By Marc Dallaire
Welding seems simple, but when you are not familiar with this field, it quickly becomes a mess. In inspection, there are several subtleties to consider. Apart from the quality itself of the welds, method of inspection is a significant requirement. On most projects, it is usually the engineers who write the specifications on scope of inspection. Sometimes, it becomes difficult to find ourselves in all the possibilities and constraints concerning inspection. Who should perform the inspection? What method should they use?
First things first. All inspections begin with a visual examination. As no sophisticated equipment is required and no special procedure has to be followed, the visual inspection remains the fastest and most effective method. Most defects can be found simply by observing the appearance of the weld. Overlaps, undercuts, porosities, and other nonconformities are very common in welds. While some may not cause any trouble in the structure, if left unrepaired, others could cause dramatic damage. That is why a certified inspector is usually required to verify that the assemblies conform to the standards. That being said, some defects cannot be found with the naked eye. Cracks, for example, can be very hard to find, especially when they are beneath the surface. It is why inspectors will use nondestructive testing (NDT) techniques.
On structural welds where cracks could jeopardize structural integrity, the magnetic particles testing (MT) becomes a helpful tool. The principle of this method is quite simple. An electromagnet induces a magnetic field in the steel on a specific area, usually a weld. If the field encounters a discontinuity in his way, like a crack, it will be locally disrupted. During this intervention, the inspector will spread floating particles of colored iron. This little cloud will concentrate itself in the disrupted zone if there is one, and the crack will be revealed. This method of inspection is very useful and fast to perform; however, it can only reveal disruption on the surface or very close to it. If we want to verify deeper, or if the material is not ferromagnetic, we must turn to other methods.
Penetrant testing (PT) is commonly used on nonferromagnetic alloy such as aluminum and some stainless steels. It helps the inspector to find cavities. In this method, a liquid penetrant is first applied on the surface. After cleaning, a developer is then applied, and pulling back the color penetrant to the surface enhances defects like cracks or porosities.
Ultrasonic testing (UT) can help you look inside the material. Similar to scans on pregnant women, we use ultrasound to inspect beneath the surface where we cannot see. A sonic wave is sent inside the assembly and comes back. When it does, the device analyzes it and informs the inspector on the continuity of the material. This method is used mostly on complete penetration welds. It is common to see a 100% ultrasonic inspection request on all complete penetration welds. For fillet welds, this method is generally not appropriate.
In some rare cases, ultrasonic testing cannot be used or would be too slow. A good alternative is radiography testing (RT). This method consists basically of generating radiation and allowing it to penetrate the piece under examination. A film receives the radiation on the other side of the assembly and produces an image, depending on the discontinuities that the radiation encounters. This method involves risks and mandates that all personnel in a defined radius be evacuated. It is often used as spot checks over an ultrasonic inspection.
In the end, you must keep in mind that all NDT methods should be performed by certified personnel, since they are very dependent of the inspector’s skills. In some cases, more than one technique can be used, and most of the time, NDT remains as sampling. Just remember, if you have any doubts, seek the help of a professional.
Marc Dallaire is president of Geni-Metal Laboratory Of Metal.
