 |
|
Volume 4, Number 1
|
|
January 2005 |
Articles one through five of this series
discussed basic equipment setup and calibration techniques commonly used
for contact angle beam testing. Remaining articles in the series are
focused on what is required once the UT operator is given a specific
part to inspect. In this article, full penetration plate butt welds and
corner and T joint welds will be used as examples. Ultrasonic testing of
nonwelded parts (castings, forgings and so on) is done in a similar
manner, though the location reference points vary depending on the
configuration of the part being inspected.
Determining
Accessibility
The first step in setting up onsite for an
inspection is to determine accessibility to the area of interest for the
weld to be inspected. In some cases, the governing code or specification
might also require ultrasonic inspection of the base material for a
short distance beyond the toe of the weld. This requirement should be
taken into consideration when selecting the wedge angle and determining
the length of the scanning area.
Common Weld
Configurations
Figure 1 shows three common weld configurations;
butt welds in a plate, a corner joint made by two plate
ends set at 90 degree angles to each other and a T joint between
two plates (typically seen in beam-column connections). The surfaces
adjacent to the weld (or in the case of corner and T welds, the face
directly behind) are the surfaces from which the weld can be ultrasonically
scanned. As shown in all three figures, these surfaces are commonly
labeled Faces A, B and C as applicable.
The two weld sketches in Fig. 1a show plates with
two weld configurations, a single-V butt weld at the top and
a double-V butt weld at the bottom. While the scanning surfaces
for both are labeled the same, the location of the root of the
weld is different and will affect the location of certain types of discontinuities.
Face A is on the plate side with the weld crown for a single-V weld
with Face B on the root side. In the case of a double-V weld, Face A
is usually the side of the weld most easily accessible.
Figure 1b shows a typical corner weld with the Face
A scanning surface on the weld crown side of the beveled plate, Face
B on the opposite side of the same plate and Face C directly opposite
the beveled plate on the reverse side of the other plate. These face
designations are the same for a typical T weld as shown in Fig. 1c.
Scanning from
Face C is done with a straight beam transducer and
is used primarily to detect laminar tears in the vertical member behind
the weld or sidewall lack of fusion at the vertical face of the weld
groove as shown. In all inspections, the face from which the weld is
being inspected should be recorded on the inspection report form.
Determining the Scan
Surface
Once the scanning face has been selected, it is
necessary to determine how much of that surface is required to perform a
full coverage inspection of the area of interest. This is determined by
material thickness and choice of the wedge angle if that choice is left
up to the operator and is not specified by code or specification. In
order to attain full volumetric coverage, it is necessary to ensure that
the weld area is scanned by both the first and second legs of the sound
beam. If an additional portion of the base metal requires inspection,
that distance must be added to the scanning area also.
Minimum Scanning Surface.
Figure 2 shows the minimum scanning surface required on each side of
a plate butt weld to provide full coverage of the weld volume. In the
figure, Face A is the scanning surface and the weld is being scanned
from both sides. Notice that at position A1, where the nose of the transducer
hits the edge of the weld crown, the sound beam only covers a small
portion of the bottom of the weld. At position B1, from the other side
of the weld, only an additional small volume that was not covered by
A1 is scanned. However, as the transducer is moved away from the weld,
toward positions A2 and B2 respectively, the sound beam will interrogate
the entire cross-section of the weld, resulting in full coverage of
the weld volume. If the governing documents require that a portion of
the base metal also be interrogated, then that distance must also be
included when the scanning surface is prepared. Operators should keep
in mind that a 70-degree probe would require a much longer scan path
than a 45-degree probe. However, a 45-degree sound beam may be too short
to get the second leg completely through the weld cross-section before
the nose of the probe hits the edge of the weld crown.
Surface Preparation.
The portion of the plate to be used as the scanning surface must be
free of any loose scale, rust or other foreign material that might cause
bad coupling and thereby prevent sound from entering the part. If weld
spatter is present, it may be necessary to use a cold chisel to remove
it. A 3 in. wide chisel is wide enough to clean a large area quickly
and is a good tool for this purpose. If used properly, it will not gouge
the plate surface. Once the scanning surfaces have been prepared, couplant
can be applied and the weld scanned.
