Nondestructive weld seam testing
for safe product use
We provide ultrasonic inspection systems for nondestructive weld inspection – manual, mechanized and automated.
VOGT Spot weld inspection
Nondestructive weld seam testing
Welds are an essential part of many construction processes. They are used in the metalworking industry, automotive, rail transport, aerospace, energy and pipeline construction, among others, and are thus an integral part of our daily lives. Quality control of such welded joints is therefore essential for the protection of people and the environment.
Nondestructive testing methods are used for setting the correct welding parameters, production control and periodic inspection of in-service components with welded joints.
These inspections of welded joints are carried out by the manufacturer with his trained personnel as well as by certified and accredited testing laboratories, which inspects the quality of welded joints according to required standards.
The main nondestructive testing methods for welds include radiographic testing, ultrasonic testing and surface crack testing (dye penetrant testing and magnetic particle testing).
A little bit of theory
Advantages of non-destructive material testing (NDT)
The quality inspection of weld seams by means of non-destructive testing (NDT) has various safety and economic advantages:
Ensuring the welding quality already before series production
Nondestructive testing of welds in the laboratory enables the welding process to be qualified and the ideal parameters for reliable welding to be determined and set. In this way, possible welding defects can be prevented before the start of series production, or their detectability can be optimized by means of non-destructive material testing.
High accuracy and reproducibility of test results
The smallest defects can be detected reproducibly using nondestructive testing methods. The test methods to be used differ in the detectability of defects in terms of their shape, position and size.
Testing in operation
Components in operation (e.g. welds on bridges, chemical plants or wind turbines) can be inspected and observed on site during their lifetime, sometimes even during operation, to prevent downtime or risks to life and limb.
Significant reduction of scrap
As its name suggests, nondestructive testing is a quality test of materials and components without destroying them. The greatest advantage of non-destructive testing is its high level of safety and cost-effectiveness. Test parts without relevant defects can be further processed and test parts with defects can be reworked if necessary.
VOGT Ultrasonics is your reliable partner
Goals for the testing of welded joints
The aim of a nondestructive weld seam inspection is to ensure the integrity of the tested component in relation to a test standard and a corresponding load capacity. It is essential that the test is performed on the component itself, which establishes a direct relationship of the test result to the usability of the component.
The quality assurance of welds is intended to ensure that the welded materials or components meet the requirements placed on them over their expected service life. Quality assurance begins even before welding. The manufacturer ensures ideal production conditions, uses welding-compatible designs and tools, materials with sufficient weldability and has the work performed by qualified personnel.
The results of the weld inspection are used to optimize the welding parameters. In most cases, non-destructive ultrasonic testing is used in correlation with destructive testing. This allows the smallest defects in the weld seam to be detected and precisely located. This information is used to optimize the entire welding process.
To ensure the reliability and safety of weld seams, destructive and non-destructive testing methods (NDT) are used. Here, the weld seam inspection is carried out directly during production, with a time delay after production, and on safety-relevant weld seams recurrently during operation. Depending on the area of application, weld seams are subject to legally defined and verifiable quality requirements.
Nondestructive testing methods such as ultrasonic testing can be fully automated and cycle-based. Ultrasonic testing systems such as PROline are used for this purpose, which can inspect 100% of weld seams in just a few seconds.
Defect detection is performed by destructive materials testing using random sampling, where these parts are then scrapped, or by non-destructive materials testing, where the components can be further used. The following typical types of defects can be detected:
- Cracks in the material can be caused by external force or stress in the material or component itself.
- Pores and pore nests can occur if there are still gases in the melt while it is solidifying. With the aid of certain inspection systems, they can be detected visually at first glance.
- Cavities or binding defects are voids in the material created during production.
- Segregations are separation of a molten mass during metal production
- Inclusions in the material, which can occur during welding.
- Insufficient welding means that the root of the joint does not reach the end of the component. A gap is produced.
- Wall thickness variations can occur directly during the manufacturing process or subsequently due to abrasion such as corrosion.
Component- and application-dependent quality criteria are defined for assessing the quality of welds. These depend on the type of weld joint and the loads and operating conditions of the welded workpieces.
If the test results deviate from the permissible values, this is referred to as a weld defect. This can be repaired or leads to rejection of the workpiece.
