How to choose Nondestructive Testing Methods for Welding?

Introduction 

It is crystal clear. 

The importance of non-destructive testing methods for welds cannot be overemphasized. Numerous non-destructive methods exist for inspecting the quality and determining the integrity of welds. Nonetheless, the type of defect you aim to unveil determines what technique to apply. Consequently, some weld inspections may require deploying more than one method. How to determine what method(s) to apply is what we plan to further buttress in this article. So, make sure to read till the end to get the full gist. We are ready when you are.

I. Why do we need Non-destructive Testing (NDT) for Welds?

Welds can be laden with countless defects from the welding process. With the use of welds in crucial industrial operations globally, defects such as slags, cracks, voids, and other metallurgical flaws could be introduced into the welds. And with such flaws, the integrity of the welds can be compromised. Hence, the need for testing. However, these tests can either be destructive or non-destructive. But with destructive testing, components have to be replaced. Non-destructive testing, on the other hand, avails you the opportunity to test without destruction. Therefore, non-destructive testing is needed to guarantee the integrity of the welds and likewise save cost.

II. What are the common methods of Nondestructive Testing (NDT) for welds? 

We’ve highlighted some of the most important methods of nondestructive testing that are also very popular in the industry. 

• Visual inspection (VT): Visual inspection is one of the best methods of non-destructive testing. This is because it makes early detection of defective welds possible. When detected early, the cost of repairs is saved and defective parts can be replaced before it is deployed in service. With the visual inspection of welds, quality is guaranteed. As production requirements of both legal and local standards are adhered to. Visual inspection also assists in shortening or reducing costs on repairs.
• Radiographic testing (RT): Radiographic testing is typically deployed for materials inspection and testing. This is a non-destructive testing method that assists trained inspection officers to view the composition of the material, and also ascertain defects that may require urgent attention to product quality. It ensures that a comprehensive viewing of the material is displayed for proper identification of voids. It also helps keep a constant record for future reference. And finally, quality is assured.
• Ultrasonic testing (UT): One of the most crucial points in production Is testing for flaws. Now, with ultrasonic testing, you’d be able to detect these flaws fast and they are pocket-friendly. Ultrasonic testing is a non-destructive testing method that helps you save time. Why is that you may ask? The accuracy and speed of ultrasonic testing are phenomenal. Additionally, the littlest or smallest flaws and discontinuities are uncovered with ultrasonic testing. And unlike the radiographic testing method, ultrasonic testing has no downtime.
• Magnetic particle testing (MT): External and internal flaws in ferrous materials can halt production and shutdown machines fast if not quickly detected. This is where the magnetic particle testing method comes in. The magnetic particle inspection (MPI) as it is popularly referred to, averts flaws that escape into the assembling line. The beneficial quality of this inspection method is that production is at optimal speed. Furthermore, there’s no fatal damage to the machines and saves resources that would have been channeled to repairs.
• Liquid penetrant testing (PT): A liquid penetrant testing method is a non-destructive method that removes external breaking elements from a machine allowing the machine to run smoothly and ease production. The liquid penetrant testing applies a liquid with high external wetting features to extract defects under UV lighting to improve test sensitivity. However, a visual inspection accompanies this form of testing to ensure all flaws are eliminated. And this helps to reduce the cost of testing with a shorter turnaround testing time.
• Eddy-current testing (ET): The eddy-current testing method is a viable method of testing conductive materials. Especially for defects like cracks and discontinuities. This form of testing deploys the current transmitted by the transducer and placed on the conductive test material. The alternating charge transmitted by the transducer combined with the test material’s surface to produce an eddy current. Moreover, the eddy current testing can easily pick up cracks and voids in the test material, differences in harness, and disparities in the material surface being tested. 
• Phased Array Ultrasonic Testing (PAUT): The proficiency of the phased array in detecting flaws and defects is second to none. As it is a more advanced technique in an inspection. Therefore, it is widely deployed in various fields, such as the construction, power generation, and oil and gas industries. It is deployable after proper consultation and it is deemed to be the testing method required for inspection. Furthermore, customized beams are created using several ultrasonic components and time delays at different focus points to accurately identify flaws.
• Time-of-Flight Diffraction(TOFD): Time-of-flight diffraction is another outstanding non-destructive method that is widely used in various sectors. It is a high-performance testing method that ensures defects are accurately identified. Defects such as slags, fusion deficiencies, and crack sizes, typically in welded elements. Time of flight diffraction is generally suitable for welds inspection in storage tanks, pressure vessels, piping, etc., as pre-service inspection. While it similarly can be employed for in-service inspections such as fatigue cracking, root corrosion, and high-temperature hydrogen attack. 

III. How to choose the proper NDT Method for welding?

When it comes to choosing the proper NDT inspection method, there are a series of factors to consider. They help to ascertain the quality of every component. 

The first consideration, however, is evaluating the defects that are likely to affect production with the chosen NDT method to be applied. Although, there are instances where the choice of NDT method to be applied is definite, and at other times more than one NDT inspection method may be required. Ensuring all defects are eliminated and production runs smoothly is often the goal. 

Secondly, identifying what point of production inspection should be initiated must be one of the priorities. Typically, inspection is slated for the latter end of operations. But then, inspection is sometimes required during production for quality control and specifications conformance. It is particularly important because some operations require two different NDT inspection methods. For instance, in the case of multiple inspections, the first ND method applied may open up other defects which the first technique might be unable to detect. Furthermore, the more defects identified in operations, the more the need for other NDT methods to be applied. Therefore, welding inspections must be thorough due to their applicability in numerous sectors.

IV. What are the requirements for Non-destructive Testing (NDT)?

An advanced testing method like non-destructive testing should establish standard requirements to guide industries and professionals in ensuring human safety and quality compliance. However, different requirements exist for different localities with some overlap between industries. Still, the essence remains the safety of users of the finished product, equipment, or facility. 

These requirements often emanate from research carried out by research and development teams within the manufacturing industries and international standard bodies like ASTM and ISO. Nevertheless, global trade associations like ASNT and ICNDT contribute to this research. These requirements often give guidelines to NDT professionals on appropriate testing methods to evaluate specific parameters. 

Also, the areas of components to test and the permissive limits for the values obtained during testing are some requirements for non-destructive testing. Additionally, the equipment requirements and set-up, testing frequency, and reporting format and storage are all part of the specifications for NDT. These requirements form the frame of reference, especially for manufacturers, airlines, and oil and gas companies, as they have some of the most complex facilities and equipment requiring NDT. 

Conclusion

In conclusion, it has been established that non-destructive testing methods offer the most appropriate inspection tools to determine the soundness of welds. Ranging from electromagnetic and radioactive waves to visual inspection and fluid application techniques to examine the surface and subsurface of welds. Additionally, it has also been verified that some techniques may be more effective than others. And some inspections may require more than one method for an efficient inspection, and fabricating quality welds.