Discontinuity
Orientation
In order to detect both transverse and axially
oriented discontinuities, the weld must be scanned in two directions
from both sides of the weld.
Axial Discontinuities.
For axial discontinuities (those that run parallel to the weld
length) the transducer is aimed at the weld and moved back and forth
as shown on side A of the plate surface in Fig. 3.
Transverse Discontinuities.
For transversely oriented discontinuities (those that run across
the width of the weld) the transducer should slide along the side of
the weld in both directions as shown on side B of the surface in Fig.
3. Both types of scan are performed on both sides A and B of the surface
to ensure that all orientations of discontinuities are found.

Manipulating the Probe
Overlap. As the probe
is moved back and forth, the sound beam is required to overlap the previous
scan by some percentage. The scan pattern shown in Fig. 4 illustrates
a scan pattern with a 50 percent overlap. If the amount of overlap is
not detailed in governing documents, a 20 to 50 percent overlap is usually
used. The easiest way to estimate the percentage of overlap is to look
at the tracks the transducer makes in the couplant on the surface of
the plate.
Oscillating the Probe. In
addition to being moved back and forth, the probe should also be oscillated
from side to side to detect discontinuities that may not be oriented
at exactly 90 degrees to the sound beam. If no oscillation requirement
is specified in the governing code or specification, a range of approximately
15 to 20 degrees can be used (see Fig. 4).
Difficulties of Probe Manipulation.
At first glance, these manipulations may appear
relatively straightforward and not a complex task. One might assume
that all that is required is to spread the couplant and then slide the
transducer back and forth far enough to ensure full coverage by the
sound beam while wiggling the probe back and forth sideways. However,
the combination of movements is more difficult than first perceived
and, in addition, probe manipulation must be done while the operator
is watching the CRT screen and not the probe. The process is further
complicated by the fact that operators routinely use probes that vary
from 0.25 in. to 1.0 in. or greater in diameter. All of these factors
make it difficult to maintain a good scanning pattern with consistent
overlap. It’s helpful to remember that proficiency improves with practice.
Scanning Rate.
Most codes or standards require that the scanning rate not exceed a
maximum travel speed, typically not more than 6.0 in. per second. The
actual scan speed will either be detailed in the governing specification
or in one of the specification reference documents. This restricted scan
rate ensures that any reflected sound has sufficient time to return to
the transducer before the transducer moves on.
Reporting
Discontinuity Locations
Using an XY Coordinate System.
The final step before beginning an inspection is to select a set of
reference points so that any discontinuities can be accurately located.
This is most commonly done with an XY coordinate system, where the X-axis
represents the distance across the weld and the Y-axis represents the
distance along the length of the weld. Figure 5 shows typical locations
for these points on different types of welds.
Locating the Zero Point. As
shown in Fig. 5a, the zero point for X on a plate weld is the centerline
of the weld and the zero point for Y is the left end of the weld. For
corner and T welds, the zero point for X is at the back of the weld
(Fig. 5b). For horizontal runs in pipe welds, it is common to place
the Y reference point (Y = 0) at the top of the pipe (Fig. 5c) and in
line with an elbow or fitting on vertical runs. The point at which X
is equal to zero is again the centerline of the weld. Note that some
codes and specifications do have specific conventions for these locations
so they should be checked prior to setting your own locating marks.
If no specific location instructions are given, the operator should
select one set of conventions and record them on the report form. It
is also smart to mark the point at which Y equals zero on the weld.
Then, if repairs are needed, the welder will know the point to measure
from.
The distance of a discontinuity from Y = 0 is
fairly obvious; the distance is measured from the left end of the weld
to the nearest end of a discontinuity. Therefore, a discontinuity that
starts at 6.0 in. from the left end of the weld and ends 8.0 in. from
the left end would be listed as a 2.0 in. indication at Y = 6.0 in.
