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Kemppi A7 TIG Orbital System 300 Manual

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A7 TIG Orbital System 300© Kemppi Oy 2016 1615
OPERATING MANUAL
3.13.1 Purging the pipe ID with inert gas
The purpose of purging is to replace unwanted air and
other vapor contaminants from the root pass by a gas
that prevents oxidation during welding. Oxidation can
produce a variety of problems, such as root oxidation
(sugar), incomplete fusion, porosity and changes in weld
chemistry, which can aect weld mechanical and corrosion
resistance properties adversely.
Purging is recommended when welding stainless steel,
nickel alloys, and most nonferrous base metals. Argon is
commonly used.
1. Isolate the weld root by means of dams or other
suitable containment devices.
2. See that there are openings through which the
purging gas can enter and exit the weld joint area
at controlled rates.
3. When argon is used, locate the gas inlet lower than
the exit opening to prevent entrapment of air to
prevent excessive pressure buildup during welding.
4. Size the exit port equal to or greater than the
size of the entry to prevent an increase in the gas
pressure.
5. If the weld joints are pre-tacked in larger
assemblies, tape each unwelded joint to prevent
excessive loss of purge gas.
The purge ow rate is based on the volume to be purged.
• A general rule of thumb: purge at ow rates
and times that produce 5 to 10 system volume
changes prior to welding.
• Recommended maximum ow rate: the volume
and shape to be purged. Typically, the purge rate
should not exceed 40 l/min.
• You can estimate reasonable purge ow rates
and times from calculations of the system volume
and by applying time factors. Divide the system
volume by the purging gas ow rate.
• Or determine purging adequacy by taking
residual oxygen measurements by using an
oxygen analyzer prior to welding to ensure a low
oxygen level.
6. After purging is complete, reduce the ow of purge
gas prior to welding until only a slight positive
pressure exists to avoid a pressure buildup from
heat due to welding. Excessive pressure can cause
root concavity and porosity in the root pass. A
purge gas ow rate of 4–12 l/min is typical for pipe
welding in the 10 cm (4”) size range.
Kemppi oers a purge plug kit that includes a selection of
plugs ranging from 10 to 100 mm ID as well as an oxygen
content meter for residual oxygen measurement.
… 41 …


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