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Mig welding mild steel using 100% argon shielding gas

yummie (Structural)
28 Jan 04 9:11
Is it ok to use 100% argon gas to weld mild steel plates ?
Plate thickness is 1/4" to 3/4".
Currently we have no mixed  shielding gas such as 80/20.
We are using solid wire .
We noticed during fit up that some of the tack welds did
not hold.
Is there a proceedure to follow that will allow us to
maintain structural integrity, while using this
combination of parent metal/ gas / & wire ?
Rich2001 (Mechanical)
28 Jan 04 9:45
Typically 100% Argon is not use for GMAW of mild steel. Argon does not have the properties to "wet out" a mild steel spray arc, by adding O2 the weld is hotter, thus slowing the cooling rate of the weld metal

Argon + 1% O2  can be used for both flat and horizontal Fillet welds - Spray Transfer mode.

Argon + 2% O2  can be used for both flat and horizontal Fillet welds - Spray Transfer mode.

Argon + 8% O2  can be used for both flat and horizontal Fillet welds - Spray Transfer mode and All position - Short Circuit Transfer Mode.

Argon + 1% O2  can be used for both flat and horizontal Fillet welds - Spray Transfer mode.

75% Argon + 25% CO2 most commonly used for short-circuit transfer - all position

100% CO2  can be used for both flat and horizontal Fillet welds - Globular Transfer mode and all position - Short Circuit Transfer mode.

Here are a few links that might be helpful
http://www.esabna.com/EUWeb/MIG_handbook/592mig1_1.htm
http://www.weldreality.com

stanweld (Materials)
28 Jan 04 10:38
In addition to the comments supplied by Rich2001,your problem with tacking does not appear to be shielding gas related. 80% Ar-20% CO2 is a standard gas mix primarily used outside the USA. What is the wire diameter, AWS classification and electrical parameters? Other items that may be causing your problems are weld gap, size of tacks (length and throat), welding position and any applied loads. Are the tacks cracking at the fusion line or center line?   
GRoberts (Materials)
28 Jan 04 10:40
If you have tack welds that are not holding, it is not a problem with the gas.  100% argon is not typically used for GMAW because the arc can wander, wich would probably make your problems worse.  The reasons you tack welds are breaking, is that they are not big or often enough for the application, or with GMAW, you may not be getting fusion with the base metal.  If that is the case, you procedure needs to be adjusted.

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