Weld is a crucial part of component construction to connect two or more metal surfaces. They are used in construction, aerospace, vehicle, railways, electricals, and machinery sectors. The welds may show signs of damage over time. It is due to varied environmental conditions or the use of low-quality welding technologies. Even small flaws can progress into larger defects over time. Hence it is mandatory to check the safety, quality, strength, and reliability of welds. There are two available methods to assess the quality of welds:
Destructive testing techniques and Non-destructive testing techniques
Destructive testing techniques (DT)
- DT techniques are time-consuming. They alter the properties of the object being tested.
- DT techniques include macro etch testing, transverse tension tests, fillet-weld break test, guided bend test, backbend test, nick break test, tensile strength break test, and free bend test.
Non-destructive testing techniques (NDT)
- These techniques can test the inner defects of welds.
- They do not alter the properties of the object being inspected
- They save both time and money.
- NDT techniques include radiography, visual testing, magnetic particle, ultrasonic, eddy current, and liquid penetrant.
In this article, let us find out how digital X-ray systems inspect the internal defects of welds. Before getting into this topic, let us first go through the types of weld flaws. You will then understand the importance of digital X-ray systems in weld defect inspection.
TYPES OF WELD FLAWS
Discontinuities/flaws are disruptions that occur in the structure of an object. They may occur in the base metal, weld material, or ‘heat affected’ parts.
There are varied types of discontinuities including:
- Cold lap – Here, the weld filler metal improperly fuses with the base metal.
- Porosity – Appearance of dark round or irregular spots singularly, in rows, or in groups on a radiograph. It arises due to entrapment of gas in the solidifying metal.
- Cluster porosity – It is similar to the porosity issue. But the spots will group together very closely on a radiograph.
- Slag inclusions – Appearance of dark, jagged asymmetrical shapes on a radiograph. It occurs due to entrapment of non-metallic solid material in weld metal or between base metal and weld.
- Incomplete penetration/lack of penetration – It is one of the most offensive flaws where the weld fails to penetrate the joint. This incomplete penetration leads to the occurrence of cracks.
- Incomplete fusion – Here, the weld filler improperly fuses with the base metal.
- Internal concavity or suck back – Here, the weld metal contracts as it cools and draws up into the root of the weld.
- Internal or root undercut – Here, the destruction of base metal occurs next to the root of the weld.
- External or crown undercut – Here, the destruction of base metal occurs next to the crown of the weld.
- Offset or mismatch – It occurs when two pieces to be welded together are not properly aligned.
- Inadequate weld reinforcement – Here, the thickness of the weld metal is less than the thickness of base material.
- Excess weld reinforcement – Here, weld metal is added excessively in an area of a weld.
- Cracks – It appears as jagged and very faint irregular lines.
Flaws in TIG welds
These flaws involve aluminum and stainless steel.
- Tungsten inclusion – Here, tungsten entraps in the weld during tungsten inert gas welding, if done improperly. It appears lighter with a distinct outline radiographically.
- Oxide inclusion – Oxides, especially aluminum, are visible on the surface of the material being welded. It appears dark and irregular radiographically.
Flaws in GMA (Gas Metal Arc) welds
- Whiskers – These are short weld electrode wires visible within the weld or on the top or bottom surface of the weld.
- Burn-Through – It occurs when excess heat causes excessive weld metal to penetrate the weld zone. These lumps of metal sag create a thick globular condition on the backside of the weld. These globs are called as icicles.
