12.2 Inclusions, Misalignment & Miscellaneous Defects

12.2 Inclusions, Misalignment & Miscellaneous Defects

Quick Answer: Solid inclusions (slag, tungsten, oxide) are ISO 6520 Group 3 imperfections. Misalignment (linear hi-lo and angular) is a fit-up/geometry problem that can invalidate the intended throat and stress path. Spatter, arc strikes, underflush, and stray-arc burns are often miscellaneous surface imperfections—easy to dismiss, but they can crack, corrode, or fail hardness/NDT rules. The IWI-S identifies, measures, records, and applies acceptance—then drives process correction.

This section completes the common non-crack defect set after porosity, LOF/LOP, and shape imperfections. Module WI1.7 expects recognition and cause thinking; WI1.8–1.9 expects you to evaluate against the quality documents you are authorised to apply.

Solid Inclusions — Group 3 Overview

A solid inclusion is foreign solid material trapped in the weld metal or between runs. Unlike gas pores (Group 2), inclusions are solid particles or films. Main types for IWI-S:

Slag inclusions

Slag is the non-metallic product of flux melting (coating, flux core, or SAW flux). If slag is not removed before the next pass, or if technique undercuts and traps slag along the fusion face, solid slag remains in the finished weld.

Causes:

• Incomplete interpass cleaning (chipping, grinding, wire brushing as required)
• Incorrect electrode angle or travel leaving a slag “shelf”
• Excessive weaving or too-wide beads that freeze slag at the toes
• Tight joint geometry with poor access for cleaning
• Undercut that fills with slag then is covered by the next pass

Appearance and detection:

• May break the surface (visible after cleaning) or be fully buried
• On radiographs, slag often appears as dark (less dense) irregular or linear indications—contrast with bright tungsten
• UT may detect larger inclusions as reflectors; interpretation needs qualified NDT personnel
• Visual alone cannot clear internal slag on multi-pass welds—that is why volumetric NDT is specified on critical joints

Prevention: thorough interpass cleaning, correct bead sequence, parameters that produce self-releasing slag where the process allows, and stop-and-clean when slag bridges are visible.

Inspector response: record location, length, and depth if known from NDT; compare to acceptance (isolated small slag may be allowed at lower quality levels; elongated/line slag is often more severe). Require removal and re-weld under approved repair procedure when limits are exceeded—do not “leave it if RT is tomorrow and hope.”

Tungsten inclusions (TIG)

In TIG/GTAW, the electrode is non-consumable tungsten (often thoriated, ceriated, lanthanated, etc., per procedure). If the welder dips the electrode into the pool, uses a contaminated tip, or sets current too high for the electrode diameter so the tip melts, tungsten particles transfer into the weld.

Recognition:

• On RT film, tungsten is high density → typically bright white spots or particles (opposite contrast tendency to slag/porosity dark spots)
• Surface tungsten may be visible as shiny hard particles after light cleaning

Causes: poor technique (dip), wrong electrode diameter for current, inadequate sharpening/preparation of the tip, contamination of the tungsten, unstable arc start.

Prevention: correct electrode size and type per WPS, proper tip grind, torch technique that avoids dipping, restart discipline after a dip (dress tip; assess whether to remove contaminated weld metal).

Acceptance: many quality levels limit or prohibit tungsten inclusions depending on size and service. Even small tungsten can be a hard stress raiser. Follow the table—do not assume “TIG is clean so RT will always pass.”

Oxide inclusions

Oxide films form when hot metal is exposed to air or when previous heat tint / mill scale / aluminium oxide is not removed. Aluminium’s refractory oxide is famous; steels also trap oxides between poorly cleaned passes or in poor gas shielding.

Inspector focus: joint and interpass cleanliness, shielding gas quality and coverage, purge where required for stainless root passes, and aluminium oxide removal procedures. Oxide films can act like lack of fusion planes—treat seriously when NDT or bend tests reveal them.

Misalignment — Linear (Hi-Lo) and Angular

Misalignment is primarily a fit-up and assembly defect, often classified under imperfect shape/dimension concepts when the finished joint is assessed.

