Weld inspection: What’s the most effective way to inspect a weld?

Wouldn’t it be great if I could give a neat, three-sentence answer to this question? It would certainly make for an easy read. 

Instead, our answer is: it depends. 

Every weld is different, and every site/ piece of equipment/ component/ circumstance is unique. There are no easy answers to this question. But, the process always begins the same way. In this article, we will look at the different weld testing solutions and explain the pros and cons of each. 

Where to start?

The first stage of a weld inspection is to do a visual check. This requires an experienced eye; not only are we looking for any visible cracks, but also any leaks, corrosion or excessive waste slag. We may use borescopes or robotic crawlers to assist with this check. We look at the quality of the weld using weld gauges to assess whether the weld is the correct shape. To check the weld’s integrity, we will look at its flatness, convexity and concavity. If there are any cracks, holes, or other defects, then these need to be repaired before we continue our inspection. 

From here, we have a number of options for our weld inspection services. Below, we explore the options and their pros and cons. Technicians are required to apply their knowledge of the testing method, along with their experience with different types of welds and materials in order to select the correct testing method. Sometimes more than one method is used. 

What are my options? 

1. Magnetic Particle Inspection

Magnetic Particle Inspection (MPI) is used to detect surface defects in ferromagnetic (or magnetic) materials. A magnetic field is put into the weld, and a magnetic ink is applied. Magnetic flux will leak through any defects, attracting the particles and visibly disturbing the ink, highlighting any defects in the material. If the weld is good then the magnetic field should pass through without affecting the ink.

One advantage is that this process is straightforward and portable, meaning it can be used on any site. It is a technique used to detect defects at the surface of the material and can identify sub-surface defects no deeper than 2mm.

However, there are limitations as the process requires surfaces to be clean before testing can start and any flaws more than 2 mm below the surface are unlikely to show up. Also, MPI cannot be used on non-ferromagnetic materials and may not be suitable for certain areas on-site due to the use of flammable liquids. 

magnetic particle inspection service technician inspecting pipe
Magnetic Particle Inspection (MPI) being performed on a weld.

2. Eddy Current Inspection 

Eddy Current Inspection (EC) is our second option. It employs electromagnetism to detect surface breaking and slight subsurface flaws.  Defects in the weld will cause a change in the eddy currents which will be picked up by a probe. EC can be used to detect shallow surface fatigue cracks, pitting and stress corrosion cracks. 

One advantage of this method is that it can be used on painted surfaces and very little surface preparation is needed. It is a portable technique meaning we can easily work on your site and get instant results. 

However, this method can only be used on conductive materials, and additional testing would be needed if any defects were identified. 

3. Ultrasonic Testing 

Ultrasonic Inspection (UT) is next. This employs ultrasonic waves to measure internal flaws in a variety of materials. UT can penetrate deep into materials and gives instant results. 

This method can identify thinning, corrosion, cracks, voids and internal planar defects. It can also be used to judge thickness through surface paint. One drawback of this method is that the probe’s beam has a single direction and has to be moved manually to cover the whole component, unlike with Phased Array. 

This process requires a clean surface and a minimum material thickness may be required. UT can only identify flaws within a material and so is often combined with surface inspection tests. 

4. Radiography

We have the facilities to carry out both Gamma and X-Ray radiography for weld inspection. We can do so in-house, in our Radiography Bay, or on-site, processing our films in our mobile darkroom. 

To carry out a weld inspection, the weld is placed between the radiation source and the film. Rays are sent through the weld and an image is imprinted on the film, which is then processed at our on-site or mobile darkroom. This process allows us to see things that would usually be invisible. Cracks and voids can be identified as well as corrosion and thinning. 

One of the advantages of radiography is that the surface doesn’t have to be clean to be tested, unlike some other processes. Very small flaws can be identified through the process. Another advantage is that we have portable radiography units which can be used on your site. 

A disadvantage of this process is that the use of radiation can cause issues testing around staff on site, and if an area needs to be cordoned off during the test period then site operations may be affected. Although, we can reduce the extent of these disadvantages significantly with our Saferad portable radiography set. 

Saferad equipment set up in bay
The Saferad equipment set up in our Radiography Bay.

5. Phased Array 

Phased Array (PA) is another technique that can be used to test a weld. It uses a set of ultrasonic testing probes made up of small elements. A beam from the phased array probe can be focused and swept across an inspection area without moving the probe itself. As the probe moves along, it sends out pulses of sound waves.  These waves bounce off any irregularities in the metal and return to the probe. These data points are recorded by the probe. Data can be collected and stored which means the process can be used for monitoring and maintenance. 

PA can identify internal defects, corrosion, thinning, cracks and voids. One advantage of this process is that it is portable and can be used in-house or on-site. As the probe can be moved electronically, PA takes less time than manual ultrasonic testing. 

However, this method requires a smooth, clean surface and materials have to have a minimum thickness for the process to work. The method will not identify surface-breaking defects for which other testing methods would need to be used. 

6. Dye Penetrant Inspection 

Finally, we have DPI. Dye penetrant inspection is used to identify surface-breaking defects during a weld inspection. A dye is used to find surface cracks which are then measured under different light sources. This can be done on-site as well as at our unit. 

An advantage of this simple method is that it is fast and low-cost. However, it requires a clean surface, is temperature dependent and can only detect surface-breaking defects. 

dye penetrant inspection 2
Dye Penetrant Inspection of a weld

Experience, experience, experience

In short, there is no one-size-fits-all approach to testing whether a weld is sound or not. To test a weld, technicians need to be skilled in a wide range of non-destructive tests and have the experience to know which test (or series of tests) would be best applied in certain circumstances. This can allow for a more efficient, less disruptive testing service, enabling site operations to return to normal more quickly. 

Responsive testing that delivers assurance that your equipment is safe.

Contact us for a quality testing service with a fast report turnaround. We can respond to any challenge.


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