Factors affecting composition and quantity of fume and gases
The quantity and composition of fume and gases generated during welding is influenced by the following:
- type of process
- welding consumable
- presence of any material coatings
- nature of MMA flux coating, or core of FCA consumable
- welding parameters
- shielding gas composition
The two most important are the welding process and the choice of consumable.
Welding processes
Gas welding
Gas
welding fume contains pollutants formed by combustion of the fuel gas.
When an oxidising flame is used, these will be carbon dioxide with
oxides of nitrogen but, for a slightly reducing flame, carbon monoxide
will also be present.
Safe practice and accident avoidance
Providing
there is good ventilation, the levels of fume and gases generated when
welding mild steel are normally well below any workplace exposure limits
(WELs). No special precautions are required.
Manual metal arc (MMA)
A
MMA rod has a suitable composition for the weld metal deposited but the
flux covering provides gases for the arc, additional alloying elements
and slag for protecting the weld pool. The composition and quantity of
the fume generated will depend on the process variables, for example,
type of consumable, polarity, voltage and current. The toxic elements in
the fume will be similar to those in the consumable, although the
proportions are likely to be different.
Safe practice and accident avoidance
For
normal welding operations, the exposure levels of fume constituents
will generally exceed relevant WELs. Local exhaust ventilation (LEV)
should be used to remove fume at source. As stainless steel fume, in
particular, causes respiratory tract sensitisation (the welder becomes
more susceptible to occupational asthma), special care should be taken
to control exposure to as low a level as is reasonably practicable.
Flux cored arc (FCA)
Continuously
fed wire in self-shielded FCA welding contains a flux which produces a
large amount of gas for the arc and protection/deoxidisation of the weld
pool. Self-shielded wires are normally used for welding outdoors. With
gas-shielded wires, which are only used in the welding shop, an
additional shield (CO
2 or argon-CO
2)
is needed to protect the weld pool. As FCA is normally employed at high
welding current levels and higher duty cycle, more fume will be
produced compared with MMA.
Safe practice and accident avoidance
Providing
sensible precautions are taken, self-shielded FCA welding, taking place
outside, will not need any fume removal measures. For gas-shielded
welding inside a building, similar precautions to MMA welding should be
used, such as LEV.
Metal inert gas (MIG/MAG)
MIG/MAG uses a solid wire and a separate gas to form the arc and shield the weld pool. The shielding gas is normally CO
2 or a mixture of argon-O
2 -CO
2
but argon can be partly replaced with helium. As well as the effect of
the welding parameters, the mode of metal transfer has a significant
effect on fume levels.
Dip transfer mode operates at a low
welding current level and has a characteristic short arc length. Fume
levels are low. Spray transfer mode which operates at much higher
current levels and at a greater arc length can generate higher fume
levels, but a spray condition at minimum voltage generates low levels of
fume.
Pulsed transfer mode operates at similar low current
levels to dip transfer but with a longer arc length and generates
intermediate fume levels depending upon the welding parameters.
Special
mention must be made of the presence of ozone which is generated by the
effect of ultraviolet radiation on the air immediately surrounding the
gas shield as shown in the diagram. Welding with an aluminium-silicon
wire, in particular, produces high ozone exposures. Over exposure to
ozone may also occur during MIG/MAG welding of aluminium, mild and
stainless steels.
Safe practice and accident avoidance
Use
LEV to remove fume and ozone at source. If ozone is not adequately
controlled by this method, the welder must wear Respiratory Protective
Equipment (RPE).
Tungsten inert gas (TIG) and plasma
The
arc is formed between the tungsten electrode and the workpiece with an
inert gas. As the filler metal is added directly to the weld pool, metal
does not pass through the arc, so there is hardly any fume generated.
In open workshop conditions, exposures will not normally exceed WELs.
Welding of stainless steel may generate unacceptable levels of ozone.
Ozone is generated by the effect of ultraviolet radiation
on the air immediately surrounding the gas shield.
Safe practice and accident avoidance
Good
ventilation does not require fume extraction but when welding stainless
steel, control of ozone using LEV or LEV plus RPE may be needed.
Submerged arc
Submerged
arc welding uses a bed of granulated flux to cover the arc. As the arc
is not exposed, there is negligible emission of fume and gaseous
pollutants, although exposure to fluorides should be considered when
using highly basic fluxes.
Safe practice and accident avoidance
Special
precautions are not usually needed but it should be remembered that
dust may be produced when filling the hopper with flux.
Exposure to welding fume and gases
The effect of process, and consumable type on exposure assessment is summarised in the Table. The
Indication of exposure levels assumes that no control systems are being used.
The
generic term Workplace Exposure Limit (WEL) is used and may apply to
one or all of the individual constituents of the fume. Assessment
indicators are the fume and gas components most likely to exceed their
respective WELs during welding.
The Welding Manufacturers
Association has produced a standard format for hazard data sheets to
enable manufacturers to comply with their legal obligations under the
Health and Safety at Work Act 1974 Section 6. The hazard sheet contains
the following information:
- chemical analysis of substances present in the fume produced by the consumable
- the appropriate WELs
For safe use of consumables, reference should always be made to the manufacturers' or suppliers' hazard data sheets.
It
should be noted that in addition to fume, where appropriate, a risk
assessment must also consider exposure to shielding gases, which may be
toxic or asphyxiant. Gases are also produced by the action of heat on
the welding flux or slag (carbon dioxide and carbon monoxide) and heat
or ultraviolet radiation on the atmosphere surrounding the welding arc
(nitric oxide, nitrogen dioxide and ozone).
Effect of process, and consumable composition on the assessment of exposure
Process | Material | Indication of exposure levels | Assessment indicators |
Gas welding | Mild steel | Below WELSs except in poorly ventilated or confined space | Carbon dioxide, carbon monoxide (reducing flame) nitrogen dioxide |
Manual metal arc (MMA) | Mild steel and low alloy steels | Above WELs | Iron oxide, Manganese |
Stainless steel | Above WELs | Hexavalent chromium or nickel |
Aluminium, copper, nickel alloys, cast iron, hardfacing | Generally in excess of WELs | Exposure to aluminium, copper, nickel, chromium barium cobalt, depending on the alloy welded. |
TIG and plasma arc | Mild and low alloy steels | Less than WELs | Shielding gas can constitute a hazard in confined spaces by reducing the available oxygen to a level which will not support life |
Stainless steel alloys | Less than WELs for fume, greater than WEL for ozone | Ozone, shielding gases in confined spaces |
Metal inert gas/metal active gas (MIG/MAG) | Mild and low alloy steels | Generally in excess of any WELs | Iron oxide, manganese, ozone, shield gases. |
Stainless steel | Greater than WELs | Exposure to chromium, nickel, ozone and shielding gases. |
Aluminium and aluminium alloys | Greater than WELs for ozone and fume | Aluminium oxide fume. Ozone levels can be very high, particularly with aluminium/silicon alloys. |
Flux-cored arc | Mild and low alloy steels | Greater than WELs | Iron oxide, manganese. Some self shielded consumables may give rise to soluble barium in fume |
Stainless steel | Greater than WELs | Exposure to Hexavalent chromium, chromium, nickel |
Publications and relevant standards
- COSHH
- Essentials for welding cutting and allied tasks
- EH 40 (updated annually)
- Workplace Exposure Limits, HSE Books.
Welding Manufacturers Association, Leaflet 236, Hazards from welding fume.
Note: When assessing fume risk, the suppliers' data sheets and hazard information must be fully consulted.
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