Ultrasonic testing of steel (UT) is a non-destructive testing method used to detect internal discontinuities without damaging the material. High-frequency sound waves are transmitted into steel, and their reflection or transmission characteristics are analyzed to evaluate possible defects such as cracks, voids, and internal separations.

Ultrasonic inspection is an important part of material acceptance and quality control, particularly for engineering steel bars, forged components, and parts intended for demanding mechanical applications. However, reliable ultrasonic testing depends on more than operating the equipment. It requires selecting the appropriate inspection standard, performing proper calibration, and evaluating results against specified acceptance criteria.

In this guide, we explain what ultrasonic testing is, how UT inspections are performed, which defects can be detected in steel, and how to read an ultrasonic inspection report.

What Is Ultrasonic Testing?

Ultrasonic testing is a non-destructive testing (NDT) method that uses sound waves above the range of human hearing to examine the internal structure of a material.

UT stands for Ultrasonic Testing. Its primary purpose is to investigate internal discontinuities without cutting, breaking, or otherwise rendering the material unusable.

Ultrasonic testing can be applied to steel bars, forged products, castings, welded joints, and other suitable metallic materials. However, the same probe, scanning procedure, or acceptance limit cannot automatically be applied to every product.

For example, inspecting the internal structure of a round steel bar may require a different procedure from examining discontinuities within a welded joint.

Quick answer: Ultrasonic testing uses sound waves to investigate possible internal discontinuities in steel. Whether the material is accepted or rejected depends on the applicable inspection standard and specified acceptance criteria.

How Does Ultrasonic Testing Work?

The working principle of ultrasonic testing is based on the propagation of sound waves through a material and changes in their behavior when they encounter boundaries with different acoustic properties.

In the commonly used pulse-echo method, a probe transmits short ultrasonic pulses into the material. As the waves travel through the steel, they may be reflected by the back wall or by discontinuities with suitable orientations.

The returning signals are detected by the equipment and displayed on a screen. The inspector evaluates their arrival time, amplitude, and other characteristics to help locate and assess possible discontinuities.

Ultrasonic inspection is not limited to pulse-echo testing. In some applications, a through-transmission technique is used, in which sound is transmitted from one side of a component and received on the opposite side.

How Do Ultrasonic Waves Travel Through Steel?

The speed of sound in steel depends on the material’s elastic properties, the type of wave, and other conditions. Longitudinal and transverse waves propagate at different velocities.

Under suitable conditions, the depth of a reflector in pulse-echo testing can be calculated using the sound velocity and the round-trip travel time.

Basic formula: Depth = (Sound velocity × Round-trip travel time) / 2. This relationship applies to a direct round-trip sound path. Angle-beam inspections and more complex geometries require additional calculations.

For example, a properly configured ultrasonic flaw detector measures how long it takes for an echo to return and uses that information to help determine the reflector’s position inside the material.

However, not every echo represents a defect. Geometric boundaries, surface conditions, and the inspection setup can also generate signals.

How Is Ultrasonic Testing of Steel Performed?

Ultrasonic testing of steel generally involves inspection planning, surface preparation, equipment setup, scanning, signal evaluation, and reporting.

The exact procedure depends on the product’s shape, dimensions, material properties, and the applicable inspection standard.

1. Surface Preparation and Material Identification

The first step is to identify the steel product being inspected. The material grade, dimensions, heat number, and relevant order information are checked.

The inspection surface is then assessed to determine whether it is suitable for probe movement and acoustic coupling.

Loose rust, dirt, heavy mill scale, or significant surface irregularities may interfere with sound transmission and probe contact.

Proper surface preparation helps improve signal quality and inspection repeatability.

2. Selection of the Probe and Testing Equipment

The ultrasonic flaw detector and probe are selected according to the material properties and inspection objectives.

Important selection factors include probe frequency, wave mode, scanning direction, component geometry, and the characteristics of the discontinuities being investigated.

Higher frequencies may provide better resolution under certain conditions. However, because ultrasonic attenuation and penetration requirements vary, the highest available frequency is not always the best choice.

3. Calibration and Sensitivity Adjustment

Before inspection, the instrument’s time base, sound path, and inspection sensitivity are adjusted using suitable reference blocks or methods specified in the procedure.

These adjustments help ensure that echoes are positioned correctly and that the specified evaluation levels can be applied.

The reference block type, artificial reflector characteristics, and evaluation technique depend on the applicable standard.

4. Application of a Couplant

A suitable couplant is applied between the probe and the steel surface to facilitate ultrasonic transmission.