In general, three evaluation groups of manufacturing quality are defined: B = high requirements, C = medium requirements and D = low requirements. In steel construction, there is an additional group B+.
Each of these groups is subject to different quality requirements in terms of type, size and number of permissible irregularities. They are defined in such a way as to permit wide application in the manufacture of welded joints.
The classification of the weld into a group is made by the designer and on the basis of a valid standard and/or the manufacturer. They include strength requirements, consequences of component failure, visual appearance and manufacturing conditions.
Definition of weld seam defect
A weld defect means a reduced production quality, which, however, does not make any statement about the suitability of the welded joint for use.
Whether a weld defect also leads to scrap depends on the type and number of permissible irregularities for the respective workpiece or material. According to the DIN EN ISO 6520 standard, an irregularity refers to a defect in the weld or a deviation from the intended geometry.
A defect is such an irregularity. Whether, how many and what kind of irregularities may be present in a workpiece or component is regulated by evaluation standards such as ISO 5817 or ISO 10042.
Steel Nickel Titanium DIN EN ISO 5817:2014-6
This standard regulates the evaluation groups of irregularities in welded joints on steel, nickel, titanium and their alloys. It is valid for manual, mechanized and automatic welding of fully penetrated butt welds and fillet welds. The principles of this standard can also be used for the evaluation of partially penetrated butt welds.
Aluminum DIN EN ISO 10042:2005
This standard regulates the evaluation groups of irregularities in electric arc welded joints on aluminum and its alloys.
Weld seam testing in practice – selection of the appropriate test method
In practice, most weld inspections are performed directly after welding or after a subsequent heat treatment. Depending on the requirements, the appropriate nondestructive testing method is selected and, if necessary, used in combination. These non-destructive weld seam testing methods are available for selection:
Ultrasonic testing – UT
Materials transmit sound. Ultrasonic testing takes advantage of this. Sound waves are partially reflected at so-called interfaces. These are transitions from one medium to another where there is a different sound wave resistance. And thus also at defects such as cracks and inclusions in the volume of weld seams.
In ultrasonic weld inspection, sound waves are emitted into the material with the aid of a probe connected to an ultrasonic testing device, and their reflection is picked up again. Based on the type and strength of the reflection and the image displayed in the ultrasonic device, conclusions can be drawn about the defect size and depth.
Ultrasonic testing is mainly used for simpler geometries (flat, round, plane, rotationally symmetrical, thicker, etc.). It is very sensitive for larger wall thicknesses and therefore has an advantage over X-ray inspection. It can detect surface flaws due to angular sound as well as volume flaws even at large wall thicknesses. For example, a small 1 mm defect is still easily detectable at a depth of 1 m.
Most ultrasonic equipment still works conventionally with an RF image (called A-scan) that shows the backscattered energy as amplitude height and the position of the amplitude as sound path. Phased array ultrasonic inspection systems are also able to generate an imaging representation of the weld seam with a B, S or even D image from these A-scans. This makes the weld quality visible at a glance as an image (e.g. top view of the component, depth position image, etc).
In contrast to conventional ultrasonic inspection, with phased array technology the weld can be inspected at different angles with only one probe and in one pass. This is a great time advantage when a standard specifies inspection at different angles.
Ultrasonic inspection and phased array ultrasonic inspection are used when defects are to be found in the volume of the weld. They can be used for unground welds from about 6-8 mm. However, it is not suitable for very thin wedge welds. In this case, a method such as PT or MT is used.
In special cases, thinner welds (e.g. laser welds ) can also be tested. However, this requires a feasibility study or corresponding experience in the selection of the right probes and their application.
Magnetic particle inspection – MT
Magnetic particle inspection can also be used to detect surface and near-surface defects.
For this purpose, the component is first magnetized with a magnet (yoke magnet or a coil). This creates field lines running parallel to the surface. A defect-free component is magnetically homogeneous. This means that every location has the same magnetic properties. If there are surface or near-surface defects in the component, these produce a change in the magnetic flux. This is due to the property of magnetic field lines to always seek the path of least resistance. If the workpiece is now dusted with magnetic powder, this collects as a “bridge” directly above the defect, thus making it surmountable for the magnetic fields. If the magnetic powder is fluorescent and a UV lamp is used to irradiate the test area, these accumulations become visible as “defects”.
