differences between welding gases 8

[xray1366]

I want to know the differences between the Backing gas, Trailing gas and the Shielding gas, Also the Standard(by details) that these words are defined in it(I quess in ASME Sec. ix)
Would you help me please!
I'm a new comer to this Website.
Thank you.

 

[metengr]

Do you have a copy of ASME Section IX and AWS SFA standards? If not, better get your hands on them if you want to learn.

For backing gas, specifically look at ASME Section IX, QW-492 Definitions. Also, look at the specific welding process within the Definitions where the function of shielding gas is described (e.g. GTAW process). Trailing gas would be considered similar to shielding based on welding process and material, but is not technically defined in Definitions.

AWS 5.32 Specification for Shielding Gases


You have all you need.

 

[racookpe1978]

Check Google and Wikipedia more carefully: I think you are looking at Plasma arc welding details.

 

[racookpe1978]

You need to read - and understand thoroughly! - this welding gas manual from Praxair:

http://www.awssection.org/uploads/longisland/files/Praxair_Shielding_Gas_Manual.pdf

 

[xray1366]

Thank you for your help. Your guidance was so useful.
I'm not looking at PAW, I just want to answer the question asked by my QC Manager, "the differences between these three". Anyway I appreciate you, the link was useful.
Thanks.

 

[Duwe6]

Took me a while to find gas definitions in Sect IX - I had to have somebody explain it to me, also.

Shielding gas is the one emanating from the GTAW 'Tig' torch, or the GMAW 'Mig/MAG' gun, to purge the area of the top of the weld puddle..

Trailing gas is additional gas flow that follows [trails] the torch/gun, and provides a longer purge time. Titanium welding requires trailing gas.

Backing gas is the internal purge of a pipe or fitting, that keeps the back side of easily oxidized metals from getting gross coats of oxudation "sugaring" [looks like lumpy brown sugar]. Stainless, Titanium, Inconel all require purging. Anly Sect. IX calls it "Backing Gas". All the 'field hands' refer to it as a purge.

And that is a pretty good reference manual racookpe1978 attached. Thanks.

 

[xray1366]

Thnak you Duwe6, but I want to know why the trailing gas provides a longer purge time?(what is the purpose?), and also which gases are used to trail?
Thanks.

 

[berkshire]

''why the trailing gas provides a longer purge time?(what is the purpose?),''

To allow the welded metal to cool to a temperature ( away from oxygen) to where it is no longer reactive.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[xray1366]

Thank you very much berkshire. do you know how it works?
and also which gases are used to trail?

 

[moltenmetal]

Reactive metals like titanium, zirconium and tantalum, react with both oxygen AND nitrogen whenever the metal is hot. The rate of reaction is significant even at 400 C (800 F). The reaction results in embrittlement which can lead to premature failures. You need to keep those areas, AND the filler metal, completely covered by an inert atmosphere until they're quite cool. You can do that either by doing the welding inside a glovebox full of inert gas, or by using processes designed to keep any hot part of the item being welded completely shielded from the atmosphere. Trailing shields are sometimes necessary as part of that.

 

[xray1366]

Thank you so much moltenmetal. It was very useful.

 

[Duwe6]

And the only fully inert purge/shielding gasses are argon and helium. Argon is heavier than air, and MUCH cheaper than helium, so it is the most common gas used when inerting is required. Like a trailing gas.

Some folks, especially Europeans, use nitrogen to purge pipes [backing gas]. Nitrogen is NOT inert, just fairly unreactive to metals. Stay with argon. At leaa than $30 per bottle fill-up, it is the obvious choice.

 

[xray1366]

thank you for your help Duwe6.

 

[gtaw]

Check AWS A3.0 for welding terms and definitions.

If a welding term or deninition is not included in the applicable fabrication code, ASME, AWS, API, and military welding standards reference AWS A3.0 for welding and brazing terms and deinitions.

Best regards - Al

Best regards - Al