Неразрушающий контроль сварных швов: Руководство 2026 года

Updated May 2026 Reviewed by ZX welding-systems engineering team

Non-destructive inspection of welds is when we look at a finished joint to check for cracks, porosity, lack of fusion, or other irregularities, without machining, etching or breaking the part. Every code-conforming pressure vessel, pipeline, or structural connection gets this signature of acceptance every time. This guide discusses the five core AWS-approved testing methods for welds – radiography (RT), ultrasonics (UT), magnetic particle (MT), liquid penetrant (PT), and visual (VT) – as well as the evolving use of AI visual inspection, the standards associated with each, and four key questions to ask when selecting your test.

What Is Non-Destructive Testing for Welds? Five Methods (and Why VT Comes First)

NDT is a suite of inspection methods that identify defects in a weld without damaging or modifying the part. Approaches span everything from a Certified Welding Inspector working with a flashlight and weld gauge, to encoded phased-array ultrasonic systems mounted on a robotic crawler.

The “five methods” most welders learn

From the American Welding Society NDT Fundamentals course to the AWS B1.10M/B1.10:2016 Guide for the Nondestructive Examination of Welds, five core methods are consistent in most code acceptance criteria: visual testing (VT), penetrant testing (PT), magnetic particle testing (MT), radiographic testing (RT), and ultrasonic testing (UT). Many CWI passing outlines list these in the not-to-miss section.

While the above “five” number is handy for everyday questions, it is not the full set. American Society for Nondestructive Testing (ASNT) recognizes 16 NDT methods in its handbook series, including eddy current testing (ET), acoustic emission (AE), thermal and infrared, leak testing, and several others used for in-service vessel monitoring. Most weld inspections focus on the five above, though a mature NDT program has a sixth in mind, as needed.

Why visual testing is always step one

AWS D1.1, the Structural Welding Code for steel, clearly states: “Welds subject to nondestructive examination shall have been found acceptable by visual inspection.”. Those under- or over-sized welds? No X-ray on them. Those buckled? No ultrasonic or shear wave to add up to an acceptance. In practice, the first test always is visual. You’re not supporting the camera and going to the magazine with your film against a warping weld until you know the rest already is good-to-go.

“Visual testing is the foundation upon which all NDT methods are built because visual interpretation is required by all the other methods. In most NDT methods, the inspector relies on instruments to perform the inspection. In visual testing, the inspector is the instrument that evaluates the part.”

— Bruce Crouse, ASNT VT Level III

What are the 5 types of non-destructive testing?

Specifically for welding, the five core methods are reactive to these families of discontinuities:

• Surface profile problems: cracks, undercut, overlap, root-pass lack of fusion, excess weld size.
• Surface-breaking discontinuities: pinholes or cold shuts from penetrant, leak paths.
• Surface and near-surface discontinuities: longitudinal cracks, footprint lack of fusion in ferromagnetic metal.
• RT reveals volumetric discontinuities like pore, slag inclusion and lack of fusion, having existed inside of the weld.
• UT and PAUT detect planar defect (cracks, lack of fusion, lack of penetration) anywhere from the back to the front, give size and depth measurement.

Below is a description of how the method works, what it can and can’t potentially discover, and when the call is right.

Radiographic Testing (RT): X-Ray and Gamma Ray

Radiographic testing (RT) involves passing ionizing radiation through the weld joint and imaging it on a piece of film, computed-radiography phosphor plate, or digital flat-panel image sensor. Discontinuities are revealed in the radiographs as areas of increased darkening because they are more permeable to radiation. RT is often thought of as “X-ray quality,” though we are likely talking about the more general term volumetric weld inspection here.

X-ray versus gamma ray: when to use which

Industrial RT uses two source families:

• X-ray tubes have a VS dial to control the voltage to suit wall thicknesses. They are used in connection with a fixed shop and are on for thinner steel (up to about 50 mm depending on tube power).
• Gamma sources are sealed radioisotopes that stay on their own, and give out constantly. Iridium-192 is the general jobbing source for steel from about 6 mm till around 75 mm, and Cobalt-60 takes the application when sections are very thick (greater than 50-75 mm) on account of higher photon energy.

