Gas mix change

By Lawrence (*****)

Date 02-01-2008 15:50

It depends.

I think it goes something like this:

Welder performance quals are fine with a gas change, if that is the only thing that is different. They will not require retest.

Procedures are different... Pre-qualified WPS will need to be edited to reflect the change in gas mixture.

If the procedures are qualified by testing... than you need to requalify the whole shebang.

By drifter57 (**)

Date 02-01-2008 16:04

Thanks Lawrence, We had a break in our gas mixer and the shop fired up on graveyard using bottles we had on hand that were the 90-10 mix, our gas is piped in usually from the mixing station. So if I understand this I could just ADD WPS for my prequalified for the 90-10 and then I would be covered if this happens again?

By Kix

Date 02-06-2008 18:23

Yes, If all the procedures you have in place at the moment are all prequalified then you can do what you asked. Sounds like your spray arcing so if you're using the right wire tensile for the steel you're welding on then your WPS's should ben prequalified. Might want to look through everything to double check.

By Goose-em (**)

Date 02-06-2008 18:55

Keep in mind that shielding gas can greatly effect the mechanical properties of the final weld. As such before making a gas change it is prudent to look up the effects that the change will have. For instance if you were welding using E70C-6M and 75/25 and decided to change to 90/10 the tensile and yeild strangth of the weld metal will increase. When tensile strength goes up ductility typically goes down as they have an inverse relationship.

In the case of 92/8 verses 90/10 the mechanical properties are similar.

One last note, anytime you decrease or increase the amount of argon present in a gas it will also change the electrical charecteristics of the arc. For instance, welding with 75/25 and then changing nothing but the gas to 90/10 you will notice that the arc seems "hotter". This is because an increase in argon allows you to run slightly lower voltages. This must also be taken into account on your pre-qualified procedure. Most wire manufacturers will have a spec available that should help.

Lastly, for a procedure to be considered pre-qualified it must utilize the electrode manufacturers recomended parameters including shielding gas. So if the manufacturer does not list 90/10 as a recomended gas the procedure would have to be qualified by testing.

By js55

Date 02-06-2008 19:00

Where is the requirement for limiting shielding gas to a manufacturer recommendation?

By mountainman (***)

Date 02-06-2008 21:43

i myself do not see that in the prequalified requirements, but it is listed as a essential variable change to a PQR that would require requalification. however i can't find where it touches on shielding gases in sec 3 prequalified requirements. now i'm confused.

JJ

By js55

Date 02-06-2008 22:14

Where does it say that the manufacturers recommendations for shielding gas are an essential variable? I see changes in mixture, changes in flow, and changes to a gas not covered by the applicable AWS spec. The only place I see mfg recommendation is amperage for SMAW.

By mountainman (***)

Date 02-06-2008 22:43

it doesn't as far as i can see. i should have been more articulate with my words. my understanding is that if the code is silent on something than it is not prohibited. the changes in parameters in amperage,or voltage, or travel speed, or shielding gas flow rate, are the only ones noted that would require a revised or new prequalified WPS. i cannot find where any change in a gas percentage mixture would warrant a new or revised WPS. (prequalified). commonsense tells me to write a new WPS to reflect a change in gas percentage changes. does this sound correct?

JJ

By aevald

Date 02-06-2008 23:01

Hello mountainman, as was already mentioned to a certain degree, gas percentage changes, particularly with some of the metal-core and flux-core gas shielded wires is where the gas changes could get one into trouble. Alloy and wire additive or modifier burn-out rates can change drastically when the gas mixtures are altered from those recommended by the given wire manufacturer. Unexpected metallurgical changes will show-up in the as-welded material deposit and could spell disaster depending on the applications. Just my $.02 Best regards, aevald

By js55

Date 02-07-2008 14:33

jj,
Its cool. I understood. I thought we were essentially approachig it from the same vantage point just saying it different. I just wanted to push the point a little harder.
I think what happens, is that people extrapolate requirements a bit. Maybe anoverzealous inspector has imposed such things, and from that point it is assumed to be code required.
And perhaps someone can clarify a condition for me. Variable 19 addresses (briefly put) a change in mixture, content, or percentage. So then wouldn't this essentially mean any change at all? Except of course for the flow rate that is covered in Variable 20.
Then we have Variable 21. I have to admit, and this is my question isn't Variable 21 just another form of Variable 19?
How can you change to a gas not covered in the stated specs without changing mixture or percentage? If there is a subtle distinction I have to admit it escapes me.
And perhaps Variable 21 is where the mgf confusion comes from.

