Back to Basics: A Guide t...

Back to Basics: A Guide to AWS
Ultrasonic Weld Inspection

How to do weld UT according to the AWS D1.1 Code

BY GORDON E. SMITH AND UWE ASCHEMEIER


The following is the first of a four-part series that will take us back to the basics of weld inspection. In this first article, Gordon Smith and Uwe Aschemeier offer a technician's guide to ultrasonic weld inspection according to the requirements of AWS D1.1:2004, Structural Welding Code — Steel. The code is industry's "how to do it" guide for the ultrasonic inspection of prequalified carbon steel welds. Smith and Aschemeier are both working AWS CWIs. Smith is an ACCP NDT Level II and III ­ UT, who was first certified more than 30 years ago. He is also an ASNT NDT Level III in MT, PT, and RT. Aschemeier has been certified in Europe and Canada, and is a CSA Weld Level III inspector. He is also a member of the AWS D1D Subcommittee 4 on Inspection.

The 1994 North Ridge, Calif., earthquake awakened many of us to the need for more effective ultrasonic inspection of welds. California, which had some of the most stringent requirements for welding and weld inspection, was found to be lacking in the face of an earthquake. Test evaluations showed that professed ultrasonic testing experts could not effectively and repeatedly perform UT to the levels required. The concept of GRR, gauge repeatability and reliability, had not been applied to our system of structural ultrasonic weld evaluation. The obvious result was that 30 inspectors produced 30 sets of results.

The recently released FEMA 355 Final Report makes recommendations for UT weld inspectors that require special performance testing and qualification in UT, after being certified as an ASNT NDT Level II — UT. Performance testing is now a requirement in the laws and regulations of many states.

Did you know that a practical examination is also an AWS requirement? Section 6.21 of AWS D1.1, Structural Welding Code — Steel, states: "The qualification of the UT operator shall include a specific and practical examination which shall be based on the requirements of the code. This examination shall require the UT operator to demonstrate the ability to apply the rules of this code in the accurate detection and disposition of discontinuities." Many of you reading this article have not taken a formal test in this area, but you must do so as a starting point for successful inspections to comply with the requirements of AWS D1.1.

While overall our national ability to perform this type of ultrasonic inspection is poor, here and there we find refreshing individuals who have come close to mastering the methodology of AWS ultrasonic testing of welds. However, many do not have the formal tests on file to support their UT activities.

So, what can you do? As it turns out, a lot, but keep in mind you are limited with regard to UT techniques to those in the D1.1 code. Using techniques beyond the practices written in the code requires a procedure from a certified NDT Level III — UT who is familiar with the code and its application of ultrasonic technology. The engineer of record may also elect in general or for specific cases to use alternate acceptance criteria. If so, for code compliance, these criteria must be in writing and should be part of the contract documents. (See D1.1 Section 6.8 on alternate acceptance criteria.)

The Technician's Guide
What follows is an operator's guide to ultrasonic weld inspection according to the requirements of the D1.1 code. All references to sections, tables, and forms are to items located in AWS D1.1. The authors' comments are in italics.

The audit-acceptable ultrasonic weld inspector should do the following:

  • Obtain training, testing, and certification as a NDT Level II — UT with VT of welds or an AWS CWI certification highly recommended.
  • Pass a UT test specific to AWS D1.1 that meets Section 6.21 requirements.
  • Check out and test your equipment. You will find the setup, field, and interval tests listed in Sections 6.22 through 6.26 and beyond. Test and document your instrument, cables, and transducers to ensure you have the required ranges and sensitivities.
  • Find the weld on the part or structure. Identify face "A" and using Table 6.3 find and mark (paint) the weld identity, the X and Y location lines adjacent to the welds — Fig. 1. Note that, according to the code, these are "shall dos." They were required 30 years ago and are still required today.
  • Identify and confirm the AWS prequalified weld type and record this information on a UT report form such as Form D-11, which is found on page 330 of D1.1. Nonprequalified welds and certain combinations of prequalified welds cannot be inspected with UT without specific NDT Level III written instructions.
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    Fig. 1 — A — Close-up of a weld X-Y line; B — weld labeled and lined for inspection.
  • Perform visual testing to Section 6 criteria followed by ultrasonic testing of the base metal. (See Sections 6.20.4, 6.25.4, 6.26.5, and 6.31.1.) Mark any visual defects and lamellar areas on the weld area and record on Form D-11. The UT area should be clean of any surface matter, slag, splatter, etc., that could hinder inspection. It is a code requirement (Section 3, Workmanship) that the welder clean the welds for inspection.

  • While you, the inspector, can fail the welds for this, it is also your responsibility to ensure that the UT area is acceptable for testing (see Section 6.26.3). The authors feel that the contour of the transducer scanning area and the back side are important. Rolling random contours exceeding ±1 deg are not acceptable for ultrasonic weld testing as the sound path will be refracted unpredictably.
    • Identify and select the proper transducer angle(s) using Table 6.3.
    • Use a code-approved couplant. The couplant should be either glycerin or a cellulose gum and water mixture of a suitable consistency. Thicker is better. A wetting agent may be added if needed. Light machine oil may be used for couplant on calibration blocks. Inspection is limited to the range of 27°­125°F without an NDT Level III-approved procedure.

