Ultrasonic Testing

Ultrasonic Testing

Ultrasonic testing sends high-frequency sound into a component and listens for what comes back. It finds flaws buried inside a weld, a forging or a casting, measures remaining wall thickness through corrosion, and does all of it from one side, with no radiation and no exclusion zone.

We provide ultrasonic inspection on site across the UK and in our Cumbria laboratory — conventional manual UT, thickness and corrosion mapping, and phased array where the geometry or the reporting demands it — to ISO 17640, ASME V Article 5 and customer specifications.

How ultrasonic examination works

A probe holds a piezoelectric crystal that converts an electrical pulse into a burst of sound, typically between 2 and 5 MHz for weld inspection. That sound travels into the material at a velocity fixed by what the material is — about 5,900 m/s longitudinally in steel — and reflects from anything with a different acoustic impedance. The back wall reflects. So does a crack, a lack of fusion, an inclusion or a pocket of porosity.

The same crystal then listens. Sound returns, is converted back to electricity, and appears on the flaw detector as a signal whose position along the trace corresponds to how long the sound was travelling. Because the velocity is known, that time becomes a distance. The instrument is not showing you a picture of the flaw — it is showing you how far away something reflective is, and how strongly it came back.

Two probe types cover most work. A compression (0°) probe sends sound straight down, which is what you want for wall thickness, laminations and plate. An angle probe introduces a shear wave at 45°, 60° or 70°, so the beam travels diagonally and skips off the back wall — which is how you get sound into a weld from the parent material beside it, without having to put a probe on the cap.

Couplant — gel, paste or water — sits between probe and surface. Air is such a poor transmitter that even a fraction of a millimetre of it reflects almost everything back. Nothing works without it, which is why surface condition matters more than people expect.

Ultrasonic testing for welding

Weld inspection is the bulk of what we do ultrasonically, and it is where the method has a specific advantage worth understanding.

A lack of fusion or a crack is planar: a flat discontinuity, often tight enough that its faces nearly touch. These are the defects that matter most, because they concentrate stress and propagate. They are also the ones radiography struggles with, since a flat flaw only shows on film when it happens to lie along the beam. Ultrasonics is the reverse — a planar flaw square to the beam is a mirror, and reflects strongly.

Radiography is better on volumetric flaws: porosity and slag show clearly on film and are harder to size ultrasonically. The two methods are complementary rather than competing, and choosing between them means asking which defect would hurt most if you missed it. If that is cracking or lack of fusion, the answer is ultrasonic.

A weld examination follows a scan plan: probe angles and positions chosen so the beam sweeps the full fusion faces and weld volume, with a parent-metal scan first to check for laminations that would deflect the beam and invalidate everything after. Sensitivity is set on a reference block with known reflectors before the job and re-checked afterwards — a machine that has drifted mid-shift makes every result since the last check unreliable.

Thickness and corrosion mapping

Point the sound straight through a wall and the time to the back wall gives its thickness, to a hundredth of a millimetre, from one side. No shutting down, no opening up, no scaffolding a vessel internally.

Single readings answer "how thick is it here". A corrosion map answers the more useful question: a grid of readings across an area, so you can see where the loss is, how it is shaped, and how it compares with the same grid taken last time. Isolated pitting, general wastage and preferential attack at a weld all look different on a map and identical as a single number.

Typical applications are pressure vessels, storage tanks, pipework and structural steel under insulation or coating — anywhere the wall is thinning where you cannot see it.

Ultrasonic pipe testing

Pipe brings its own difficulties. Curvature means the probe rocks unless the shoe is contoured to the diameter, and small-bore work needs the probe profiled to sit properly. Girth welds are examined from both sides where access allows; where a pipe runs against a wall or through a penetration, the scan plan has to achieve coverage from whatever is reachable, and the report should say so honestly rather than implying coverage that was not achieved.

We inspect girth and seam welds, branch connections and pipe supports, and carry out thickness surveys along runs to find where wall loss is concentrated. On thin-wall and small-diameter pipe the geometry can put conventional UT at its limits, and that is usually the point at which phased array or radiography earns its cost.

Where ultrasonic inspection is the wrong answer

Being straight about this saves everybody a wasted mobilisation.

Coarse-grained material scatters sound. Austenitic stainless weld metal, cast austenitics and some nickel alloys have a grain structure that scatters and bends the beam, raising the background noise and steering it off course. It is not impossible, but it needs specific techniques and procedures qualified for that material — a standard carbon-steel procedure applied to an austenitic weld produces a confident, clean-looking result that means very little.

Surface condition limits it. Heavy scale, weld spatter, rough as-cast surfaces and loose paint break the couplant path. Preparation is often the difference between a valid inspection and a wasted day, and it needs costing before mobilisation rather than discovering on arrival.

Geometry can defeat it. A shape that returns strong reflections from its own corners and changes of section can mask a real flaw. Nozzles and complex joints often need phased array, or a different method entirely.

It depends on the operator. Conventional manual UT produces a trace that the technician interprets in real time, and unless it is recorded, the record is their report. That is exactly why certification level and procedure qualification matter here more than in most methods — and it is one of the strongest arguments for phased array, which stores the data so somebody else can look at it afterwards.

Certification and reporting

Our technicians hold PCN Level 2 in ultrasonic testing for welds and for wrought products, and we provide PCN Level 3 services for writing and approving procedures, qualifying techniques and independent review of other people's results.

Examinations are reported against the acceptance criteria named in your specification, with indications located, sized and characterised, and coverage stated including anything access prevented. Reports come from our digital reporting system, so repeat inspections of the same asset build a comparable history — which is what turns a thickness survey into something you can plan maintenance from.

Tell us what you need examined — the material, the joint, the thickness and what you are worried about — and we will tell you whether ultrasonic inspection is the right method, and what it will and will not see.