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Back Purging Requirements for Stainless Steel Welding

Back Purging Requirements for Stainless Steel Welding

(OP)
Hello,

We are fillet TIG welding stainless support pads to 316L 4''sch10S pipe.

It is fillet weld but is back purging argon through pipe still required ?

Thx.

RE: Back Purging Requirements for Stainless Steel Welding

If heat tint on the pipe ID surface is not permitted beneath the weld region on the OD surface you may need to protect the thin wall pipe ID surface from oxidation due to the heat of welding. In this case, you may need to use Argon to prevent ID oxidation, otherwise only shielding gas is required for a fillet weld.

RE: Back Purging Requirements for Stainless Steel Welding

If you care about the corrosion resistance of the ID then you need to back purge, and you should be checking the color afterward.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Back Purging Requirements for Stainless Steel Welding

If the ID of the pipe contains an electrolyte and subject to corrosion, purging is recommended. If the fluid is dry gas or hot gases or if the fluid is not corrosive, the heat tint is probably not a problem. However, any scaling could be a problem if the scale detaches and contaminated something down stream.

So, the answer is dependent on the service conditions and the application.

Best regards - Al

RE: Back Purging Requirements for Stainless Steel Welding

You're welding Sch 10 pipe(typical wall thickness of 3mm or 1/8").Heat tinting may occur easily, depending on the welding parameters used.Why not use a mock up piece and see the extent of heat tint on the ID.If there's access to the ID use of rotary brush-to clean the tinted ares would help.
Depending on the extent of heat tint and the process fluids, heat tinted areas may corrode: https://www.nickelinstitute.org/~/media/Files/Tech....

Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299
All provided answer are personal opinions or personal judgements only. It's not connected with any employers by any means.

RE: Back Purging Requirements for Stainless Steel Welding

Kripton,
Very good info from all the previous posters.
Will add a little bit more as I have done a huge amount of this type of work years ago on Sch 10 & Sch 5 stainless (without purge).
Support pads are almost always thicker than the actual pipe material so direct all of the heat from the TIG torch to the pad and not the parent material.
In a normal fillet weld you would aim the arc towards the junction of the pad and pipe.
Aim the arc as far away from the pipe as you can and the molten S/S filler metal will flow down and fuse with the pipe material without causing the pipe material to oxidise or get a heat tint.
You couldn't do the same with C/S but it is one of the pleasures of working with S/S,
Hope that helps,
Cheers,
DD

RE: Back Purging Requirements for Stainless Steel Welding

If purge is required to protect the ID surface of the pipe, you could use nitrogen

RE: Back Purging Requirements for Stainless Steel Welding

(OP)
Very good inputs from all of you.

Thank You All.

RE: Back Purging Requirements for Stainless Steel Welding

If the inside of the pipe can be reached , you could also use pickling paste to remove the heat tint, but it is better not to have it there in the first place.
B.E.

You are judged not by what you know, but by what you can do.

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