Spray Method

By Niekie3 (***)

Date 01-06-2004 19:15

Hi jfwi

I do not have any direct evidence, but as a general rule of thumb, spray transfer allows the metal being transferred across the arc to react with the shielding gas. In shor-arc, this is not the case.

The result is then that with a relatively high CO2 content in the shielding gas, (obviously it can't be too high if you are welding spray transfer) some of the Si in the wire (ER70S-6 has a high Si content.) reacts with the CO2 and is "stripped out", leaving the weld somewhat weaker and more ductile than would be the case with shot-arc. If you were using a low CO2 content to weld with, which is often the case for spray transfer of thinner base metals, then there should not be a significant difference.

Another factor to keep in mind is that welding with higher heat inputs will generally result in a lower weld metal and HAZ strength, due to grain growth effects. Generally spray transfer will be a higher heat input process than short arc.

Hope this helps.

Regards
Niekie Jooste
Fabristruct Solutions

By dee

Date 01-07-2004 03:06

Niekie,
I thought the caramel-colored, vitrious deposits appearing intermittantly along the weld bead was Si, and took it to indicate the Si was therefore beyond saturation in the weld metal. My impression of the function of Si in the filler was primarily to serve as a cleaning agent is based on the presumption that ER70xx electrodes all share similar physical properties which is, apparently, inaccurate.
Where am I wrong? Are those vitreous deposits indeed Silicon, and if so, how'd it come to be liberated from the S6 filler? Is my recollection that both the s3 and higher Si s6 fillers both test at 60-70ksi leading me to the wrong concfusion?

Does the amount of CO2 in my 180-200 Amp short arc shield gas (85%Ar/15%CO2) relate exactly to what you were saying about CO2 in the shield?
Unfortunately I can only achieve practical spray on my little single phase power supply with smaller, 035 wire, and at that an Ar rich mix is essential. I'm unaware of what a spray arc weld looks like with a 350 or 400 Amp spray arc gas mix (if any "glass" exists)

What I believe I have been seeing is an electrode with essentially silicon to burn.
Lacking reference and anything more than what I presumed I had been observing (detailed characteristics of both spray and short-arc welds in low C steel)... perhaps presuming too much... I seem to find my opinion does not agree with your position. If you could shed a little more light where you think I need it I'd appreciate it. Note that I am carefully defining my position as an opinion; I'm not prepared to defend it as fact.

Ive got a feeling you know something I dont- I have never seen you post opinion without the ability to thoroughly substantiate it and it kinda makes me feel subject to correction.

I would have said that for the low carbon steel I work with there is little or no practical difference between strength of either spray or short-arc transfer GMAW welds... can this be a case to support the practice of bona-fide laboratory testing back here?
I agree with issues with HAZ. I dont find them as serious as one may initially expect because of the faster travel rate (I guess computation of the actual amount of heat put in might be in order), Issues pparently become more pronounced on very cold steel (around 32f) with better results as the temperature rises closer to 70-100f (its allways easier to weld in the summer)... but all else equal I believe the penetration profile is a bit wider with spray (which I generally wellcome and interpret as a good thing)

I've got no comment on welding higher alloys.

It may be that we're both right and the operative word here describing the difference is "practical"?
I am not building pressure vessels or space ships but my impression was that spray is superior to short, in the final analysis, for issues of practicality- dealing with fusion & penetration and deposition rate.

You do build pressure vessels, I think. Inasmuch as none of them have taken off like space ships I am inclined toward discretion :)
I wasn't aware of any change in the filler alloy; your reasoning for it becomes counterintuitive for me. S6 seems to bleed Si; unless those "vitreous" residues are NOT Si, or some other error on my part, I have trouble with the silicon logic. (logic be damned, analizing the weld metal deposits would close the issue)

Regards
d

By bmaas1

Date 01-07-2004 04:16

The caremel colored substance you see is what some refer to, or at least me, as glass is a residual effect of the silicon and other agents that help remove impurities from the molten weld metal. This is pretty common to see.

Brian Maas

By dee

Date 01-07-2004 04:27

brian
Thanks for the tip.

My point: if this Si based compound is bleeding out of the weld metal how can strength be based on reduced Si in filler alloy? It's excess in either mode or impurity-dependant.
D
Off-point: "vitrious" describes a glass-like material, and the caramel residue probably truely IS a silicon glass

By Niekie3 (***)

Date 01-07-2004 05:42

Hi d

The Si addition within E70S-6 is indeed for the purpose of removing impurities in the weld pool. None-the-less, Si has the effect of increasing the tensile strength of the weld metal. The effect is about 15ksi per 1% of Si. (I hope that my conversion was correct - it is 10kg/mm2) As one could potentially run between just over 1% Si, when none of the Si is removed, to probably around 0.5% or less if some is removed, we see that we could have a difference of around 7ksi in tensile and yield strengths. This I believe to be significant. (10% of UTS)

We have to remember that the 70 ksi value for the filler, is a minimum value. It is very typical to get around 80 ksi (or higher) tensile strengths with a E70S-6 filler.

As a general rule then, giving all other factors as equal, an E70S-6 would have a higher tensile strength than an E70S-2.

I have to also just emphasize that I have not done any comparative tests myself, but the opinion given was based on the generally accepted "theory" of what should happen.

Regards
Niekie Jooste
Fabristruct Solutions

By dee

Date 01-07-2004 06:53

Hello Niekie,
How are things on the warm end of Mother Earth today?

Accepted theory of what should happen should be acceptable.

