17.3 Visual Appearance of Discontinuities
Key Takeaways
- Grazing light shows texture and tight openings; frontal light shows color; silhouette shows profile — use more than one angle before you name a line a crack.
- Undercut is a melted groove at the toe; overlap is an unfused rolled lip; underfill is a sunken weld face; a crack is a tight fracture.
- Surface-breaking incomplete fusion looks like a dark line or step at the fusion boundary, not like a rounded pore or a shy weld face.
- Product form changes the picture: cold shuts and hot tears on castings, laps and seams on forgings, regular tool lay on machined parts, process faults on welds.
- Arc strikes remelt parent metal; spatter sits on it; mechanical damage is directional with displaced metal; pitting is a cavity a pit gauge can read.
The ASNT NDT Level II visual-testing general exam lists Visual Appearance of Discontinuities as official VT topic 7. The items are not asking you to recite an acceptance table — that work sits in evaluation, reporting, and codes (the next chapter). This topic asks whether you can name what you see: how a crack looks versus undercut, overlap, porosity, underfill, surface-breaking incomplete fusion, an arc strike, spatter, corrosion, pitting, mechanical damage, and misalignment, and how that picture changes with product form and lighting angle.
An indication is what you see. A discontinuity is an interruption in the typical structure of the material. A defect is a discontinuity that fails the named acceptance standard. Topic 7 lives in the first two words. Candidates who jump from a dark line to "reject" have skipped the work this topic tests.
Lighting angle: texture versus profile
Lighting is not a comfort item. It is how appearance is created.
| Lighting | What it reveals | What it hides or fakes |
|---|---|---|
| Grazing / low-angle (beam nearly parallel to the surface) | Texture, tight cracks, laps, seams, shallow undercut, tool marks, grinding scratches | Overall color and heat tint; can over-dramatize harmless ripples |
| Frontal / diffuse | Color, contrast, stains, heat tint, general corrosion, printed marks, spatter as discrete blobs | Tight linear openings that do not cast a shadow |
| Silhouette / grazing along a profile | Reinforcement height, underfill as a valley, undercut as a notch, hi-lo mismatch, an overlap lip | The interior of a pit |
| Oblique from two directions | A real groove — the shadow moves and the opening persists — versus a stain that never becomes a groove | — |
Rule: use at least two lighting directions before you name a tight linear mark a crack. A grind scratch lights up from one direction and dies from the other. A crack that is open stays a dark, often jagged line more persistently and does not follow the regular lay of a grind or a tool.
Coaxial light down a borescope can wash texture. If the only light is on-axis, a tight toe crack can vanish. Add a grazing component, or rock the tip, before you write NI.
How weld discontinuities look
Cracks
A crack is a fracture. Visually it is a tight, linear opening, often jagged or branched, sometimes with a slight mismatch of the two faces. It can be extremely fine. Confirm with 5–10× and grazing light from more than one side.
Locations teach the name:
- Toe crack — leaves the fusion line into the HAZ or along the toe.
- Throat or centerline crack — along the weld axis, often a hot / solidification crack.
- Crater crack — star-shaped in an unfilled crater at a stop.
- Root crack — on an accessible root, a tight line at the root face or HAZ.
- Underbead crack — may be invisible to VT until it breaks the surface. Absence of a surface line is not proof the HAZ is uncracked.
Do not confuse a crack with slag lines (wider, often pickable), grind marks (parallel, uniform, shiny), or weld ripples (regular, rounded). Product-form cousins — a cold shut, a forging lap, a seam — can look linear. Name them from origin and shape, not from the single word "line."
Undercut
Undercut is a groove melted into the base metal at the toe (or at an accessible root). It looks like a dark shadow line along the fusion boundary and, in silhouette, like a notch. An undercut gauge or a pit-style bridge seats across the adjacent plate and reads depth. The bottom is a melted groove, not a fracture through solid metal.
Continuous or intermittent. Not underfill (the weld face itself is shy). Not a crack (tight fracture, no melted groove). Not hi-lo (member offset).
Overlap
Overlap (cold lap) is weld metal that rolls over onto the parent without fusing. It looks like a rolled lip or a shelf. Lighting from the parent-metal side throws a shadow under the lip. A fingernail or a thin feeler may enter the crevice. That crevice is a ready-made crack starter. VT should catch overlap because it is a surface profile condition, not a subsurface plane.
Porosity
Porosity is trapped gas. On the surface it looks like rounded pinholes or craters, scattered, clustered, or aligned. Piping (worm-hole) porosity is elongated, often nearly perpendicular to the face. A cluster is a field of holes, not a crack. Do not promote a line of pores into a linear crack without seeing a fracture that connects them.
Underfill
Underfill is a weld face or accessible root that sits below the adjacent parent surface. The joint looks sunken — a valley. A straightedge rocks; silhouette lighting makes the valley obvious. It is the weld metal that is shy, not a groove melted into the plate beside the weld.
Incomplete fusion at the surface
Incomplete fusion (lack of fusion) that breaks the surface looks like a tight dark line or a step at the fusion boundary — unfused bevel, unfused land, or unfused interpass that you can see. A thin feeler or a probe may enter. Distinguish it from a toe crack by location (unmelted face versus a fracture through melted metal or HAZ) and from a pore by shape (linear step versus a rounded cavity).
