Volume 3, Number 4
October 2004
Article four of this series, IIW Based Angle Beam Calibration, discussed angle beam calibration using the IIW block and some of its derivatives. With the IIW block, the reference reflector is a single reflector that results in a single screen trace, or if the sound path is long enough, in a second trace farther down the time line. Most codes and specifications that specify IIW calibration rely on a simple formula to account for attenuation or loss of sound that occurs as the sound beam travels through the part being inspected.An alternative method of angle beam UT calibration for welds uses a calibration block with three side-drilled holes called a basic calibration block. All three holes are of the same diameter and are used to determine the amount of sound energy returned from a reflector of the same size at different sound paths. Basic calibration blocks are referenced in Article 5 of Section V: Ultrasonic Examination Methods for Materials and Fabrication of the ASME Boiler and Pressure Vessel Code and thus are often referred to as ASME Cal Blocks. The block discussed in this article is shown in Fig. 542.2.1 in Article 5 (1999 Addenda) of this code. A simplified version is recreated here in Fig. 1.
The Basic Calibration Block
The thickness of the weld being inspected determines the size of the calibration block to be used. One of the benefits of using the basic calibration block is that Section V of the ASME Boiler and Pressure Vessel Code permits using a block of a particular thickness to cover a range of weld thicknesses. For example, a 3/4 in. thick block can be used for welds with a thickness of 1 in. or less; a 1 1/2 in. thick block can be used for welds in the 1 to 2 in. range; a 3 in. thick block can be used for welds in the 2 to 4 in. range, and so on. In all cases, a block the same thickness as the weld being inspected can be used. The major advantage of blocks that cover a range of weld thicknesses is that an inspection agency can manufacture two or three blocks that will generally cover the full range of weld thicknesses that will normally be inspected.
As shown in Fig. 1, most blocks have a standard 6 in. width and a minimum length of 3t, where t is the block thickness. When determining block length, the fabricator should consider which transducer wedge angles will be used for the applicable range of weld thicknesses. Since a 70 degree wedge angle utilizes the longest sound path, a common choice is a block length slightly longer than the distance required to accommodate a full skip distance for a 70 degree probe. A block this length can be used for 45 and 60 degree transducers as well. Weight can become a consideration on thicker blocks. For example, a 5 in. thick carbon steel cal block that is 6 in. wide and 15 in. (3t) long will weigh about 150 pounds, making it awkward to handle and dangerous if dropped. It is not uncommon to see a handle welded on one end of blocks of this size to facilitate lifting with a crane.
The three side-drilled holes are located at three distances (1/4t, 1/2t and 3/4t) from the scanning surface. Diameters for these side-drilled holes are described in a table (refer to Fig. 542.2.1 of Section V) for each calibration block thickness. As the blocks increase in thickness, hole diameter also increases creating a larger reflector that results in lower sensitivity. Thus, in thicker blocks, a discontinuity in a thin weld that might be rejectable may not be rejectable in a thicker weld.
Nominal hole sizes for the various block thicknesses are: a 3/32 in. diameter hole for a 3/4 in. block, a 1/8 in. diameter hole for a 1 1/2 in. block and a 3/16 in. diameter hole for a 3 in. block. As block thickness increases in 2 in. increments, hole diameter increases by 1/16 in. Hole diameter tolerance is ± 1/32 in. for all blocks. Minimum hole depth is 1 1/2 in.
Another benefit of basic calibration blocks is that their fabrication is relatively simple and can be done in-house using a band saw, a drill press and several different sized drill bits and reams. A piece of acoustically similar steel of the correct thickness that has been properly heat-treated (refer to Article 5 of the ASME Boiler and Pressure Vessel Code) should be scanned using a straight beam transducer to determine that the piece is free of internal defects and laminations. It can then be cut to size using either a vertical or horizontal band saw.
Once the block is sized and soundness is confirmed, holes can be drilled using a bit with the proper diameter in a drill press. The holes must then be reamed to ensure a smooth reflecting surface for the sound beam. Care should be taken when choosing the size of the drill bit. Most reamers remove 0.002-0.003 in. of material from each side of the hole thus requiring that a slightly smaller bit be used.