Let us go through non-destructive testing techniques in detail:
Ultrasonic testing (UT) and Radiographic testing (RT) are the two common nondestructive testing methods to determine the discontinuities within the internal parts of welds. Both these methods help in showing the weld’s internal integrity without damaging its components. Still, there are a few differences between these two methods. Let’s summarize them in detail:
DIFFERENCE BETWEEN UT AND RT IN WELDING
|Characteristics||Radiographic testing (RT)||Ultrasonic testing (UT)|
|Nature||Slow than UT||Comparatively quicker|
|Source||X-ray/gamma rays; X-ray produced by an X-ray tube; gamma rays by a radioactive isotope||Ultrasonic waves produced by laser|
|Material||All materials||All materials especially metals and plastics. Can be used on ferrous and nonferrous materials.|
|Geometry||Appropriate for complex weld||Need for special probes in varied geometries|
|Principle||Highly penetrating rays are passed through the weld on to photographic film. This in turn creates an image of the weld’s internal structure on the film. The amount of energy absorbed by the sample depends on its density and thickness. The energy not absorbed by the sample will appear darker in the film. Hence the areas of the weld where the thickness got changed due to discontinuities will be displayed as dark outlines on the film.||A beam of high-frequency ultrasonic energy is directed to the sample object. This beam penetrates the object with negligible energy loss, except when it is stopped and reflected to its origin by a flaw. The starting signal, returned signals, and the signal of the object is displayed on the screen of an oscilloscope.|
|Types of defects||All types of flaws like surface, and subsurface defects. Not for very fine defects||All types of flaws like deep finer, thicker section, surface, and subsurface defects|
|Applications||Detect cracks, inclusions, porosities, and voids in the welded pipeline. RT can detect scattered porosity, unlike UT.||Control spot welding, detect SAW (Submerged-Arc Weld) in the pipeline, detect FSW (Friction Stir Weld). UT can measure the thickness of the material. UT can detect lamination-like defects, unlike RT.|
|Examples: Weld process||Laser beam welding, Electric resistance welding, Gas metal arc welding, Electron beam welding||Friction stir welding, Resistance spot welding, Electron beam welding, Laser beam welding|
|Advantages||Detect the type and position of defects. Can automate||Detect the length, type, exact position, and location of defects. Portable.|
|Disadvantages||Inaccurate defect size. Poor resolution. Require access to both sides of the part. A two-dimensional image of the defect is easier to interpret. The film is too sensitive to humidity, pressure, and temperature.||Defect direction. Requires access to both sides or ends of the part. The surface must be accessible for ultrasound transmission. Requires a coupling medium to promote the transmission of sound energy into the object. It is difficult to detect thin, non-homogenous, rough, and irregular shaped small materials. It is also difficult to inspect cast iron and other coarse-grained metals due to high noise signals and low sound transmission. UT may not detect linear defects parallel to the sound beam. Require reference standards for calibrating equipment and characterizing flaws.|
|Regarding interpretation||Requires an expert eye to detect the weld defects||Requires a trained professional to sense the spikes and amplitude-tuned data shown on the screen.|
Here, we are going to study radiographic X-ray testing in detail:
WHAT IS THE RADIOGRAPHIC X-RAY WELDING TEST?
Radiographic Testing (RT) is a non-destructing testing (NDT) method that can detect the defects without damaging the components of the object. RT is used to inspect most of the materials and products such as welds, castings, and composites. RT can assess the welded joints by accessing from both sides. To ensure the best operational quality, RT shows changes in thickness, assembly details, as well as both internal and external defects. Though this process is slow and expensive, RT can detect cracks, inclusions, voids, and porosity in the internal parts of the welds.
Radiographic Testing uses X-rays or gamma rays to produce a radiographic image of the target object. X-rays are emitted by an X-ray tube. Gamma rays are emitted by a radioactive isotope.
Principle of radiographic X-ray welding test
The radiographic inspection of weld materials is similar to the principle of medical radiography.
- Keep the radiographic film on the remote side of the target object.
- Transmit the radiation (X-ray/gamma rays) emitted by an X-ray tube/ a radioactive isotope respectively, from one side to the remote side where the film is placed.
- The radiographic film determines the radiation and measures them in varied quantities over the entire surface.
- Process this radiographic film in darkroom conditions and view the final image on a special light-emitting device.
- You will find the image in different degrees of black and white according to the density of radiation.
- The difference in the density of the processed film is due to the discontinuities in the target material.
