Ultrasonic flaw detection, as a modern nondestructive testing technique, is characterized by its ease and speed of operation and high detection accuracy, and has been widely applied in engineering construction projects.

In light of this, this paper explores the application of ultrasonic testing in nondestructive evaluation of steel structures, with the aim of providing a reference for related work.

1. Ultrasonic Testing Technology and Principles

With the continuous advancement of science and technology, ultrasonic testing has emerged as a cutting-edge nondestructive testing technique that is widely employed and plays a crucial role.

The scientific and rational application of ultrasonic technology enables the detection of slag inclusions and cracks in steel structures, thereby ensuring thorough assessment of construction quality.

This technique offers a high safety factor and is both convenient and efficient. Ultrasonic flaw detection primarily consists of an ultrasonic flaw detector, transducers, and couplant. During ultrasonic wave propagation through a medium, various wave modes can occur; in the course of inspection, the most commonly encountered modes include shear waves, longitudinal waves, plate waves, and surface waves.

The underlying technical principle is as follows: an ultrasonic probe is used to emit ultrasonic waves, which propagate rapidly through the material being inspected. If porosity, slag inclusions, or cracks are present in the material, some of the ultrasonic waves will be reflected and detected by the ultrasonic receiver, with the reflected signals displayed on the screen. By processing and analyzing these echo signals, the actual condition of the inspected material can be accurately determined.

2. Defects in Steel Structures

Even though steel structures, like many other materials, possess irreplaceable characteristics, their use is accompanied by numerous drawbacks that can severely compromise structural quality. These drawbacks primarily encompass the following aspects.

2.1 Design Deficiencies in Steel Structures

Current steel structure design reveals that, due to a shortage of qualified designers in this field and a lack of practical experience, the design process relies heavily on data calculations, with limited application of design expertise and insufficient manual intervention. Moreover, the current emphasis on minimizing construction costs has resulted in reduced structural reliability, with human factors being the primary cause of these issues.

2.2 Material Defects in Steel Structures

Steel structures are made from a wide variety of materials, and the relative proportions of different elements significantly influence the material properties. Prior to steelmaking, careful selection of raw materials is essential. During rolling and smelting, limitations imposed by process equipment can adversely affect steel quality, most notably through the presence of cracks in the material. In such cases, rigorous supervision and inspection of materials are indispensable.

2.3 Defects in Steel Structure Connections

The steel structural system that constitutes a building is an assembly of various types of steel products in different configurations, with welding being the primary method used to connect the different components of the steel structure. This approach effectively minimizes connection space and ensures rigid connections, thereby enhancing structural integrity. However, factors such as the welding environment and the level of welding technology can lead to internal damage within the steel structure, resulting in defects like porosity and slag inclusions, which in turn compromise the stability of the steel structure.

3. Application of Ultrasonic Nondestructive Testing Methods

Due to its deep penetration, long detection range, light weight, and compact design, ultrasonic flaw detection enables rapid inspection.

Typically, ultrasonic flaw detection does not require precise identification of defect nature or defect type; however, through prolonged practical experience, the characteristics of various ultrasonic wave paths can be systematically characterized. Based on the type of welding material and the configuration of the welded structure, the severity of defect-related damage can then be assessed.

3.1 Preliminary Flaw Detection

Upon receipt of a nondestructive testing assignment, the first step is to specify the relevant technical requirements for weld quality in the drawings and to carry out all operations in accordance with the acceptance criteria for steel structures, thereby preventing any uncontrolled or arbitrary procedures.

At the same time, it is essential to acquire expertise in multiple disciplines. For instance, if the welding quality grade for steel structures is specified as Level I, the corresponding quality assessment may be conducted at Level II, ensuring the full application of ultrasonic testing techniques; this approach can be extended iteratively until the quality standard reaches Level III.

During the initial flaw detection operation, particular attention should be paid to the echo signals displayed on the oscilloscope screen. If any echoes exceed the evaluation threshold, detailed records must be maintained to lay a solid foundation for subsequent quantitative defect assessment.

3.2 Precise Flaw Detection

This nondestructive testing method should ensure high detection accuracy. The specific procedure is the same as that used for preliminary inspection; the only difference is that the entire operation must be performed more slowly, with meticulous scrutiny throughout the testing process to prevent any missed defects.

If a defect is detected during the first inspection, a second inspection must also be conducted to identify the highest echo beam that caused the defect and to document the findings accordingly. This approach will facilitate improvements in defect management.

During nondestructive testing, it is important to calculate the inspection ratio based on the percentage of the length of each weld.

For welds that require localized nondestructive testing, if such defects are permissible, the inspection length shall be extended at both ends of the defect, with the extended length accounting for more than 10% of the original inspection length.

