12.1 Undercut, Overlap & Shape Imperfections

12.1 Undercut, Overlap & Shape Imperfections

Quick Answer: Undercut is a melted groove in the parent metal at the toe (or root) left unfilled by weld metal. Overlap is excess metal that covers the parent without fusion. Both are ISO 6520 Group 5 shape imperfections. Together with excess weld metal, excessive convexity/concavity, and incomplete filled groove, they are accepted or rejected against ISO 5817 levels B/C/D (or the contract code)—not by “looks bad.”

Chapter 11 covered cracks, porosity, and lack of fusion/penetration. This section continues WI1.7–1.8 geometry and profile defects that visual testing (VT) catches every day. Shape imperfections rarely look dramatic on a radiograph the way a crack does, yet they dominate shop NCR lists and fatigue performance in service. IWI-S candidates must name, measure, and link them to process causes and acceptance tables.

Where These Sit in ISO 6520

ISO 6520-1 groups imperfections by nature. Cracks are Group 1; cavities Group 2; solid inclusions Group 3; LOF/LOP Group 4; imperfect shape and dimension are Group 5; miscellaneous items (spatter, arc strikes, etc.) are Group 6.

Key Group 5 identities for this section:

You do not need every numeric code memorised for the Standard exam, but you must not confuse undercut with lack of fusion, or overlap with a smooth fillet toe.

Undercut — Definition and Why It Matters

Undercut is a groove or channel melted into the base metal along the weld toe (most common) or sometimes at the root, that is not filled by weld metal. The toe is the junction where the weld face meets the parent surface. Undercut reduces local section thickness and creates a notch—a stress concentration that is especially damaging under cyclic (fatigue) loading.

Inspector recognition:

• Continuous undercut runs along a length of weld; intermittent undercut appears in patches
• Depth is measured from the original parent surface (or drawing reference) into the groove; length along the weld axis is recorded separately
• Root undercut (where accessible) is a different location but the same geometric idea: parent metal removed and not replaced

Not undercut: a neat, blended toe with full fusion; a shallow paint line; grinding that intentionally creates a smooth transition under a repair procedure. Is undercut: a finger-nail catch at the toe where base metal was melted away and the bead failed to wet back.

Causes of undercut

Typical process drivers:

1. Excessive current or too high travel speed — the arc digs the toe and freezes before fill
2. Wrong electrode/torch angle — arc force directed into the parent rather than into the joint
3. Excessive weaving or dwell at the wrong point of the weave
4. Long arc length (especially MMA) — unstable pool and edge melting
5. Incorrect voltage / wire-feed balance in MIG/MAG or SAW that produces a diggy arc
6. Poor position technique (e.g. vertical-down on thick plate with wrong parameters)

Prevention and inspector checks

• Verify WPS parameters (amperage, voltage, travel speed, heat input band) are followed
• Watch travel angle and work angle during surveillance
• For multi-pass work, ensure toe blending and that subsequent passes do not re-melt a continuous notch without filling it
• After welding, measure depth and length with a weld gauge, depth gauge, or calibrated profile method as required by the ITP
• Distinguish short vs long imperfections when the acceptance standard does—many ISO 5817 tables treat continuous length differently from isolated short indications

Overlap — Definition and Confusion Trap

Overlap is excess weld metal that protrudes over the parent metal surface without fusing to it. From a distance it can look like a “fat” toe; up close, a probe or sharp edge may catch under a cold lap of metal that never fused. Overlap conceals the true fusion line, can trap slag or dirt, and is often not permitted or is tightly limited because it is a form of incomplete fusion at the surface.

Contrast with undercut:

Causes of overlap

• Too low travel speed or excessive deposition for the position
• Wrong torch angle pushing metal ahead of the pool without fusion
• Low heat input relative to mass deposited (cold weld)
• Contaminated or cold parent surface that rejects wetting
• Poor technique on horizontal fillets (metal sags or rolls onto the vertical leg)

Prevention

Correct heat input and travel speed, clean joint faces, proper angle so the arc melts both members and the previous pass, and stop “piling” metal to hide undercut—overlap is not a repair for undercut.

