Welding is a widely used procedure in manufacturing and can turn dangerous if there aren’t sufficient quality control checks. The welded joints may fail over time through wear and tear; hence the need for periodical tests. Weld tests are vital not just to ensure the quality of welds but to also validate the position, presence, and coverage of welds. Weld testing and analysis can be destructive or non-destructive methods. X-ray welding test is one of the most used non-destructive tests for identifying weld flaws.
This article is about x-ray welding inspection and how to use digital x-ray systems for welding inspection. But before we discuss why and how to use X-ray welding test, it is necessary to understand the different types of flaws that occur in welds.
Type of flaws in welding
Several flaws or discontinuities can occur in welding. These refer to the disruptions to the typical structure of the material. Discontinuities can occur either in the base metal, weld material, or the heat-affected zones. Discontinuities that fail to meet the requirements of the codes or specifications used to invoke and control inspection are termed as defects.
Welding discontinuities might be:
• Cold lap - where the weld filler metal does not properly fuse with the base metal or the previous weld pass material
• Porosity - gas entrapment in the solidifying metal
• Slag inclusions - solid non-metal trapped in weld metal or between weld and base metal
• Incomplete penetration - when the weld metal fails to penetrate the joint
• Incomplete fusion - the weld filler metal does not properly fuse with the base metal
• Internal concavity or suck back - the weld metal has contracted as it cools and is drawn up into the root of the weld
• Internal or root undercut - the erosion of the base metal next to the root of the weld
• External or crown undercut - the erosion of the base metal next to the crown of the weld
• Offset or mismatch - a condition where two pieces being welded together are not properly aligned
• Inadequate weld reinforcement – a weld area where the thickness of weld metal is less than the thickness of the base material
• Excess weld reinforcement - an area of a weld that has weld metal added more than specified
• Cracks – often detected in an X-ray only when they spread in a direction that produces a change in thickness that is parallel to the x-ray beam
Why do you need X-ray inspection for welding?
As mentioned earlier, X-ray inspection is a common mandatory test to comply with international quality standards. All relevant welds must be tested in compliance with international standards such as the EN-17636, ASME, ISO, and many more. This is especially applicable to pipes and tanks/boilers used in the oil and gas industry as well as the aerospace industry with its oddly shaped parts.
X-ray inspection for welding is an image-based test. It can be manual or digital. The welding inspection is performed based on x-ray images or video, or machine vision. Popularly known as radiographic weld inspection, it is used to detect discontinuities within the internal structure of welds.
X-ray inspection uses the images created by a radioactive isotope. When a radiation passes through the weld onto a photographic film, it produces an image of the object's internal structure. The object absorbs energy based on its thickness and density and any energy not absorbed will cause exposure of the radiographic film.
These areas will appear dark when you develop the film. Conversely, areas of the film exposed to less energy appear lighter. So, wherever the thickness has changed – due to discontinuities such as porosity or cracks – it will appear as a dark outline. All discontinuities can be detected by viewing the shape and the variation in the density of the processed film.
As technology progressed, the traditional x-ray films have been replaced by digital radiography where the images are captured and stored digitally.
Advantages of digital X-ray systems for weld inspection
• Faster inspection process through Digital Radiography (DR)
• Detect typical welding flaws like cracks, porosities, undercuts, and inclusions among others
• Compliance with international standards (EN-17636, ASTM, ASME Section 5)
• Double or single wall inspection
• Highest accuracy to fulfil the resolution and contrast demands
Important considerations in X-ray testing of welds
The advantages of x-ray tests don’t mean you should opt for an X-ray welding inspection in every situation. The best way would be to ask the following questions before you decide to go for an x-ray welding inspection.
Are the results accurate and reproducible?
Any test that doesn’t give accurate and precise results isn’t worth the effort. While accuracy refers to the nearness of the result to the true value, precision is about the replicability of the results. To get the best results, the instruments used in the testing process should be regularly calibrated to a known standard.Do you need a combination of NDT methods?
It is also important to answer the question of whether you need more than one NDT method to test the weld. Assembled metal components such as the ones in pipes, joints, and bends, might need a combination of tests rather than a single test. This will ensure the consistency of the outcome and completeness of inspection.
Several industries such as petroleum, automotive, and aerospace opt for more than one testing method. Hence, apart from the x-ray welding test, they also perform other tests such as ultrasonic inspection, dye penetrant testing, and more.What are the strengths and weaknesses of the inspection methods?
Different methods of testing have different benefits and weaknesses. Further, different types of defects may exist in any given sample. It means it might need different inspection methods to identify the problem. So, before deciding on X-ray tests, ask whether it is the right choice for that material. For instance, de-laminations and planar cracks may not be detected in x-ray testing or other radiographic test systems. In this case, ultrasonic and acoustic-based methods are more suitable to find the defects.
Once you have answered the above questions and have reached the conclusion that the X-ray welding test is the best choice, go ahead with the test. You can choose from a wide range of non-destructive testing solutions available for digital X-ray welding tests.