Selection of sources influences field logistics. Given the 74-day half-life of Ir-192, the practical source for a project source will be exhausted by a protracted shutdown; Co-60 has an extremely long half life but requires more shielding. The United States Nuclear Regulatory Commission working-safely-in-gamma-radiography guidance packs details of both source types and the operational failings that resulted in overexposure.

What RT does well, and where it is blind

RT is unsurpassed on locating dispersed porosity and slag inclusions and provides a permanent film record which goes home after the job – a significant practical bonus when a contract specifies a film file to keep. It does require a practical spotter to interpret while the image itself flushes the whole wall thickness onto a single image – opening the potential to over-quantify the porosity (all voids shown if Table One porosity is actually bushed out front to back) and it is poor on planar indications which lie along the direction of the film, such as a lamination or a tight lack of fusion line. (And this limited blind-spot eventually lead the codes to permiss ultrasonic alternatives.)

Ultrasonic Testing (UT) and Phased Array (PAUT)

Welds are inspected using ultrasonic testing. High frequency sound (usually 2-5 MHz for a carbon-steel weld) is pumped into the joint by a piezoelectric transducer. Detecting echoes returns from internal reflectors, an electronic timing circuit interprets the returned energy and displays a depth and amplitude reading on an A-scan, B-scan or a C-scan display.

UT instruments cover surface, close-to-surface, and deep-to-surface defects on the same display.

Conventional UT, phased array, and TOFD

Three flavors are common in modern weld work:

• Traditional pulse-echo UT is ‘single probe, single energy source and receiver’. Operators scan manually with couplant gel between the probe and the metal — cheap, portable and requires a lot of skill.
• PAUT transducer is composed of between16 and128 small elements. These are fired in groups in precisely control led time delays to electronically control the beam angle and focus. One pass will scan the entire weld cross-section and save the data as a file.A recording can then be examined by the inspector at a later time.
• Time-of-flight diffraction (TOFD) employs a pair of transmit/receive probes to detect the amount of diffraction from crack tips. It is extremely accurate in determining through-wall flaw height and if the flaw height technique is employed it is normally used in conjunction with PAUT.

ASNT’s ultrasonic-testing reference explains each technique and the C-scan, B-scan, and A-scan formats that present the data; a senior practitioner sums up the value of advanced UT this way:

UT results carry useful information for planning repair and replacement of machinery and equipment. Today, conventional and advanced UT techniques are applied widely, so current personnel qualification and certification is essential for NDT investigators.

— Huidong Gao, ASNT UT Level III

Can phased array UT replace radiography?

Yes, in a growing number of code-accepted instances. A peer-reviewed ASME paper, “Review of Ultrasonic Phased Arrays for Pressure Vessel and Pipeline Inspection“, documents exactly how new pressure-vessel codes — most especially ASME Code Case 2235 — have allowed PAUT instead of RT to become accepted practice for pressure-vessel welds. ASME Section V Article 4 has incorporated UT for weld inspection into the main code body since 2010, with the use of a code case no longer necessary in many cases. API 620 and API 650 both include Appendix U, which details using ultrasonic in place of RT, and the US Nuclear Regulatory Commission has allowed PAUT rather than RT for safety-related piping at stations such as Florida Power & Light’s Turkey Point station, under filing ML17208A058.

Practical motivations: no radiation shield, electronic data rather than film, the option of sizing flaws by wall thickness, and instant feedback to the welder. Reddit’s r/nondestructivetesting community reaffirms this in plain English — one practitioner claims RT’s remaining advantages are “permanent record images and less subjective interpretation” while PAUT crawlers are now “industry standard” on many pipeline projects.

Magnetic Particle Testing (MT / MPI)

Mag: Find surface and near surface discontinuities on ferromagnetic materials – carbon steel, low-alloy steels, ferritic stainless – by excited a magnetic field to draw on iron-oxide particles. Where an imperfection breaks the field, the leakage flux pulls the particles into a visible indication that follows the flaw.