Alternatively, black magnetic powder can be made visible on a layer of white paint previously applied thinly to the component.
The magnetic particle inspection is regulated in the standard DIN EN ISO 9934 and especially for welded joints in DIN EN ISO 17638 as well as DIN EN ISO 23278.
Dye penetrant testing – PT
The dye penetrant inspection can also be used to detect the smallest surface defects in weld seams, e.g. hairline cracks, pores, binding defects.
For this purpose, the component is first cleaned in the test area, i.e. free of oil, scale, paint, rust, etc.. Then a liquid penetrant is applied by spraying, brushing or dipping the component.
The test medium has a red color or is fluorescent and penetrates cracks and other narrow surface damage due to physical capillary action. This agent dwells for approximately 10 – 30 minutes to penetrate any imperfections such as fine cracks. After that, the penetrant is washed off without leaving any residue. Care must be taken not to wash the penetrant out of imperfections with a sharp jet of water.
In the next step, a developer is applied to cover the component with a white layer. The developer extracts the red or fluorescent penetrant from the flaws so that they are clearly visible on the surface in the corresponding red or also fluorescent coloration. When using the red penetrant, this is commonly referred to as “bleeding the cracks”.
To see the fluorescent defects, a darkened viewing room and UV illumination are required.
The colored penetrant inspection is regulated in the standard DIN EN ISO 3452 as well as in the standard DIN EN ISO 23277 especially for the weld seam inspection.
Eddy current testing – ET
Eddy current testing can be used to detect surface cracks and, in the case of certain materials, cracks close to the surface in the weld seam. The requirement is the electrical conductivity of the material. For this purpose, the weld seam is scanned with an eddy current probe. This has an energizing coil and a receiver coil. The energizing coil generates electromagnetic currents in the material. If there are defects in the material or in the weld seam, the eddy current lines are deflected and change their course. These changes are picked up by the receiver coil in the form of a current, which enables the inspector to recognize the irregularities in the display of his testing device. Eddy current testing is regulated in the DIN EN ISO 15549 standard.
Visual testing – VT
In the visual inspection of welds the weld is tested for defects from the outside with the armed and unarmed eye. Here, the inspector can be assisted by optical instruments, e.g. a magnifying glass. Based on the appearance and geometry of the weld seam, conclusions are made regarding its quality. The DIN EN ISO 17637 standard is used here, which regulates the procedure and requirements for the visual inspection of fusion welded joints. This inspection requires an experienced inspector
Radiographic testing / Digital radiology -DR
Radiographic testing is a nondestructive, imaging technique for visualizing differences in materials. Like ultrasonic testing, it is able to check the volume of the weld seam for internal defects.
When welds are inspected, they are examined with the aid of a suitable radiator, e.g. an X-ray tube or gamma radiator. The beams pass through the weld and create an image of the weld on a suitable carrier, e.g. X-ray film or a digital image detector. Different material thicknesses and densities can be identified by the degree of blackening. The thicker and denser the weld, the less radiation can pass through it and the brighter is its image. This makes it easy to detect flaws such as cracks, pores, inclusions. Weld inspection using this method is regulated by the DIN EN ISO 10675-1 standard.
Compared to ultrasonic testing, radiographic testing is always used when the components are very thin or cannot be ultrasonically inspected well due to their geometry.
We help you
Vogt Ultrasonics inspection systems for weld seam testing by means of ultrasound
Within the scope of ultrasonic testing, VOGT Ultrasonics GmbH supplies inspection systems for weld seam testing. These are mainly inspection systems for flat but especially also rotationally symmetric components, which are automatically inspected and evaluated by inspection systems.
The ultrasonic inspection systems are networked with production ( Industry 4.0) and offer unmanned inspection of components with ultrasound.
It is often friction welds, laser, electron beam welds of thin but also thicker components to detect surface cracks or even volume defects.
But also spot welds or short step welds (e.g. Arplas®) are inspected in the automotive sector by VOGT Ultrasonics inspection equipment and systems.
Solutions for your industry
As a service provider with all nondestructive testing methods and also as a producer of our PROline and PHAsis ultrasonic inspection systems and devices, we can offer our customers the ideal weld seam and spot weld inspection solution for individual requirements.
Optimization of the welding process and reduction of scrap through ultrasound
Automated inspection of welds with ultrasound can be performed with two objectives: Fast component sorting or precise defect location.
