Ultrasonic Testing of Welds
Updated: Feb 27, 2020
It is not uncommon for a nondestructive testing ultrasonic technician to inspect structural welds according to the American Welding Society D1.1: Structural Welding Code – Steel (AWSD1.1), at least a few times in his or her career, and some inspectors use it daily. Completing a professional AWSD1.1 inspection, however, without the proper code specific training can be daunting. This article is designed to give the reader an introduction to the 2008 edition of Ultrasonic inspection per AWSD1.1 inspection procedure. The reader needs to be aware that some important (but routine) steps such as equipment qualifications have not been addressed here to allow the article to fit in available space. It is assumed that the reader has accomplished those tasks before proceeding on to transducer and wedge selection.
Transducer and Wedge Selection
AWSD1.1 recommends the weld metal be inspected using a 45o, 60o, or 70o degree shear wave in steel. The frequency range required by the code is 2-2.25 MHz. The active element inside the transducer must be square or rectangular with width in the 5/8” to 1” inch range and 5/8” to 13/16” in height. Major ultrasonic transducer suppliers sell complete AWS base and weld metal transducer kits that comply with the codes.
The next step is to determine the proper wedge for the weld inspection. The testing angle is selected using Table 6.7 in AWS D1.1 Chapter 6. The selection process requires knowledge of the weld type and plate thickness. Part of Table 6.7 is shown in Figure 1 to illustrate the wedge selection procedure. Supposing the inspection was performed on a 1.625” thick T-weld, the table shows the inspector “1” and “F” or “XF” in the Material Thickness column. The 1 is the shear wave angle from the table’s legend. In this case a 70 deg. shear wave is required for inspection of the top quarter, middle half, and bottom quarter of the weld. The “F” specifies the weld – base metal transition zone must be inspected further using a 45 deg., 60 deg., or 70 deg. degree wedge, whichever generates a shear wave closest perpendicular to the weld fusion line. The “X” specifies that inspection is required from Face C, which is defined in the legend of AWSD1.1 Table 6.7 for butt, corner, and T-joints.
Before the inspection of the welds could begin, there are a few simple steps that need to be taken to identify the area to be inspected using ultrasonic inspection and accurately report the location of any detected flaws.
Marking Skip Distance: The skip distance defines the zone of inspection. It is provided by a simple formula: Skip Distance = 2*T*tanθ, where T is the thickness of the part inspected and θ is the refracted angle of the shear wave in steel (45o, 60o, or 70o). Measure the thickness of the part that is to be inspected. Using a calculator, the skip distance can be determined easily. Mark the end of the skip distance from the edge of the heat affected zone (HAZ). The scanning distance is not measured from the weld centerline but rather from the edge of the HAZ. Identifying the skip distance from the edge of the HAZ ensures complete coverage of the weld when performing the angle beam ultrasonic inspection.Reference System: The reference system for flaw location must be setup before scanning with the calibrated unit so that the location of flaws may be reported accurately. First, the inspector should mark out the X- axis, which is the centerline of the weld as shown in Figure 2. Next, an edge or another well-defined reference point should be selected as the Y-axis. The inspector must select the +Y and –Y directions as well. In the example shown, +Y and –Y are to the left and right of the weld centerline, respectively.
Once the appropriate transducer for the inspection has been chosen, the technician will see AWSD1.1 calibration is relatively straight forward using the IIW block. Screen Range: It is important that appropriate screen range is selected on the UT flaw detector display. It is chosen in such a way that the minimum sound path of two legs (or the V-path) can be seen on the screen. The sound path of two legs can be calculated from the formula: Sound Path = 2*T/cosθ.Range Calibration: Couple the chosen transducer (and wedge) to the IIW block. The IIW block has two radii of 2” and 4”. With the chosen screen range, the reflections from the different radii of the IIW block should be at 2” and 4” on the flaw detector screen display.Beam Index Point (BIP) Verification: This is performed to ensure the ultrasonic beam from the transducer exits the wedge at a point where it is designed to be. Place the wedge-transducer on the index point on the IIW block. Peak the signal on the UT flaw detector display by moving the transducer back and forth, about the index point and verify that the BIP (also known as exit point) on the wedge matches that of the IIW block. Note: Beam Index point verification and screen range calibration may be performed at the same time.Refraction Angle Verification: The angle of the wedge transducer needs to be verified within +/- 2 degrees. Maximizing the reflected signal from the IIW block’s 2” diameter Plexiglas reflector helps in performing this step. Place the transducer at the angle it is supposed to send the refracted shear waves (say, 70 o). Slide the transducer back and forth about this angle marking until the reflected signal is maximized. Sensitivity Calibration: This calibration step is achieved by observing the reflected signals from the 0.060” SDH in the IIW block. The reflected signal is maximized and the amplitude is then adjusted to 50%-75% of the full screen height. The location of the 0.060” SDH, relative to the BIP, should always be double checked during this step using the trigonometry functions of the flaw detector.
