TIG WPS ASME Section QW 408.9 1

[Nashanas]

Hello everyone,

I am haivng a problem with WPS for GTAW. The WPS format used by my company has a column for "Flowrate" in the section QW408 for Gas declaration. In that section for backing gas, we have declared our glas flow rate, but I noticed that according to QW-256 for GTAW the declaration of gas flow is 408.3 which is not essential, instead 408.9 is essential, which is "- backing". Which to my understanding means the deletion og backing gas. How do I declare it in the WPS?

I have seen that almost all industries declare gas flow rate but they dont declare when do they delete the backing gas during the welding process. Which units should be used here? Should it be a percentage or a weld depth? Or am I understanding everything wrong. I am new to this area, so I apolgize if the question is stupid.

 

[DVWE]

Should it be a percentage or a weld depth?

Usually it is weld depth. The variable is clarified somewhere in the WPS, usually in the "Gas QW-408" section.

"Backing gas shall be applied for at least 1/4" (6mm) of weld depth, or 2 layers minimum."

The devil is in the details; she also wears prada.

 

[Nashanas]

Thank you for the response. Is there any standard depth for the backing gas cutoff? As you have written 6mm or 2 layers. If a layer is 2mm in that case the cutoff depth can be 4mm?

One other thing, my welding operator is saying that the backing gas is used only in case of tubes. If the material is a solid plate, he never applies the backing gas. I dont know if I am interpreting it wrong, but ASME Section IX is not making this differentiation, and backing gas should be there whether plate or tube.

 

[DVWE]

Typically, the material (tube or plate) doesn't matter. What matters is whether the weld joint is open groove or not.

The devil is in the details; she also wears prada.

 

[gtaw]

If the base metal is austenitic stainless steel, the mechanical properties will probably be acceptable with or without backing gas. The need to use backing gas is to prevent root oxidation. If the root side of the joint is exposed to a corrosive environment, it probably makes a different. If the system is dry, corrosion is usually not an issue.

I've read a number of posts and article that are simply inaccurate. One article claimed that welding without backing gas "burns the chrome and nickel out" and converts the stainless to carbon steel. It scary to think that particular article was published in a technical publication. It is simply wrong. I've also read posts on the internet that claimed all the carbon will be "burn out" of the stainless and that's what causes "sugaring". Again, simply wrong and sits along side the wive's tale that preheating is needed to "drive moisture" out of the steel. There are a host of wive's tales that are simply wrong.

The root side shielding (purging) gas is needed until the thickness deposited is thick enough that the root surface isn't heated hot enough to form heat tint (oxidation). That's all heat tint is, oxidized material due to being exposed to oxygen while it is hot enough to react with the oxygen. It only affects the the surface and is very thin except for the most extreme cases. It is a concern if the oxidized surface is or will be exposed to a corrosive environment that can initiate galvanic corrosion. Sensitization is not due to the failure to shield the root side of the joint, it is a different problem.

Best regards - Al

 

[weldstan]

Typically plate will be welded from both sides but that is not always the case, especially for thin plate.

 

[EdStainless]

gtaw, recommended for any Stainless Steel.
We always took as 'at least 2 passes or 6mm, whichever is greater'.
I rarely work heavier than 10mm in tube and pipe, and we require backpurge for all passes.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Nashanas]

We have to perfrom a 3F welding. The plates are 5mm thick. Now, the operator is asking me how to actually perfrom the back purge? Because the sheilding gas comes from the weld gun. He told me that in case of pipes and tubes it is easy to perform back purge because you mount a plug in the pipe fill it with Ar and then perfrom the welding. But in case of 3F fillet welds, how can we provide a back purge? Should it be a seoerate Ar cylinder with a nozzle pointing at the back of the plate?

 

[gtaw]

You can make a fixture or you could take a length of small diameter tubing drilled along it's length with small holes that can be directed toward the root side of the joint. Argon can be directed to the tube via a small flexible hose. The flow rate should be controlled with a separate flowmeter and if possible from a separate cylinder of argon.

You didn't tell us whether this is a "one of a kind" where it will be used once or if this is a production application where hundreds or thousands of parts will be fabricated. Likewise, you didn't tell us whether the joint is 25 mm, 125 mm, or longer.

A little information produces responses with little value.



Best regards - Al

 

[weldstan]

Fillet welds in stainless steels are most often not made with an inert gas b ackpurge. You might want to qualify a fillet weld procedure w/o back purging to resolve the problem or you can do as recommended by gtaw.

 

[Nashanas]

Actually the production is not one of a kind. And this is the problem, because the welder told me that he he can not perfrom welding with a backpurge tool in line which he has to continuously adjust. One way is like "Weldstan" said that I can requalify the WPS without backpurge. But here I wanted to ask a question, I saw in ASME Sec 9, that you can use one PQR to qualify multipe WPS, and also that you can ammend WPS without the need of requalicifcation if the variable you are touching is non-essential. Since for P8 materials the backpurge is not essential, can I play this game? I can make another WPS referring to the PQR with backpurge, but removing the backpurge in WPS, and since bakcpurge is not essential, I will not need to ask a certifying body to re-qualify my WPS?

 

[DVWE]

Since for P8 materials the backpurge is not essential, can I play this game?

EDIT:

You actually can do this, according to QW-408.9

 

[gtaw]

The one question that hasn't been asked is what is the application? Is the back side of the weld exposed to a wet environment or dry. If it is dry, corrosion isn't likely a concern, so backing gas isn't necessary (assuming there is no danger of melt-through). If the base metal is thick enough, the back side will not suffer heat tint.

A welder with a little grey stuff between his ears can probably figure out a way to provide backing gas if it is needed.

The addition or deletion of the backing gas isn't going to affect the mechanical properties of the welded joint.

Best regards - Al