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Tuesday, 24 October 2017

Shielding gases in Welding

Shielding gases for ARC welding (MIG & TIG)(GMAW & GTAW)

shielding gas for fcaw,gmaw,mig welding

Topics covered

  • What is shielding gas used for arc welding process?
  • Purpose why shielding gas used in welding?
  • Properties of shielding Gas
  • Types of shielding Gas
  • Commonly used shielding gases
  • Effects of shielding gases on Welding
  • Applications of shielding gas in welding process
  • Commonly used Shielding gas mixtures used for welding of metals.

What is shielding gas used in arc welding process?

shielding gases are semi-inert or inert gases that used in the process of mig welding and tig welding(GMAW & GTAW) to protect welding from oxygen and water vapour.
shielding gas cylinder for welding.

Purpose of shielding gas for welding?

  • To protect weld puddle from oxygen and water vapour.
  • Protect welding from atmospheric gases(mainly oxygen,hydrogen and nitrogen) because atmospheric gases makes welding process difficult also affects weld quality.
  • Determine weld penetration profiles
  • Helps to maintain ARC stability
  • Helps to achieve desired mechanical properties in completed weld.
  • Affects transfer of filler metal from ARC to welding joint.
  • Increases efficiency and quality of weld
  • Helps to Determine weld bead appearance and weld toughness and strength.

Properties of shielding Gas

  •  Thermal conductivity and heat transfer properties are very important properties of shielding gas.
  • Density also plays important role because more the density lesser flow rates required for welding and vice versa.
  • ionizing property(important for LASER and Plasma welding).
Welding gases can be used as a pure or blend of 2 or 3 gases. 

List and explain welding shielding gases

Argon as a shielding gas - Argon is most common used gas which is used as base for most gas blends.For non ferrous metals(aluminum)generally pure argon  is used.
Color coding for argon cylinder is Blue

Helium as a shielding gas - It's lighter than air so larger flow rates are needed to blanket the weld.It is a inert gas not reacting with  molten metal.It requires higher voltage to start arc as its not easy to ionize.Due to higher ionization it provides hotter arcs at higher voltage.Its good for aluminium,copper and other non ferrous metals.Blends of helium with addition of 5–10% of argon and 2–5% of carbon dioxide ("tri-mix") can be used for welding of stainless steel.Pure helium cant be used for steel as it creates erratic Arc.helium and CO2 are first gases used for welding.
because of cost in-effectiveness argon is preferred instead of helium. 
Helium cylinder color coding - Brown .

CO2 as a shielding gas - Carbon dioxide is least expensive but poorly performing gas.It negatively affects arc stability.Can forms droplets (spatters).Is commonly used with mix with argon which reduces surface tension of molten metal.
Color coding of CO2 cylinder is green grey.

Hydrogen as a shielding gas - It is generally referred for thicker pieces as it improves fluidity of molten metal.It also helps in improving cleanness of surface.Generally  it added to argon typically under 10%. 
However it causes hydrogen embrittlement for many alloys and specially for carbon steels.So its application is limited only for some special stainless steels and nickel.
Hydrogen also can be added in argon-CO2 blend to reduce oxidizing effects of carbon dioxide.Addition of hydrogen cause narrowing of arc and increasing arc temperature which leads to very good welding penetration.It can also be used for conducting materials like Copper but it should not be used for welding of aluminum,steel because it causes porosity & hydrogen embrittlement which may cause cold cracks.

Color coding of hydrogen cylinder is  Red.

Oxygen as a welding gas- oxygen is also used as shielding gas for welding process but in rare cases and in very small amount.Generally 2-5% in argon.It improves Stability of arc and decreases surface tension of molten metal.Oxygen generally applicable in spray transfer type welding process of low alloy,stainless steel and mild carbon steel.It causes oxidation of weld so cannot used for welding aluminium.Excess content of oxygen may cause brittleness in heat affected zone(HAZ).
For austenitic stainless steel argon-oxygen blend is used in which argon is 98-99% and oxygen is 1-2%.In this type of steel generally CO2 not used because it requires low carbon content in weld.

Color coding of Oxygen cylinder is White.

These color codes combined if the gas is blend of two or three.
ex. if Argon-Co2 used then the cylinder colour will be full blue with patch of green.

Dichlorodifluoromethane - Rarely used with argon.For aluminium-lithium alloy for reducing porosity.