Incomplete penetration at an open root looks like a leftover land or a dark centerline groove — the joint was not consumed. Different name, same visual idea: you can still see the original joint.
Arc strikes and spatter
An arc strike is a localized remelt of parent metal off the intended weld (or beside it). It looks like a small crater with a halo of heat tint, and it may contain a crack in the crater. It is not a blob sitting on the surface.
Spatter is discrete globules of weld metal stuck to the surface — scattered BBs, sometimes with a tiny neck. Frontal light shows them. Dense spatter can hide a toe. Spatter sits on the metal; an arc strike remelts it.
Reinforcement and profile
Excess reinforcement is extra metal on the face — a high convex bead, obvious in silhouette. Insufficient fillet throat is a concave or undersize profile that a fillet gauge reads. Those are geometry conditions. They are not cracks.
Corrosion, pitting, mechanical damage, misalignment
Corrosion
General corrosion looks like rust, scale, loss of mill finish, color change, and roughness. Localized corrosion is a patch, under-deposit attack, or crevice rust at a lap or a bolt. VT sees color, texture, and corrosion product. It does not measure remaining wall unless you add a pit gauge, a remaining-thickness tool, or another method. Heat tint on stainless is a color map, not automatically metal loss.
Pitting
Pitting is a discrete cavity, often with an undercut mouth or a dark or bright bottom. A pit gauge bridges on surrounding metal and drops a stylus into the pit. Do not confuse service pits with as-welded surface porosity (rounded, in weld metal, from the process) or with gas holes on a casting (solidification, often toward the cope).
Mechanical damage
Gouges, nicks, clamp marks, grind gouges, tool chatter, scoring, and handling dents are directional. They often show displaced metal or a bright cut. Grind marks are parallel and regular; a crack is not. A grind that erases a weld toe can also erase the evidence of undercut — record the repair, do not pretend the toe was never there.
Misalignment
Hi-lo / mismatch is one member standing above the other at the joint. It looks like a step. A hi-lo gauge reads it. Related pictures: out-of-plane distortion, angular misalignment, a root gap that is too tight or too wide. Misalignment is geometry. It can hide or exaggerate undercut and can look like overlap if you only glance.
Product-form differences
The same English word does not look the same on every product. Chapter 3 sorted origin families; this topic is the photograph.
| Product | What VT is usually hunting | What it looks like | What it is not |
|---|---|---|---|
| Weld | Process faults above | Toes, faces, roots, HAZ, adjacent parent | A plate lamination will not look like a weld-toe crack |
| Casting | Surface porosity, cold shuts, hot tears, misruns, sand inclusions, chaplets, parting-line mismatch, scabs | Cold shut: a smooth, often oxidized line where two streams met and did not fuse. Hot tear: ragged, oxidized, at a change of section. Misrun: incomplete fill, rounded cold edges. Parting line: a mold match, not a fracture. | Do not call a parting line a crack. Do not call as-cast shrink a service pit. |
| Forging / wrought | Laps, seams, bursts, scale pits | Lap: a fold of metal, often with scale inside, a crease parallel to the working direction. Seam: a tight longitudinal line from an elongated inclusion or rolled-out crack. | A lap can look like a crack until you see the folded lip and the scale. |
| Machined | Tool marks, chatter, scoring, nicks, burrs, leftover stock | Regular, periodic marks matching the tool; chatter is wavy; a nick is local | Regular tool marks are not cracks. A crack crosses the lay. |
Service damage can sit on any product. Fatigue cracks are tight, often at toes, keyways, and section changes. They need grazing light and 5–10×. Early fatigue is a poor radiographic target; it is a visual and surface-method target once it breaks the surface.
Realistic exam scenarios
A dark line at the toe is jagged, opens under grazing light from two directions, and does not accept an undercut-gauge seat as a melted groove. Appearance: crack, not undercut.
A groove is melted into the plate at the toe, the shadow is a notch in silhouette, and an undercut gauge reads a depth. Appearance: undercut, not overlap and not underfill.
A rolled lip at the toe throws a shadow underneath and a feeler enters. Appearance: overlap.
A straightedge rocks on a sunken weld face; the plate beside the weld is intact. Appearance: underfill.
On a sand casting, a smooth oxidized line sits where two streams met. Appearance: cold shut — a casting confluence, not a weld-toe crack and not a machined tool mark.
On a forging, a crease with scale runs parallel to grain flow and shows a folded lip. Appearance: lap, not a fatigue crack and not weld overlap.
On a machined shaft, regular spiral marks follow the tool. Appearance: tool lay. A tighter line that crosses that lay is the crack candidate.
Topic 7 language is short: name what it looks like, which light revealed it, and which product form can even produce it. The table that accepts or rejects it is the next chapter.
A groove is melted into the base metal along the weld toe. Grazing light throws a shadow in the groove, and an undercut gauge seats and reads a depth. What is the appearance-based name?
How should lighting be used to tell a tight surface crack from a shallow grind scratch?
On a sand casting, a smooth, often oxidized line appears where two streams of metal met. What is the expected visual call, and why does product form matter?