Common Welding Defects and Visual Inspection: Causes and Corrections

A professional welder should be able to recognize abnormal weld conditions, think through possible causes and decide what can be corrected safely. The wrong approach is to see every visible irregularity as a “bad weld,” or to grind away anything suspicious before an inspector or supervisor can evaluate it.

This guide explains common welding defects and inspection as a practical diagnostic process. It covers visual checks before, during and after welding, then reviews porosity, undercut, incomplete fusion, incomplete penetration, spatter, cracks, distortion and arc strikes.

The word imperfection is important. A visible feature becomes unacceptable only when it exceeds the acceptance criteria that apply to the job. ISO 5817, for example, defines quality levels for imperfections in many fusion-welded joints, but the project specification determines the applicable acceptance level. A welder should therefore describe what is observed rather than invent an acceptance decision.

These are practical skills that may help technicians prepare for competency-based work and assessment. Saudi Skill Test is an independent preparation resource and is not affiliated with the Saudi government, Takamol or NSDC.

Safety: Never remove a suspected critical crack or other significant indication simply to improve appearance before authorized evaluation. Grinding, gouging and repair welding introduce additional hazards and may require an approved repair procedure.

Quality vs Appearance

A smooth, shiny weld is not automatically acceptable, and a rough-looking weld is not automatically rejectable. Visual appearance gives clues, but weld quality depends on the applicable requirements.

The welder should separate three questions:

1. What do I observe? Example: a narrow groove is visible along the weld toe.
2. What might have caused it? Example: travel speed, arc length, angle, current or joint geometry may have contributed.
3. Is it acceptable? That answer comes from the project acceptance criteria, not from appearance alone.

This distinction prevents two common errors: rejecting sound work based on preference, and accepting defective work because the bead “looks nice.”

Visual Inspection Before Welding

ISO 17637 covers visual testing of fusion-welded joints and can also be applied to visual checking of the joint before welding.

Before welding, inspect:

• material and joint identification;
• edge preparation;
• cleanliness;
• fit-up and alignment;
• root condition;
• tack welds;
• backing or purge arrangement where required;
• access for the specified position and sequence;
• obvious damage, laminations or surface conditions that require review.

Many welding problems are cheaper to prevent at this stage than to repair after completion.

Scenario: contaminated groove

A prepared groove looks dimensionally correct, but oil is visible near the root.

A weak response is to increase heat and “burn through it.”

A better response is to stop, identify and remove the contamination using the approved method, reassess the joint, and confirm that cleaning has not altered the required preparation.

Inspection During Welding

The welder should observe the process while the weld is being made. Useful clues include:

• arc stability;
• shielding behavior;
• unusual sound or spatter;
• puddle control;
• slag behavior;
• bead placement;
• visible contamination;
• interpass cleaning;
• joint movement and distortion;
• overheating or equipment problems.

If the process suddenly changes, do not automatically compensate with random machine adjustments. Check the WPS, consumable, work connection, gas system, contact tip or electrode condition, and the joint.

Inspection After Welding

A post-weld visual check can include:

• weld location and completeness;
• bead continuity;
• surface cracks;
• porosity or visible cavities;
• undercut;
• overlap or abnormal profile;
• crater condition;
• excessive spatter where relevant;
• arc strikes outside permitted areas;
• distortion and alignment;
• required weld dimensions;
• cleanup and identification.

Visual inspection cannot reveal every internal condition. Some jobs require additional non-destructive testing (NDT) or destructive testing under the project inspection plan.

Porosity

Porosity is gas trapped in the solidifying weld metal, producing pores or cavities. It may appear as isolated pores, groups or elongated indications depending on the cause and process.

Possible causes

Possible causes include:

• contaminated base metal;
• moisture, oil, paint or other surface material;
• contaminated or improperly handled consumable;
• inadequate or disturbed shielding gas;
• leaks or restrictions in the gas system;
• excessive drafts;
• unsuitable technique;
• process-specific conditions identified by the WPS or manufacturer.

