Hardness testing of welds in pipes and boilers is essential to ensure long-term reliability under pressure and heat. Microstructural changes in the heat-affected zone can lead to cracks over time, making early detection critical.
This blog explores the advantages of the UCI method for weld seam evaluation, particularly in challenging environments. Learn how portable devices like the SONODUR 3 bring precision and efficiency to on-site inspections.
Certain microstructures within the heat-affected zones (HAZ) of weld seams in pipes and boilers are prone to cracking due to stress relaxation during welding – caused by the combined effects of heating and subsequent rapid cooling. High-alloy steels commonly used in pressure piping and boiler systems, particularly in power plants, often undergo precipitation hardening along the weld line within the HAZ. This process increases localized mechanical stress and susceptibility to cracking.
The occurrence and nature of these cracks are strongly influenced by several factors, including the welding technique, filler material, and the specific composition and alloying elements of the steel. Notably, such cracks often develop only after prolonged service, under the influence of operational stresses such as pressure, temperature, vibration, and material aging.
To detect early signs of potential damage, hardness testing is one of the most reliable diagnostic methods. This involves measuring and documenting the hardness profile across three critical zones: the base material (BM), the heat-affected zone (HAZ), and the weld metal (WM). It is essential that specific hardness limits are not exceeded—particularly in the front and rear regions of the HAZ and within the weld metal itself. Furthermore, the hardness of the weld metal should remain within a defined range of the base material’s hardness, as stipulated in standards such as DIN EN 288-3 (see Fig. 1).
Figure 1: Diagram of heat-affected zones showing base material, heat-affected zones and weld seam
In some cases, additional treatment is required—or alternatively, a (costly) rewelding of the seam must be carried out.
Details on weld seam inspection procedures are specified in DIN EN 1043-1.
Portable UCI (Ultrasonic Contact Impedance) hardness testing has proven highly practical in this field. It is especially well suited for use on pipes and boilers, enabling manual, multi-directional testing directly on the component surface. The protruding Vickers diamond tip ensures precise probe positioning (see Fig. 2), allowing accurate testing of component edges (e.g., cut edges per EN ISO 1090, HV10) before welding, as well as detailed scanning of narrow HAZ zones using the extremely small indentation.
UCI hardness testing is also very fast—with a measurement cycle of about 2 seconds—since the hardness value is determined digitally during load application. This eliminates the need for visual inspection of the indentation and allows immediate display of the result.
Figure 2: Precise positioning using a Vickers diamond tip enables accurate scanning of component edges and heat-affected zones (HAZ) during UCI hardness measurements.
Therefore, UCI hardness testing can be carried out directly on the component—on-site and immediately after heat treatment or hot forming of structural steels – enabling quick detection and response to any deviations from the target material condition.
When the UCI probe’s Vickers diamond tip contacts the material, it produces a characteristic frequency shift in the vibrating resonant rod. This shift primarily depends on the contact surface area (i.e., the hardness), assuming the elastic modulus is known. For example, the SONODUR UCI tester is calibrated for steel with an elastic modulus of approximately 210 GPa, making it suitable for most steel grades without recalibration.
During manual load application, once the nominal force is reached, the hardness value is precisely calculated. Small angular deviations (up to ±5° from perpendicular) have minimal influence on the result, thanks to consistent coupling of the probe to the material. In contrast, classical Vickers testing is significantly more sensitive to angular misalignment—with notable errors starting at deviations as small as ±2°, due to difficulties in optically measuring the deformed indentation.
On thin-walled pipes near or below the 3–5 mm range, unwanted resonances may affect results. In such cases, signal analysis, such as that provided by the SONODUR 3, can help identify unreliable readings or assess the suitability of the UCI method.
Other portable hardness methods – like the Leeb rebound technique (developed in 1974) – are only partially suitable for pipe and boiler construction. This is due to the relatively large indentation size, which makes it unusable in narrow HAZs and sensitive to wall thickness variations. Although the method was once considered groundbreaking, many devices were unfortunately used in applications beyond their optimal range, leading to confusion about measurement accuracy.
That said, when applied appropriately – for example, using the Leeb sensor on the SONODUR 3 (model SONO D) – the method can deliver accurate, repeatable results with simple operation.
Hardness testing systems based on the Rockwell principle, or traditional portable Vickers devices with low test forces, have seen limited adoption in pipe and boiler inspections. Their practical limitations on curved surfaces, uneven weld zones, constrained positions, or difficult-to-access areas make them unsuitable for many real-world applications.