At this point of fabrication, optional notches in the top and bottom surfaces of the block can be machined into the block if needed (refer to Fig. T-542.2.1, Section V, Article 5 of the ASME Boiler and Pressure Vessel Code for notch dimensions and positioning). The block is now ready for production calibrations.
The DAC Curve
A distance amplitude correction (DAC) curve is used to determine attenuation or degree of sound loss that occurs as the ultrasonic sound path increases in length. A reflector of known size close to the transducer (short sound path) returns more sound to the transducer than a reflector of the same size that is farther away (long sound path). This is due in part to the fact that a sound beam spreads as it travels away from the transducer, much like the cone of light from a flashlight. Some attenuation or loss of sound power also occurs as the sound beam travels through the material of the test object. Any change in the wedge, transducer or coaxial cable can affect the amount of sound generated and would therefore require a new DAC curve to be generated.
Loss of sound power as sound path length increases is not linear. Use of a simple formula such as that used with the IIW block will not accurately show the amount of sound returning to the transducer. The concept of using three side-drilled holes of the same diameter at different sound paths was developed to compensate for any distance error in amplitude that appears on the monitor.
Calibration Based On Hole Depth
Two techniques are commonly used in calibrating the basic calibration block. The first uses the actual sound path to each hole to establish screen width. The second uses the hole depth or distance from each hole to the scanning surface. The hole depth technique is the more simple of the two calibration methods. Figure 2 shows correct positioning of the transducer to locate the three holes using the first leg of the sound beam. Couplant is first applied to the surface of the block and the screen trace representing the nearest hole (at the 1/4t depth) is maximized. The left side of the trace is positioned above the first major graticule on the screen baseline. Amplitude or screen height of the trace is set at 80 percent of full screen height or FSH. The gain setting for this amplitude is recorded as the reference level. The top of the trace is marked on the monitor with a marker. (This can be marked electronically in newer equipment. See user’s manual).
Next, with no change to gain setting, move the transducer backwards on the scanning surface until the trace from the 1/4t hole (or 7/4t) in the second leg is maximized. Position the screen trace on the baseline at the seventh major graticule. Then, switching back and forth between the first and second leg signals, use the range and delay controls to set the two traces over the proper graticules. (Controls for range and delay may vary on newer equipment. Consult user’s manual.) When this has been done, the maximized screen traces from the 1/2t (2/4t) and 3/4t holes should show above the second and third graticules for the first leg and above the sixth and fifth graticules for the second Leg. When locating the 3/4t hole, do not mistake the signal from the corner of the block, which may show up as a larger trace just to the right of the hole trace. The operator should then go back to each hole location, maximize the signal on the screen, mark the top of each respective trace with the grease pen and finally connect the dots in as smooth a line as possible. This line becomes the DAC curve for that block and equipment (Fig. 3). Note both ends of the curve have been extended to include both the first and eighth graticules. In some cases, when the 1/4t hole trace is set at 80 percent FSH, it may not be possible to see the traces at the sixth and/or seventh graticules. If this occurs, set the trace from the 1/2t hole to 80 percent FSH as shown in Fig. 3 and use that gain setting as the reference level. Be sure to report that the 1/2t hole was used to set the reference level.
The technique may seem confusing but Fig. 4 can be of help. The full thickness of a block is 1t, which can also be written as 4/4t. The three holes, if kept in 1/4t units, would be at 1/4t, 2/4t and 3/4t depths. Full thickness or 4/4t would be the bottom surface of the block. In Fig. 4, a regular block is shown in solid lines with the standard three holes. Directly below is a reverse or mirror image of the block that shows where those holes would be if the block were twice as thick. Total thickness is now twice that of the normal block and is represented as 2t or, if expressed in 1/4t increments, 8/4t.
The second leg sound paths in Fig. 4 are also shown as if they continue straight on into the mirror block instead of reflecting back from the bottom surface. In this way, the second leg signal for the 3/4t hole appears to be at the 5/4t depth in the mirror block. Similarly, the 1/2t (2/4t) hole appears at 6/4t and the 1/4t hole appears at the 7/4t position. A full skip distance, two thicknesses, would be at the bottom of the mirror block or at the top of the regular block.
