Weld seam inspection
Weld seam inspection refers to various methods for checking and evaluating weld seams.
Table of contents
What is weld seam inspection?
Wherever welded joints are used, the quality of weld seams is a key issue in quality assurance. The purpose of weld seam inspections is therefore to evaluate the quality of the weld seams and to ensure they meet the applicable standards and requirements.
During weld seam inspection, the seams produced while joining two workpieces are checked for compliance with specified quality criteria. They must, for instance, be free of cracks, shrinkage cavities and other defects so they are fully fit for purpose. Both destructive and non-destructive inspection methods are used to check these aspects.
In this way, it is possible not only to assure quality, but also to draw specific conclusions about the potential for optimisation of the welding process and the welding technology. In line with the importance of weld seam inspection, there exist numerous standards that differ in the requirements placed on the component.
Why is weld seam inspection necessary?
During welding, two objects are permanently joined together using heat. The objects could be made of steel, aluminium, copper, glass or plastic. Here joints are to be produced that ensure the product can meet the requirements to be expected over a predicted lifetime. This is only possible if the weld seams meet quality criteria or other quality characteristics in accordance with requirements. However, it is difficult to generalise about these criteria because they are dependent on the requirements the related workpiece must meet.
Defects in welded joints can cause the failure of the component, which may involve serious consequential damage. By means of weld seam inspections, cracks and other flaws are reliably detected in accordance with standards based on specified tolerances and evaluation groups. In this way it can be ensured that only a component capable of withstanding the full load without failure is used and faulty components are excluded.
For this reason, in many sectors the checking of weld seams is essential to ensure the quality and safety of products with welded joints. Welding defects can degrade the load carrying ability and the durability of the joints. These problems can arise because, during welding, material characteristics may change unexpectedly due to the application of heat and reduce the plasticity, ductility and corrosion resistance of the material. There is also the risk of the formation of cracks with subsequent material failure.
Weld seam inspection
at Quality Analysis
Along with the inspection of the weld seam geometry and hardness testing over the entire area of a weld seam, we offer our customers a detailed microstructure analysis of each weld seam. Our experts employ both damage analysis and the latest technical equipment during the quality control of your weld samples.
Our accreditation in accordance with DIN EN ISO/IEC 17025 gives you the certainty that every weld seam inspection is undertaken in our laboratory with the highest precision and in accordance with recognised standards in relation to technologically correct implementation, as well as in relation to cracks, pores and other defects. You receive the results of your weld seam inspections in a comprehensible test report. Your questions are answered and you receive specific recommendations for the rectification of defects.
Weld seam analysis at Quality Analysis is accredited by Deutsche Akkreditierungsstelle GmbH (DAkkS) in accordance with DIN EN ISO/IEC 17025; this means we meet national and international standards and requirements.
You can read more about the advantages our accreditation offers you here:
DIN EN ISO 5817 as the standard for the inspection of a weld seam
Specific requirements are placed on every weld seam. These requirements are based on the required strength and visual appearance, as well as other criteria. However, the severity of the consequences of a possible failure of the component are also of importance. Based on this information, every weld seam is assigned to an evaluation group from B (high requirements) to D (low requirements).
Depending on the evaluation group, DIN EN ISO 5817 permits certain tolerances for irregularities on a weld seam. If these tolerances are exceeded, the term weld seam defect is used. DIN standard 5817 applies to the evaluation of weld seams on components made of steel, titanium, nickel and alloys of these metals. It applies to manual, mechanised and automated welding in all welding positions. Surface irregularities are assessed based on a visual inspection and internal weld seam defects are evaluated using a metallographically polished specimen. For welded joints in aluminium, on the other hand, DIN EN ISO 10042 applies. It is also to be noted that, as a rule, each component has weld seams in various evaluation groups.
Methods for weld seam inspection in practice
During weld seam inspection, a differentiation is made between destructive and non-destructive inspection methods. Destructive inspection methods include, tensile testing in accordance with DIN EN ISO 4136, bending testing in accordance with DIN EN ISO 5173, as well as various test methods for hardness. Non-destructive inspection methods include visual inspection, radiographic inspection, eddy current inspection and ultrasonic inspection, as well as magnetic powder inspection (DIN EN ISO 17638).
