Penetration (Lack of Penetration - LOP)

Incomplete penetration occurs when the weld metal fails to extend completely through the joint thickness, leaving an unfused channel along the root face or failing to fuse the backing strip.

• Causes: Root gap too narrow, root face (land) too thick, incorrect electrode diameter, or insufficient welding current.
• MT Detectability Boundary: If the weld root is physically accessible (e.g., inside an open pipe or double-welded vessel), MT provides outstanding detection of LOP, forming a continuous linear indication down the root centerline. However, if the joint is single-welded with an inaccessible root (e.g., small-bore piping), MT conducted from the weld crown will completely miss root lack of penetration, because the flaw is buried deep beneath the crown reinforcement beyond the penetration reach of the magnetic field.

6. Undercut

Undercut is a groove or channel melted into the parent base metal adjacent to the weld toe or weld root that is not backfilled by deposited weld metal.

• Causes: Excessive welding current, excessive arc voltage, incorrect torch travel angle, or improper weaving technique.
• Risk Factor: Undercut creates an abrupt change in cross-sectional geometry at the exact boundary where tensile residual stresses and metallurgical notch sensitivity are highest, serving as the prime initiation site for subsequent service fatigue cracks.
• MT Indication: Forms a continuous, broad linear indication running parallel to the weld toe. Inspectors must take care to differentiate true undercut indications from non-relevant geometric flux leakage created by steep weld reinforcement angles.

7. Porosity (Gas Cavities)

Porosity consists of spherical, cylindrical, or elongated voids formed by gas entrapment (hydrogen, nitrogen, carbon monoxide) in the weld pool during rapid solidification.

• Types:
  • Isolated / Scattered Porosity: Individual spherical pores dispersed through the bead.
  • Clustered Porosity: Groups of pores concentrated at arc strikes or weld starts.
  • Linear Porosity: A string of pores aligned parallel to the weld axis along a pass boundary.
  • Piping Porosity (Wormholes): Elongated cylindrical gas channels extending toward the surface caused by severe gas evolution.
• MT Indication: Surface-breaking pores produce distinct, rounded particle accumulations. Subsurface pores within 1 mm of the surface create broad, faint, fuzzy rounded spots with low contrast.

8. Slag Inclusions

Slag inclusions are non-metallic solid materials (oxides, flux residue, or deoxidation products) trapped within the weld metal or along the fusion interface.

• Causes: Incomplete inter-pass slag chipping during multi-pass flux-cored (FCAW) or shielded metal arc welding (SMAW), or improper puddle manipulation.
• MT Indication: Non-magnetic slag trapped at or near the surface produces irregular, elongated or rounded indications with moderate edge definition.

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Weld Examination Techniques and Magnetization Protocols

The AC Articulating Electromagnetic Yoke

The portable articulating AC electromagnetic yoke is the industry standard instrument for non-destructive weldment examination per ASME Section V Article 7, AWS D1.1 (Structural Welding Code - Steel), and API 1104.

• Operating Physics: An AC yoke induces an alternating magnetic field into the ferromagnetic weldment via magnetic induction. Due to the electromagnetic skin effect, AC magnetic flux is concentrated within a shallow surface layer ($1\text{ to }2\text{ mm}$ depth). This surface concentration maximizes the surface tangential field strength ($H_t$) and imparts exceptional vibrational mobility to magnetic particles, optimizing detection of fine surface-breaking cracks.
• Elimination of Arc Strikes: Because the yoke transfers magnetic energy inductively through laminated pole legs without passing electrical current directly into the steel, it completely eliminates the catastrophic risk of electrical arc burns associated with prod testing.
• Lifting Power Calibration: Governing codes mandate that an AC yoke must demonstrate a minimum lifting force of 10 lbs (4.5 kg) at the maximum pole spacing to be used, verified with a calibrated steel test weight.

Articulating Leg Positioning and Field Coverage

Welded surfaces present complex surface profiles, including raised crowns, rippled beads, and angular plate transitions. Articulating double-jointed yoke legs must be positioned so that both pole faces make intimate, flush mechanical contact with the steel.

