MetFab - Air Products Welder's Handbook - Flipbook - Page 25
Gas backing
When weld metal penetrates through
the root in a butt joint, it is exposed to air
and oxidises. This can cause poor quality
welds in steels especially stainless steel
and reactive metals such as titanium.
Contamination can be avoided by
protecting the weld root with a gas shield
known as a backing or forming gas.
Material
Backing gas
Carbon, alloy and
stainless steels
Argon
Nitrogen/hydrogen
Nitrogen
Aluminium
and alloys
Argon
Argon is most commonly used as a backing
gas, however, advantages will be seen,
particularly when welding stainless steels
if mixtures of hydrogen in nitrogen are
selected. N5NH5 (5% hydrogen in nitrogen)
or N5NH10 (10% hydrogen in nitrogen)
are recommended for backing gas when
welding stainless. The hydrogen scavenges
the oxygen in the gas-protected area around
the root often reducing purge times and the
amount of backing gas required, resulting
in a brighter weld finish (less oxidation).
Purging should be
completed in 3 stages:
• Slowly displace the air before starting to
weld. Oxygen levels should be below 50
parts per million (ppm).
• Maintain the purge during welding. A
minimum flowrate should be selected to
maintain oxygen levels below 50 ppm
• Maintain the purge after welding. A
minimum flowrate should be selected to
maintain oxygen levels below 50 ppm until
temperature has dropped below 250C.
The purging technique is determined by
the density of the backing gas:
Lighter than air
• Helium
• N5NH5
• Nitrogen
• N5NH10
Heavier than air
• Argon
• R1ArH5
Top Tip
Use backing gas for 1st , 2nd and 3rd
passes as well on wall thicknesses
greater than 10mm.
21
