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  Welding is
the process whereby metallic pieces are joined together by means of heat
and/or pressure, with or without the addition of another metal (transfer
of metal).
In order to obtain a resistant and technically acceptable welding, the
smelting area must be protected from oxidizing phenomena and the smelted
metal must be purified from slag in order to allow for a weld bead
without faults.
Welding processes are usually divided into two main groups:
- Autogenous welding: this is obtained without the transfer of metal or
with the transfer of the same metal of the pieces to weld. It includes
gas welding and electric-arc welding.
- Heterogenous welding: this involves the melting of a transfer metal,
different from the metal of the pieces to weld, whose melting point must
be lower than that of the material to weld.
Gas welding
In gas welding the heat source is represented by a flame stoked by a gas
generally made up of oxygen and acetylene mixed in equal parts. Such
welding is employed for light metals or for metals requiring a low
gradient of temperature. When the oxygen share is increased, the flame is
able to cut weakly bound steel.
Electric-arc welding
Electric-arc welding can take place:
- with a covered meltable electrode
- in a protected atmosphere
- submerged-arc welding
In the first instance, smelting is achieved by making an electric-arc
between the base metal and the covered electrode.
The electric-arc welding carried out in a protective atmosphere can be
further divided into:
- TIG (Tungsten Inert Gas), where smelting is achieved by making an
electric arc dart between the metal to weld and an electrode made of
tungsten, nonconsumable, in an inert gas atmosphere (helium or argon);
- MIG (Metal Inert Gas), where smelting is achieved by making an electric
arc dart between the metal to weld and a meltable electrode, in an inert
gas atmosphere;
- MAG (Metal Active Gas), where a small amount of carbon dioxide is added
to the helium or to the argon and reacts with the base metal;
- Plasma-arc welding, that differs from the TIG because the inert gas
mixture is made conductive by ionization and is then forced through a
nozzle so that its temperature increases.
The submerged-arc welding is achieved by making an arc dart between the
manufactured article and a electrode, both covered by a layer of a
granular conductor (dust flux) that shields the arc from the air.
Braze welding and brazed joint (soft soldering and solder brazing)
By means of braze welding different metals can be welded together. Brass
and bronze, with a smelting point of around 900°C, are employed as
tansfer of metals.
In soft soldering, alloys with lead and tin, that smelt at a temperature
below 450°C, are employed.
In solder brazing, alloys with copper and zinc, that melt at higher
temperatures, are used; to lower the smelting temperature, alloys with
silver and cadmium are employed.
Welding entails risks that can be classified as risks deriving from
physical agents (such as radiations, heat, electricity, noise) and risks
linked to the inahalation of fumes, vapour and gases that are released
during the welding process because of the high temperature.
A faulty insulation of the electric cables during the change of the
electrodes can expose the worker to electrocution.
The heat coming from the smelted metallic material and the projection of
incandescent particles may cause burns. Oxyacetylenic welding constantly
entails a risk from infrared radiations, whereas electric-arc welding
entails a risk both from infrared and ultraviolet radiations. The decomposition
of degreasing agents, lubricants and paint that are present on the pieces
to weld can give origin to carbon monoxide, ammonia and phosgene.
Gases that are released during the welding process originate from
acetylene's combustion, from the electrodes' coatings and from the
modifications undergone by oxygen and nitrogen during the process. Nitrogen
oxides and ozone are the determining factors in the respiratory risk of
welders (welders' lung). Nitrogen oxides originate from the oxidation of
air nitrogen and the main one is nitrogen peroxide (NO2). Ozone
originates from the action of ultraviolet rays on air oxygen. Carbon
monoxide formation is bigger in the MAG welding with CO2. Welding
operations carried out in confined spaces without appropriate ventilation
may entail a risk of acute intoxication from such gases. Welding fumes
are made up of metallic vapour that is released in the smelting area; the
presence of such fumes is bigger in the case of electric-arc welding. These
fumes are mainly made up of iron and its oxides when welding involves
ordinary steel, but they contain also big quantities of chrome, nickel
and manganese in the case of special steel.
For additional information
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