      Now we are ready to begin UT of the identified prequalified weld with ID numbers and X and Y markings.

    • Check and set your calibration for sensitivity and range. Your field calibration block shall be traceable to your company's or outside service master's IIW block (Section 6.23.1).
    • Your range display should show two or three UT legs on the CRT. Figure 2 shows acceptable AWS screen displays for ultrasonic weld testing.












    Fig. 2 Acceptable screen displays for ultrasonic weld testing. A — Soundthickness display for two legs. Weld grooves are drawn on the screen for reference. path display for three legs; B — weld

    • Apply couplant along the weld to be inspected using a brush to prewet large areas. This will increase your ability to remain in good sound contact.
    • If part of a weld is found to be inaccessible to testing in accordance with the requirements of Table 6.6, due to laminar content or adjacent welds, the UT is to be conducted using an approved alternative procedure as necessary to attain full weld coverage.
    • Welds are to be tested using an angle beam search unit conforming to the requirements of 6.22.7 with the instrument calibrated in accordance with 6.25.5 using the angle as shown in Table 6.6 and the scanning motions in Section 6.24. Following calibration and during testing, the only instrument adjustment permitted is the sensitivity level adjustment with the calibrated gain control (attenuator). Note that use of the display zoom function is not considered an instrument adjustment.
    • The reject (clipping or suppression) control is to be turned off for UT inspection.
    • Sensitivity must be increased from the reference level for weld scanning in accordance with Tables 6.2 or 6.3, as applicable.
    • All butt joint welds "shall be tested from each side of the weld axis."
    • Corner and T-joint welds are primarily tested from only one side of the weld axis.
    • All welds are to be tested using the applicable scanning pattern or patterns shown in Fig. 6.24 as necessary to detect both longitudinal and transverse flaws.
    • It is intended that, as a minimum, all welds be tested by passing sound through the "entire volume of the weld and the heat-affected zone in two crossing directions," wherever practical.
    • Discontinuity indications shall be evaluated as specified in Sections 6.26.6.3 through 6.26.6.6. Indication level "a" is the decibel (dB) difference between the reference level "b" and the indication. Reference level "b" can be found in Section 6.25, and the attenuation factor "c" is obtained by subtracting 1 in. (25 mm) from the total sound path. The indication rating "d," from Annex D, Form D-11, represents the algebraic difference in decibels between the indication level and the reference level with correction for attenuation as indicated in the following expressions:
      Instruments with gain in dB: a ­ b ­ c = d Instruments with attenuation in dB: b ­ a ­ c = d

        Alternately, indications exceeding the reference levels from a distance amplitude correction (DAC) curve within written instructions from an ASNT NDT Level III using AWS D1.1 Appendix K are considered rejectable if the flaw length exceeds acceptance levels in accordance with 6.31.2 and 6.26.8.
    • The length of flaws is determined in accordance with procedure 6.31.2. This is often called the 6-dB drop method. An alternative name is the K9.3 length.

       
      The Importance of the 6-dB Drop in Flaw Evaluation

      Fig. 3 — The 6-dB drop. A sound beam is shown traversing a flaw.
      As the UT transducer beam is passed over a flaw, a reflection or UT indication may be produced. With the flaw indication maximized, the reflective area is centered in the UT beam. By moving the UT transducer in either the X or Y direction, the beam will decrease. When the decrease is equal to 6 dB, the beam has moved halfway off of the defect's reflective area. By traversing the defect and measuring 6-dB drop dimensions, the flaw reflector dimensions may be approximated for evaluation.

      Note that the surface of most flaws — with the possible exception of an unwetted braze or weld surface — is not a flat perfect sound reflector and that flaws only return a frequency response in relation to their composition and structure. When the flaw is very small in relation to the scanning sound beam wavelength, the reflector will have no discernable shape in any dimension. When inspecting steel with a 2.25-mHz transducer, it would be hard to give flaws below 0.025-in. diameter a specific dimension. This should be far below the reference that is specified. Figure 3 illustrates the 6-dB drop.

       



      Fig. 4 — A comparison of flaw types and flaw reflectors.
    • Ultrasonic testing indications found to be within ± 1/16 in. (4 mm) of a weld fusion plane are to be evaluated with 45 to 60 or 70-degree transducers to characterize the type of flaw — Fig. 4. Do not forget to do this from the opposing weld inspection face. Your comments would then be recorded in the comments section of Form D-11.

    Basis for Acceptance or Rejection
    Each weld discontinuity is accepted or rejected on the basis of its indication rating and its length, in accordance with Table 6.2 for statically loaded structures or Table 6.3 for cyclically loaded structures, whichever is applicable.

