gas for stainless???

By pjseaman (**)

Date 09-20-2005 00:32

Depends on the process! Tig or mig? Tig argon or helium. Mig tri-mix is the ideal gas, but it is expensive.

By - Date 09-20-2005 02:07

Yes, it depends on the process. Argon is the best shielding and purge gas for the austenitic stainless steels. For MIG, I recommend 98%Argon-2%oxygen. Tri-mix, 90He-71/2Ar-21/2CO2 is acceptable, but helium is 10X ligher than argon. Why would you want to shield with a gas that is lighter than air?

CM

By backpurge (*)

Date 09-20-2005 12:14

Depends on thickness and process

TIG up to 2mm - argon. Over 2mm - argon & 2 to 5% hydrogen, backpurging - nitrogen & 10% hydrogen

MAG - argon and 2% CO2 wil give far better results than argon & 2% O2 or around 40% helium, 2% CO2 balance argon

By - Date 09-20-2005 12:32

Nitrogen above 3% can have adverse effects on the tungsten causing possible tungsten inclusions.

By backpurge (*)

Date 09-20-2005 12:46

Agree totally, that's why we only use it for root protection where the results you get when you mix it with hydogen are far superior to anything you can achieve with straight argon

By - Date 09-20-2005 13:38

Hydrogen is not recommended for welding the ferritic and martensitic stainless steels due to the disolved hydrogen and high ferrites can cause hydrogen cracking. Additions of more than 5% hydrogen can cause porosity in the weld pool.

Shielding gases for TIG welding:

Ferritic and martensitic--1st choice: argon, 2nd choice:argon + 30% helium

standard austenitics (304, 316, etc)--1st choice:argon, 2nd choice:argon + 30% helium, 3rd choice:argon + 2% hydrogen

Fully austenitics (254 SMO, etc)--1st choice:Argon, 2nd choice:Argon+ 2%N2, 3rd choice:Ar +30% He+2% N2

Duplex (2205, 2507)--1st choice:Ar, 2nd choice:Ar +2% N2, 3rd choice: Ar+30%He+2%N2

Nickel based alloys (625, 800)--1st choice:Ar, 2nd choice:Ar+30%He

Chuck

By JCaprarulo (**)

Date 09-25-2005 15:49

What about 100% Nitrogen for backpurging in austenitic SS.

By - Date 09-25-2005 17:26

Nitrogen is fine for a backpurge on SS, but one needs to realize that when doing a root pass using the TIG process, the pure nitrogen will cause possible breakdown of the tungsten causing very likely tungsten inclusions. 100% nitrogen is also not recommended as a back purge for the Duplex SS. Nitrogen, being a strong austenite former, will lower the ferrite levels to possible unsatisfactory levels. It's just hard to beat good ole argon for a backpurge for SS, even the Duplex grades.

Chuck

By jon20013 (*****)

Date 09-20-2005 13:27

Chuck hit it right on the button.

By backpurge (*)

Date 09-20-2005 15:05

Totally agree with your advise Chuck although in the UK argon/hydrogen would always be picked ahead of argon/helium for austenitics, it's not a price thing either as both mixes cost around the same, more down to welding speed and quality

Cheers - Bob

By - Date 09-20-2005 15:30

Hi Bob,
I guess I failed to mention that here in the USA, many distributors do not like to mix hydrogen with anything. Unfortunately, there is a big misconception that hydrogen, in any amount, is dangerous. As we know, that is not the case, but still, here in the USA it is rarely used as a purge or shielding gas. Even the use of helium needs to have considerable consideration before using it, too. Helium is primarily used for it's higher arc energy, but even that has it's drawbacks, especially when welding thinner thicknesses and when welding the fully austenitic SS which has a higher tendency for precipitation of secondary phases. The same applies to hydrogen. When mixing gases, there is always the possibility of gas separation after extended periods of time and improper mixing. Hydrogen has limited uses (not recommended for all stainless steels) so the process and service needs to be considered before using it.

Chuck

By medicinehawk (**)

Date 09-25-2005 19:59

I only know from past experience that an Argon/Hydrogen mixture for welding stainless does affect the GTAW (or TIG) process in a positive way. I work for a contractor who specializes in automatic tube welding or Orbital welding where we use the GTAW fusion process to join thin wall stainless steel tubing, usually 316L.
Years ago, I notice a competitor who used a 92/8 % mixture of argon/hydrogen and I ask the welder what the advandage was of using this bi-mixture. He said it made the(weld) puddle hotter while allowing of a thinner weld bead while maintaining penetration. Casual observation told me this was probably true.
I went to the QA/QC(of my company) and asked them about the mixture and they where aware of it, but stated that it was more expensive considering the minor affects of quality as compared to pure argon. I guess this makes sense, so I didn't need to push the issue as I knew our quality was very high, indeed.
A couple of years later we had a job where they(The Client) wanted us to manually fuse 316L, schedule-10 pipe for the root pass (sizes ranged from 1/2" - 10" diameter). The kicker was that they did not want any color other than a very slight blue or straw (perferably NO color AT ALL) on the interior of the pipe.
WE have routinely acomplished this when we Orbital weld for the Biotech industry (thin walled tubing).......anyway, they wanted the same criterion for pipe welding as is applied to tubing.
This was not a problem with pipe diameters from 1/2" up to 6", but sizes like 8" and 10" proved to be a challange as the wall thickness is greater than smaller bore pipe.
To make a long story short, we used pure Argon for the purge gas and some of us use used 92/8% argon/hydrogen in our tig torches which allowed us to use less amperage while maintaining complete penetration and achieving a colorless root (on the ID) of the pipe. There is more to it than that, but I have already given too many details I think, but anyway....I liked using that bi-mixture and I truely believe quality was inhanced because of it.