I'm reading 15ksi as 15,000lbs/square inch equal to 10,000grams/square mm... (it's too late for me- my face is in my keyboard abd I'm nearly seeing double)

I don't disagree from a point of ignorance; I merely need to get the concept grounded to where I can get my brain around it.
Questions remain. I cannot relate to the amount of CO2 necessary to liberate a given amount of Si from the filler and I've no feel for the amount of impurities required to liberate a like amount of Si. Finally one remaining point remains undefined... 10% of UTS is indeed significant on its own, however, of what practical significance is even more than that where the deposit remains stronger than required even after the loss of Si...

Apparently it seems the mode of transfer needs to be considered when selecting filler.

Thanks for your patience and the handy information.

Regards
d

By Niekie3 (***)

Date 01-07-2004 18:56

Hi d

Things are indeed nice and warm around our end of the planet. In fact, we could do with a little rain right about now!

I could not give you any definitive answers regarding the amount of impurities or CO2 content that will result in any given amount of Si loss. I have not seen any such information from the filler metal suppliers either. They merely give the guidelines as I have also given. (Also in SFA5.18 appendix.)

Regarding the thoughts that the loss of strength is not significant, I must agree, as long as the other factors such as heat input etc. do not also lead to a loss in properties. The combination could just be enough to result in a value lower than the minimum required.

At any rate, in South Africa we almost always use ER70S-6. Hardly ever have I been involved in a job where ER70S-2 is used. In fact, most of the local filler metal catalogues do not even list any other alternative to S-6! I guess we prefer to err on the high strength side.

Just as a side issue, there are times when there can be quite a build-up of Si, which theoretically could lead to cracking under some circumstances. If my memory serves me well, this issue has in fact been discussed on the forum already.

Hope this answers your query.

Regards
Niekie Jooste
Fabristruct Solutions

By H.Dibben (*)

Date 01-09-2004 18:11

As a side bar to this discussion, the new Canadian spec for filler metals requires the shielding gas to have a minimum oxygen equivalent (= % 02 + .5 x % CO2) of 4 % for ER70S6 filler. The theory (as much as I understand it) is that the oxygen is required to remove some of the Si and Mn from the weld metal. Without the oxygen in the gas, the strength of the weld metal could become too high with a consequent loss of ductility.

This would certainly imply that the shielding gas has a significant affect on the weld metal composition.

By jfwi

Date 01-15-2004 19:28

Hi,

As I replied to Niekie after I first posted, research I found gave me much the same answer as you said, the shielding gas if reactive has a big effect on the final product.

Thanks for the answer, Jerry.

By jfwi

Date 01-15-2004 19:13

Niekie,

Thanks for the information, I have read all the entry that were made on this subject and consider all helpful. The shielding gas that I have seen used are 75% argon - 25% CO2 and 98% argon - 2% oxygen the second choice recommended by a supplier. The research I made gave me the impression that the reactive gases, CO2 and O2 may have larger effect on the end product. I was under the impression that the O2 was used as a "scrubbing agent" much like H. Dibben mentioned, giving ductility.
I just like trying to find answers that are not normally given.

Thank you for you response, Jerry

By Niekie3 (***)

Date 01-15-2004 21:42

Hi Jerry

Just keep in mind that 25% CO2 is a bit high to be getting spray transfer with. This gas is really for dip transfer, or FCAW.

Regards
Niekie Jooste
Fabristruct Solutions

By Lawrence (*****)

Date 01-15-2004 22:20

This is a good point!

Stable spray transfer cannot be consistantly accomplished with 75/25 and 0.35/0.09mm mig wire (mild steel). 15-20 argon/Co2 or tri mixes are a better choice

However you can achieve spray transfer with 75/25 and 0.045/1.2mm mig wire at:
34 volts @ 330 amps at the low end
39 volts @ 450 amps at the high end

For 0.052/1.4mm mig wire with 75/25
33 volts @ 350 amps on the low end
36 Volts @ 440 amps on the high end

Mill scale (if present) is also be a big factor and should be a consideration in gas selection and voltage settings.

As far as the mechanicals go I'll leave that to deeper swimming fish.

By jfwi

Date 01-16-2004 16:44

Sweet, I love numbers.

By bmaas1

Date 01-16-2004 17:30

If memory serves me, you need an argon percentage of at least 80% to achieve a true spray transfer.

Brian J. Maas

By dee

Date 01-17-2004 02:08

Brian,
Actually the correct response to that is yes in practical terms, but "no" in actual terms.

Spray transfer is a phemoneon of physics. It occurs at energy levels determined by current density (machine settings/electrode diameter) and the ionization potential of the shielding gas. The harder it is to ionize the gas, as well as the larger the diameter of the electrode, the more energy is required. [Magnetic?] effects upon arc stability, operator comfort, and equipment tend to establish the practical limits.

He, O2, and CO2 serve to raise the ionization potential of Ar

Drop in to see Ed Craig for more detail than you need: http://www.weldreality.com

Regards
d

By brande

Date 01-17-2004 06:10

Mill scale has a great deal of silicon in it, so I am told. Could easily cause numerous "slag islands" as we say around here, or the "glassy deposits" we all see in our hard wire welds.

Good Luck

brande

By jfwi

Date 01-16-2004 16:44

Thanks this is very good information. When I did use spray method the 98% argon - 2% oxygen. The 75-25 was used by a contractor that I was questioning at the time. Thank you again.

By bmaas1

Date 01-07-2004 00:16

It depends on the carbon content of the material. Low carbon or high carbon. With low carbon the difference would probably be negligible. With high carbon steel then we start having significant grain growth which could be a problem. It also depends on what the cooling rate will be. A quick cooling rate will result in a harder, stronger but less ductile weld compared to normal cooling rates.

Just my thoughts,

Brian J. Maas