Weld Inspection and NDT Explained: How to Know a Weld Is Sound

Weld inspection is the set of checks that proves a welded joint is sound before it goes into service. It starts before the arc is struck, continues while the welder works and ends with visual and non-destructive tests on the finished weld. For piping, boilers, tanks and steel structures, it is the main defence against leaks and failures.

A weld can look neat and still hide a serious flaw. So plant owners should understand what good weld inspection involves, even if they never hold a torch. This guide explains the stages, the test methods and the paperwork in plain terms.

Why does weld inspection matter?

A welded joint carries pressure, weight or both. If it fails, the result may be a steam leak, a spill, a fire or a collapse. In addition, repairs on a live plant cost far more than checks in the workshop.

There is a commercial reason as well. Codes, insurers and inspectors ask for evidence. Without records, a pressure system may not pass its statutory inspection. Therefore, good inspection protects people, production and the owner’s investment at once.

The three stages of weld inspection

Inspection is not one event at the end. In fact, most of the value comes earlier.

Before welding

First, the inspector checks the foundations of quality.

• The procedure. A welding procedure specification, or WPS, sets out the process, filler, current, joint design and heat treatment. A procedure qualification record, or PQR, proves by test that the procedure works.
• The welder. Each welder must hold a current qualification for that process, material and position.
• The material. Plates, pipes and fittings need certificates that match the drawing.
• The consumables. Electrodes must be the right type and dry. For example, low hydrogen electrodes need oven storage.
• The fit-up. The joint needs the correct bevel angle, root gap and alignment, with clean surfaces.

During welding

Next, the inspector watches the work itself. He or she checks preheat where the procedure requires it, the welding parameters and the cleaning between passes. The root run deserves special attention, because faults there are hard to reach later.

After welding

Finally, the finished weld gets its examination. Visual inspection comes first, and then the non-destructive tests that the code or the client specifies. After that, some systems also need a pressure test.

What does visual weld inspection look for?

Visual examination is the cheapest and most useful test. A trained eye, good light and a few gauges find most problems. The inspector looks for the following.

• Size and profile. The weld must have the specified leg length or reinforcement, with a smooth shape.
• Undercut. A groove melted into the parent metal beside the weld weakens the joint.
• Surface porosity. Small holes show that gas was trapped.
• Cracks. Any crack is unacceptable.
• Overlap and spatter. Both point to poor technique.
• Incomplete fill. The weld must fill the joint completely.
• Misalignment. The two parts must line up within tolerance.
• Arc strikes. Stray strikes outside the joint can start cracks.

However, the eye sees only the surface. For that reason, critical welds need more.

The main NDT methods used in weld inspection

Non-destructive testing, or NDT, examines a weld without damaging it. The table compares the common methods.

Dye penetrant testing

The technician cleans the weld and sprays on a coloured liquid. The liquid seeps into any surface crack. After a set time, the technician wipes the surface and applies a white developer. Then any crack shows up as a red line. This method suits stainless steel, where magnetic testing does not work.

Magnetic particle testing

Here, the technician magnetises the area and applies fine iron particles. A crack disturbs the magnetic field, so the particles gather along it. As a result, the flaw appears as a clear line. It works only on carbon and low alloy steels.

Radiographic testing

Radiography passes X-rays or gamma rays through the weld onto a film or a digital plate. Voids and slag absorb less radiation, so they appear as dark marks. It is the traditional test for butt welds in pipe. However, the area must be cleared of people during each exposure.

Ultrasonic testing

A probe sends high frequency sound into the steel. When the sound meets a flaw, part of it bounces back, and the instrument displays the echo. Ultrasonic testing finds planar flaws, such as lack of fusion, very well. In addition, it needs access to one side only.

How much of the welding gets tested?

The code or the specification decides. Engineers match the extent of testing to the risk.

For example, high pressure steam and gas lines often need radiography or ultrasonic testing on all butt welds. Lower risk systems may need only a sample, such as ten percent of the joints. In that case, the inspector picks the sample, not the welder.

