CO2

Hi Guest,

During welding temperatures are achieved which would cause the constituent gases of air to react with the metal and produce an inferior seam.

CO2 in reasonably inert in this instance, and also cheap to produce.

The CO2 pushes other gases away from the hot zone, so that no unwanted impurities are allowed to merge with the weld material.

For stainless steel, CO2 is not good enough, and argon is used instead.

To weld most successfully, the heat should be as localised as possible, so any cooling effect the shroud gas has helps to stop the heat spreading across the material which can easily cause twisting and warping.

Although I like your answer, I fear you lost the OP with your first sentence.

In the same aspect as the fire extinguisher you are flooding the atmosphere around the weld to keep other gases from contaminating the molten pool until it cools. The air has all kinds of impurities in it that the heat of the molten pool would ignite . With out an oxidiser they can't burn. With fire extinguisher you are doing it to remove the oxygen and snuff the fire.

As far as being a refrigerant most of the cooling effect has been done. It was done at the regulator when the pressure was reduced from the bottle to a working pressure. May get some cooling effect but it's minimal. Even if it wasn't you could compensate with adjustments on the welder. Increasing the heat of the arc.

CO2-shielding gas-keeps impurities out of the weld.

co2 adds carbon to the weld and weakens it,

argon and helium hold heat in and sheild the weld,

helium works best for aluminium as it disapates heat

so fast that the sheild gas needs to be best insulator,

always clean alum with stainless steel brush not

steel brush or sand paper, contaminates the weld

Mitch the retired peugeot mech

Carbon di-oxide is widely used for welding of mild steel as it is inexpensive, gives sound weld deposits, deeper penetration compared to argon and helium and adequate mechanical properties at high welding speed.

CO₂ has higher hot-cracking resistance compared to SAW, SMAW and TIG processes. It also improves weld metal ductility and toughness, CO₂ with Argon is well suitable in spray arc type welding used in MAG/CO₂ process. This mixture also give higher Tensile & yield strength compared to welding with pure CO₂ gas.

To answer your query about non freezing during welding, it is to be understood that, the arc temparature is nearly 5,000-6,000⁰C and is nearly three to four times more than normal fire temp.

Further, CO₂ cylinder contains liquid and gaseous carbon di-oxide in equilibrium at high pressure. As the gaseous CO₂ is withdrawn, from the space at the top of the cylinder, liquid CO₂ evaporataes to replace the gas removed, thus maitaining the equilibrium between gas and liquid.

If the gas is taken off too fast, the evoporating liquid is unable to obtain sufficient heat from the environment and the temparature of the system falls with a decrease in the CO₂ top pressure.

If the process is carried out still further, the cooling may be so intense, that the liquid CO₂ remaining in the cylinder freezes solid and the pressure pushes it out before it happens... This is what happens in fire extingusher with CO₂ gas and thus able to extinguish the fire to large an extent immediataely.

To preevenat it happening during welding, CO₂ is not allowed to be drawn off at a rate exeeding 6 lb/hr (48 ft³/hr) and preferably no faster than 4 lb/hr (32 ft³/hr). If more CO₂ is required, manifold system with minimum of 4-6 cylinders are used, in which case, the withdral of CO₂ can be up to 16-24 lb/hr (128 -192 ft³/hr).

This is why the CO₂ used for welding do not freeze out and it is due to slow withdrawl of the gas and welding is no affected. It is prevented further by using a heater before regulator, which provides a latent heat to pevent cogging of the gas at the outlet of the cylinder.

Such arrangement is not available in the fire extinguisher and free flow of high pressure gas in the absence of a regulator, extinguses the fire immediately.

Sridhar.

co2 is chemically inert gas.it does not react with o2.so, it doesn't cause fire while using for high heat application.the main reason for using co2 in arc welding as shielding gas and as fire extinguisher is to prevent the entry of o2 from the atmosphere to the process.