    Only those discontinuities that are rejectable need be recorded on the test report, except for welds designated in the contract documents as being fracture critical. Acceptable ratings within 6 dB, inclusive, of the minimum rejectable rating are recorded on the test report.

    • Identification of Rejected Area. You should indicate each rejectable discontinuity on the weld by placing a mark directly over the entire length of the discontinuity. The depth from the surface and indication rating are then noted on the nearby base metal.

    Welds found unacceptable by ultrasonic testing are to be repaired by the methods permitted in Section 5.26. Repaired areas are then retested ultrasonically with results tabulated on the original form (if available) or additional report forms.

    Preparation and Disposition of Reports
    At the time of inspection, the ultrasonic inspector must complete a report form that clearly identifies the work and the area of inspection. The report form for welds that are acceptable need contain only sufficient information to identify the weld, the inspector (signature), and the acceptability of the weld. As mentioned previously, Form D-11, which is found on page 330 of AWS D1.1, is an example of such a form.

    Before a weld subjected to ultrasonic testing is accepted, all report forms pertaining to the weld, including any that show unacceptable quality prior to repair, are to be submitted to the owner's inspector.

    A full set of completed report forms, including any that show unacceptable quality prior to repair, are to be delivered to the owner upon completion of the work.

    Basics of Indication Acceptability
    For specifics, you should always refer to the AWS D1.1 code and project contract documents and drawings; however, in general (static), acceptability is as follows:

    • "Class A (large discontinuity): Any indication in this category shall be rejected regardless of length.

    • "Class B (medium discontinuity): Any indication in this category having a length greater than 3/4 in. shall be rejected.

    • "Class C (small discontinuity): Any indication in this category having a length greater than 2 in. shall be rejected.

    • "Class D (minor discontinuity): Any indication in this category shall be accepted regardless of length or location in the weld."

    Relationships the
    UT Inspector
    Should Know


    If you know the transducer angle and the plate thickness, you can easily calculate the skip distance.
    For
    • 45 deg, the skip distance = 2 ¥ the thickness = 2 legs
    • 60 deg, the skip distance = 3.5 ¥ the thickness = 2 legs
    • 70 deg, the skip distance = 5.5 ¥ the thickness = 2 legs

    The distance back from the weld centerline is proportional to the depth of the flaw.

    The distance back from the centerline of the weld to the transducer index point is proportional to the indication depth.

    To find flaw depth, divide the distance from the weld centerline by the skip distance factor by 2.

    Example: T = 1 in., angle = 70 deg, SF = 5.5, distance = 1.875 in. Therefore, flaw depth = (1.875/(5.5/2)) = 0.682 in.

    Checklist for Personnel and Equipment for AWS Field UT Inspection
    1. NDT Level II UT (AWS qualified) certified operator.
    2. Instrument meeting code requirements.
    3. Tested cable/transducer assemblies with current documentation of sensitivity and range, and with wedge couplant (not defined in AWS D1.1:2004).
    4. AWS D1.1 code acceptance criteria (static/dynamic).
    5. Written approved engineering UT test requirements.
    6. Written approved instructions for "special" UT inspection conditions.
    7. AWS D1.1 instructions or written practice.
    8. Calibration block traceable to an accepted master IIW block with offset dB value, if any.
    9. Inspection couplant meeting AWS requirements. (Special couplants require NDT Level III approval/written procedure for use.)
    10. Marking pens, ruler, tape measure, paper, reporting forms.
    11. Drawing with weld identification numbers and locations.
    12. Cleanup materials such as rags, soap, paper towels, etc.
    Additional suggested equipment: digital camera and laptop computer.

    Steps for
    Doing the UT inspection
    1. Locate welds to be inspected, complete heading information on report form.
    2. Paint identification on each weld joint.
    3. Paint/mark the long axis and "skip" distances on each weld.
    4. Test the base metal for laminations from the appropriate faces.

  • Note that butt joints are to be tested from two different crossing sides.
  • Mark on the base metal all rejected areas.
  • Sketch reject base metal areas and attach the sketch to the weld inspection report. 5. Select the appropriate angle transducer to begin the inspection.
    6. Perform setup calibrations; record on UT report form.
    7. Perform in-process UT system checks on schedule. Record system checks on UT report form.
    8. Switch to opposite side for welds in butt joints.
    9. Evaluate flaw reflectors with little or no transducer movement using amplitude.
    10. Evaluate flaw reflectors with definite transducer movement using the 6-dB drop method to define the edges of the reflector in both the X and Y dimensions.
    11. Record/evaluate reflectors as applicable on report form.
    12. Mark reflector areas on base/weld metal. Include reflector IDs with each marked reflector.
    13. Evaluate reflectors for location on weld interface.
  • If on the weld interface, reevaluate with other shear wave transducers.
  • Use most critical amplitude measurement angle to define reflector location.
  • Determine the most likely flaw type(s) from reflectors and indicate this on the report. 14. Mark depth of reflector(s) on the base/weld metal; record on report.
    15. Complete the UT report form; submit for review and approval, if required.