In cycle-based, production-accompanying inline inspection, rapid sorting into good and bad is required for cost reasons. For this purpose, an ultrasonic inspection system performs a short single-line scan and then feeds the component into the further production chain or sorts it out. It is a fast process without deeper defect analysis with the question of why. The rejected components are repaired or scrapped or subsequently subjected to a second inspection to determine the extent of the defect.
Fig.: PROline ultrasonic testing system for weld seam inspection in production
For longer cycle times, the weld seam can also be completely inspected with more inspection tracks. With extensive documentation of the results, the inspection data from the automated inspection can be subsequently evaluated and the welding process adjusted accordingly. A second inspection to determine the extent of the defect can often be omitted.
In contrast, in the laboratory it is important to detect the smallest defects and locate them precisely in the weld seam. The aim is to optimize the welding process on the basis of the information obtained and to minimize rejects.
We supply the appropriate and proven inspection system for both areas of application. Both ultrasonic systems are capable of independently evaluating the weld. The automatic evaluation, test report generation and result archiving saves time and creates test reliability.
Weld seam testing on electro-rotors
For a supplier to the automotive industry, we delivered an ultrasonic inspection system for electron welds on rotors for asynchronous motors. The requirements were a high availability of the system in 24/7 operation, speed and the use as a fixed part of a production line as a fully automated inspection system.
Fig.: PROline inspection system for weld seam inspection on rotors
The copper part to be inspected is to be tested on both end faces for connection defects of a size of up to 0.35 mm. A robot feeds the component to the inspection system. The system independently inspects the part from both sides and communicates the inspection result directly to the customer’s central control system. The inspected component is then passed on to the next stage of production or sorted out.
The developed inspection system uses ultrasonic phased array technology, which achieves a cycle time reduction by a factor of 30 compared to conventional inspection.
Weld seam inspection in shipbuilding
In shipbuilding, X-ray inspection is often still the method of choice when it comes to weld seam inspection. However, by regulation, it requires large-area barriers and work interruptions in adjacent production islands. Butt welds are tested on all support structures in the area of the ship’s hull for plates made of normal and high-strength steel with wall thicknesses from 8 to 30 mm.
A very good alternative for the inspection of such butt welds is phased array ultrasonic testing. It not only allows work interruptions to be avoided, but also makes inspection faster and more efficient.
This is mainly due to the possibility of the so-called sector scan of a phased array ultrasonic testing device. Here, the inspector can scan the weld seam with only one probe and in one pass at different inspection angles. In addition to a significant reduction in inspection time, this also provides a more global view of the seam quality.
if there are still questions
Depending on the quality demand on a weld seam and thus the area of application, the weld seam may be assessed by a trained welder, a welding supervisor or, in the case of safety-relevant weld seams, a certified inspector according to a defined standard. Non-destructive weld seam testing usually has high demands on the testing technology as well as on the tester himself.
Thus, weld seam inspections are generally only approved by ultrasonic inspectors with certified Level 2/ Stage 2 to inspect and evaluate weld seams.
Depending on the use of the weld, it is subject to certain quality requirements, which are regulated in product and application standards.
Weld seams must be inspected if regulations define this, if the failure of the weld seam endangers human life or component or plant safety, and if the weld seam inspection is intended to demonstrate consistent component quality.
Weld seams are tested destructively as spot tests or non-destructively.
Destructive testing methods include, for example, metallographic examinations (mechanical and chemical microstructure) or mechanical-technological tests such as notched bar impact tests, tensile tests, folding tests, corrosion tests, microstructure examinations, heat treatments, etc. In all methods, the component can no longer be used afterwards.
Non-destructive testing methods include ultrasonic testing (UT), digital radiography (DR), visual testing (VT), eddy current testing (ET), dye penetrant testing (PT) and magnetic particle testing (MT), where the component remains intact and can be used after the tests.
NDT methods are non-destructive testing methods. They are used to check the quality of materials and components without destroying them. This means that the components can be reused after a positive test and no unnecessary rejects are produced.
The tasks of a materials inspection are (1) to determine the technological material properties in order to verify suitability for production, (2) to monitor and ensure the quality of a material or component, e.g. by inspecting welds for cracks, and (3) failure analysis in case of component failure.