handbook-gas-shielding-11.html
AGA CW Handbook A4_32130-UK
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12 the work environment
ozone
is formed of the oxygen in the air as the uv radiation generated by the
arc
collides with an oxygen molecule which is split into oxygen atoms. these will react further with oxygen molecules forming
ozone
, with the sum reaction being as follows: 3o 2 -> 2o 3 uv radiation at wavelengths of 130–175 nm generates the most
ozone
. a majority of
ozone
is formed in the immediate vicinity of the
arc
.
ozone
exits the
arc
region along with the hot plume of smoke rising from the
welding
spot. the amount of
ozone
emissions depend on how much
ozone
was origi- nally formed and how much of that amount is reduced back into oxygen (o 2 ) in the surrounding smoke plume.
ozone
is reduced in the smoke plume in the following three ways: 1. thermal reduction in the zone nearest to the
arc
, where the tem- perature is 500°c or higher. 2. catalytic reduction, where the metal oxide particles in the smoke plume act as the catalyst. 3. chemical reduction as the
ozone
reacts with other gases in the smoke plume. the most efficient reaction is between
ozone
and nitrogen monoxide (no) as follows: no + o 3 -> no 2 + o 2 the 0.03% nitrogen monoxide addition in mison ® shielding gases is sufficient to effectively reduce the amount of
ozone
generated during
welding
. 0 20 40 60 the effect of mison ® shielding gas on mag
welding
. when mison ® shielding gas is used, less
ozone
is generated. with standard gas mixtures,
ozone
emissions are lowest in the globular
arc
region, where fume emissions are at their maximum. cf. the graph in section 3.3. the probability of exposure to the effects of
ozone
concentrations of over 0.1 ppm (over two times the htp 8h values) in
welding
work; the source material is the rese
arc
h of prof. ulvarsson in 1978. using mison ® shielding gases, the exposure probability is significantly reduced. affecting factor effect
welding
process the combined effect of the filler material, shiel- ding gas and
welding
parameters. more nitro- gen oxides are formed during mma
welding
, resulting in lower
ozone
generation than in mig/ mag or tig
welding
. filler material cored wires generate somewhat more smoke, and
ozone
generation is lower than when solid wires are used. base material mig
welding
of aluminium results in the largest
ozone
emissions.
welding
parameters higher
arc
energy = more
ozone
. pulse
welding
= less smoke but more
ozone
. shielding gas more fumes = less
ozone
. low o 2 or co 2 content = lots of
ozone
. spatter more spatter = more fumes = less
ozone
. other more nitrogen oxides = less
ozone
. the effect of different factors on
ozone
generation and the work environment. unalloyed steel, wire diameter 1.0 mm short
arc
globular
arc
spray
arc
argon gas mixture mison ® gas mixture 0 100 150 200 250 300 1 2 3 4 0
ozone
(ml/min) current (a) percent % the probability of exposure of
ozone
concentrations mma
welding
- unalloyed steel 0% mag
welding
- unalloyed steel 20% tig/mag
welding
- stainless steel 20% tig
welding
- aluminium alloys 10% mig
welding
- aluminium alloys 50%
nitrogen-monoxide-dioxide-13.html