Locating a defect on the X-axis requires a little
more thought. Centerline indications are easy because they are at X
= 0, as shown by indication 1 on the shaded weld in Fig. 6. Indication
2 is 0.5 in. from the centerline away from the inspector and would be
recorded as being at X = +0.5 in. Because indications 3 and 4 are both
on the operator side of X = zero, they would be recorded at X = –0.5
in. and –0.25 in., respectively. There is one note of caution when scanning
the weld from the other side; the operator should make sure that the
±X convention is not accidentally reversed.
Sizing the
Discontinuity
Six Decibel Drop.
When sizing a discontinuity for length, the operator should refer to the
governing documents for direction. However, if a specific method of
determining the end point is not given, it is common practice to use the
six decibel drop method for making that determination. If used,
note of its use should be recorded. The six decibel drop method refers
to the fact that a 6 decibel (dB) decrease in gain results in a 50
percent decrease in screen amplitude. The operator manipulates the
transducer to maximize the signal from an indication. When this is
achieved, the maximized signal is then set to 80 percent full screen
height (FSH) and the transducer is moved slowly along parallel to the
axis of the defect until the screen signal drops to 40 percent FSH, or
50 percent of the maximized signal (a 6 dB drop). A mark is then made at
the centerline of the transducer to denote that end of the
discontinuity. The transducer is then moved back toward the other end of
the discontinuity until the signal peaks at 80 percent FSH and again
drops to 40 percent at the other end of the indication. This end is
marked as before using the centerline of the transducer. The distance
between the two marks is considered the length of the discontinuity.
Irregularly Shaped Discontinuities.
Note that since many discontinuities are irregular
in shape, the operator should continue the sizing scan past the point
at which a 50 percent amplitude drop is first seen. It is possible that
a discontinuity will have a varying orientation that will drop off in
amplitude at one point then increase in amplitude again farther away
from the center of the discontinuity. If this condition occurs and the
scan is stopped when the signal first drops to 50 percent, the recorded
length will not reflect the actual discontinuity length. If the acceptance-rejection
criteria being used is based on a combination of length and decibel
rating, miscalculation of the length could have serious results.
Discontinuity Depth
The depth of a discontinuity can be calculated
using a trigonometric function based on the sound path or surface
distance and the inspection angle. This can be read directly from a
graphic UT calculator. On newer machines, depth calculation is an
integral part of the programming and can be done by simply pushing a
button. Regardless of how depth is calculated, the operator should bear
in mind that depth calculation is based on the nominal wedge angle (45,
60 or 70 degrees). In addition, most codes and specifications allow the
transducer to vary within a range of ±2 degrees. As a result, a three
decimal place depth number may not be as accurate as the operator might
think. It should also be noted that the welder that must dig out the
defect identified by the UT operator will not be able to remove the weld
in 0.001 in. increments when using a 0.375 in. diameter carbon air arc
rod or 36 grit grinding wheel.
It has been the author's habit to report the
defect depth to the next lower full 0.0625 (1/16) of an inch. Reporting
a defect in this manner is done for two reasons. Repeated handling of
the transducer results in more wear at the nose of the wedge face, thus
causing the angle to decrease over time. In addition, digging a little
deeper the first time ensures removal of the defect and is quicker and
less expensive than if a second attempt is required. 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>.
Copyright © 2012 by the American Society for Nondestructive Testing, Inc. ASNT is not responsible for the authenticity or accuracy of information herein. Published opinions and statements do not necessarily reflect the opinion of ASNT. Products or services that are advertised or mentioned do not carry the endorsement or recommendation of ASNT.
IRRSP, NDT Handbook, The NDT Technician and www.asnt.org are trademarks of the American Society for Nondestructive Testing, Inc. ACCP, ASNT, Level III Study Guide, Materials Evaluation, Nondestructive Testing Handbook, Research in Nondestructive Evaluation and RNDE are registered trademarks of the American Society for Nondestructive Testing, Inc. ASNT exists to create a safer world by promoting the profession and technologies of nondestructive testing.