Importance of Shielding gases
Welding shielding gases are an essential part of a successful sound weld deposit. In general, we study basic information about shielding gases. In this post, I have dug deeper for a detailed insight into the important properties of the shielding gases that affect and influence the welding output in gas shielded arc welding processes. The following are the main properties of the shielding gases:
- Ionization Potential
- Dew Point
- Thermal conductivity
- Specific gravity, and
- Gas density
Ionization Potential for welding shielding gases
The ionization potential (measured in electron volts (eV) or kJ/mol) for a shielding gas is the voltage required to remove an electron from an atom from the shielding gas and convert that removed atom to an ion. In simple words, its tells, how easily the shielding gas can form electrically conducting Plasma?
Arc stability & arc ignition are highly dependent on the ionization potential of the shielding gas in the welding arc. E.g, an argon shield has a much better arc ignition and smoother arc than helium shielding due to the low ionization potential of argon.
Usually, the higher the ionization potential of the shielding gas, the hotter the welding arc be. This is due to the reason that ionized particles return to the job surface and release higher ionization energy. Also, as the ionization potential increase, it requires higher voltage to ignite the arc. The higher arc voltage of helium produces a stronger arc with high heat input, so creating a weld with more penetration.
Below, a table of the ionization potential of shielding gases is given. The ionization potential for argon is 15.7 eV, Hydrogen 13.5, Helium 24.5eV, Nitrogen 14.51, Carbon dioxide 14.4, oxygen 13.2. This is why argon requires a lower voltage than helium during welding.
Dew Point for welding shielding gases
The dew point of the shield gas should be very low. The dew point of a gas is the temperature at which the moisture in the gas condenses as water vapor. Welding shield gas must be very dry (low dew point) as the moisture (water) in the gas is converted to hydrogen and oxygen as it passes in the welding arc (2H2O = 2H2 + O2). Moisture leads to the porosity of the weld metal. The dew point of the shielding gas must be 104°F or 40 ° C or less.
The minimum gas purity, maximum moisture, and maximum dew point at 1 atmospheric pressure are shown in the below table. For Argon gas a welding shielding gas, the minimum purity level is 99.997, CO2 99.8, helium 99.995.
Determining the Dew Point for shielding gas
Sample gases for dew point analysis shall be drawn from the individual cylinder, vessel, or gas outlet source. Any standard dew point measurement method may be used.
Dew point may be expressed in °F at one-atmosphere pressure (14.7 psia), [°C at 760 mm of mercury], or in ppm. The Dew Point Conversion Chart, shown in the below table may be used to convert dew point measurements to or from °F, °C, or ppm. Results of the dew point test shall meet, or exceed, the requirements of Table 1 for the gases being analyzed.
Thermal conductivity for welding shielding gases
The thermal conductivity of gas means its ability to conduct heat during welding. A gas with good thermal conductivity improves the thermal conductivity of the work. The thermal conductivity of the shielding gas affects the shape of the weld bead, defines the depth of weld penetration.
Helium offers excellent thermal conductivity in welding. In TIG or MIG welding with helium, the arc disperses the heat in the weld pool and produces a wider and smooth weld bead appearance. Argon, on the other hand, has a low thermal conductivity, which narrows the arc column and thus gives a wine glass shape weld penetration profile. The thermal conductivity of carbon dioxide lies between argon and helium, which creates a good weld bead between the two gases but high weld spatters.
Specific gravity for welding shielding gases
The specific gravity of the shielding gas depends on the velocity required to generate an effective gas shielded arc and is also affected by the specific density of the protective gas. The lower the density of the shielding gas, the higher the gas flow rate required to produce an effective gas shielding. For example, the flow rates of helium and hydrogen as gas shielding need to be significantly higher than argon for welding.
Gas density for welding shielding gases
Gas density is the weight per unit volume of gas. Heavier shielding gases such as CO2 or argon produce better coverage than low-density gases (For example Helium), as well as lighter gases, require more gas flow rate because they will easily get dispersed in the atmosphere due to low density. Argon gas is 1.5 times heavier than air and 10.5 times heavier than helium, so it requires a flow rate of 1.5 times that of helium, but the purpose in the welding is the same. The density of important welding shielding gases is shown in the above table.