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From Steel to Tube
An overview of welded tube manufacture (continued)
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Making the tube (continued) |
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The strip edges are heated to a welding
temperature by the high frequency welder. The welder is a large radio
frequency oscillator producing and alternating current of 200.400 kHz which
resistance heats the strip edges. Due to the two phenomena associated with
radio frequency electric current (i.e. The skin effect and the proximity
effect) this current concentrates in the surface of the strip edge. |
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At the weld head the two heated
edges are brought together and pressure applied to form a forged weld. All
previously liquid metal is expelled together with any oxides and the plastic
areas behind the heated edges upset. The geometry of the weld area is very
important and the ingoing and outgoing circumferences are measured to assess
the amount of metal, which has been pushed out. The symmetry and dimensions
of the heat pattern are regularly checked by cutting a sample from the tube,
polishing and etching a cross section of the weld and examining the
microstructure under a microscope. Subsequent heat treatment
completely removes the heat pattern resulting in a uniform structure in the
weld region. |
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After welding, the internal
and external weld flash or bead is planed from the tube. The strip edges are
thickened in the fin passes before welding which allows the internal and
external bead planning equipment to marginally cut into the parent metal
without reducing the tube wall below the nominal thickness, (Figure 14). The
weld bead is regularly checked for uniformity and integrity. The weld line
is then water cooled to lower the temperature of the pipe as it enters the
sizing mill. The sizing mill rounds up the pipe and marginally reduces its
diameter to give the required finished dimensions. Within the sizing mill
the weld line is inspected continuously using ultrasonic shear wave and
surface wave techniques together with eddy current testing. |
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The sizing mill is also
used to produce a straight pipe by adjusting the final restraining roll pass
at the end of the sizing section. The rotary cut-off is used to cut the
continuous pipe into the required lengths unless stretch reduction is
required when the pipes are left in lengths of up to 122 metres. |
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