What is non destructive testing in welding?
Non destructive testing is the name applied to several methods of testing welded joints after the welding process has been completed, whether immediately after the welding has been performed or on components already in use. Destructive testing methods usually involve cutting through the welded surface and inspecting it to assess the strength of the weld and the presence of any discontinuities or defects. While this can be helpful to determine if the welding methods being employed are correct, it is completely unsuitable for testing welding on equipment or structures already in use as it involves destroying the weld itself. As a result, there are several methods of non destructive testing that can assess the integrity of a given weld without invasive measures, leaving the weld completely unharmed after inspection.
A welding discontinuity occurs when there is an interruption in the consistent thickness of a weld or the base metal it is attached to. This lack of material can lead to a weak join between the welded surfaces. While a certain small amount of discontinuity will occur in most welds, there are codes and standards that dictate what is the acceptable limits for this. In industries such as nuclear power production or the petrochemical industry, weak welds in pipelines can have catastrophic outcomes, or weak joins in the metal structure of any building or construction can put all its users at risk. Welding defects are usually caused by contaminants such as slag getting into the weld metal, or a porous or cracked weld, again causing weakness in the join. Porosity and cracking, caused by problems with the rate of cooling after welding are not always obvious immediately after welding and some cracks can take weeks to appear, meaning the welded pieces could have been fitted and already be in use before the problem becomes obvious. Welding is a difficult skill to perfect, and discontinuity and defects can occur even when the work is carried out by an expert and non destructive testing is the only way to be sure of the integrity of the weld.
Types of non destructive testing
There are three main types of non destructive weld testing that will be discussed here:
- Visual Inspection – Visual inspection of welding is an effective method of checking for externally obvious flaws such as cracks or porosity. Visual inspection does not merely consist of viewing the weld with the naked eye, it can involve equipment such as borescopes, flexible infra red and fibre optics for internal examination of pipelines and other equipment and use video surveys for a more complete evaluation of any flaws. The main drawback of visual inspection is the inability to view any interior defects in the weld and for this the following methods are more suitable.
- Radiographic testing – Radiographic testing of welding is very similar to the methods employed to X- ray the human body in a hospital. There are two types used, firstly using X-Rays produced by an X-ray tube and secondly using gamma radiation emitted by a radioactive isotope. The radiation produced is passed through the weld and onto a photographic film and the resulting permanent image can be inspected by a professional with suitable training to assess any interior defects. The penetrating radiation will cause the radiographic film to develop as it strikes and any intervening substance will absorb the radiation to different degrees depending on its density. Where undeveloped the film will remain white and areas struck by the radiation will darken depending on the level of exposure. This will give an accurate picture of the internal structure of the weld, as areas of low density such as pores or cracks, or contamination by slag, will allow more radiation to pass through and will show up as darker areas on the film. Conversely, areas of contamination by high density substances such as tungsten will show up as lighter areas than the main body of the weld. Although this is a time consuming and expensive method, it is very effective in producing an accurate picture of any defects in the weld. It must always be carried out by trained professionals as the radiation used is invisible to the naked eye and be extremely harmful to the human body on prolonged exposure.
- Ultrasonic Testing – Ultrasonic testing is a useful and versatile method of non destructive testing with a superior level of penetration for thicker welds that gives an extremely accurate picture of internal and external defects. A pulser/ receiver emits high frequency sonic waves above the range of human hearing and these ultrasonic waves pass through the weld material via a coupling medium, usually a layer of oil applied to the surface to be tested. Where there is a discontinuity or defect, part of the energy will be deflected back to the receiver and converted by a transducer from sonic to electrical energy and the results displayed on a screen. By measuring the time between the pulse emission and receiving the reflected energy, the ultrasonic testing equipment can measure the size, orientation and other physical characteristics of the fault in far more accurate manner than other methods. Unlike radiographic testing, only one surface has to be accessible to perform the inspection and automated systems can produce instantaneous, detailed results, but an extensively trained professional still needs to conduct the test. In phased array ultrasonic testing, the equipment has multiple pulser elements that can pulse individually at tiny intervals. This allows testing of a far larger area without any need to move the pulser to give a larger area of inspection in a shorter time.
Format NDT are an independent business with over forty years experience of providing a wide range of non destructive testing and services. Our extensive team of engineers provide services across a range of industries to businesses across the UK, twenty four hours a day. If you are looking for the ideal partner for all your non destructive testing needs who will always go the extra mile and get it right every time, contact us today.
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