How the field is generated

Two fields-generation techniques dominate weld work. A yoke is a portable hand-held electromagnet pinned to either side of the weld, producing a longitudinal field through the gap between its poles. Prods push direct current through the part to generate a circular field. Field direction can be critical: a crack only produces an indication when it runs at an angle to the lines of force, which makes 90 degree rotation required by ASTM E709 and ISO 17638 necessary.

Wet versus dry, visible versus fluorescent

Particles are applied dry (colored powder blown from a globe) or wet (suspended in a kerosene-class or water carrier). Dry particles work better than wet on rough as-welded surfaces – wet particles are finer, and detect tighter cracks. Fluorescent particles read under UV light at levels of sensitivity not attainable with visible-light particles – which in the aerospace industry is why fluorescent wet-method MT is the norm for engine components.

MT is quick, inexpensive, and acceptable on as-welded surfaces with minimal scale. Its shortcoming is penetration: it will only reveal surface flaws, and roughly the first 3 mm below. For anything beneath that, RT or UT are called for.

Liquid (Dye) Penetrant Testing (PT)

Liquid penetrant testing detects surface-breaking flaws on all non-porous materials – steel, stainless steel, aluminum, copper, titanium, even glass and ceramics. Where MT cannot magnetize a stainless weld, PT is the fix.

The five-step PT process per ASTM E165

1. Pre-clean: degrease and dry the weld – oil, slag residue, or paint will inhibit the penetrant.
2. Apply penetrant: spray, dip, or brush a contrasting or fluorescent dye onto the weld surface.
3. Dwell: wait the required time – generally 5-30 minutes for welds per ASTM E165, longer for tight fatigue cracks.
4. Clean excess: wipe the surface clean using the carrier’s specified remover (water, solvent, emulsifier) – do not flood, as this flushes the dye out of the cracks.
5. Develop and inspect: dust on a developer powder; the developer draws dye from any flaw, emphasizing the indication. Visible-dye indications can be viewed in white light; fluorescent dye requires UV-A.

PT remains affordable (a basic three-can spray kit is sub-$50), portable, and tolerant of surface geometry—you can shoot it on a fillet weld, a tube T, or an ever-hopeful overhead joint. Its weakness is brutal—only surface-breaking faults will be revealed. A subsurface lack of fusion 2 mm below the cap won’t be visible, no matter how long the dwell.

AI Visual Inspection: Where Machine Vision Fits in Weld NDT

AI visual inspection of welds applies convolutional neural networks–the same family of computer-vision architectures used on robots and in radiology–to images of the weld surface. A camera, often combined with structured-light or laser-line pro filing, takes an image of the as-welded bead; the model classifies bead geometry as acceptable or one of several defect categories (porosity, undercut, spatter, missed seam, irregular cap profile).

What AI vision sees that human VT misses

Vendors focus on consistency, not miracle-working vision. A trained model looks at every bead on a robotic cell, according to a single criterion, never gets tired halfway through its shift, and produces a time-stamped record of every weld image for downstream inspection. Predictive systems also feed bead-shape data back into the welding controller to recalibrate voltage, wire feed, or travel speed. Investor signal is loud — venture funding is actively flowing—most commercial vendors demonstrate full-bead defect detection at the 90-plus percent level on their reference applications, with continued investment through 2024 and 2025.

What it cannot replace

An AI camera does not produce a magnetic field in a weld, it does not transmit sound through it, and it does not generate a radiograph. This is—Physics-ly—a visual strategy, with identical surface-only restrictions as human VT. AWS D1.1 still requires VT by a qualified individual, and code-stamp acceptance is approved by a flesh-and-blood machine operator, not an AI engine. A practical camera placement is simply a supplementary one: AI visual inspection is a 100% weld-inspection layer running inside production, with human CWIs and the volumetric methods (RT/UT) conducting the code-mandated acceptance examinations.

Where it integrates with robotic welding cells

One ideal fit for AI visual inspection is within the welding cell. A camera on a fixed second arm or the torch-lead arm simultaneously takes images of every weld in the inspection zone right after welding takes place, while the part remains clamped in the fixture and an incorrectly-placed bead has at least one more opportunity to be corrected without unloading and loading parts from the fixture. This advantage matters most in robotic welding cells used for power-industry pressure-vessel fabrication, where 100% inspection is contractually required, and where the cost of a missed flaw (a returned vessel, a code re-test) vastly exceeds the capital cost of a camera and associated software.

