TPT September 2012

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Technology News
September 2012 69
PHI Tube/Pipe End Finishing Machines
help you achieve the Shape and Finish you are seeking!
Model 2CPVE Model 3CPVE
Operations: Squaring, Deburring, Flaring, Beading, Flanging, Expending
Materials: Annealed Ferrous and Stainless Steel, Nonferrous.
Tube Diameter, mm: 2CPVE: 3 to 50; 3CPVE: 3 to 75
Standards: SAE: AS4330 (MS33584), AS33583 (MS33583), AS5131
Features: Pneumatic clamping, Variable Speed
Tooling: Per Customer’s Specification
Industries Served: Aerospace, Automotive, Defense, Manufacturing, Consumer Products
14955 E. Salt Lake Ave.
City of Industry, CA 91746-3133
Tel: 626/968-9686
Fax: 626/333-3610
E-Mail:
Web Site:
Post weld purging
WHILE most welders recognise the
need for inert gas purging when making
quality joints it is not obvious why
purging should be continued after the
weld has been completed. The answer
is obvious really; hot metal oxidises in
the presence of air so the weld and any
metal surrounding it will discolour until
it has cooled. Welders know to keep
the inert gas flowing from the torch to
prevent oxidation around the surface.
Why do they overlook the underside?
While nominal oxidation, ie a light straw
shade, is quite acceptable in some cases
and in any event can perhaps be cleaned
at a later stage, some applications require
absence from contamination of this type.
With a little care it can be avoided. Simply
continue the flow of inert purge gas over
the hot metal until it has cooled – typically
to below 150°C ( 300°F).
Effective gas seals need to be provided
on either side of the joint and these need
to be far enough away to be unaffected
by the temperature rise during welding.
Gas can be admitted through the seal or
between the seal and the pipe or tube
wall. Gas exit should be available so
that the lighter air is fully expelled by the
denser argon. Note that if helium is being
used it is lighter than air and the air exit
should be at the bottom.
Having recognised the need to use
gas purging, many welders think that
just passing inert gas through a pipe or
tube is adequate. This is poor practice.
Even with small diameter tubes, the cost
of continuously passing gas through is
often greater than the cost of effective
seals. Increasing flow rate simply leads
to turbulence, trapping air around the
joint.
It is a fallacy that using crumpled
paper or discs of cardboard or wood
gives a good seal. Even if these
apparent solutions appear to be a good
fit, the chances of leaks are high. There
is also the possibility of burning if the
weld line is too close. Bear in mind
also that all three materials probably
contain contamination and some
residual moisture – it is all bad for the
welding procedure. Proprietary sheet
plastics have been developed which
are contaminant free and can even be
used when making joints for use for
example in nuclear and pharmaceutical
engineering. These are water soluble
and can be removed effectively and
easily after welding but care is needed
during application. See Huntingdon
Fusion Techniques’ Technical Note TN
14 ‘Purge Film’.
Huntingdon Fusion Techniques
manufactures a unique re-usable range
of purging equipment to meet every tube
and pipe diameter from 25 to 1,800mm
(1 to 72").
They have all been designed
specifically to meet exacting sealing
demands, use the minimum quantity of
inert gas, are easy to install and remove
and greatly speed up the overall welding
procedure.
Huntingdon Fusion Techniques – UK
Email:
Website: