Ultrasonic Testing of Steel Plate, Bar and Forgings

Ultrasonic testing finds internal defects that no surface inspection can see. This guide explains how pulse-echo UT works, what the EN 10160 and EN 10308 quality classes mean, and where the method reaches its limits.

How it works

A probe emits short pulses of high-frequency sound into the steel through a couplant (usually water or gel). The pulses reflect from the back wall of the product — and from anything inside that interrupts the metal: a lamination, a cluster of inclusions, a crack, a void. By timing and sizing the echoes, the operator maps where the discontinuities are and how large their reflecting area is.

What UT finds

In plate: laminations (flat separations parallel to the surface), inclusion clusters and internal cracks — most of them inherited from the slab and invisible on both faces.
In bar and forgings: internal cracks, flakes, porosity, large inclusions and segregated centre zones.
In welds (a separate discipline with its own standards): lack of fusion, slag and cracks.

UT answers the question surface methods cannot: whether the inside of the section is sound enough for the service — critical where a plate will be cut into a pressure part, or a bar machined into a highly stressed shaft.

Standards and quality classes

Acceptance is organized in classes, with the higher classes imposing smaller permissible imperfections and more complete scanning:

EN 10160 covers flat products (plate and wide flat). It defines body quality classes S0 to S3 and edge classes E0 to E4: S0/E0 is the most lenient, S3/E4 the strictest.
EN 10308 covers steel bars.
EN 10228-3 and -4 cover forgings (ferritic/martensitic and austenitic respectively), again with graded quality classes.
Common ASTM counterparts are A578 for plate and A388 for forgings.

A purchase order that simply says "UT tested" without naming standard and class leaves both the scan coverage and the acceptance limits undefined — the single most common cause of UT disputes.

Where UT reaches its limits

Dead zone. The first few millimetres under the probe are hard to evaluate; near-surface defects need a second technique or a dual-element probe.
Defect orientation. A planar defect reflects best when it is roughly perpendicular to the sound beam. Straight-beam testing of plate finds laminations well but can miss steeply angled cracks; angle-beam probes cover other orientations.
Coarse grains. Cast structures and austenitic stainless scatter and absorb sound, reducing penetration and sensitivity. UT of coarse-grained material needs tailored procedures and lower frequencies.
Sizing is approximate. Acceptance rules are based on echo amplitude and indication extent compared with reference reflectors, not on a direct measurement of the true defect. Two standards can judge the same indication differently.
Geometry and surface. Thin sections, complex shapes and rough or scaled surfaces all restrict coverage; the standard defines how much of the volume must be scanned for a class to be claimed.

Specifying UT well

Name the standard and class, state whether scanning is 100% or on a grid, and agree in advance how indications at the acceptance boundary will be handled. That turns "UT tested" from a slogan into a verifiable requirement.