Linepipe Manufacture
06.03
The pipe-making process comprises three key processes: steel
production, pipe manufacture and pipe coating. Within each of these steps, a
number of processes are carried out to ensure the manufacture of high quality,
functional pipe. BlueScope Steel, Orrcon Steel, OneSteel and Bredero Shaw have
opened their factory doors to share the secrets of the pipe-making process with
readers of The Australian Pipeliner.
Sourcing steel
The ingredients required for the production of line pipe steels
come from all corners of Australia, with iron ore from Western Australia and
South Australia, coal from the Illawarra and limestone from Marulan in New
South Wales all brought to BlueScope Steel’s Port Kembla operation.
Before the production of steel can begin, high-quality iron –
the basic ingredient for steelmaking – needs to be produced. The iron-making
process is carefully controlled by blending various grades of iron ore to
produce a mix which is exact in its content. Fines of iron ore and coal are
also mixed and fused together in a sintering process to form a lumpy feed for
charging into the blast furnace, and constitute approximately 60 per cent of
the charge.
The last components of the reduction process are coke and
limestone. Coke, a strong porous product, is formed by burning coal in ovens
and is charged into the blast furnace to support the iron ore and sinter. It is
coke which provides the carbon necessary for reducing the iron ore to iron.
Limestone is used as a flux or cleaning agent in the reduction process.
These products are charged into either one of two blast
furnaces, which are as tall as a 27 storey building and heated by the injection
of air and gas to a temperature of 2,300 degrees Celsius. As the charge melts,
the molten material makes its way to the bottom of the furnace where it is
taken off and stored in refractory-lined ladles awaiting transfer to the next
stage of production, where the liquid iron will be converted into steel at a
production facility called the BOS or basic oxygen steelmaking.
Prior to this, there is one more important step which is key to
the production of high-quality pipeline steel grades – the process of
desulphurising the molten steel iron. Sulphur is an undesirable element in pipe
steels as it reduces the cleanliness of the steel, decreases the ductility and
restricts the weld properties of the final pipe. Once desulphurised, the molten
iron is ready for conversion into steel.
In the BOS, iron is converted to steel around 50 times a day,
and the Port Kembla operation produces in excess of 5 MMt/a of steel. In the
process, the iron, which has a carbon content of around 4 per cent, is refined
to levels of less than 1 per cent. It is this process that differentiates iron
from steel. A vessel containing a mixture of approximately 300 t of charge,
made up of 50 t of scrap and 250 t of iron from the blast furnace, is converted
to steel by injection of pure oxygen, which creates a chemical reaction and
increases the temperature to 1,700 degrees Celsius. Fluxes are added to absorb
the impurities, the scrap melts and the carbon content is lowered to form
steel.
The molten steel then moves to the vacuum degassing station
where precise additions of the necessary alloys such as niobium, manganese,
aluminium and titanium are made to meet the tight specification limits of pipe
steels. Then, to further enhance the cleanliness of the steel, the molten steel
is injected with a calcium silicide powder, which combines with some of the
last remaining impurities and removes them from the molten steel. At this point
the final chemical composition of the pipe steel grade is set and the molten
steel is transferred to the slab casting facility.
In the slab caster, the steel is poured into a water-cooled
mould and drawn through a series of segments, which chill the outer surface and
slowly solidify the steel to a stage where it is fully hardened. The continuous
strand of steel is then cut to length by automated gas torches to individually
designed lengths or slabs. At this stage, the slab is approximately 12 m long
and 230 mm thick.
After cooling, the slabs are inspected for any defects that may
have been induced during the casting operation. Once the slabs have passed this
quality assessment they move onto the final major processing stage where they
are converted to a hot rolled strip to the exact dimensional and mechanical
property requirements of the customer.
The purpose of rolling the cast slab is two-fold; firstly to
achieve the final product dimensions and secondly to produce a finer grain
structure; thereby giving the steel greater toughness and strength. In the Hot
Strip Mill the slabs are reheated prior to rolling the steel down to its final
desired thickness by passing it through a number of rolling stands. BlueScope
Steel’s Port Kembla Hot Strip Mill uses six finishing stands to achieve the
final desired strip thickness. The strip is then water-cooled on a long
‘run-out’ table prior to coiling. Precise control of slab reheating and rolling
temperatures, scale removal, width and thickness ensure that the final strip
meets all the dimensional, strength and toughness requirements of the ordered
steel grade.
