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From Steel to Tube
An overview of welded tube manufacture (continued)
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CONTROL OF PIPE QUALITY AT THE WELDING
MILL
The quality of EW weld is assessed by standard
destructive tests such as cone expansion and flattening and by inspecting
the microstructure of the weld area. This testing augments the
non-destructive testing and provides a regular feedback to the mill on weld
quality.
The objective is to ensure that the weld area is as strong, if not
stronger than the body of the pipe. The microstructure is inspected to
ensure that the heat affected zone is symmetrical and satisfactory with
regard to width and structure, that there is adequate diversion and that all
the previously liquid metal and oxides have been removed from the weld
region. Having established that the welding conditions are correct and that
the weld is satisfactory it is then essential to ensure that the conditions
under which the sample was taken do not change.
There are many aspects of the pipe making process, which affect
weld quality. These include setting of the forming mill, strip presentation
to the forming mill, consistent width dimensions of the ingoing strip and
control of the welding temperature.
The two main variables, which can affect the weld temperature are
the strip speed and thickness of the strip edge. The welding power is
proportional to strip speed and strip thickness and the welding power
requirement is measured using a constant called the energy factor. The
energy factor is the welding power in watts per mm thickness per metre, per
minute speed (watt min/mm²).
Strip speed variation is dealt with by measuring the strip speed
with a wheel driving a measuring instrument, which runs on the strip and is
used to control the welder power output. Any variation in strip speed is
therefore directly compensated for.
Strip edge thickness is governed by the thickness of the slit strip
plus thickening occurring in the fin pass rolls, which is impractical to
measure.
The most effective way of compensating for edge thickness and other
variations is by keeping the temperature of the weld region constant. This
is measured immediately after the formation of the weld with a two colour
pyrometer which is unaffected by steam and water. The temperature
measurement is used in a computerised control loop to adjust the speed power
system to maintain a constant weld temperature. |
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STRETCH REDUCTION AND HEAT TREATMENT
For pipe diameters of 139.7mm or less and wall thicknesses of 7.9mm
or less, a 168.3mm diameter pipe is produced at the EW Mill and stretch
reduced to the required finished size.The 168.3mm as welded pipe is heated
in a 122m long gas fired barrel furnace followed by a 10m in line 10
megawatt induction furnace to about 1000°C, and stretch reduced through a
series of water cooled rolls to the required size. After cooling, the
pipes are cut to the required length and either sent to the visual
inspection tables or for quenching and tempering. All pipes of
diameter greater than 139.7mm or thickness greater than 7.9mm are formed to
dimensions close to the finished size on the welding mill and heat treated
using one of the following three methods. The weld line only may be heated
to the normalising temperature using in line induction heaters, the complete
pipes may be normalised in a separate furnace or for higher strengths, the
pipes may be quenched and tempered. The pipes passing on to the quench
unit are heated rapidly in a gas fired barrel furnace to 850-950°C and are
water quenched on the outside surface by passing through 20 in-line spray
rings as soon as they leave the furnace. To aid even heating and
cooling, pipes are rotated on passing through the unit and a pyrometer is
used to monitor the temperature at the furnace exit prior to quenching to
ensure temperatures within the required range are obtained. Hardness’s are
measured on the transverse faces of rings cut from each quenched batch to
ensure sufficient through wall hardening has occurred. Normalising and
tempering is done in gas fired walking beam or electric roller hearth
furnaces where the pipes are maintained at an accurately controlled
temperature for about 10 minutes and are then removed from the furnace to
cool in air. |
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INSPECTION
The bore and the outside surfaces of each pipe are visually
inspected and the reasons for rejection recorded and analysed. The diameter,
thickness and degree of ovality of each end of each pipe are checked. The
pipes are bevelled and hydrostatically tested using the conditions laid down
in the API specifications. If the pipes are to be subsequently threaded this
test may be done at a later stage. The pipes are then sent for rotary probe
ultrasonic testing, |
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