Chapter 18 Weld Inspection and Repair
271
of the ability of short wavelength radiations, such as
X-rays or gamma rays, to penetrate material that is
opaque to ordinary light.
X-rays are a form of electromagnetic radiation
that penetrates most materials. An X-ray test is similar
to a photograph. A machine in a fi xed location trans-
mits X-rays through the material being tested. A fi lm
or sensor on the other side of the material is exposed
by the X-rays that pass through the test material.
Any defects or inconsistencies in the metal change the
amount of X-rays that are able to pass through. Because
more or fewer X-rays pass through those locations, they
look different in the developed fi lm, or display.
The area surrounding the X-ray machine may
be lead-shielded to prevent the escape of radioac-
tivity. An X-ray testing station usually includes all
of the support equipment, such as fi lm-developing
machines. The end result is a radiograph made in a
minimum amount of time. Recently, more portable
X-ray equipment has been developed and is becoming
very common in the pipeline industry. This equip-
ment consists of a small machine that is sent down the
center of the pipe to X-ray each weld.
Gamma rays are electromagnetic waves that
are similar to X-rays, but with a shorter wavelength.
Gamma rays are produced from radioactive mate-
rials such as cobalt, cesium, iridium, and radium.
These radioactive materials must be contained in a
lead-shielded box and transported to the job site for
in-place radiographs.
The fi lm that is exposed by these rays is called a
radiograph. Film is placed on one side of the weld, and
the radiation source is placed on the other side of the
weld. The radiation passes through the test material
and exposes the fi lm, revealing any inconsistencies in
the weld. Different types of radiation sources are more
or less powerful. The thickness of the material usually
determines the type of radiation source used for the test.
A radiograph inspection of a fusion weld is shown
in Figure 18-10. The fi lm is developed for viewing on
a special viewer. The radiograph must be compared
by a skilled technician to a specifi cation that defi nes
discontinuities. The fi lm must have sharp contrast
for proper defi nition of the weld and identifi cation of
any defects. (Contrast is the degree of blackness of the
darker areas compared with the degree of lightness of
the brighter areas.)
To ensure sharp images on the fi lm, image
quality indicators (IQI), also called penetrameters,
are used to indicate the quality of the radiograph. A
hole-type penetrameter consists of a thin shim of the
base metal, usually with a thickness equal to 2% of the
weld thickness. One, two, or more holes with various
diameters are drilled into the metal shim. The shim is
laid next to the weld before being x-rayed. The ability
of the radiograph to show defi nite-sized holes in the
penetrameter establishes the radiograph quality. The
resolution of the X-ray is indicated by the smallest hole
that is visible.
Another type of IQI is a wire type. A wire IQI is
a series of wires embedded in plastic. The wires have
decreasing diameters. The quality of the radiograph is
determined by the thinnest diameter wire that can be
seen on the image. Figure 18-11 shows a wire-type IQI.
Radiographs are expensive; however, they
provide a permanent record of the weld quality. It is
often useful to compare a radiograph created when the
weld was fi rst made with later radiographs made after
Gamma Ray
Film
Film Intensifying
screen
Lead
X-ray
Figure 18-10. Radiographic test operation.
Wire set
Figure 18-11. This radiograph used a wire-type IQI to
monitor clarity in the development process.
(VJ Technologies)