By - Date 09-25-2005 20:25

Hydrogen and helium work much in the same way concerning arc energy and weld penetration. Hydrogen, with an atomic weight on 1.01, and helium, with an atomic weight of 4.0 both allow less "restrictiveness" in the weld arc. In some cases this is an advantage, but in some cases this is a disadvantage. Hydrogen is definitely a strong reducing agent and minimizes the amount of oxidation on the weld bead surface. Like helium, hydrogen has a higher ionization potential and thermal conductivity, which results in a higher arc energy. This higher arc energy results in a higher heat input which needs to be considered when welding this guage steels and fully austenitic stainless steels. Also, hydrogen amounts above 5% have a much greater propensity for porosity in the weld metal than 5% and less. Many fabricators say that when welding large bore pipe, that the Ar/He mix or the Ar/H mixture cost much more and they do not get that much added extra benefit from the less expensive straight argon. I guess it is up to the fabricator and how much he is pinching his pennies.

By medicinehawk (**)

Date 09-26-2005 07:26

I appreciate the technical data concerning the reason "the arc is Hotter" as all I know is from doing. Thanks Chuck.
In the most General aspect, straight Argon works fine and I guess the cost coefficient makes it the way to go. I fully agree.
What I was saying in my previous post was that the root pass (including the HAZ) had to have NO color other than the silver/blue (very light straw) of larger bore schedule 10, 316L SS (6"-10" Dia.) pipe.
Have you ever tried this????
It is more a matter of ID purge than heat input. To me, Argon/Hydrogen mixture worked well in using less amperage than straight argon with the tig torch. I saw no instances of porosity in the work being done, here.
But if it took 135-150 amps (DCSP) with straight Argon to sink a root
as opposed to 110-125 amps (DCSP) using this Bi-mixture.....I would conclude the later is the better choice concerning quality as the less heat input the better.
Like I said......this was an isolated job where the Client wanted the interior of the welded pipes to have no colour.
In all other instance straight Argon works fine.
Thanks,
Hawk

By backpurge (*)

Date 09-26-2005 08:37

The main reasons Ar/H2 mixtures are so popular in the UK for welding austenitics are.
Productivity is increased dramatically, in some cases manual welding times can be halved, this is mainly when welding plate above 2mm thick, although good improvements are also noted when welding pipe. These improvements far outweigh the increase in gas price, about 3X that of argon (if you're welding twice as fast you only use 1/2 as much gas anyway)

Reduced post weld cleaning and removal of oxides.

Better weld width to penetration depth ratios, hydrogen narrows the arc and concentrates the "hot spot" whereas, although helium increases weldpool temperature it also broadens the arc considerably

By - Date 09-26-2005 11:24

You are correct in your statements, but it seems that the UK and the USA have different feelings on gas mixtures. Apparently, the use of hydrogen in the USA is no where near as popular as in the UK, for whatever reason. Lastly, all gas mixtures, including Ar/H mixture, has it's advantages and disadvantages. The only thing that matters is what the fabricator decides is best for his application. Regardless of the gas used, post weld clean up is necessary to restore the SS back to it's natural anti-corrosive state. Whatever gas is used, the heat from the welding process has burnt away the chromium portective layer that is necessary for corrosion portection.

By Jim Hughes (***)

Date 09-26-2005 17:39

Hey Chuck, looks like for the most part prayer worked for Texas. Victoria only recieved 1.5 inches of rain.

I'm seeing more Ar/Hydrogen use in the work I have been doing. My first exper. was working with Laburdi/Diametrics machines on some canister lids. We achieved better penetration with the 3% Hydrogen mix. But I did also learn that here is the nuclear industry you could only use up to 3% because of safety and storage issues. I don't know if that was just for this project or if that is accross the US. But I have nver heard of anyone using Ar/Hydrogen mix for purge gas. I don't see what advantage that would be.

Thanks
Jim

By backpurge (*)

Date 09-26-2005 17:46

Ar/H2 gives a better result when purging because of the reducing effect of the hydrogen, you don't need to get the residual O2 level down as low as you do with Argon to get really clean penetration beads so you use a lot less purge gas.

We've just completed a job on 316 st/st and used nitrogen with 10% H2 for the purge. This gas is a lot lighter than air so it sits right under the weld seam forcing any remaining O2 out of the penetration zone, when completed the penetration bead looked like it was chrome plated!!! saved a load of gas and no need to passivate

By - Date 09-26-2005 18:18

Hi Bob,
"This gas is a lot lighter than air...." That is one of my drawbacks of using helium or hydrogen as a shielding gas. Argon, 40X heavier than hydrogen and 10X heavier helium, requires more CFH to adequately shield the SS than the MUCH heavier argon. Again, all of the gases we have been talking about has it's advantages and disadvantages. For a company that welds all families of SS, argon is unbeatable. Hydrogen is not recommended for the ferritic and martensitic grades for reasons stated earlier, so less amount of cylinders will have to be stored for fabricators that weld all families of SS. The Formier gas (90%He-10%H) is much more popular in the UK than over the pond. Hydrogen gas in any mixture is in the very low percentages here due to the perception that 1) is is more dangerous, and 2) it does not warrant the extra cost. I'm not saying I agree with that perception, I'm just saying what is true here in the USA