The Orthogonal Two-Pass Rule

Fundamental Level III Mandate: A complete weld examination requires two separate, mutually perpendicular (orthogonal) inspection passes to ensure 100% directional flaw coverage:

1. Pass 1: The Straddle Pass (Transverse Magnetization):
   • The yoke legs are positioned straddling the weld crown, with one pole on either side of the weld bead in the adjacent base plate.
   • Magnetic flux lines travel across the weld bead, oriented perpendicular ($90^\circ$) to the longitudinal axis of the weld seam.
   • Flaws Detected: Longitudinal centerline cracks, longitudinal fusion line cracks, lack of side-wall fusion, and weld toe undercut/cracking.
2. Pass 2: The Longitudinal Pass (Parallel Magnetization):
   • The yoke is rotated $90^\circ$, placing both pole legs directly along the centerline of the weld crown (or immediately adjacent along the HAZ).
   • Magnetic flux lines travel parallel to the longitudinal axis of the weld seam.
   • Flaws Detected: Transverse weld cracks, transverse HAZ cracks, and crater cracks.
3. Scanning Overlap: To prevent uninspected dead zones immediately beneath the pole contact footprints (where flux enters vertically with zero tangential leakage capability), successive yoke placements must overlap by at least 25 mm (1.0 inch), or 20% of the active pole spacing.

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Prod Testing: Mechanics, Risks, and Controls

Prod testing involves passing high-amperage direct electrical current (typically 90 to 125 A per inch of prod spacing) directly through the component via two handheld copper- or aluminum-tipped electrical contact electrodes.

• Magnetic Field Geometry: Current flowing between prods creates a circular magnetic field around each prod and an intense, combined elliptical field in the zone between them. A prod spacing of 6 to 8 inches (150 to 200 mm) is standard.
• The Severe Risk of Arc Strikes: When prods are placed or lifted while electrical current is flowing, or if contact pressure is inadequate, electrical arcing occurs between the prod tip and the steel surface.
  • Metallurgical Consequence: The intense heat of the electrical arc instantly melts a localized spot of base metal. The massive cold surrounding steel acts as a severe heat sink, rapidly self-quenching the melted spot to form untempered, brittle martensite containing high residual tensile stresses and micro-fissures.
  • Copper Contamination: Arcing can melt copper from the prod tips directly into the grain boundaries of the molten steel pool, inducing liquid metal embrittlement (copper cracking).
• Code Restrictions on Prods: In critical aerospace alloys, nuclear components, and high-strength quenched and tempered structural steels (e.g., ASTM A514), prod testing is strictly prohibited due to arc strike hazards.
• Required Level III Controls (Where Prods are Permitted):
  1. Use of lead, aluminum, or braided copper contact pads rather than bare pointed copper prods.
  2. Mandating remote trigger switches: current must only be energized after firm mechanical contact pressure is established, and current must be de-energized before lifting prods from the surface.
  3. Mandatory post-inspection dressing (light grinding and etching) of all prod contact sites to verify absence of martensitic arc strikes.

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Surface Preparation and Joint Profiling

Effective magnetic particle inspection requires intimate mechanical access to the steel surface:

• Weld Spatter: Globules of molten weld metal spattered onto adjacent base metal act as physical particle traps, creating false indications and lifting yoke legs off the surface. All spatter within 50 mm (2 inches) of the weld must be scraped or ground smooth.
• Slag and Oxide Scale: Multi-pass weld slag must be removed by wire brushing or needle scaling. Tightly adhering mill scale must be removed if it exceeds $0.05\text{ mm}$ ($2\text{ mils}$) thickness.
• Weld Reinforcement Profile and Flush Grinding: Steep weld crowns with abrupt reinforcement angles create non-relevant magnetic flux leakage at the weld toe. For critical aerospace joints, rotating machinery welds, and high-cycle fatigue connections, weld crowns must be ground flush with the base plate to eliminate geometrical flux leakage and optimize detection of micro-cracks.