Interpretation of radiography test for welding
The amount of energy absorbed by the target material depends on the density and thickness. The non-absorbed energy leads to exposure of the film. These areas will appear dark while developing the film. The areas exposed to minimal energy will appear lighter. The discontinuities like cracks or porosity with respect to the changes in thickness will be shown as dark outlines. Inclusions of low density like slags will be shown as dark areas. Similarly, inclusions of high density like tungsten will be shown as light areas.
A qualified and well-experienced inspector can interpret the final images in terms of the location and the type of defect in the target object. He can also study the entire internal structure of the target object. Remember that only the qualified and well-experienced personnel should only conduct the radiography testing and interpret its results. False readings prove to be expensive and can seriously harm productivity.
Advantages of radiographic X-ray welding test
- Apart from the above applications, RT can detect the volume of the target object.
- RT is an incredible quality control method that can detect both surface and subsurface flaws.
- The results can be stored as a permanent record.
There are two types of radiography test for welding:
- Film Radiography (RT)
- Digital Radiography (DR)
There are few differences between these Film and Digital radiographic methods. Let’s summarize them in detail:
DIFFERENCE BETWEEN FILM AND DIGITAL RADIOGRAPHY
|Characteristics||Film Radiography (RT)||Digital Radiography (DR)|
|Image source||RT uses film to capture the image||DR doesn’t need a film as a source. It uses a flat panel detector (FPD) to capture the image|
|Principle||The initial rays pass through the target object exposing the film.||The initial rays create a digital signal directly or indirectly. Direct signal is emitted when the FPD converts the initial rays into charge pattern. Indirect signal is emitted via a scintillator that emits light when exposed to rays. This is detected by a photosensitive diode layer.|
|Processing of the result||The latent image created on the film can be seen after processing in a dark room.||Data appears on the computer screen at once.|
|Exposure control||No control of exposure in real-time.||You can also control the exposure in real-time.|
|Final image||The resulting image can be viewed in a film using a lightbox viewer||The resulting image can be manipulated to get the best image of the sample.|
|Final image||The final image is made of silver halides, where the excess silver is removed by processing the film.||The final image can be annotated, stored, and transferred. Data will also remain unaltered for future references.|
|Codes and standards||RT has many codes and standards covering the entire aspects of its applications and quality control.||DR has codes and standards recently for their applications.|
|Interpretation support||You need a large pool of experienced technicians for interpretation.||No need of a large pool of experienced technicians for interpretation. Training/qualifications are required specific to the technologies due to operational differences.|
|Features||RT takes a long time to process and interpret. The quality of the final image is comparatively low. The film can be used only once. RT is comparatively less expensive. There is a chance of human error while interpreting the results. Physical transfer of the film||Shorter exposure time with quick interpretation.
Digital images are of high quality. It requires a large amount of data storage. DR is repeatable and reproducible. DR is more expensive. DR reduces the chance of human error. Electronic transfer of data
|Applications||RT detects damages or flaws in the weld||DR can identify flaws, foreign objects, weld repairs, and corrosion under insulation.|
HOW TO USE DIGITAL X-RAY SYSTEMS FOR WELD INSPECTION?
Noise has an effect on the quality of the image. Higher the image noise, the lesser the image quality. Hence, the new technology was focused on removing the noise sources such as X-ray source, CCD camera, test object, imaging screen, controller circuits, etc. This is where, digital radiography successfully improves the image quality by eliminating the noise.
The digital X-ray system is an advanced technology, where the X-ray image of the target object is directly visualized on a computer screen without any need for intermediate scanning or chemicals. This system can directly evaluate the defects in a three-dimensional manner.
Principle of digital X-ray welding test:
The incident X-ray beam converts into an electric charge. This in turn gets converted into a digital image by passing through a sensor detector. Here, the flat panel detector is used to provide high-quality digital images with better signal to noise ratio.
Flat panel detector work in two ways such as direct and indirect conversion.