By mountainman (***)

Date 02-07-2008 16:42

i agree that variable 21 is basically a branch to variable 19, that's how i see it, however i don't know why the information is given that particular way. these particular variables ( 19 and 21) only pertain to a PQR as i see it. and variable 20 pertains to both PQR's and prequalified WPS's per 3.6, if i am correct than why wouldn't a prequalified WPS need to be revised or a new one produced if a change in shielding gas mix percentage has taken place? i'm tryin to stay caught up, but i'm learnin, learnin, learnin.

thanks,
JJ

By bmaas1

Date 02-07-2008 18:39

If you change gas mixes or composition percentage then you would need to revise your WPS or requalify your procedure if it is not prequalified.

Brian

By Goose-em (**)

Date 02-07-2008 20:26

Table 4.5 PQR Essential changes Requiring WPS Requalification for SMAW, SAW, GMAW, FCAW and GTAW.

19) A change in shielding gas from a single gas to any other single gas or mixture of gas, or in the specified nominal percentage composition of a gas, or to no gas

So if a procedure is qualified with 92/8 and it is decided to use 90/10 you need to requalify the procedure.

Perhaps I worded it the wrong way when I said any change to the manufacturers recomendations. Perhaps I should have said any change in shielding gas for any WPS requires requalification based on table 4.5.

Hope this answers your question.

By Lawrence (*****)

Date 02-07-2008 22:31

Goose...

You said "So if a procedure is qualified with 92/8 and it is decided to use 90/10 you need to requalify the procedure."

This is only true of procedures that *must* be qualified by testing.

With a pre-qualified procedure the wps must be revised and that's it. If all other elements in the above senerio fall within the pre-qualified range... a change from 98/2 to 90/10 does not require requalification by testing, Just an update of the WPS itself.

By Goose-em (**)

Date 02-08-2008 18:22

Not to belabour the point here but......

If you are using a pre-qualified procedure it still has to meet certain requirements. If a change in gas composition matters when the procedure was tested then it stands to reason that the same would apply to a prequalified procedure.

If this were not the case then I suppose a person could weld wqith 1 volt and 600 amps because the procedure was prequalified? Or perhaps we changed from 90/10 to 100% oxygen, I suppose this would still be considered prequalified? Or what about 90/10 to 100% helium, still prequalified?

Just because a procedure is prequalified does not mean that a person can change the variables willy nilly in order to suit thier needs. If any variable falls outside the PQR that the prequalified procedure is based on then it must be retested.

And yes a prequalified procedure has at one point been tested and does have a PQR. At some point a PQR was made and A WPS based off of it. It worked, so at that point somebody at AWS said this procedure is prequalified if you follow these instructions. It doesn't mean that someone just woke up one day and labled something prequalified. The very meaning of the word pre-qualified dictates that the procedure was qualified previously. Therefore even a pre-qualified procedure is subject to the same rules that apply to a procedure that was written based on a PQR as the prequalified procedure was based on a PQR.

All a prequalified procedure does is allow the user the ability to skip the PQR process because someone has already done it. In the end we need to use good engineering judgement when making any change to any variable present in a pre-qualified or qualified by testing (all weld procedures are qualified by testing some are just not qualified by the manufacturer) as changes to amperage, voltage, welding gas etc. can have detrimental effects on the final weld joint, including the base metal.

By Lawrence (*****)

Date 02-08-2008 18:55

Goose,

Its good to belabor the point, until we get it right :)

I think you may be confusing a SWPS (Standard Welding Procedure Specification) which does indeed have historically verifiable PQR's directly tied to it, and a Pre-Qualified WPS which does not.

D1.1 Section 3 is not a list of pre-qualified procedures that each have a PQR... They are Groupings of Materials, Processes, Joint designs and other parameters that have indeed been proven over time by experience.

What you can't do is open up D1.1 to section 3 and grab a pre-qualified procedure that is ready made and has been validated by testing.. Which is exactly what you are suggesting. Again.. There are no pre-qualified procedures to be found in AWS D1.1

If you switch 90/10 to oxy 100% you will not achieve spray transfer... No spray transfer.. No pre qualified status. In this case the process is what is prequalified.

Maybe somebody else can say this a bit more clearly ??

By jwright650 (*****)

Date 02-08-2008 19:10

To add just a reference to Lar's reply....
C-3.2.1...the first paragraph of this in the Commentary basically repeats what Lar has stated in his reply.