Welding gases explained: the right gas for oxy-fuel TIG, MIG, MAG, plasma and laser work, and what the ISO 14175 codes mean

Every welding and cutting gas does one of two jobs. In oxy-fuel work a fuel gas burns in oxygen to make a flame hot enough to melt or burn steel. In arc work a shielding gas replaces the air around the arc and the weld pool, and the arc does the heating. Choosing the right gas means first knowing which of the two jobs it is doing, then matching it to the metal and the process.

The trade-offs within the MAG column, argon against carbon dioxide, are set out on choosing a shielding gas.

Acetylene's flame is hot because acetylene carries a positive heat of formation, released on combustion on top of the normal heat of burning. Edwards, Smith and Brinkley of the US Bureau of Mines calculated 3,086 °C at the stoichiometric ratio of 2.5 volumes of oxygen to one of acetylene, and 3,114 °C at equal volumes. That is why oxy-acetylene remains the gas process for fusion welding of steel, covered in India by IS 1323:1982, and the fastest gas for piercing and short cuts. LPG burns cooler and needs more oxygen per cubic metre, which suits long continuous cuts on thick plate; the comparison is on acetylene vs LPG cutting.

Acetylene comes with limits that no other welding gas has. Free acetylene may be used at not more than 0.612 atm, and at 1.021 atm only with clearance from the Chief Controller of Explosives (IS 14814:2023 cl 7.2.1), because above that it can decompose without any oxygen. Gas quality matters too. IS 308:2024 limits phosphorus compounds to 0.01 % v/v and sulphur compounds to 0.02 % v/v, which is what keeps a weld free of phosphide and sulphide inclusions; KIGL's own specification adds a minimum of 98.0 % acetylene. The oxygen should be named by grade: IS 309:2023 Grade 2 is 99.5 % minimum. Both lines need flashback arrestors to IS 11006:2011 at the regulator and the torch; the full technique is on oxy-acetylene welding and cutting.

ISO 14175:2008 classifies every gas and mixture for fusion welding by a letter for its chemical behaviour and digits for its composition. The groups used most in fabrication are:

A mixture outside these ranges is class Z. Helium may partly or wholly replace argon in groups M1 to N, and two Z mixtures with the same code need not be interchangeable. The designation on a cylinder label reads in a fixed order, for example ISO 14175 – M21 – ArC – 18 for 18 % carbon dioxide in argon. TWI's published examples of commercial MAG blends, collected by W. Lucas in 2001, include 98 % argon with 2 % CO₂, 92 % argon with 8 % CO₂ and 93 % argon with 5 % CO₂ and 2 % oxygen, and for TIG, argon with 2 or 5 % hydrogen. When ordering gas mixtures for welding, quote the full ISO 14175 designation rather than a trade name.

ISO 14175:2008 sets a floor on each group, and moisture matters as much as purity, because water in the arc gives hydrogen in the weld.

Mixtures in groups M1 to M3 may also carry no more than 1,000 ppm nitrogen. Table 3 sets the blending tolerance: a component above 5 % may vary by ±10 % of its nominal value, so 18 % CO₂ may lie between 16.2 and 19.8 %; a component of 1 to 5 % may vary by ±0.5 percentage point. KIGL fills welding-grade argon at 99.99 % or better, which meets the group I floor.

Flow rates come from the nozzle, not from the cylinder. TWI's Standard Data for Arc Welding, Volume 2, first published in 1975, found 5 to 10 litres per minute from a 12 mm nozzle the most satisfactory range for manual TIG, with a back purge of 2.4 to 5 litres per minute, and gave 10 to 30 litres per minute for semi-automatic CO₂ welding depending on gun design. The same work used a gas heater on CO₂ equipment where needed, because carbon dioxide cools sharply as it expands. Above the useful range, extra flow turns turbulent and draws air into the shield, so porosity gets worse, not better. The regulator and flowmeter for each gas are covered on regulators for each gas.

Density explains why the gases behave differently in the shield. ISO 14175:2008 Table 1 gives argon at 1.784 kg/m³, 1.380 times as dense as air, and carbon dioxide at 1.977 kg/m³, 1.529 times; both settle over a downhand joint. Helium, at 0.178 kg/m³, rises away from it and needs more flow to cover the same pool.

Argon, carbon dioxide and nitrogen are odourless and give no warning, and the two denser gases collect in tanks, pits and trenches. A welder inside a vessel with gas running is in an atmosphere that is losing oxygen. Ventilate at the low point, monitor oxygen, and treat entry as a permit procedure; inert gas and confined space entry sets out the steps.