Depending on the application, a gel, oil, or water-based medium may be used. The couplant reduces the effect of air gaps that would otherwise interfere with sound transmission.

Insufficient coupling may result in signal loss and inaccurate interpretation.

5. Scanning and Signal Analysis

The inspector moves the probe over the material according to the specified scanning plan. Signals are monitored, and indications meeting the relevant evaluation threshold are examined in accordance with the procedure.

Information such as signal position, amplitude, extent, and other relevant measurements may be recorded.

For long steel bars, the inspection coverage and scanning pattern are particularly important. Inspecting only a limited surface area does not mean that the entire product volume has been examined.

6. Evaluation and Reporting

Finally, recorded indications are evaluated against the acceptance criteria specified in the purchase requirements or inspection procedure.

The inspection report should identify the material, inspection method, equipment, inspection coverage, results, and other required information.

Acceptance or rejection is not determined merely by the presence of an echo on the screen. It depends on the applicable evaluation procedure and acceptance limits.

What Equipment Is Used in Ultrasonic Testing?

Ultrasonic testing equipment affects measurement reliability and the practical application of the inspection procedure. The main components are summarized below.

An ultrasonic flaw detector alone cannot establish material conformity. Reliable results depend on proper equipment selection, appropriate settings, and competent personnel.

A-Scan vs. B-Scan vs. C-Scan in Ultrasonic Testing

Ultrasonic inspection results can be presented in different display formats. Common examples include A-Scan, B-Scan, and C-Scan.

A-Scan is widely used in conventional pulse-echo inspection. B-Scan and C-Scan can provide more extensive visualization when suitable data acquisition and positioning systems are available.

Important distinction: A C-Scan does not automatically provide a three-dimensional image. It generally displays a two-dimensional area map of a selected depth range or signal parameter.

What Defects Can Ultrasonic Testing Detect in Steel?

Under suitable inspection conditions, ultrasonic testing can help identify different types of discontinuities inside steel.

However, detectability depends on the discontinuity’s size, orientation, location, surface conditions, probe characteristics, and the material’s acoustic behavior.

Not every discontinuity is automatically an unacceptable defect. A discontinuity is a variation in the material’s structure, while its classification as a rejectable defect depends on the applicable acceptance criteria.

Similarly, ultrasonic testing cannot guarantee the detection of every microscopic defect. Very small discontinuities, unfavorable orientations, and inaccessible inspection regions may limit detection.

Why Is Ultrasonic Testing Important for Steel Bars?

Steel bars are widely used as semi-finished products in the manufacture of shafts, fasteners, gears, machine components, and other industrial parts.

Internal discontinuities may create difficulties during subsequent machining or heat treatment. Evaluating internal material integrity is particularly important for components designed to operate under demanding loads.

When performed according to an appropriate technical specification, ultrasonic testing can support material inspection before manufacturing begins.

Ultrasonic Inspection of Round Steel Bars

When planning UT inspections for round steel bars, factors such as diameter, surface condition, material grade, and the volume to be examined must be considered.

Probe movement and scanning coverage are determined according to the discontinuities being investigated.

Inspecting only a limited section of the product does not mean that the entire cross-section has been examined.

Therefore, if ultrasonic inspection is required during procurement, the applicable standard, inspection coverage, and acceptance level should be clearly specified.

UT Inspection of Hot-Rolled and Cold-Drawn Steel

The applicability of ultrasonic testing to hot-rolled and cold-drawn steel products depends on their geometry and surface conditions.

Although cold-drawn products may have smoother surfaces, this does not guarantee better ultrasonic inspection results in every situation.

The inspection method, material characteristics, and technical requirements must be evaluated together.

Ultrasonic inspection does not replace mechanical testing or chemical analysis. It is a separate inspection method that supports the evaluation of internal discontinuities.

Which Standards Apply to Ultrasonic Testing of Steel?

The appropriate ultrasonic testing standard must be selected according to the product type and inspection objective.

Different technical standards may apply to steel bars, forged components, and other industrial products.

EN 10308 addresses ultrasonic testing of steel bars within its defined scope using the pulse-echo method. It covers products with a diameter or equivalent thickness not exceeding 400 mm. The use of mechanized or automated techniques may be subject to agreement between purchaser and supplier.

ISO 16810:2024 establishes general principles for ultrasonic testing. However, it does not independently specify the inspection coverage and acceptance criteria for every type of product.

Therefore, a statement such as “ultrasonic testing performed” is not sufficient by itself. The applied standard, inspection procedure, and acceptance requirements should also be clearly identified.