Diagnostic sequence

Do not simply turn up gas flow. Check:

1. Is the material clean and dry?
2. Is the correct consumable being used?
3. Is the correct shielding gas connected?
4. Are hoses, fittings and torch parts sound?
5. Is there strong air movement around the arc?
6. Does the setup match the WPS?
7. Has the consumable been stored correctly?

A single symptom may have more than one cause.

Undercut

Undercut is a groove melted into the base metal adjacent to the weld toe or root that is not adequately filled by weld metal.

Possible contributors can include:

• excessive current for the actual condition;
• long arc;
• excessive travel speed;
• incorrect torch or electrode angle;
• poor manipulation;
• joint geometry or position;
• unsuitable parameter combination.

The correction depends on the verified cause. Slowing travel alone is not always the answer, and adding weld metal without an approved repair method may create a different problem.

What the welder should report

Instead of saying “the weld failed,” report the observable condition: location, length, approximate severity if measurement is permitted, and whether it repeats. The inspector can compare the indication with the governing acceptance criteria.

Incomplete Fusion

Incomplete fusion occurs when weld metal does not fuse adequately with the base metal or a previous weld bead.

Possible causes include:

• insufficient heat at the fusion face;
• poor torch/electrode angle;
• excessive travel speed;
• slag or oxide trapped between passes;
• poor joint access;
• incorrect bead placement;
• unsuitable parameter selection;
• inadequate preparation.

This condition may not always be visible from the surface. That is one reason a good-looking cap cannot prove full weld soundness.

Prevention mindset

The welder can reduce risk by ensuring:

• proper joint preparation;
• correct procedure parameters;
• effective interpass cleaning;
• controlled bead placement;
• correct manipulation for the position;
• adequate access to fusion faces.

Incomplete Penetration

Incomplete penetration describes a condition where the weld does not achieve the required penetration through the joint root or specified depth.

Possible causes include:

• incorrect root preparation;
• root opening outside procedure;
• excessive root face for the procedure;
• insufficient welding energy;
• poor electrode/torch access;
• incorrect angle or technique;
• mismatch or joint movement.

The key lesson is that penetration problems often begin with joint preparation. A welder should not treat them as machine-setting problems only.

Excessive Spatter

Spatter consists of droplets of molten metal expelled during welding that do not form part of the intended weld.

Some processes naturally generate more spatter than others. Excessive spatter can point to setup or technique issues such as:

• incorrect voltage/current relationship;
• unstable wire feeding;
• incorrect polarity;
• poor work connection;
• excessive arc length;
• gas or consumable problems;
• contaminated surface;
• unsuitable transfer conditions.

Spatter is usually a clue, not a complete diagnosis. Examine the entire process.

Cracks

Cracks are among the most serious welding indications and require careful handling. They can occur in weld metal, heat-affected material, crater areas or other locations and may form during or after welding.

Possible contributing factors vary widely and can involve:

• material composition and hardenability;
• hydrogen;
• high restraint;
• rapid cooling;
• incorrect preheat/interpass control;
• crater termination;
• unsuitable filler;
• stress concentration;
• procedure deviations.

A general training guide should not tell a worker to grind out a crack and reweld it. The correct response is to stop, preserve the indication, report it and follow the authorized inspection and repair process.

Scenario: fine line near the crater

A fine line appears at the end of a completed bead. It may be a harmless surface mark—or a crater crack.

Do not polish it away to see whether it disappears. Mark the location if site practice permits, stop further work that could hide the indication, and obtain the required review.

Distortion

Welding heats and cools material unevenly. Shrinkage can cause angular distortion, bowing, twisting or dimensional change.

Factors can include:

• heat input;
• joint design;
• weld size;
• sequence;
• restraint;
• part geometry;
• number of passes;
• deposition balance.

Distortion control should begin before welding. Fit-up, tack sequence, balanced welding, fixtures and planned sequence may help when permitted by the procedure.

