gas

By - Date 01-27-2006 04:24

If I may, why do you feel that a 98-2 Ar/CO2, or even a tri-mix (90%He/7 1/2% Ar/ 2 1/2% CO2) mixture is better than the 98-2 Ar/O2 mixture for spray mode of transfer? The CO2 in the 98-2 mixture serves as a stabilizer of the arc with very little penetration additive. In the spray mode a 98%Ar/2%O2 is the recommended gas. I fully agree with your owner that the 98%Ar/2%O2 is the best gas for the spray mode of transfer for stainless steel. Respectfully, what do you feel you can achieve by using a 98%Ar/2%CO2 mixture over the recommended mixture of 98%Ar/2%O2 mixture? In the spray mode of transfer, both the CO2 and the O2 are added to the gas for arc stability. Using a CO2 mixture in the spray mode will possibly allow you to pick up unwanted carbon associated with the gas. This will not happen with the O2 mixture.

By cryogenicshaun (**)

Date 01-27-2006 11:08

with the o2 mixture it leaves an oxide coated blackish grey color which we spend alot of time post weld cleaning. all the oxygen does is produce a hotter arc correct? i thought the purpose of the inert gas was to eliminate the oygen? with the co2 wouldnt it eliminate the rough oxide coating of the weld afterwards? therefore require less clean up after welding? would we need to use a pulse spray mode to make a difference. im just looking to improve the quality of the weld because right now they look terrible. i dont know everthing i need to yet im still new in the qc aspect of the business. so most of this ive gathered from reading and talking to different people in the gas industry. his way might be the best, but i figured id give this a shot and see what i came up with.

By Lawrence (*****)

Date 01-27-2006 12:38

The reason for the oxide on the top of your weld beads is that the gas coverage does not remain on the weld metal until it is fully cooled.

A custom shield gas trailer is one option.

FCAW is another option that will provide a better post weld appearence. Slag removal may be easier than you think. The price of the FCAW wire will have to be weighed against the labor to remove the oxide you now find offensive.

Pulsed spray transfer while providing slightly less heat input is still a spray transfer mode and will leave about the same oxide as traditional spray and you will have spent ten grand on a new power supply.

As far as color of the finished bead, the difference in post weld color due to gas changes will be negligable, and as Chuck rightly noted you will be straying from procedures generally accepted as standard.

By - Date 01-27-2006 14:05

The purpose of a shielding gas, as you said, it to keep the atmosphere from the molten metal. Oxygen is the primary oxide former, but the atmosphere also contains other substances such as nitrogen, which you should also try to eliminate as much as possible. In the 98%Ar/2%O2 gas mixture, the oxygen at such a low percentage will not effect the properties of the weld. It is used only to stabilize the arc.The oxidation you see on the surface of the weld, whether it is a dark color or a pretty rainbow color, is a chromium depleted oxide layer. Chromium, the primary element in SS that protects it from corrosion, is burned off during the high heat of the welding process. That is the discoloration you are seeing. To restore this protective layer, this oxide layer must be fully removed in order for the SS to automatically self-heal itself and re-form the impervious, or invisible layer of chromium that is necessary to provide corrosion protection. The oxidation in the air, working with the chemistry of a SS will automatically do this.

By Lawrence (*****)

Date 01-27-2006 15:57

Chuck,

I appreciate your constant focus on the *best* way to produce welds.

I'm not sure if this is a point of conflict or the instance of one code's acceptance criteria being good enough for the intended service and another code having a higher level due to service requirements.

The first example that comes to mind is AWS D17.1 and the acceptence criteria found on table 6.1. The criteria for discoloration of stainless steel is that "All oxidation colors" are acceptable in class "A," class "B," and class "C." welds. The rational that was given to me for this was that the oxide layers left after welding operations were so tenatious as to actually aid in extending the service life of hot section engine componants.

Now I know we did have exceptions in our procedures to remove oxide for multi-pass operations and of course Titanium is another conversation all together.

There are other codes for biotech and food service that require all oxides to be removed by chemical or mechanical means.

So I guess all that blather helps me ask you this; Is it your opinion that *all* stainless welds really ought to have post weld oxide removal as a "best practice" ?

By - Date 01-27-2006 17:39

Hi Lawrence.
My previous response was not intended to recommend the best way to produce welds, but to suggest "after care" once the welds were already produced. While it is a metallurgical fact that the oxide layer is highly detrimental to the anti-corrosivity of SS, that does not mean that it is necessary in every application. Leaving the oxide layer on a weld in totally controlled, sterile, non-corrosive environment is not detrimental. Pickling is recommended as a way to restore the chromium layer that is necessary for maximum corrosion protection of all SS products, regardless of the surroundings. Is it necessary in every instance? No. But, to maximize the anti-corrosivity of SS, then, yes it is necessary. I will not comment on what AWS D17.1 will or will not accept regarding oxidation on welds. I will only say from my point of view, and based on documented metallurgical tests, that pickling is the most advised way to FULLY remove the oxide layer to allow SS to self-heal itself. The pickling follows the recommendations of ASTM A-380.

By Lawrence (*****)

Date 01-27-2006 17:49

Thanks Chuck

I've been trying to get my brain around this whole stainless post weld appearence + corrosion resistence equation for a whole bunch of reasons.