The code specifies the amplitude of the known reflector should be adjusted to 50 to 75% FSH using the gain control. The % FSH should be noted and the reference gain should be entered as the Reference Level in Column b of the reporting template shown in Figure 3. During the scanning process, the gain of the calibrated unit will be increased by 14 to 39 dB above the reference for compressively loaded structures based on the length of the sound path distance. The scanning level dB is set based on the chart provided in the AWS D1.1 code.
When a reflector is detected in the weld metal or HAZ, the inspector must maximize the reflection through small rotations and translations of the probe. The reflector must then be located relative to the X- and Y-axes. Starting with the distance from the weld centerline, the inspector will record the surface distance from the BIP to the flaw using the instrument’s trigonometry (TRIG) functions. Then the distance from the BIP to the weld centerline will be measured and recorded. The actual distance between the flaw and the weld centerline will be the latter minus the former. The sound path in the Figure 2 example is 1.75” and corresponds to a surface distance of 1.64”. In this example, the distance from the BIP to the X-axis is approximately 1.4”. Therefore, the “Distance From X” is entered as -0.25” since the flaw was determined to be on the –Y side of the weld. Sizing the flaw is performed using the standard 6 dB sizing method. The inspector must maximize the signal from the flaw and move the transducer until there is a 6 dB drop and mark it, and move the transducer in the opposite direction, until the flaw echo drops 6 dB, or 50 %, again, and mark the position. In Figure 3, the first 6 dB point is at 6” and this is recorded in the “Distance From Y” column in the inspection report. The second 6 dB point determines the flaw length and is entered as “Length” column. In this example it is 1”.
The severity of the flaw must also be determined and reported. Recall that during the sensitivity calibration, the inspector set the SDH reflection at 50 to 75% FSH and recorded the reference gain which in this example is 48 dB. Starting with the maximized reflection from the weld reflector, the inspector must adjust the gain until the reflector is at the same % FSH as the SDH reflection during the calibration step. This is recorded in the “Indication Level” column in the report and is 42 dB for this example. Upon completion of this step, the inspector is ready to move ahead with the reporting part of the inspection for this weld reflector.
Each company usually has its own AWSD1.1 template but they all closely resemble the model presented in the code. Starting from left-to-right in Figure 2, the first column is simply the line number. The numbers in this column are typically included with the template and do not need to be filled in. In the second column, 1 is entered since the reflector in this example was the first one detected during the inspection. The Transducer Angle in column 3 is the shear wave angle used. Enter the face from which the inspection was made in the fourth column, From Face. In the fifth column, enter the Leg in which the flaw was detected. In this example, the flaw was in the first skip and is therefore in Leg 1. The Indication and Reference Levels discussed above are entered in to Columns a and b, respectively. The Attenuation Factor in Column c is calculated by subtracting 1”inch from the sound path and the remainder is multiplied by 2. The result is rounded to the nearest dB value. For this example, 1” is subtracted from the 1.75” sound path. Now,0.75 inch is multiplied by 2 to give 1.5 inch and then rounded to 2. This number is entered in to Column c. The Indication Rating in Column d is calculated by the following simple formula d = a – b – c. In the example calculation, the answer would be -8 dB (column d). The flaw length, sound path, and depth are entered into the next three columns. The distance from X and Y are entered in their respective columns.
Acceptance/Rejection is determined by cross-referencing the Indication Rating with Table 6.2 and the respective legends. As shown in Figure 4, this value is used to determine the Discontinuity Severity Class in Table 6.2. Since the indication is -8 dB, the Discontinuity Severity Class is Class A. The Legend states any indication in this category shall be rejected regardless of length. Therefore this weld is rejected based on this indication rating and Reject is entered in the Discontinuity evaluation column. Finally, the inspector may enter remarks if deemed necessary in the last column on the right.
Summary and Remarks
This technical note provides a general overview of AWSD1.1 procedures to ultrasonic weld inspection. The selection of transducer, calibration procedure, inspection layout, inspection and reporting steps of the weld inspection procedure has been discussed. It is important for the reader to notice and acknowledge that certain routine but important steps such as equipment qualification have not been discussed to limit the article’s length.
AWS D1.1 is different from most major UT specifications in that, it does not use a direct comparison with a reference reflector but relies on reference to various tables and calculations. The discussion on origins and evolution of these tables and formulae are beyond the scope of this article.
Technicians would be well advised to realize that the information provided in the reports must be accurate and be subject to intense scrutiny such as that by a lawyer in a liability trial.
ANSI/AWS D1.1/D1.1M: 2008, Structural Welding Code - Steel, Second Printing, Edition: 21st, American Welding Society, 02-Jul-2008, Chapter 6.