A common mistake is to use excessive restraint without understanding the stress it introduces. Fixtures and strongbacks should be used under the fabrication plan, not improvised blindly.

Arc Strikes

An arc strike is an unintended arc outside the designated weld area. It can create a localized metallurgical and surface condition that requires evaluation.

Prevent arc strikes by:

• placing the work connection correctly;
• controlling the electrode holder or torch when not welding;
• avoiding accidental contact on finished surfaces;
• following approved strike/start practices.

Do not simply grind an arc strike until it disappears unless the project repair method allows that action and required inspection follows.

Corrective-Action Decision Tree

When a weld looks abnormal, use a consistent sequence.

Step 1: Stop the condition from getting worse

If the process is unstable or a serious indication appears, stop safely rather than continuing a long weld hoping the problem will disappear.

Step 2: Identify the observable symptom

Examples:

• surface pores;
• toe groove;
• unstable arc;
• heavy spatter;
• irregular bead;
• suspected crack;
• unexpected distortion.

Step 3: Check simple, non-destructive causes first

• correct WPS?
• correct consumable?
• correct polarity?
• good work connection?
• correct gas and gas path?
• clean joint?
• equipment damage?

Step 4: Compare with the procedure

Check process parameters and controlled variables. Avoid “dial chasing.”

Step 5: Decide whether welder correction is authorized

Minor setup correction may be within normal work. Repairing a completed weld, removing metal or rewelding may require supervisor, inspector or welding-engineering authorization.

Step 6: Verify after correction

A repair is not complete just because the surface looks better. Required inspection must be repeated according to the project plan.

Defect → Clue → Possible Causes → Prevention

This table supports diagnosis; it does not replace engineering evaluation or acceptance criteria.

When Supervisor or Inspector Review Is Required

Escalate when:

• a crack is suspected;
• the joint or weld is outside a controlled drawing/WPS condition;
• an acceptance decision is unclear;
• repair welding is required;
• base material has been damaged;
• arc strikes require evaluation;
• dimensions are outside tolerance;
• repeated defects suggest a process or equipment problem;
• required NDT or hold-point inspection is due.

A skilled welder knows not only how to weld, but when not to continue.

What to Study Next

Continue with:

• W1 — Welding Processes, Machines and Consumables for setup causes;
• W2 — Weld Joint Preparation, Welding Positions and WPS Basics for upstream fit-up causes;
• W5 — Practical Welding Work Sequence and Skill Readiness for end-to-end practice;
• the Welder practice tests for original competency-oriented questions.

Key Takeaways

• Appearance and acceptance are not the same thing.
• Visual testing starts before welding and continues during and after the weld.
• Porosity, undercut, fusion problems and spatter can have multiple causes.
• Do not diagnose by changing several settings at once.
• Cracks and other critical indications should be preserved for authorized evaluation.
• Acceptance or rejection must come from project criteria, not a generic article.
• Good troubleshooting begins with simple checks: procedure, material, consumable, polarity, work connection, gas and cleanliness.

Technical References

1. ISO 17637:2016 — Non-destructive testing of welds — Visual testing of fusion-welded joints — https://www.iso.org/standard/67259.html
2. ISO 5817:2023 — Welding — Fusion-welded joints in steel, nickel, titanium and their alloys — Quality levels for imperfections — https://www.iso.org/standard/80209.html
3. ISO 15607:2019 — Specification and qualification of welding procedures for metallic materials — General rules — https://www.iso.org/standard/71495.html
4. ISO 4063:2023 — Welding, brazing, soldering and cutting — Nomenclature of processes and reference numbers — https://www.iso.org/standard/75108.html
5. Saudi National Council for Occupational Safety and Health — industrial hazards guidance — https://ncosh.gov.sa/en/knowledge-center/rules-regulations/administrative-systems/

Editorial note: This guide deliberately avoids universal defect acceptance limits. Acceptance depends on the applicable code, quality level, drawing, contract and inspection plan.