For additional technical information, see the official ISO 16810:2024 standard page.

How to Read an Ultrasonic Testing Report

An ultrasonic testing report is a technical record documenting the inspection method, scope, and results.

When reviewing a UT report, first verify that the inspected product has been correctly identified. Then examine the inspection standard, scanning coverage, equipment settings, indication records, and acceptance criteria.

The following table is a hypothetical example designed to illustrate how a UT report can be interpreted. It is not an actual inspection result or conformity certificate.

The word “accepted” in a report may not be sufficient without additional context. It should be clear which standard, scanning coverage, and acceptance level were used.

For example, a successful inspection of a limited area must not be interpreted as proof that the entire steel bar was examined.

How Are Acceptance and Rejection Criteria Determined?

Acceptance and rejection criteria establish the conditions under which detected indications are considered acceptable or unacceptable.

These requirements may be defined by an applicable product standard, customer specification, technical contract, or approved inspection procedure.

Depending on the procedure, evaluation may consider signal amplitude, indication position, extent, and other specified parameters.

For example, an echo exceeding a particular reference level may require additional evaluation or recording. However, this does not necessarily mean that the material must be rejected immediately.

Similarly, the absence of a significant indication on the screen does not prove that the entire material volume is free from defects.

Important: There is no single universal “ideal echo value” or acceptance threshold applicable to every steel product. Acceptance criteria depend on the product, inspection method, and technical specification.

Advantages and Limitations of Ultrasonic Testing

Ultrasonic testing offers important advantages for evaluating internal material integrity. However, understanding its limitations is essential for interpreting results correctly.

For these reasons, ultrasonic inspection may be used alongside other non-destructive testing methods and material tests when required.

Relationship Between Ultrasonic Testing and Material Certificates

An ultrasonic testing report and a steel material certificate may document different technical information.

A material certificate may include product identification, heat number, chemical analysis, and required mechanical test results. An ultrasonic inspection report records the method and results of an internal discontinuity inspection performed within a defined scope.

Having an MTC or MTR does not automatically mean that a steel product has undergone ultrasonic testing.

If ultrasonic inspection is required, the requirement should be specified during procurement and verified through appropriate documentation.

For further information, see Uyar Çelik’s article How to Read a Steel Material Certificate? MTC and MTR Guide.

Frequently Asked Questions

What is the ultrasonic testing method and how is it used?

Ultrasonic testing is a non-destructive inspection method that uses high-frequency sound waves to investigate discontinuities within a material. The material is scanned using a suitable probe and equipment settings, and the resulting signals are evaluated against specified criteria.

What is an ultrasonic testing device?

An ultrasonic testing device, often called an ultrasonic flaw detector, generates, processes, and displays ultrasonic signals. It is used in non-destructive testing for discontinuity detection and, in certain applications, thickness measurement.

What does an ultrasonic sensor measure?

Depending on the application, ultrasonic sensors may measure sound-wave return time or other signal characteristics. In industrial UT equipment, these measurements help determine reflector positions and evaluate signal properties inside a material.

Does ultrasonic testing damage steel?

No. Conventional ultrasonic testing is a non-destructive inspection method that does not compromise the material’s structural integrity. However, surface cleaning or preparation may be required for proper probe contact.

Can ultrasonic testing detect every crack?

No. Detection depends on the crack’s size, orientation, depth, material properties, and inspection technique. Every inspection method has practical detection limits.

Is ultrasonic testing mandatory for steel bars?

There is no universal ultrasonic testing requirement for every steel bar. Inspection requirements depend on the applicable product standard, intended application, contract, and purchase specification.

Does a passed UT inspection mean that steel is completely defect-free?

No. A passing result means that the material meets the specified requirements within the defined inspection scope and acceptance criteria. It does not guarantee the absence of every possible defect.

What information should an ultrasonic testing report contain?

Depending on the applicable procedure, a UT report should include material identification, inspection standard, test coverage, equipment information, relevant settings, indication records, acceptance criteria, and inspection results. Personnel identification and approval details should also be documented as required.

Why Should Ultrasonic Inspection Be Considered in Steel Procurement?

In steel procurement, selecting the correct material grade alone may not be sufficient for every application. Mechanical properties, chemical composition, delivery condition, traceability, and, when necessary, internal material integrity should be evaluated together.

For components intended for demanding strength or reliability requirements, defining the appropriate inspection procedures during material acceptance can help reduce downstream manufacturing risks.

Specifying ultrasonic inspection requirements during procurement also helps ensure that the applicable standard, inspection coverage, and acceptance criteria are clearly communicated to the supplier.