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Thickness Variation in Seamless Tube |
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Seamless tube is formed by piercing a hole along the axis of an ingot or
bloom followed by one or more processes of elongation. Precise measurement
and control of the outside dimension of solid rolled steel sections poses a
severe problem, but for the seamless
tube maker, controlling the shape and concentricity of the inside of the
tube is an even more challenging problem. To help solve this problem,
ultrasonic equipment has been developed which will measure the wall
thickness of tubes over their entire surface at a sufficiently high speed to
enable the whole output of a plant to be monitored. The tube thickness
variations can also be analysed to provide information highlighting areas
where adjustment of manufacturing equipment or process techniques may give
even better thickness control. |
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Fig 1 |
Manufacture of seamless tube above about 150mm diameter, uses the rotary
forge process shown above. Ingots which have been specially cast, carefully
inspected and dressed to remove surface imperfections are heated in a
furnace and a hole is punched along the centre of the ingot by a hydraulic
press. A rotary elongator spins the tube over a mandrel to give a first
stage of elongation. The tube is then processed in a rotary forge mill where
specially shaped rolls forge the tube to its final length and wall
thickness. The pierced ingot or bloom is threaded on to a mandrel and pushed
into the rolls. An eccentric lobe on the rolls rotates to meet the incoming
bloom and bites into it. The rolls rotate further, rolling the bloom back
out of the mill and swaging the steel in the bite over the mandrel. Further
rotation of the rolls brings an open portion or gap round to the bloom
enabling it to be pushed back into the mill ready for the next bite. This
process continues with the bloom advancing through the roll gap on each
cycle. Typically, the mill rolls rotate at about 60 r.p.m. causing the
forging cycle to occur once per second.
If the various machines used during each stage of tube manufacture
are not set up and maintained correctly, or if the ingot is not uniformly
heated, variations in wall thickness will occur. For example, if the ingot
passes through the furnace too quickly, the centre will still be hard and
the punch in the piercer may wander off centre giving an eccentric hole in
the back end of the bloom. The subsequent process may not entirely remove
this eccentricity. For this reason, the thickness of every pipe is measured
over the entire surface of the tube by automatic equipment.
The data obtained are processed in two ways:
1. By checking against maximum and minimum tolerance
limits to ensure that no tube
delivered to customers has a wall thickness outside the range of the
specification in any part of the tube (Inspection).
2. By analysis of wall thickness variations within the
limits of tolerance to determine patterns of variation, which can be related
to the process so that corrective action can be taken to trim process
adjustments (Quality Control). |
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Fig 2 |
Automatic measurement
An ultrasonic compression wave technique is used to measure the
tube wall thickness as shown in Fig.2. A transmitter (TX) energises a probe
containing a piezo electric element, 1000 times per second, to generate a
burst of high frequency ultrasound (5 MHz) which then travels down a column
of water (a) As it reaches the tube wall, part of the energy is transmitted
into the steel (b) and part is reflected (c).
The transmitted pulse (b) is reflected from the tube inner surface
(backwall) and returns to the outside of the tube where part of the energy
passes back into the water column (d), and part is reflected back to bounce
once more off the backwall. Each time the pulse returns to the outside of
the tube, part of it is passed back into the water column, where it travels
back to the transducer and is converted into an electrical signal. These
echo signals are amplified in a receiver (RX) to produce a train of pulses
(c, d, e, f, etc). The time between each, pulse if proportional to the tube
thickness. The third and sixth echoes (f & g) are selected electronically
and the time between the echoes is measured; a check is made to ensure that
this period is approximately equivalent to three times the period between
the third and fourth echoes. This technique gives an accurate and reliable
thickness measurement.
To enable thickness measurement to be made over the entire tube,
four probes are mounted on a rotor, which rotates at approx. 600 r.p.m.
around the tube whilst the tube moves slowly through the rotor. Each probe
thus trances a helical path along the tube. Using four probes, the entire
surface of a tube, 500mm diameter and 10m long, can be examined in about two
minutes, resulting in approximately half a million individual thickness
measurements. The same rotor also carries probes which use different
ultrasonic techniques to search for small cracks and other imperfections.
For inspection purposes, the thickness data are checked
electronically against preset minimum and maximum tolerance limits and any
area of the tube which is outside these limits, can be automatically marked
using a paint spray. A chart showing the location and severity of any thick
or thin section and any imperfections is also produced.
For quality control purposes, techniques have been developed
whereby thickness data are recorded on disc for analysis by computer. This
enables powerful mathematical analysis techniques to be used to identify
trends and repetitive patterns in the thickness variations. |
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