Typical defects that weld seam inspection detects
During a weld seam inspection, a check is made for both internal and external defects. Weld seam defects open to the surface are termed surface defects. Surface defects can be documented with high precision by means of a visual inspection, dye penetration inspection and magnetic powder inspection. The most common surface defects are cracks.
Weld seam defects inside the weld seam are termed volume defects. These are documented reliably using ultrasonic or radiographic inspection. The most common defects on weld seams are cracks, lack of fusion, pores (gas inclusions), insufficient penetration and solid inclusions (slag).
Non-destructive weld seam inspection
In many sectors it is necessary to check the quality of weld seams without destroying the related component. For this purpose, there exist numerous methods that either permit direct insight into the interior of the seam (e. g. using X-rays) or indirect conclusions about damage. In view of the large number of methods for non-destructive weld seam inspection, the following list is restricted to a short overview of the most important methods.
1
Visual inspection of a weld seam
One of the most important methods for inspecting weld seams is the visual inspection of weld seams. An experienced inspector can determine the quality of welded joints simply from their appearance. With the aid of weld seam gauges, it is also possible to evaluate the geometry of the seam (e. g. the throat).
2
Magnetic powder inspection
If a workpiece is magnetised, the field lines run parallel to the surface of the workpiece, provided it is free of cracks and other surface defects. If there are defects, a stray field is produced where the magnetic powder (e.g. iron powder) collects.
3
Eddy current inspection
Using this method, eddy currents are induced by means of induction. At the points at which the amplitude and/or phase of the eddy currents changes, damage to the material can be expected because flaws and contamination affect the electrical conductivity.
4
Radiographic inspection
During radiographic inspection, components are subjected to X-ray or gamma radiation. The brighter a point appears, the thicker the material is at this point (because it lets less radiation through). As such this analysis also renders visible any flaws hidden from the exterior. This type of material inspection is therefore a standard method, particularly on safety-relevant parts.
5
Ultrasonic inspection
The acoustic characteristics of sound waves change at surfaces within a medium. If sound waves in weld seams are incident on, for instance, cracks or shrinkage cavities, the sound waves are deflected in these zones or their impedance changes. These characteristics of ultrasound are used for weld seam inspection.
Destructive weld seam inspection
Some characteristics of weld seams can only be analysed reliably using destructive inspection methods. This is, for instance, the case while testing hardness, tensile strength or ductility.
1
Cut and etch weld inspection
With this method, a cross-section of the weld seam is polished and etched to make the inner microstructure visible. This preparation makes possible detailed assessment of the microstructure under a microscope. Cut and etch weld inspection is crucial for the evaluation of the quality of the welded joint because it provides information about important aspects such as the particle size, the distribution of the phases and the presence of microdefects.
2
Geometric measurement of the weld seam in a cross-section
With this method, a cross-section of the weld seam is analysed thoroughly to acquire important geometric parameters such as seam width, depth and profile. Geometric measurement helps to ensure that the weld seam is not only free of defects externally, but that its structure also corresponds to the specific requirements.
3
Light and scanning electron microscopic analysis
During light and scanning electron microscopic analysis, an etched specimen of welded joints is analysed. This analysis is undertaken, e.g., using a special weld seam analyser. These microscopic techniques make it possible to identify and evaluate very fine details in the microstructure of the weld seam such as particle size distribution, porosity and inclusion morphology.
4
Hardness testing
Why is hardness testing important for weld seams? Welding applies heat to the base material, as a result the hardness of the material can change. Hardness testing is used to ensure the hardness in the area of the weld seam is not excessively high or low. Vickers hardness testing is used for testing weld seams. It is also possible to create a hardness map, here the entire cross-section of a weld seam including the heat-affected zone and the base materials is measured completely.
5
Tensile test
Tensile testing is used to determine the tensile strength perpendicular to the weld seam. This test ends with the fracture of the test specimen because, when the weld seam fractures, it is possible to draw conclusions about irregularities in the weld seam.
6
Bending test
During a bending test, the workpiece is folded to a pre-defined bending angle at an ambient temperature that is also pre-defined. During this movement, no cracks are permitted to appear on the workpiece or in the weld seam.
In summary: weld seam inspection
The term weld seam inspection refers to non-destructive and destructive methods for checking the quality of weld seams. The methods include both visual assessment and the use of modern technical equipment such as X-ray machines or computer tomographs.