When implementing flaw detection work in practice, it is essential to have a precise understanding of the structural characteristics of the steel and to accurately assess each defect.

3.3 Repeated Flaw Detection

Repeated flaw detection serves as a verification and inspection of the previous two inspections, with essentially the same detection methods. Since this round of flaw detection builds upon the work of the first two inspections, it is necessary to accelerate the process in order to save time and effort.

4. Defects, Causes, and Countermeasures

Weld defects in all steel structures are typically classified as incomplete penetration, slag inclusion, lack of fusion, porosity, and, in some cases, cracking.

To date, there remains no reliable method for making perfectly accurate assessments in ultrasonic flaw detection. Current inspection practices rely solely on the comprehensive analysis of waveforms reflected by ultrasound, upon which relatively precise conclusions can be drawn.

4.1 Slag Inclusion

When welding steel structures, slag inclusions are inevitably present to some extent, which can compromise the quality of the steel and the weld. The main issues associated with slag inclusions include the following:

In the case of spot-like slag inclusions, echo signals similar to those of porosity will be generated, at which point the inspection personnel must meticulously document a series of observations. Furthermore, when linear slag inclusions are present, the ultrasonic waveform will exhibit a jagged amplitude profile.

Finally, once the waveform takes on a tree-like morphology, the amplitudes of the reflected waves will vary depending on the direction of detection.
The primary causes of this defect are as follows: during the welding process, the welding current is too low and the welding speed is relatively high, which prevents slag inclusions from being expelled in a timely manner; moreover, the purity of the weld metal is insufficient, with impurities such as sulfur present.

To prevent this from occurring, it is necessary to use the correct welding current, reduce the welding speed, and thoroughly remove any contaminants before commencing welding.

4.2 Lack of Full Penetration

In steel structures, the absence of welds is also a common issue, which can be readily detected using ultrasonic testing. When such defects are present, the ultrasonic echo amplitude is typically high; in severe cases, it may even produce a “Raven” echo. The primary causes of these defects include excessively slow welding speed or insufficient welding current during the welding process, as well as inconsistent electrode manipulation angles.

To avoid such defects, proper welding procedures are typically employed to effectively ensure the quality of steel welds, maintain the steam turbine in normal operating condition, and thereby enhance productivity.

4.3 Porosity During Welding

As the size of the porosity varies, the ultrasonic waveform changes accordingly; for instance, isolated pores, though producing a relatively stable waveform, correspond to single, discrete gaps.

Due to differences in the direction of detection, varying results will be obtained; therefore, the detection process must be carried out in both forward and reverse directions, which helps reduce errors during detection.

The causes of porosity include the following: when these steels are welded manually, excessive or insufficient welding current, inadequate removal of impurities, or excessively high voltage can all lead to such defects.

Reducing the weld area will correspondingly decrease operational efficiency. To prevent this type of defect, it is essential to avoid the presence of extraneous impurities, properly treat rusted steel, and, when performing welding, select appropriate current, voltage, and welding speed.

4.4 Crack Detection During Welding: For personnel with sufficient work experience, cracks will be detected by ultrasonic testing, characterized by relatively broad waveforms and high amplitudes.

Compared with other defects, cracks pose a greater hazard; therefore, effective crack prevention is of paramount importance.

The primary cause of crack formation is non-uniform heating during the welding process, coupled with rapid cooling under external loading, which prevents the weld metal from fully bonding. As a result, cracks of varying sizes develop.

To prevent the formation of these cracks, it is necessary to select steels with low manganese and sulfur content, thereby enhancing the weld’s ability to accommodate thermal expansion and contraction. The application of advanced welding techniques can effectively reduce the likelihood of crack initiation.

4.5 Incomplete Fusion During Welding

When performing ultrasonic inspection, defects that are not fully fused produce relatively stable waveform reflections; consequently, during inspection of the welds on both sides, it is highly likely that no inspection results can be obtained on one side.

If the welding speed is too high or the welding angle is incorrect, incomplete fusion may occur.

When dealing with such issues, it is necessary to select the appropriate welding method and probe angle, while also controlling the current magnitude.

5. Conclusion

In summary, as modern construction projects continue to evolve, the application of advanced science and technology provides reliable assurance for the quality inspection of construction materials.

Today, ultrasonic testing—a nondestructive evaluation technique—is widely employed for quality inspection of steel structures commonly used in construction projects. This method enables the timely and rapid detection of defects within steel structures, followed by thorough analysis of the identified issues, thereby enhancing the structural stability of steel components in construction works.

It is hoped that the present study will provide recommendations for enhancing the application of ultrasonic testing in the inspection of steel structures in the future.

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