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Physical Boundaries: MT vs. Radiographic (RT) and Ultrasonic (UT) Testing

Level III personnel must understand the fundamental physical capabilities and limits of MT relative to complementary volumetric non-destructive testing methods:

Electromagnetic Skin Depth Limitation

The alternating magnetic field induced by an AC yoke decays exponentially with depth ($z$) beneath the surface according to the classical electromagnetic skin depth equation: $\delta = \sqrt{\frac{\rho}{\pi f \mu}}$ Where $\rho$ is electrical resistivity, $f$ is frequency (60 Hz), and $\mu$ is magnetic permeability. In structural carbon steels, skin depth ($\delta$) is approximately 1.0 to 1.5 mm (0.04 to 0.06 inches). Flux leakage from defects located deeper than $2\text{ mm}$ beneath the surface drops to near zero.

Even when using penetrating Full-Wave Rectified Direct Current (FWDC) with dry powder, the maximum reliable depth of flaw detection in structural steel weldments rarely exceeds 6 mm (0.25 inches) under ideal laboratory conditions, and is typically $<3\text{ mm}$ in field conditions.

Volumetric NDT Mandate

Because MT is physically blind to deep internal discontinuities:

1. Internal Lack of Fusion / Buried Slag: Planar side-wall lack of fusion buried at mid-thickness in a 25 mm (1.0 inch) thick multi-pass weld produces zero surface flux leakage and is 100% invisible to MT. Volumetric detection requires Ultrasonic Testing (UT)—specifically shear-wave angle-beam or Phased Array Ultrasonic Testing (PAUT).
2. Internal Volumetric Voids: Subsurface porosity clusters, piping porosity, and slag inclusions trapped beneath the cap pass require Radiographic Testing (RT) or Ultrasonic Testing.
3. Inaccessible Weld Roots: In complete joint penetration (CJP) pipe welds welded from the outside only, unbacked root lack of penetration (LOP) cannot be detected from the outer crown surface by MT. RT or internal angle-beam UT is mandatory.

Conclusion: Magnetic Particle Testing is the premier, most cost-effective method for verifying surface integrity and catching crown, toe, and HAZ cracks. However, MT can never replace volumetric UT or RT for full-thickness weldment qualification.

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Summary Comparison Table: Welding Discontinuities

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Practical Level III Engineering Scenario and Exam Traps

Scenario: A fabricator completes a multi-pass complete joint penetration (CJP) groove weld on 38 mm (1.5 inch) thick ASTM A514 Grade B quenched and tempered high-strength structural bridge steel. The welding procedure specification (WPS) mandates 100% MT of the finished weldment. The inspection contractor uses a portable AC yoke, executes a single straddle pass across the weld crown with dry visible black powder, and signs off the joint as conforming.

Level III Technical Audit and Findings:

1. Missing Orthogonal Pass: The contractor executed only the straddle pass (transverse field). While this pass detects longitudinal cracks and side-wall lack of fusion, it is completely blind to transverse cracks and crater cracks running crosswise to the bead.
2. High-Strength Steel Risk: ASTM A514 is highly susceptible to delayed transverse hydrogen cracking in the weld metal and coarse-grained HAZ. A single straddle pass creates a massive risk of missing transverse cracks.
3. Timing of MT Examination: The contractor inspected the joint immediately after the weld reached room temperature. High-strength steels require a mandatory post-weld holding period of at least 48 hours before final MT acceptance inspection to permit delayed hydrogen cold cracking to manifest.
4. Volumetric Flaw Blindness: The contractor's sign-off implied full-thickness joint integrity, ignoring the reality that AC MT only inspects the outer 1 to 2 mm skin. Deep internal lack of side-wall fusion at mid-thickness remains uninspected.

Mandatory Corrective Action: The Level III rejects the inspection report. The weld must be quarantined for 48 hours post-welding. The weldment must then undergo a qualified two-pass orthogonal MT examination (straddle pass and longitudinal pass) using wet fluorescent MT or dry powder with white contrast paint. In addition, angle-beam ultrasonic testing (UT) or phased array UT must be performed to inspect the internal volumetric cross-section and unfused root boundaries.