Sulfur hexflouride -same application as dichlorodifluoromethane.

Nitric oxide - Reduce ozone and stagbilize arc in high allooy stainless steel welding application.

Main Types of shielding Gases

  • Inert gases (not reacts with other substances)
  • Semi-inert gases

Commonly used shielding Gases Mixture

Commonly used (inert)noble Gases are argon and helium because only argon and Helium are cost effective gases.
These inert gases are used in Tungsten inert gas welding also called as GTAW as well as metal inert gas welding also called as GMAW  
Pure argon and helium used in case of non-ferrous metals.
Among helium and argon is mostly used due to its cost effectiveness and lesser flow rate required as compared with Helium.
Some semi-inert shielding gases like CO2,oxygen,Nitrogen and hydrogen would damage weld if used in large quantities and if used in proper and controlled quantity can improve the weld characteristics. 

Shielding gas blends used for welding.

1-Argon-carbon dioxide.

It is very popular shielding gas for welding of steels.also called as orgoshield.

C5 - C denotes CO2 percentage (5%)and remaining is Argon (95%). is used for pulse spray transfer and short circuiting low alloy steel like ferritic stainless steel.

C10 - (90% argon - 10% CO2) - Low spatter and good weld penetration supported for many steels.Sufficient for ferritic SS.

C15-(85% argon -15% CO2) - This gas mixture is generally used for carbon and low alloy steels.Its suitable for thicker plates.Suitable for high deposition rates.Has lower tendancy to burn  through than higher co2% gas.Suitable for Globular,short circuit,pulse and spray transfer type welding.

C20-(80% argon -20% CO2) - Used for welding of carbon steel with short circuiting and spray transfer.

C25-(75% argon -25% CO2) -Used for welding of low carbon steel.In small scale production.Only be used with short circuiting and globular transfer welding.

C40-(60% argon -40% CO2) - It gives better weld penetration than  C25 grade shielding gas,Used for Some flux cored arc welding (FCAW).

C50-(60% argon -40% CO2) -Used for welding of pipes with short arc.
 here argon-CO2 gas grades list completed for all types of steel generally shielding gas used from C grade.

Is oxygen is used as shielding gas?
Yes in some cases.I also shocked first time i heard this because we use shielding gas to shield weld puddle from nitrogen ,oxygen and water vapour and here oxygen used as shielding gas Fu**.
but its true is used in small content because of its arc stabilization property.

Argon-Oxygen shielding gas
O1-(1% oxygen and 99% argon) - Used for stainless steel helps in stabilizing the arc.
O2- (2% oxygen and 98% argon )-Used for spray arc on stainless steel,low alloy steel and carbon steels.More wetting and darker and oxidized welding than O1.

O5 - (5% oxygen and 95% argon)- This is generally used gas for carbon steel welding.Oxygen helps in higher speed of welding.
more than 5% oxygen causes oxidizing of electrode which results in porosity in weld.

A25 - (25% helium and 75% argon) - Gives higher heat input and good weld appearance,used with some non ferrous base.
A50 -(50% argon and 50% helium) - Used for non ferrous metals thinner than 20mm.used for high speed welding automation.

A75 -(75% helium and 25% argon) -Use for welding of thick section in mechanized welding.Reduce weld porosity in copper.

Argon-Hydrogen shielding gas
H2 (2% hydrogen)
H5 (5% hydrogen)
H10(10% hydrogen)
H35 (35% hydrogen)
(Remained argon 98%,95%,90%,65% respectively)
Other shielding gases for Mig welding process
  •  Argon with 2-5% CO2 and 2-5% Nitrogen in short circuiting mode it increases welding speed also give very good weld shape and color of weld metal.In pulsed spray and spray transfer modes it  is nearly same to other tri-mixes.Gives better weld penetration,Improves arc stability and reduces distortion.
  • 2-5%  CO2, 5-10% argon & 85-95% helium is also used for short circuit welding of Carbon steel.
  • For Austenitic stainless steel Argon-CO2 with 1-2% hydrogen is helpful.Lowers amount of oxide on weld surface.
  • Argon-CO2-Oxygen
  • Argon-helium-hydrogen-CO2
  • Argon-helium-hydrogen.
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  1. This comment has been removed by the author.

  2. thank u ketan for loot of knowledge about inert gases.

    1. welcome sadanand....for your kind support to our work.

      Engineersworld team



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