TPT May 2016

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50 May 2016
T E CHNOLOG Y
Monitoring tube production
to find scarfing defects
By Cameron Serles, Xiris Automation Inc, Canada
RECENT advancements in camera and
optics technology have come together
to make a new type of inspection device
able to see defects related to the forming
and welding area of a tube or pipe.
The new type of inspection device is a
laser-based triangulation system that
measures the outside contour of a tube
or pipe in the vicinity of its weld. The
result is improved quality assurance and
process control on the production line.
scarfing is a process of removing
excess weld bead on a pipe or tube to
create a desired shape. It is done by
longitudinally planing or grinding the
welded tube or pipe right after the welding
process. If it is performed perfectly, the
resulting profile will match the ideal
shape of the parent material. However,
if the scarf tool is set to plane or grind
too much material, or not enough, the
resulting profile could appear too deep
or leave a weld bead above the parent
material.
In Figure 1, the scarf tool has not cut
deep enough into the weld bead, creating
a flat surface along the top of the weld
bead, but narrower than the bead width.
In Figure 2, the scarf tool has cut too
deeply, removing some of the parent
material as well as the top of the weld
bead, producing a much wider scarf
width.
Typically, NDT systems are used to
detect defects on a tube and pipe mill.
However, some defects, such as those
caused by scarfing, cannot be easily
detected. In contrast, a laser inspection
system can scan the surface of the tube
near the weld zone. This lets operators
know how well their scarfing tool has
been set to match their process, allowing
them to perform corrective action before
significant scrap occurs. this ability is
especially helpful for one of the most
common defects found across all types
of tube and pipe manufacturing: scarf
defects.
Detecting if the scarf tool is properly
adjusted is a difficult task to do on a
continuous basis. In particular, some
thick walled tube and pipe mills make
precision end products with multiple
scarfing tools that operate sequentially
to make successive cuts to the weld
bead down to the parent material. If not
correctly adjusted, the tube or pipe could
end up with an incorrect profile shape
where the scarf occurred.
A way to make sure the right amount
of material is removed from the welded
tube is important. If placed after the
scarfing process on a tube mill, a laser-
based inspection tool can measure the
scarf width and bead height left behind
after a scarfing process to provide
quality control of the scarfing process
itself.
the scarf width is defined as the
length of the ‘flat’ portion of the tube that
appears after the weld bead has been
removed. Scarf width measurement can
change quickly during production as the
pipe or tube moves up or down, causing
the scarfing tool to cut deeper or not as
deep into the weld bead. Therefore it is
best to average the scarf width over a
number of inspections, in order to make
the measurement stable.
Laser-based inspection systems
typically use a sensor head incorporating
a laser line and a camera whose optical
axis is offset to the axis of the laser line
by an ‘offset angle’. The sensor head
creates a visible cross-section of the tube
by projecting the laser line on to the tube
and capturing an image of the line using
the camera. The resulting image shows
a profile of the tube surface as if it were
cut in cross section. If a tube is ideally
round, the laser image will represent a
section of an ellipse, and any anomaly
such as a cut or out of round profile
can be mathematically detected. The
best laser-based inspection systems
base all of their measurements on the
differences between
the actual laser
profile line seen by
the camera, and the
ideal mathematical
profile based on the
tube parameters.
Using the position
of the actual laser profile, the ideal
mathematical profile and the size of the
pixels in the image, a laser inspection
system can rapidly determine the
scarf width – a task that is difficult to
accomplish using traditional NDT quality
control tools such as eddy current or
ultrasonic testing techniques.
Figure 4 shows an image taken from
a laser-based inspection system. a laser
profile is shown of a welded tube, after
the scarfing operation. the area of the
scarfing does not reflect the laser line as
well as the area outside of the scarfing,
so the scarfed area can be easily
detected and measured. As the scarf
cuts deeper into the material, the scarf
width will increase; as it cuts shallower,
the scarf width will decrease. In such a
way, by measuring the width of the scarf
zone, the depth of the scarf cut can be
inferred.
Laser-based inspection systems
offer an effective way for tube and pipe
producers to monitor the quality of their
scarfing process on a real-time basis.
Xiris Automation Inc – Canada
Fax: +1 905 331 6661
Email:
Website:
Figure 1: Result of a scarf tool not cutting
deep enough
Figure 2: Result of a scarf tool cutting
too deeply
Figure 3: The scarf width, where
w = the width of the scarf cut
Figure 4: Image of an ideal scarf zone