Excess Weld Metal, Convexity, Concavity & Incomplete Fill

Excess weld metal (butt welds)

The cap stands higher than necessary above the parent. Some reinforcement is normal and often required within limits; excessive reinforcement wastes metal, can create abrupt toes, complicates NDT, and may be limited by code for fit-up or hydrodynamics (pipe internals). Measure height of reinforcement against the quality level table.

Excessive convexity (fillets)

A fillet face that is too bulged for its leg size gives a sharp toe angle and poor stress flow. Designers and ISO 5817 care about profile as well as leg length. A very convex fillet may meet leg size on a gauge yet still fail profile/toe rules.

Excessive concavity / incomplete filled groove

The face sinks below the required level, or a groove weld is not filled to the parent surface (or to the specified cap). Incomplete filled groove reduces throat in butt joints and is a clear dimensional nonconformity when below acceptance. Distinguish from undercut: concavity is about the weld face height; undercut is a groove in the parent at the toe.

Incorrect weld toe

Even without measurable undercut depth, a sharp toe angle (abrupt transition) concentrates stress. Some specifications require grinding or TIG dressing of toes on fatigue-critical members—only when the procedure allows and NDT follows.

ISO 5817 Quality Levels B, C, and D — Limit Concepts

ISO 5817 specifies quality levels for imperfections in fusion-welded steel joints (companion standards cover other materials). Levels are not “good / average / bad craftsmanship labels” in the abstract; they are acceptance severity bands chosen by the design/contract:

Linear limit concepts the inspector must understand (always read the actual table in the referenced edition for numbers):

• Undercut depth h is limited as a function of parent thickness t, often with a maximum absolute depth (e.g. tighter absolute caps at Level B than at Level D)
• Length of undercut along the weld may be restricted; continuous undercut may be treated more harshly than short local undercut
• Excess weld metal height limits are smaller at B than at D
• Overlap is frequently not permitted at Level B and restricted or not permitted at C depending on the edition and imperfection type—treat as high-severity surface fusion discontinuity
• Incomplete filled groove and excessive concavity have explicit height/depth limits tied to thickness and level

Exam and shop rule: Level letter alone does not accept a weld. You need the applicable standard edition, the imperfection type, short vs long, thickness, and sometimes joint type. Product standards (pressure equipment, structural Eurocodes, client specs) may tighten ISO 5817 or replace it.

Imperfect ≠ automatic reject. A small, short undercut within Level C limits on a Level C job is acceptable. The same undercut on a Level B fatigue detail may be rejectable. Chapter 12.3 develops this evaluation logic fully.

Measurement Practice for VT

1. Clean the area enough to see the true toe (remove loose slag/spatter that hides geometry—without grinding away evidence before recording if dispute is possible)
2. Identify type: undercut vs overlap vs excess reinforcement vs concavity
3. Measure depth/height and length; note location (toe side, root, start/stop, which pass)
4. Compare to the acceptance table for the quality level and thickness
5. Photograph or sketch if the ITP/NCR process requires it; quantify in the report

Use fillet gauges, cam-type undercut gauges, and straightedges as appropriate. Do not “estimate by eye” when the limit is 0.5 mm and the code cares.

Process–Imperfection Links (High-Yield)

Inspector Mindset

Shape imperfections are where skill, parameters, and acceptance tables meet. IWI-S does not redesign the joint on the spot, but does:

• Stop work or raise NCR when limits are exceeded and authority allows
• Feed root-cause data to welding supervision (not only “undercut—grind and weld” without cause)
• Ensure repairs follow an approved procedure and re-inspection path
• Never accept overlap as a substitute for fusion or as a cover-up for undercut

Bridge to the Rest of Chapter 12

Section 12.2 covers inclusions, misalignment, spatter, and arc strikes. Section 12.3 consolidates acceptance evaluation, recording, escalation, and the boundary between ISO 5817 decisions and advanced ECA. Master the geometry language here first—most oral and practical VT scenarios start with undercut, overlap, and profile.