The steel coils are now allowed to cool before being strapped
and labelled for despatch to the pipe manufacturing mills.
Producing pipe
Before steel is released from the steel mill, it is required by
pipe manufacturers Orrcon Steel and OneSteel to have passed various quality
tests, including: composition, strength, toughness, surface condition, and coil
shape. When steel is despatched from the steel mill, a test certificate is also
sent from the steel mill’s data system. When each coil arrives at these plants,
checks are performed to ensure there has been no transport damage and that the
identity of the coil matches its paperwork, markings and labels. Each coil is
then unloaded into the warehouse, and its identity carefully recorded at
OneSteel using a barcode process, or at Orrcon Steel using a traceability
system called OrrTrace. The coil is then ready to be loaded onto a mill to make
pipe.
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After identifying the coil, the key stages involved in the pipe
making process include:
·
Loading the coil into the mill;
·
Forming;
·
Welding;
·
Cut-off;
·
Bevelling;
·
Hydrostatic testing;
·
Ultrasonic inspection; and,
·
Marking.
According to OneSteel Piping Systems Quality Assurance Manager
Dr John Piper these are merely the manufacturing processes and much of the
company’s effort is spent verifying the quality of each pipe as it is made via
a multitude of test and inspection processes. These include visual inspection
of the coils, weld visual monitoring, cooling process control, preliminary
inspection of dimensions and surface quality, mill control ultrasonic testing,
sampling for strength, composition and toughness testing, flattening and
hydrostatic testing, and the final inspection process. This final process
consists of ultrasonic testing, length and mass measurement, visual and
dimensional inspection of the weld and body, and the bevel inspection. The
outcome against the unique identity of each pipe is recorded to ensure that all
pipe has passed all tests before being placed to order.
Orrcon Steel employs approximately 100 people at its Wollongong
API Pipe Mill. This includes technical and quality, operations, administrative,
maintenance, laboratory, logistics and other support staff. At OneSteel, the
number of employees required at the plant is dependent on market demand and
whether the company is running one, two or three shifts.
According to Mr Piper, the number of man hours required to make
a pipe depends on the section and diameter, which range from 6–18 m in length
and 168.3–508 mm in diameter. A production crew at the plant typically produces
between 100 and 400 pipes per shift depending on the diameter, thickness,
length and grade of the product being produced. Large diameter heavy wall pipe
is produced at a slower rate than small diameter light wall pipe, however large
diameter heavy wall pipe delivers a much higher output on a tonne per shift
basis.
Orrcon Steel Technical Manager – Pipelines and Infrastructure Dr
Cameron Dinnis says “Depending on the pipe size, the welding speed will be
anywhere from 14–21m per minute. The pipe spends 10seconds at pressure during
the hydrostatic test and will spend about one minute being ultrasonically
tested. All of this means that the total man hours that go into a pipe can
range between 1.5 and 5 hours.”
Welding the seam
As an electric resistance welded (ERW) pipe manufacturer, Orrcon
Steel is essentially a manufacturer of weld seams.
To make the seam, the company uses a precise machine to trim the
edges of the steel coil in a process known as edge milling, then heats the
edges using high frequency electrical current and pushes them together to form
the weld. The external and internal weld bead, which is the material that is
ejected from the weld area when the strip edges are forced together, is then
trimmed off by carbide cutting tools, leaving the weld seam flush with the
surface of the pipe. The weld seam is then immediately subjected to an initial
ultrasonic inspection. The seam is then heat-treated to ensure the
microstructure is suitable for service. Following this, the pipe is tested to
ensure that the weld seam and the pipe body have structural integrity and are
within dimensional tolerances.
Specifically, the seam is subjected to a hydrostatic test and
the full length of the weld seam is subjected to an ultrasonic inspection from
both sides by 36 differently angled beams. The inspection frequency is such
that a 0.5 mm long defect will be detected five times by each of the 36 angles
in the time the ultrasonic probes traverse across it.
OneSteel achieves the weld seam using a similar
process. “The integrity of the weld seam is of course, critical to the end use
of the pipe so we are very careful in both process control and inspection,”
says Mr Piper.
Sumber:
http://pipeliner.com.au/news/pipe_production_101/043584/
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