Linear misalignment (hi-lo)

Linear misalignment (often called hi-lo in piping) is offset between the centre lines or surfaces of two members that should be flush or within a specified offset. Example: pipe ends where one ID/OD stands higher than the other at the root.

Why it matters:

• Reduces local wall or creates a step stress raiser at the root
• Disturbs root penetration and LOF risk
• Affects flow in pipes and fatigue at the step
• Can invalidate the assumption that full penetration restores a smooth bore

Limits: codes and ISO 5817-type tables often express allowable offset as a fraction of thickness with a maximum mm cap. Measure with rulers, hi-lo gauges, or internal alignment tools as appropriate.

Angular misalignment

Angular misalignment is an angular deviation between members that should meet at a specified angle (e.g. plate flanges not coplanar, T-joint legs not square to drawing). It changes load path and effective weld geometry.

Inspector duties at fit-up (before welding):

1. Check drawings/WPS for permitted hi-lo and angular tolerance
2. Measure and hold until corrected if outside limits—welding “to pull it in” is not a universal fix and can increase residual stress
3. Record as-found and as-released fit-up when the ITP requires
4. After welding, re-check for distortion-induced misalignment that appeared during fabrication

Miscellaneous Defects — Spatter, Arc Strikes, Underflush, Stray Arc

Spatter

Droplets of weld metal stuck on adjacent surfaces. Common in MAG/MMA with wrong parameters or poor gas. Spatter can:

• Hide the true toe for VT
• Create corrosion initiation sites if not removed where coatings must bond
• Be limited by appearance specs or client cleanliness standards

Usually removed by chipping/grinding/brushing without underflushing the parent. Prevention: parameter tuning, anti-spatter where allowed, correct transfer mode.

Arc strikes (stray flash)

An arc strike is an accidental arc on the parent metal outside the intended weld zone—from a stray electrode touch, poor earth clamp, or starting on the plate. Arc strikes can create hard spots, microcracks, and local HAZ damage on hardenable steels. Many structural and pressure codes prohibit arc strikes on base metal or require blend grinding plus NDT/hardness checks.

Inspector response:

• Treat arc strikes as reportable surface damage, not cosmetics
• Locate, mark, and assess per procedure (remove, blend, MT/PT, hardness if required)
• Investigate earth return quality and welder practice to prevent recurrence

Underflush (excessive dressing)

Underflush is removal of parent or weld metal below the required surface level during grinding or repair—local thinning. It is the opposite problem of excess reinforcement: over-enthusiastic dressing. Measure remaining thickness; underflush can reject a part that was dimensionally fine before grinding.

Stray arc damage and earth-clamp burns

Poor work return (earth) clamps that arc at the clamp location burn and pit the plate. Same family as arc strikes: local thermal damage, possible cracking, coating failure. Inspect clamp locations on finished structures; require repair of damaged zones per procedure.

Inspector Response Framework

For inclusions, misalignment, and miscellaneous defects, use a consistent loop:

1. Identify the ISO 6520-type category (do not call slag “porosity” or tungsten “slag”)
2. Measure size, length, depth (or NDT extent), and location relative to welds and members
3. Compare to the contract acceptance standard and quality level
4. Disposition: accept as-is, rework (clean/grind), repair weld, or escalate to engineering when fitness is unclear
5. Correct process: cleaning discipline, TIG electrode control, fit-up gauges, earth practice—not only scrap and reweld forever
6. Re-inspect after repair with the same or upgraded NDT as the ITP states

Typical exam traps

• Assuming all dark RT indications are cracks (could be slag or LOF)
• Assuming all bright RT spots are “good density” rather than tungsten
• Ignoring hi-lo because the cap looks pretty
• Treating arc strikes as “just marks” on quench-sensitive steels
• Grinding underflush below minimum thickness to erase undercut without thickness check

Link to Evaluation (12.3)

Finding an inclusion or arc strike is only half the job. WI1.8–1.9 require you to apply defined acceptance criteria, record the evidence, and know when the decision is beyond pure table lookup—into engineering critical assessment, which is introduced next and deepened at IWI-C.