All hardness tests are carried out in accordance with approved procedures, including guidelines for calibration, surface preparation, and removal of interfering layers (e.g., paint, oxides, or decarburized surfaces). These steps are particularly critical in demanding applications such as testing steels in the P91 class.
A well-equipped testing laboratory – with an in-house welding workshop, mobile and stationary hardness testers, and other destructive testing tools – is highly beneficial. This setup supports both modern steels (by comparing mobile and stationary measurements) and components in long-term service.
Proper surface preparation is essential. Typically, a flap wheel is used to level the weld bead, followed by removal of approximately 0.5 mm of base material to eliminate mill scale and decarburized layers, exposing the true material structure.
This involves a stepwise grinding process, starting with 80-grit and progressing through 120, 240, 320, and up to 400 grit. The goal is to reduce surface roughness to allow clear visual differentiation of weld zones using etching agents like Adler or Nittal, which is key to accurate UCI hardness profiling. For high-alloy or heat-resistant steels and very narrow HAZs, further refinement up to 600 grit or the use of polishing emulsions may be required.
Figure3: Cross-section, used for early UCI hardness measurements, showing hardness zones
UCI hardness measurements are often performed manually, using a probe that scans through a defined sequence of zones: base material 1 (BM1), heat-affected zone 1 (HAZ1), weld metal (WM), heat-affected zone 2 (HAZ2), and base material 2 (BM2). On pipes, measurements are typically taken at least at two opposing points around the circumference (e.g., at 180° top and bottom for horizontal pipes). Additionally, the exact location of the test and the sequence or direction of measurement – such as the flow direction of the medium within the pipe – must be recorded.
In the past, one technician would handle the probe while another recorded the values manually. This process made result evaluation cumbersome, especially when incorrect measurements required manual recalculation for quality assessment.
Modern UCI systems like the SONODUR 3 streamline this process by providing immediate graphical feedback on the hardness profile across the weld zone. With built-in documentation features and statistical analysis, operators can quickly verify suspect values by adding nearby test points. Anomalous or invalid data can be excluded as needed.
Multiple readings are taken in each zone, but only those relevant according to applicable standards are included in the final evaluation. The “worst-case approach” is used – reporting the most critical values, which must remain within the permissible hardness limits for the specific steel. Isolated outliers may be tolerated, but overall compliance is required. The summarized results are typically submitted to an overseeing authority for approval.
A notable accessory is the SONO MS2 magnetic stand with x-y adjustment (Fig. 4). Its mechanical stability makes it the only fixture capable of guiding HV10 UCI measurements precisely across a weld seam. It enables accurate, repeatable measurements by mapping the weld topography on flat surfaces.
Figure4: SONO MS2 Magnetic Stand with X-Y Adjustment for SONODUR UCI Probes
The SONODUR 3 has proven highly effective for UCI hardness testing in weld inspections on pipes and pressure vessels. Designed for robust field use, it supports a wide range of inspection scenarios.
It offers the broadest selection of probes: handheld versions with standard test forces of 10 N, 49 N, and 98 N; stand-mounted probes with the same forces; and motorized probes with 1 N, 3 N, or 8.6 N. This versatility allows adaptation to various component geometries and hardness levels. Handheld probes come with interchangeable tip designs to access hard-to-reach areas, while motorized probes ensure excellent repeatability – even on very hard materials –due to consistent force application. Special probe feet allow use on both flat and curved surfaces.
Stand-mounted probes can be equipped with custom-designed probe adapters that precisely match the curvature of pipes or bends in boiler components (see Fig. 5), ensuring stable positioning and accurate measurement results.
The SONODUR 3 also supports conversion of UCI hardness values (HV10) into tensile strength in accordance with EN ISO 18265:2019, enabling fast, standards-compliant evaluations. Stored measurements are automatically compared to preset tolerance thresholds to highlight any critical values.
All original data is permanently stored for traceability, while selected results can be saved separately or transmitted via Wi-Fi or Bluetooth for efficient reporting.
Figure 5: Example setup for measuring the heat-affected zones of a weld seam using the UCI method
Repetitive tasks – such as predefined test runs, adjustment factors, or tolerance settings – can be saved and recalled. This is particularly useful when working with reference pieces tested in the lab, which may have different elastic moduli in individual zones.
With SONODUR 3, FOERSTER delivers a mobile, high-performance solution for UCI hardness testing in demanding field environments. Its wide selection of connected probes – from lightweight handheld options to precision motor and tripod probes – covers a full range of test forces and geometries. Whether you're working on curved boiler walls or narrow heat-affected zones, the SONODUR 3 adapts seamlessly to your inspection needs.