It is important for welders and welding inspectors to understand the various types of flaws that can be present in a weld. Knowing the flaws and their causes can help prevent these flaws from occurring or help accurately repair the weld. Below is an explanation of three types of weld flaws- porosity, slag inclusion, and incomplete fusion- and their common causes.
Porosity is small cavities of gas or air present in a weld. Porosity occurs when dissolved gases and gases released during the welding process do not have enough time to escape before solidification. Surface porosity occurs on the surface of the weld. It can be seen, and it looks like small holes in the material. Subsurface porosity occurs within the weld itself. It can only be detected with technology such as phased array or industrial radiography. Elongated porosity is when the flaw is stretched. The most common causes of weld porosity includes impurities on the surface of the metal, excess shielding gas, damp electrodes, and backing bar material that doesn't match the metal being welded.
Slag refers to foreign materials trapped inside the weld. Slag is the residue of the flux coating in MMA welding, which is a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. A common cause of slag is the presence of a coating on the component being welded. For example, aluminum is often coated with aluminum oxide. During the welding process, the aluminum oxide can become trapped inside the weld. In order to prevent slag, cleaning of the component before beginning the welding process is required.
Incomplete fusion occurs when the weld material and the parent piece are not completely fused together. Welds with incomplete fusion are weak and unlikely to pass code inspection. Incomplete fusion is commonly caused when not enough weld material is deposited to fill the weld joint, there are gaps between weld beads at the root, and the surface of the component is dirty.
Active thermography works by heating the component that is to be inspected in a controlled environment then using an infrared camera to record an image of the temperature differences across the object. The results of thermography inspection are recorded as high resolution images, making data easy to be stored. Some benefits of active thermography inspection include the speed of the test, data is easily stored, and the ability to inspect large areas. There are a few different types of active thermography inspection including pulse thermography, lock-in thermography, and vibrothermography. The methods differ mostly in how the component to be inspected is heated.
Pulse thermography is also referred to as flash thermography. With this method, a short pulse of energy in the form of light is applied to the measuring object. The part cools after flash heating, and any flaws present will cause changes in the cooling rate. The cooling rate is recorded by the infrared camera as an image. Differences in temperature are depicted in the image and reviewed to identify any flaws present in the test object. This method is commonly used on components with thin walls or layers. An advantage of pulse thermography is the inspection time is only a few seconds long, at most. Also, the depth of the flaw is able to be determined using pulse thermography.
During lock-in thermography inspection, the surface of the test object is energized with periodical harmonic-modulated energy. The source of this energy could be derived from a halogen lamp or hot-air blower. Heating of the test object with this method is more controlled because energy can be continuously applied to the object. Consistent heating ensures that the infrared camera will be able to pick up and record any temperature differences. With other active thermography inspection methods, the test object is not continuously heated. Therefore, the part may cool too quickly and the camera will not be able to capture a usable image. The measuring time for this method is slightly longer than other active thermography methods.
In vibrothermography inspection, power ultrasound is used as the energizing source. For this inspection method, the ultrasonic equipment must be in contact with the surface of the test object, and couplant must be applied to the test object before the start of inspection. The ultrasonic energy is converted into heat by rubbing on defect locations. This rubbing creates friction. Any defects present act as an internal heat source while the surface of the test object shows almost no increase in temperature. An advantage of this method is the defects are shown very clearly on the resulting image.
There are multiple techniques that can be used for nondestructive testing using infrared thermography. The two main techniques are passive and active thermography. When selecting the appropriate technique, there are several factors that need taken into account. These factors include thermal characteristics of the test part, type of indication to be detected, and the type of infrared camera being used.
Both passive and active thermography offer several advantages over other nondestructive testing techniques. Infrared thermography inspection is fast, and contact with the object being inspected is unnecessary. Large components can be inspected easily with the proper thermography equipment.
In passive thermography, the component is inspected during or directly after a thermal cycle. The thermal cycle is the component’s normal operations. When in use, the component will heat; testing takes place during or directly after use.
An example of a time where passive thermography is used is in inspecting aircraft for water ingress immediately after landing. The aircraft will be hot from use, but the water will be cooler then the aircraft. The infrared camera will be able to detect these differences in temperature. Once the aircraft cools down, the camera will no longer be able to detect the temperature variations.
A benefit of passive thermography is components do not need to be taken out of use in order to be inspected. It is most effective when inspecting for strong thermal indications, such as water ingress.
In active thermography, the component being inspected is heated and cooled in a controlled environment. The component is continuously monitored while it is heated and cooled.
There are a variety of ways to create a controlled heating and cooling environment for the component. Heat lamps, heated blankets, and hot air guns are heat sources that can be controlled by the operator. Another way is to set the component in the sun for a specified period of time to heat, then move it into the shade to cool.
A benefit of active thermography is the operator has control over the thermal cycle. This control also allows for the component to be consistently heated and cooled multiple times. The operator has control over the thermal variables in this method of infrared thermography inspection
There are a few different types of active thermography testing, including flash thermography, lock-in thermography, and vibrothermography.
I was on a job site recently assisting a coworker with performing phased array ultrasonic inspection of welds. One of our tasks was to inspect two 16" pipe welds. One of the welds required repairs, and we were able to explain to the welders where the problem area was. As we were conversing with the welders, I realized almost all of them have similar questions about phased array. They want to know how it works, and how to interpret the results.
How Phased Array Works
The picture above is of a phased array scanner. This model is magnetic and will stick to the pipe. The operator will move the scanner around the pipe, and results are shown on the equipment computer. The part of the scanner that is circled in the picture above is called the transducer. Ultrasonic sound waves are emitted from the transducer, and the results are a visual image of the reflection of the sound waves.
Couplant is coated on the pipe to ensure there is no space between the transducer and the pipe. Without the couplant, the technology would not work properly. Sound waves travel in their directed position through the couplant where they would not if there was any space between the transducer and the pipe.
Using the equipment computer, technicians are able to set beam parameters such as angle, focal distance, and focal point size. Technicians are also able to adjust the angle of the sound beam, or set it to emit sound beams in multiple angles. Adjusting the angles of the sound beams allows for accurate flaw detection of components.
Since the technology utilizes ultrasonic sound waves to inspect components, there are no safety hazards associated with it. It is also a quick inspection method which can be used effectively on pipe welds of large sizes.
How To Interpret The Results
The most basic way to interpret the results for someone who does not have much knowledge of phased array is blue is normal and red is bad. Skilled technicians are able to interpret the results more specifically. They can locate areas with porosity and slag, and phased array allows for the technician to measure the size of the indications.
The images above are two different ways of looking at the weld. The image of the left is as if you were looking through the weld sliced in half. The dark blue area on the far left, near where the scan is narrow, indicates the root. As you move right through the picture, it is as if you are moving through the weld. The surface of the weld is where the blue ends and solid white begins. In this view, technicians are able to measure the depth of a flaw.
the image on the right is as if you are looking directly on the weld. The root is at the bottom of the picture and the surface of the weld is at the top. In this view, technicians are able to measure the length of a flaw.
Skilled phased array technicians have taken multiple training courses on the technology and have thousands of hours of in the field experience with phased array. The technology is difficult to understand at first. We were happy to converse with the welders about phased array while on the most recent job site. NDT technicians understand that the technology can be confusing, so we are happy to take a few minutes to explain it. The next time you have questions about phased array, do not hesitate asking the NDT technician on site.
Hello! My name is Melanie Boop, and I am the Communication Specialist at AER.