Now consider the screen positions of the six traces that make up the DAC curve. In the first leg, the 1/4t trace is at graticule 1, the 2/4t trace is at graticule 2, the 3/4t trace is at graticule 3 and there is no trace at graticule 4 (the bottom of the block). In the second leg, the 5/4t trace is at graticule 5, the 6/4t trace is at graticule 6, the 7/4t trace is at graticule 7 and there is no trace at graticule 8 (top of the actual block or bottom of the mirror block). These 1/4t designators are shown below the baseline in Fig. 3 and to the right of Fig. 4. There are several codes that refer to the second leg points on a DAC curve as the 5/4, 6/4 and 7/4 locations.
Calibration Using Sound Path
When calibrating using the sound path method, couplant is again applied to the surface of the block and the signal returning from the nearest hole (at the 1/4t depth) is maximized. Then the operator applies the trigonometric formula:
sin θ = opposite side ÷ hypotenuse
where θ is the the complementary angle of the wedge angle, opposite side is the distance from the scanning surface to the center of the hole, and the hypotenuse is the sound path. This is shown graphically in Fig. 5. Remember that the wedge angle is the angle formed by a line normal (90 degrees) to the scanning surface. For a 70 degree wedge, θ is 20 degrees and for a 60 degree wedge, θ is 30 degrees. With the 45 degree wedge, both wedge and complementary angles are 45 degrees.
Once the sound path has been calculated, the left side of the screen trace for that hole is placed on the screen at the distance that represents that sound path. The amplitude (screen height) of the trace is set at 80 percent FSH. The gain setting for this amplitude is recorded as the reference level. Then, with no changes to the gain setting, the same operation is performed on the 1/2t hole, maximizing the reflector, calculating the sound path and locating the trace at the proper location on the monitor. In most cases, it will be necessary to go back and forth between the 1/4t and 1/2t holes to properly align both screen traces in their proper screen locations using the range and delay controls. The 3/4t hole trace and the second leg reflectors from all three holes are then set up on the screen in a similar manner. This sets the screen width for the holes in the block being used.
As with hole depth calibration, the operator may find that the screen traces for the last two positions, 1/2t and 1/4t holes in the second leg of the sound path, may not be visible at the recorded reference level. This usually means that the 1/4t hole in the first leg was in the near field and cannot be used to develop the reference level. When this occurs, it is necessary to go back to the 1/2t hole (first leg), set that screen trace to 80 percent FSH and use that gain setting as the reference level. If the 1/2t and 1/4t holes in the second leg can be seen on the screen, calibration can be completed. Note that when the reference level is changed, sound path locations of all holes should be rechecked to ensure the sound path is unchanged. If the last two hole traces are still not visible, it may be necessary to change transducer sizes and/or frequencies to reduce the near field to allow proper calibration.
Once traces from the six hole locations (three in the first leg and three in the second) are all properly located, the operator should go back to each hole, maximize the screen trace and mark the top of the trace on the surface of the monitor. This is repeated for all six traces and the six points are then connected to form a curve on the screen that represents the DAC curve for that block and equipment being used.
Using a DAC Curve
Scanning is performed at a gain setting higher than the reference level. This level is typically dictated by a governing code or specification. When a screen trace is seen, the gain setting is set back to the reference level and the signal is maximized. In most codes and specifications, any discontinuity that creates a screen trace at a reference level that exceeds the distance amplitude correction curve is rejectable. If the maximized signal is below DAC but is greater than 50 percent of DAC height, the indication is usually recorded and any signal greater than 20 percent of DAC should be interrogated. These are general rules, and the operator should refer to the governing code or specification to determine the actual requirements. TNT
*Jim Houf is Senior Manager of ASNT’s Technical Services Department and administers all ASNT certification programs. (800) 222-2768 X212, (614) 274-6899 fax, <jhouf@asnt.org>.
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