- Indirect conversion flat panel detectors use a photodiode matrix of amorphous silicon.
- Direct conversion flat panel detectors use photoconductors such as amorphous selenium or cadmium telluride on an electrode plate. When photons impact upon the photoconductor, they get converted to electric signals directly which would be amplified and digitized. In the direct conversion procedure, photons do not spread laterally as the scintillator is absent. This step ensures the delivery of a sharper image with the greatest resolution.
The thin-film transistors read the information on both these detectors.
Digital X-ray system is used in both the labs and the field. It is used to inspect the weld and casting quality, insulated or uninsulated pipings, pipelines, and aerospace components.
There are two types of digital radiography techniques to inspect in the field:
- Computed Radiography (CR)
- Real-time Radiography (RTR)
Computed Radiography (CR)
Here, the X-ray image is captured on a reusable and flexible phosphor-coated imaging plate. Next, this plate is scanned using a laser to produce a digital image that can be uploaded, edited, and shared via computer. This CR system is easily retrofitted into film-based systems. Hence, there is no need to process the lab film or use any chemical.
Real-time Radiography (RTR)
Real-time Radiography (RTR) is otherwise called as fluoroscopy testing. RTR emits rays into one side of the material. These rays are then converted into the light on the other side with the help of sensors. The resultant digital image reveals internal or external defects as well as corrosion in real-time. The RTR procedure is quick and safe. Hence, there is no need for a dark room to produce images.
Applications of digital X-ray welding test:
- Digital radiography is not only used to inspect the welds and castings. There are few other applications:
- Digital radiography can detect the presence of foreign materials in the target object.
- Digital X-ray system can inspect composites and fiber-reinforced components.
- It can detect the flow accelerated corrosion.
Advantage of digital X-ray over conventional X-ray
This is where digital radiography automates the flow by taking the digital images and detecting the flaws. It helps in storage, management, and analysis of digital images (especially the weld flaws) easily and quickly. There is no need for chemicals or consumables. There is no need for any battery changes or darkroom technicians. It is scalable and eco-friendly. Similarly, digital radiography helps in noise removal, contrast enhancement, and vision improvement.
- Reduced exposure time
- Increased efficiency in detecting more details
- Reduced inspection time
- No film processing is required
- Highly productive and portable
- Deliver immediate results
- Safe for the environment as there is no usage of chemicals for film processing
- Increased data storage
- Smaller exclusion zones
- Deliver enhanced digital images
- Improved dynamic range. Hence, it is an easy tool to inspect multiple thicknesses in one shot.
- Reduced human errors during result interpretation
Compared to the conventional X-ray system, the digital X-ray system is more expensive. Still, the digital X-ray system proves to be safer, quicker, detailed, and more reliable in the radiography test for welding.
Considerations before using X-ray inspection machine
The usage of welds has become critical in the manufacturing sector. Hence, they should be inspected well to ensure that they follow international standards before they are put into service. But before opting for radiographic inspection of welds, it is recommended to consider the following three steps:
- Calibrate the instruments used in quality check to a known standard. Calibration helps in the accuracy and precision of the test results.
- Check whether if you require a combination of NDT techniques. Usually, companies use several methods to assess assembled metal components like joints, pipes and bends. Automotive, petroleum, and aerospace industries not only opt for X-ray testing. They also use ultrasonic inspection, magnetic particle inspection, and dye penetrant testing. This is to ensure the comprehensiveness of the examination and reliability of the results.
- Before opting for combinational NDT techniques, check the pros and cons of all the methods. Varied defects may require varied inspection methods. For example, you need ultrasonic and acoustic-based tests to evaluate de-laminations and planar cracks. It can be difficult to use only radiographic methods. Similarly, the size of the surface area may also determine the testing type. For example, you need dye penetrant testing to detect the large surface area at a low cost. But there is one disadvantage. This method requires proper surface preparation and chemical handling precautions.
Hence, digital X-ray is the fast, accurate, and reliable tool used to inspect internal weld defects.