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Fabrication of Flawed Blocks for Ultrasonic Testing in Lieu of Radiographic Testing

by Ronald Kruzic*

Construction codes and standards sometimes, with restrictions, permit substituting ultrasonic testing for radiographic testing. This "Back to Basics" article paper discusses requirements for creating flawed welds to be used for such ultrasonic testing. The minimum number of flaws, and their types, locations and sizes, are discussed, along with concepts important in fabricating a flawed block standard. David Kupperman Associate Technical Editor

 

Background

Many of today's construction codes and standards allow for the substitution of ultrasonic testing (UT) for radiographic testing (RT) as long as certain conditions are met. For example, Sections I and VIII of the ASME Boiler and Pressure Vessel Code (2006), including Code Case 2235-9, allow the substitution of UT for RT if the procedure is demonstrated on a qualification block as being able to detect discontinuities that are of an acceptable size for the thickness involved, and have an indicated length equal to or greater than the actual length (that is, discontinuities significant enough to be considered flaws). Under Appendix U of the API 620 (2008) and API 650 (2007) standards, the substitution is allowed if the operators pass a practical examination for UT on a blind test plate that includes having operators testing elements that are contained in the Appendix.

This paper discusses a methodology that can be used to construct flawed welds to be utilized for these purposes if acceptable to the parties involved.

It is suggested that weight be given to the more critical points
such as sizing, categorization and grouping calls.

Initial Considerations

For either an ASME procedure demonstration or API operator practical examination qualification, the same basic initial information is required to be accumulated: namely, base material and its heat treatment, thicknesses, welding processes and joint details. Since the ASME (procedure based) and API (practical based) qualifications are so different, they will largely be discussed separately.

For an ASME based examination, before the decision as to the exact number of flaws and their size, orientation and location can be determined, a number of factors must be decided upon:

• the ultrasonic technique(s) used: pulse/echo, time of flight diffraction, phased array and so on
• whether or not the block can be flipped during the qualification process (the need for similar weld details in both the inner and outer diameter, and the presence of cladding or overlays present)
• the number of individual setups (zones) necessary for complete coverage of the required examination volume.

For example, suppose it were determined that for a particular weld it was necessary to utilize four time of flight diffraction setups to provide adequate coverage of the examination volume, and a weld overlay was involved so that the block could not be flipped. Per code, a minimum of three flaws are required, but in this case the minimum would be four, as at least one flaw per zone should be considered as the new minimum. See Figure 1 for an example.

Figure 1 — Minimum flaws for four-zone testing.

For an API based examination, the minimum number of flaws is not specified in the Appendix. The testing program details must be by agreement with the purchaser. It is felt that a minimum of 10 flaws should be considered. This could yield a reasonable number of flaws, both individual and grouped, so as to create an adequate grading scheme for the operators (Figure 2).

Figure 2 — Ten flaw layout example.

The ten flaws in Figure 2 would not only allow for 10 individual location, sizing (length and height) and categorization (surface or subsurface) calls, but also three grouping calls (flaws 1 and 2, 4 and 5, and 8 and 9), for a total of 56 grading points (note that a flaw need not be surface-breaking to be categorized as a surface flaw). Another grading point could be the surface breaking confirming magnetic particle/liquid penetrant testing (MT/PT), thus upping the potential points to 66. It is suggested that weight be given to the more critical points such as sizing, categorization and grouping calls. See Table 1 for an example of a possible scheme with weight factors. Also needed to be factored in would be any miscalls or false calls made by the operators.

Suggested Flaw Type

Since these flawed welds are for new construction codes and standards and not for inservice or fitness-for-purpose type examinations for specific service-induced flaws or failure mechanisms, and to keep the costs involved in block fabrication to a reasonable level, it is felt that electric discharge machined (EDM) slits are a reasonable choice and would be representative of the more critical planar type flaws (that is, cracks and nonfusion).

Additional details concerning the flaws must be considered:

• ASME requires that the demonstration weld's flaws be oriented to simulate flaws parallel to the production weld's fusion line. One surface flaw per surface (inner and outer diameter) and one subsurface flaw are the minimum called for. (A surface flaw is defined here as a flaw whose separation distance from the nearest surface is less than or equal to one-half its depth for ASME or less than one-half its height for API; a subsurface flaw is one whose separation distance from the nearest surface is greater than one-half its depth for ASME or equal to or greater than one-half its height for API.) See Figure 3 for examples.
• API requires the test plates to have not only both surface and subsurface flaws, but have both acceptable and unacceptable flaws. They also must include groupings of flaws. The weld flaws should probably also have the slits orientated in the same fashion as ASME's, though this is not so stated in the Appendix.

Figure 3 — Flaw/joint examples.

Block Creation

Once the number, categorization, size, orientation and locations of the EDM slits have been determined, then it becomes a matter of creating the flawed block. A methodology for this is to first partially weld together two plates with the same joint details and filler metal to be utilized in production. The plate material shall meet the same requirements as that for a UT calibration block per Section V, Article 4, T-434 of the ASME Boiler and Pressure Vessel Code. Portions of the weld are left uncompleted (underfilled) for the EDM slits to be embedded, and the remaining areas completed for EDM slits that are to be left open to the surface (Figure 4).

Figure 4 — Joint pre-welding example.

For the weld embedded flaws, their final desired depth (in ASME's language) or height (in API's) must be increased by the amount of weld penetration expected when the unwelded portions are completed during the selected embedding welding process (by depth or height, the texts refer to the through-wall flaw dimension, drawn normal to the inside pressure retaining surface of the component). Then a machinist burns the notches and the incomplete portions of the weld are completed (Figure 5). A radiograph is recommended to further document the flaw locations as well as to identify any flaws that may have been unintentionally introduced during welding.

Figure 5 — EDM slits.

The last thing required is to decide what is the flaw size to be utilized for the procedure demonstration or operator practical examinations for the newly embedded flaws. Three sides of the EDM slit are normally unaffected by the embedding process; therefore, its length is accurately known and only the depth (or height) to be used needs to be decided upon. If it is agreed that the expected amount of penetration can be utilized, then the original desired flaw depth (height) can be utilized plus this amount added. If it is decided that the amount of penetration must be determined, this means testing it with ultrasonic techniques that have been proven successful at doing so. Figure 6 illustrates an estimated penetration example.

Figure 6 — Estimated penetration example.

Conclusion

Whether the aforementioned methodology is utilized for flawed block fabrication or other schemes, it is highly recommended, due to the cost and fabrication time, to always first come to a agreement with the user or purchaser as to not only the block fabrication scheme, but also the number, size and locations of the flaws prior to fabrication.

References

API, API 620: Design and Construction of Large, Welded, Low-Pressure Storage Tanks, 11th edition, Washington, DC, American Petroleum Institute, 2008.

API, API 650: Welded Steel Tanks for Oil Storage, 11th edition, Washington, DC, American Petroleum Institute, 2007.

ASME, ASME Boiler and Pressure Vessel Code, New York, American Society of Mechanical Engineers, 2007.

 

* Chicago Bridge and Iron Company, 14105 South Route 59, Plainfield, IL 60544; (815) 439-6000; fax (815) 439-6006; e-mail rkruzic@cbi.com.

 

Copyright © 2008 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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