Oxyfuel gas welding includes any
welding process in which the source of heat for welding is the exothermic
chemical combustion of a fuel gas with oxygen. While natural gas/methane,
propane, propylene, butane, or other hydrocarbon gases, or even hydrogen, can
be used. oxyacetylene welding, which uses acetylene gas as the fuel, is the
most commonly used oxyfuel gas welding process due to its high flame
temperature (i.e., intense source energy). Oxyacetylene welding (OAW) derives
the heat needed to cause melting of the substrates and, almost always, filler
from two stages of combustion. In the first stage, known as primary combustion,
the acetylene fuel gas partially reacts with oxygen provided from a pressurized
gas cylinder to form carbon monoxide and hydrogen:
C2H2 + 02 (cylinder) = 2CO + H2
This reaction is exothermic and is responsible for about
one-third of the total heat generated by the complete combustion of acetylene.
The dissociation of acetylene to carbon and hydrogen releases 227 kJ/mol of
acetylene at 15°C, while the partial combustion of the carbon to form carbon
monoxide releases 221 kJ/mol of carbon. No combustion of the hydrogen takes
place at this stage. The total heat released by the primary reaction is 448
kJ/mole (501 Btu/ft3) of acetylene.
In the second stage of oxyacetylene or other fuel gas
welding, known as secondary combustion, which occurs immediately after the
primary combustion, the carbon monoxide resulting from partial combustion of
the carbon dissociated from the acetylene (or other fuel gas) reacts further
with oxygen, this time from the surrounding air, to form carbon dioxide, while
the hydrogen from the primary combustion dissociation of acetylene (or other
fuel gas) reacts with oxygen in the air to form water:
2CO + O2(air) = 2C0
H2 + 0.502 (air) = H2O
These reactions are also exothermic and are responsible for
two-thirds of the total heat generated by burning the dissociation products of
the acetylene completely. Burning of hydrogen to produce water vapor releases
242 kJ/mol of hydrogen, while further oxidation of carbon monoxide releases an
additional 285 kJ/mol of carbon monoxide, or 570 kJ/mol for the reaction. The
total heat released by the second reaction is thus 812 kJ/mol (907 Btu/ft3) of
acetylene.
The actual primary and secondary combustion reactions occur
in the gas flame of an oxygen-acetylene torch in two distinct regions, as shown
in Figure. Primary combustion occurs in an inner cone, while secondary
combustion occurs in an outer flame. Although only accounting for one-third of
the total heat of the overall combustion reaction (448 kJ/mol out of 1260
kJ/mole), the inner cone tends to be more concentrated in volume, and so is
hotter (i.e., the energy is more dense). Thus, the welder tends to work with
the tip of the inner cone near the workpiece to cause melting, using the outer
flame to provide a degree of shielding of the molten weld metal and hot, newly
formed weld by the carbon dioxide, to provide preheating to aid in initial
melting and to slow down cooling once the weld has been made (thereby sometimes
avoiding adverse post-solidification or heat-affected zone transformations.
Similar combustion reactions can be written and energy
balances performed for other fuel gas mixtures with oxygen, with different
amounts of energy being liberated and different flame temperatures being
produced for each.