When a sampled weld fails, the rules usually require more tests on that welder’s work. So one bad result widens the inspection. That is fair, because a fault rarely happens only once.

Common weld defects and their causes

Understanding the cause helps to prevent the repeat.

Many of these start with conditions, not skill. For instance, rain, wind and damp electrodes ruin welds on open sites. Therefore, a good contractor provides shelter, ovens and clean, dry storage.

What happens when a weld fails inspection?

A rejected weld is not the end of the job. First, the welder removes the defect by grinding or gouging. Then the inspector confirms that it has gone. After that, the welder rewelds the area to the approved procedure.

Finally, the repair gets the same test again. The contractor records the repair in the weld log. Repeated repairs in the same place can damage the metal, so codes limit how many attempts are allowed.

Which standards govern weld inspection?

Each industry uses its own code, although the principles are shared.

• Pressure vessels and boilers. The ASME Boiler and Pressure Vessel Code, with Section IX for welding qualifications.
• Process and power piping. ASME B31.3 and B31.1.
• Structural steel. AWS D1.1 from the American Welding Society, or the equivalent ISO and European standards.
• Storage tanks. API 650 for welded tanks.

In addition, the people who test must hold qualifications. NDT technicians usually carry certification to recognised schemes at Level 1, 2 or 3. Level 2 technicians can perform and interpret tests, so ask to see the certificates.

Which documents should you receive?

Paper proves quality after the weld is covered by insulation or paint. A complete handover pack includes:

• approved welding procedures and their qualification records
• welder qualification certificates
• material test certificates
• a weld map that numbers every joint
• a weld log that links each joint to its welder and its tests
• NDT reports, signed by a qualified technician
• repair records
• pressure test certificates
• calibration certificates for test equipment

Together, these give full traceability. If a problem ever appears, the owner can trace it to a joint, a welder and a date.

How does an inspection and test plan help?

An inspection and test plan, or ITP, lists every check in order. For each step, it states who inspects, what standard applies and what record results.

It also marks hold points. At a hold point, work stops until the client or inspector signs. For example, fit-up before welding and pressure testing are common hold points. As a result, nothing important gets buried before somebody has seen it.

So ask for the ITP before the job starts. A contractor who cannot produce one probably does not have a quality system.

What should owners ask a contractor about weld inspection?

A few questions reveal a great deal.

1. Are your welders qualified for this material and position, and may I see the certificates?
2. Which welding procedures will you use?
3. Who performs your NDT, and what level are they?
4. What percentage of welds will you test, and by which method?
5. How do you store electrodes on site?
6. Will I receive a weld map and a full data pack?
7. How do you handle repairs?

Clear answers suggest a contractor who treats quality as routine. Vague answers suggest the opposite. Our guide on how to choose an engineering contractor in Nigeria covers the wider selection process.

Frequently asked questions about weld inspection

What is the difference between weld inspection and NDT?

Weld inspection is the whole process, from procedures and fit-up to the final checks. NDT is one part of it. It covers the test methods that examine a finished weld without damaging it.

Which NDT method is best for pipe welds?

Radiography and ultrasonic testing are the usual choices for butt welds in pipe, because they find internal flaws. Dye penetrant or magnetic particle testing suits fillet and socket welds.

Can a weld pass a pressure test and still be defective?

Yes. A pressure test shows that the joint does not leak today. However, it may not reveal internal flaws that could grow into cracks under repeated loading.

How do I know a welder is qualified?

Ask for the welder’s qualification record. It states the process, material, thickness range and positions that the welder passed, along with the date and the test results.

Insist on proof, not promises

Sound welds keep steam, gas and product inside the pipe. Weld inspection is how you know they are sound. So require procedures, qualified people, agreed testing and complete records on every job.

Maven Technical Services employs certified welders and works under a zero-defects quality standard, with inspection and testing documented for each project. See examples on our projects page, read our guide to stainless steel process piping, or talk to our engineers about your next fabrication job.