Welding is a one of the metal joining processes and is widely used in the manufacturing industry. According to AWS, a weld is a localized coalescence of metals or non metals produced either by heating the materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone and with or without the use of filler metal. Welding process is employed in manufacture of various critical equipment’s such as boilers, pressure vessels, and nuclear vessels, piping applications, aero space applications etc. hence effective methods of testing are a need to ensure soundness of welds. hence effective quality program is required to determine the suitability of a given base metal or a weld to perform in intended service.
One major disadvantage of employing the destructive testing methods is the test object is destroyed in the process hence various non destructive testing methods are used as they permit non destructive evaluation of the metal or component. Non destructive test provide indirect, valid, leave the test objects unchanged and ready to be placed in service if found acceptable.
The suitability of a particular nondestructive test for a given application is determined by considering of the following factors:
- A source of probing energy/medium
- A discontinuity must cause change or alternate of the probing energy
- A means of detecting this change
- A means of indicating this change
- A means of observing or recording this indication so that an interpretation can be made.
Over the years numerous nondestructive test methods have been developed. Each one having various limitations and advantages. it would be difficult to mention each one of them in one blog. Hence we will concentrate on most common nondestructive test methods which are commonly used for the evaluation of base metals and weldments which are as follows.
- Visual Testing (VT)
- Penetrant testing (PT)
- Magnetic Particle Testing (MT)
- Radiographic Testing (RT)
- Ultrasonic Testing (UT)
Visual testing should be the first nondestructive test method applied to an item in order to locate surface anomalies in most materials and subsurface discontinuities in translucent materials.
Visual testing is the observation of a test object, either directly with the eyes or indirectly using optical instruments, by an inspector to evaluate the presence of surface anomalies and the object’s conformance to specification.
While visual inspection is usually considered to be less complicated than other nondestructive testing methods, which does not imply that just anyone can effectively perform this operation. Whoever performs visual inspection must be knowledgeable in numerous area of expertise.
Also various tools are used to carry out weld inspection easily and effectively. This method can be used effectively when applied at every step of fabrication process otherwise certain discontinuities can be missed.
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Penetrant testing also known as dye / liquid penetrant testing is a versatile nondestructive test method used for the detection of OPEN TO SURFACE DISCONTINUITIES in a wide variety of solid, non-porous materials.
It basically involves the application of Penetrant on the testing surface, allowing it on the test surface for a period of time and application of the developer after cleaning the Penetrant completely from the test surface. The test results indicating the presence of Discontinuities not visible by unaided human eyes.
Usually penetrant testing is applied
- Edge PT of setup joints to find any possible open to surface discontinuities that can propagate or increase in size due to heat applied during the welding process.
- Root run PT in order to find out the a sound root is deposited without any incomplete lack of penetration, lack of fusion, deposited slag and porosities.
- PT of backchip welds to ensure that backchipping is carried out upto the sound metal.
- Final PT of welds after visual inspection to locate discountnuities that cant be seen with unaided eyes.
- PT after stress relieving- to locate cracks are generated during the stress relieving process.
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Magnetic Particle Testing (MT)
Magnetic particle testing is used to detect surface and shallow subsurface discontinuities in ferromagnetic materials (no other type of material) such as iron, nickel, cobalt, and some of their alloys.
This method includes magnetizing the part using direct/indirect method by applying a magnetic field into the part and detecting the flaws by application of fine metallic particles.
Magnetic particle testing is capable of revealing surface discontinuities including those that are too small or too tight to be seen by the unaided eye. Magnetic particle testing is used for ferrous welds to locate surface and subsurface discontinuities. Usually carried out on backchip welds and final welds after visual inspection.
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Radiographic Testing (RT)
Radiography is a volumetric NDT test method used to determine internal flaws in castings and Welds. RT includes passing of X-rays or Gamma Rays through the test item from one side and recording the rays on an imaging media on the other side giving a permanent visual record of the internal structure of the test items.
It is used to find out internal flaws of welds such as porosity, slag , heavy inclusions and incomplete penetrations of weld seams that cannot be inspected by Visual Inspection. It is usually carried out once the visual and either of the surface tests (PT/MT) is carried out.
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Ultrasonic Testing (UT)
UT is a family of NDT techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials.
One of the drawbacks of RT is that lack of fusion is not detected in it hence soundness of the weld joint is not ensured completely. Hence UT is many times used in replacement or followed by RT. Moreover UT also gives length and location along with depth of the discontinuities hence it is preferred by the manufacturers.
UT is usually carried on final welds and at setup stage of the weld joints to detect laps or laminations which can open up due to heat applied during the welding process.
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Applicable codes and standards
Now in the Industry in order to meet the standard requirements, all Non-Destructive tests are carried out as per standard procedure to ensure Reliable test results. These procedures are derived from applicable CODE, STANDARD, and Specifications.
ASME BPVC Section V contains requirements and methods for nondestructive examination (NDE) Which are Code requirements to the extent they are specifically referenced and required by other Code Sections or referencing documents and is referred largely in the boiler and pressure vessel industry.
- Article 9 – Visual Examination
- Article 2 – Radiographic Examination
- Article 4 – Ultrasonic Examination Methods for Welds
- Article 6 – Liquid Penetrant Examination
- Article 7 – Magnetic Particle Examination
Moreover ISO standards are also used as reference world wide to perform NDT of welds some of them are :
- ISO 17637:2016 – Visual testing of fusion-welded joints
- ISO 17636-1:2013 , Non-destructive testing of welds – specifies techniques of radiographic examination of fusion welded joints in metallic materials using industrial radiographic film.
- ISO 17638:2016 – Non-destructive testing of welds – specifies techniques for detection of surface imperfections in welds in ferromagnetic materials, including the heat affected zones, by means of magnetic particle testing.
- AWS – Welding Inspection Technology
- ASME Sec- V
- feature image : Photo by Christopher Burns on Unsplash