Choosing the Right Method: Defect Coverage, Cost, and Speed

Moving forward with the ideal method doesn’t correspond to whichever available solution exhibits the greatest combined sensitivity and specificity in an advertiser’s demo video. It’s the method that appropriately identifies the flaw classes the code concerns with the materials on hand, within the radiation, training, and budget limitations of the project. Most teams arrive at the right answer thanks to two tables and a four-question evaluation strategy.

Defect-by-method coverage matrix

Which NDT method is best for surface cracks?

Surface-only cracks. The answer depends on the base material: For ferromagnetic steel use MT—yet a yoke plus dry powder will give a clear indication in less than a minute. For stainless or aluminum use PT—the field will not seat in non-magnetic alloy. For tight fatigue cracks below the visible-dye threshold fluorescent PT under UV works well. UT also finds surface cracks but for non-artificial flaw surface work it’s overkill in comparison to MT or PT, and operator skill more difficult to justify.

The 4-Question NDT Selection Framework

Codes, Standards, and Acceptance Criteria

Method coverage tells you what each NDT method can do. Codes tell you what your industry mandates. Most projects come under one of five governing documents.

Inspector certification

Two certification systems dominate the field. AWS Certified Welding Inspector (CWI) credentials the visual inspector under AWS D1.1 and the like; the test covers weld processes, code training, and inspection practice. ASNT SNT-TC-1A is a recommended-practice outline used by employers to approve Level I, II, and III inspectors in each individual NDT method. Most pressure-vessel and pipeline projects ask for both: a CWI visual inspector and either Level-II or III NDT cadre.

Industry Outlook: NDT for Robotic Welding and Where AI Vision Goes Next

The global non-destructive testing and inspection market in 2025 will be worth USD 14.99 billion climbing to USD 22.34 billion in 2030 at a compound annual rate of 8.3 percent according to MarketsandMarkets own 2025 NDT and Inspection report. Three trend lines matter most for the welding side of that market.

PAUT continues to displace RT in code-stamped fabrication

The standards change is mostly done: ASME Section V Article 4 has been covering UT on welds directly since 2010, the Code Case is mature, API 620 and 650 have their Appendix U and the NRC has approved PAUT in all but name instead of RT for safety related piping at many US plants. The work in progress remains inside fabricators – especially for pipeline spool work and pressure-vessel longitudinal seams where PAUT crawlers now appear as standard equipment. If you are scoping a fabricator for 2026 and beyond, plan your process to take encoded PAUT files instead of radiography exclusion zones.

AI vision moves from screening to closed-loop control

The first wave was about detecting the weld defect after it already occurred. A second wave, some of whose users are already shipping, takes these bead-shape parameters and feed them back in real time to the welding controller so that the voltage, wire-feed speed, or travel speed before the marginal weld is downgraded to a fall-out. This is the logical evolution of in-line inspection in robotic welding systems engineered for code-quality power-industry welds, where the inspection loop protects the ASME-stamp check and the control loop protects throughput.

Non-intrusive inspection (NII) reduces confined-space entry

Related to this is the trend of upward adoption of NII for in-service ships: PAUT, guided-wave UT, thermography. Scaffolding, gas freeing and confined-space permits are eliminated by doing the work without opening up the piece. Anecdotal reports by practitioners over on r/nondestructive testing bore out what the asset-integrity groups say is happening-they—clients—are actually asking for NII proposals rather than CSE-based ones.

For weld owners it’s a matter of allocating on-shop PAUT capability as well as compilation-stage UT.

Taking action for fabricators with plans for 2026 capex: where your weld scope is limited by ASME Section V, AWS D1.1, or API 1104 the most audience-affecting NDT expenditure is registered PAUT feasibility with AI optical verification as a production-line filter hat sits in front of it. RT is still a reasonable choice for one-off thick-section outriders, but the wagons are departing:

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