The more we discuss, the more I understand.

By Zeke

Date 01-27-2006 16:15

Chuck,

I hope you have forgiven me for my assumption. I never heard back from you. I hope that you truly beleive that everything I said was related to what I read in your posts. I would not criticize a post unless I thought there was something wrong with it. Obviously I assumed too much, and never thought you would take it personal.
I agree with everything you just said, but wanted your take on something related. Why is it that every SS Flux Core manufacturers rep that comes through the shop focuses on "self-peeling slag" and says nothing about the oxidation left on the weld surface. It seems to me that the oxidation would be more detrimental than slag issues. There are a couple of wires where the slag does not peel (curl) off. It comes off in a straight form, very easily, and there is virtually no oxidation on the surface of the weld. Granted, there are some where the slag will not come off at all and that is unacceptable. I just cannot understand why the sales pitch on self peeling slag, when the weld looks like crap. An optimum slag will stay tight till about 400 F, and then come off easily in straight pieces. What is your opinion?

Zeke

By - Date 01-27-2006 18:08

Zeke,
We're cool. No problems here. I was going to e-mail you, but you did not post your e-mail address. I have mine psoted and thought I might hear from you.

Naturally, the sales pitch of most SS FCAW reps is to point out the best attributes of their products. Self peeling slag, or the ease of removal are a big concern, naturally. You know, Bismuth is the chemical alloy added to the flux of the FCAW wires that allow easy removal of the slag. Too little and it will be hard to remove, or may come off in pieces. Too much Bismuth and the weld will tend to have a greater chance of hot cracking due to picking up Bismuth form the coating into the weld. So, there is a fine line there. Some fabricators request a FCAW with no Bismuth to avoid the possibility of cracks. Then, they have the problem of slag removal being less than easy. Others do not concern themselves with the Bismuth. Heck, some fabricators, supervisors, or QC people may not even know what Bismuth is or what it is added to the flux for. I don't know. The oxidizing agents in the flux play a big part in the appearance of the oxide layer after the flux it removed. Granted, different welding process produce different degrees of oxide layers. It is not uncommon to see one brand of SS FCAW with a heavier oxide layer appearance than another manufacturer. Strictly my personal opinion is that I look for a FCAW that has self-peeling or is very easy slag to remove. The less the oxide layer, he better protection you have, in my opinion. But, the bottom line is that regardless of the circumstance, the heat has burned off the chromium protective layer. Like you said, possibly a sales rep would rather have yoou focus on the ease of slag removal than focus on the ugly weld. You know how that goes...An easy slag removal FCAW with "minimal" oxidation shows me that the formulation of the slag if right where it should be. Have a good day, my friend..

By jon20013 (*****)

Date 01-27-2006 18:58

Hey Chuck, isn't Bismuth in North Dakota? lol ;-)

By - Date 01-27-2006 19:53

Either that or that famous German battleship....

By jon20013 (*****)

Date 01-27-2006 19:55

LMFAO!!!! Oh? Well, tell ya what, it's Friday lets all get gassed!

By cryogenicshaun (**)

Date 01-27-2006 22:59

hey guys thanks for all of your help. like i said im still learning. but the more things i read from these post the more knowledge i gain. so, once again thank you

By Zeke

Date 01-28-2006 14:54

Chuck,

I heard of an instance where a large valve was fabricated with 308L flux cored filler. For some unknown reason, a stress relief treatment was made after welding. The thing nearly fell apart with cracks. Bismuth was blamed. I believe Bismuths melting point is only about 500F. 308H was used on the next one which was also was put under the same PWHT. It was fine. Many people in the US think that in a 308H filler, H stands only for high Carbon. I understand the reasoning for this as all 308H fillers have increased Carbon for high temperature tensile strengths. But obviously from the slag removability, 308H material is made without Bismuth and probably a few other elements that are non-desirable in a high temp application. I wonder how many Stainless jobs have failed due to using the wrong filler materials. I can guarantee that the general welding public has no clue on this issue. I am surprised that the specs don't bring this issue out more clearly. Maybe because Bismuth is used in such a small amount, it is not required to be reported. I do think it deserves at least a footnote somewhere in 5.22.

Zeke

By - Date 01-28-2006 17:50

Zeke,
Your points are very well taken. I can only assume, which sometime I should never do, that the filler metal specifications relating to SS do not list the "trace elements", or what we consider to be trace elements. Unless I'm mistaken, Bismuth is considered a trace element and is measured in PPM instead of percentages. Such could be said for the rare earth elements. I have also heard some engineers, QC, and others sometimes try to consider the stabilizing elements (Cb, Nb,Ta) as trace elements. I, personally, disagree with that statement strictly because columbium (Cb), niobium (Nb), and tantilum (Ta) are dependent upon the percentage of carbon in the formula in virtually all cases. I guess that the specification does not have a min/max when it comes to requirements of Bismuth in the SS FCAW. Most FCAW manufacturers have already done their homework regarding Bismuth and fully realize the pros and cons of it even in very small amounts. The specifications are requirements of chemistry of the filler metal and don't specify requirements for flux chemistry, such as oxidizing agents and our old favorite, Bismuth. Yes, it would be interesting to see every single element that is involved in producing every grade of wire, trace element or not.