Stainless Steel Mig Welding Procedure

[RobsVette]

Hi Guys,

I have to make up a Stainless Steel Mig welding procedure for a project we are doing. The project is the field assembly of a 316 Demin water storage tank.

The thought process at our company is to Mig weld the tank seems in order to be able to weld faster.

I was hoping someone coudl provide some clarification regarding a couple of different items and make sure my thinking is correct and add any feedback on items I may be over looking.

The project is going to involve welding 316L stainless plate together to make a square water tank. The tank is about 40' long x 20' wide x 20' tall and the plates are coming in about 8' x 8' sections. From what I understand some of the weld will be X Rayed, since the weld seems are going to be open butt for the most part. Here are my concerns.

1) Backing Gas - Since I have a decently thick piece of steel (3/8") I feel that the right gas is the 90% Helium, 7.5% Argon, 2.5% CO2 mix. I say this because I know that the Helium leads to a hotter weld and since I have some good thickness to work with I dont think it would be a problem. My concern here is that the higher heat from the helium will sensitise the 316 stainless and cause problems with Rust and the weld turning black. I think the alternative to the tri-mix is the 98% Argon, 2% O2 since it welds cooler, but I thought that was more for thinner metals. Any insight here would be appreciated.

2) MIG Weld Type - I believe the correct method of welding is to use the Spray Arc method. This is because it minimizes the spatter and with stainless this would be very difficult to clean. Also I think this would produce the cleanest weld, since it would be a consistent transfer of metal (unlike short circuiting)

3) Wire Thickness - From what I understand the wire thickness and stiffness can become an issue with stainless. I have read that the stainless mig wire is stiffer than the carbon steel so it tends to hang up in the hose very pretty consistently. With our carbon steel mig we usually use .045" wire, so with the stainless I think we should drop this to .035" to be on the safe side.

4) Dis-Colorations - I have heard of Mig welds turning black after welding. I assume the blackness is from the heating and cooling of the weld, but what specifically causes the blackness? I have heard all sorts of stories of stainless Mig Welds that rust in the heat affected zone and obviously with demin water this is more of a concern.

Any other thoughts or insight will be appreciated. I hope this is in the correct forum. If not, (moderators) please feel free to move this to where you think is most appropriate.

Thanks in advance for any help.
Rob

 

[ornerynorsk]

http://www.prest-o-sales.com/other_...dingGases/Manuals/GMAW of Stainless Steel.pdf

Here is a pretty thorough manual that I have referenced in the past.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[metengr]

I would agree with the above reference from Praxair. Carefully read the information from Praxair. Also, here are two other web sites worth visiting;

http://www.weldreality.com/stainlesswelddata.htm

http://www.brazing.com/techguide/procedures/stainless.asp

Here are additional pointers;

Cleanliness is most important. Keep cleaning tools designated for stainless steel.

Keep the interpass temperature below 300 deg F.

Make sure the welders are comfortable and experienced with GMAW because you can have issues with LOF defects. I would run welder qualification for this specific job regardless if welder qualification paper work is in order from previous assignments.

Last, I would hire a CWI to provide oversight for this job if you have limited welding experience with austenitic stainless steels.

 

[gtaw]

This is an application for GMAW-P. Pulsed spray offers the advantage of enabling the welder to weld in all positions with sufficient heat input to mitigate the problem associated with fusion type defects so often experienced with short circuiting transfer. The spray transfer mode is not suitable for the vertical or overhead positions. Even grooved joints in the horizontal position are not viable using spray transfer.

I have used Ar-98%/O2 - 2% and I have used helium additions. The added cost of helium is something of a personal like or dislike. Both gas mixes worked.


The discoloration is simply the product of the oxidation of chrome, iron, and nickel. It has little affected on the mechanical properties, but can be detrimental to the corrosion resistance of the weld and HAZ. If the welds are ground to bright metal, the corrosion resistance is the same as the base metal.


If the water is chlorinated you might consider the problem associated with sensitization. Use low carbon grades of austenitic stainless or use a stabilized grade. If the water is fresh water, i.e., no chlorides, I would not be overly concerned if the interpass temperature is controlled.



Since this is a field built tank, don't overlook using FCAW.




Best regards - Al

 

[RobsVette]

gtaw, can you please elaborate on the following things

- Why is the spray arc not suitable for grooved joints? I had thought that this was the cleanest mig process. We use short circuiting on carbon steel in our weld shop, but I believe that is only because we can deposit more metal faster, without any determintal effects on the weld.

- Using the pulsed spray - would that not cause spatter and issues with filling the weld? I have thought that since stainless is much stiffer than carbon steel, the wire tends to bind up in the hose. Adding in either short circuit or pulsed spray would only exagarate this as they are not an even consistent flow of metal like the spray arc is.

I ask these questions because the tank will hold demineralized water. Sensitization will not be a huge problem because we are using low carbon stainless (316L.) And the water will have minimal chlorine since it has already been demineralized by the time it gets to the tank.

- Why would you suggest the use of a FCAW procedure as opposed to a GMAW here? What advantages would that provide?

Thanks to everyone for you help so far. The articales have been extremely helpful.

Rob

 

[gtaw]

You mentioned that it was a field assembly. There are several FCAW electrodes that are self shielded and require no shielding gas.

I didn't say the spray mode was not applicable to groove welds. Spray mode transfer produces a very fluid weld pool. The fluid weld pool is fine when welding grooves in the flat positions or fillets in either the flat or horizontal positions, but the weld tends to "run" when welding the groove in the horizontal position.


Pulsed spray produces little spatter if the parameters are correct.


I have several clients welding stainless in all positions (pipe) using GMAW - pulsed spray transfer.

Best regards - Al

 

[Guest102023]

With all respect, you are trying to learn everything about welding stainless steel on here when you should bring in a welding specialist (he/she need not be a PE).

I do agree that pulsed GMAW is a suitable process for this task. Manual GTAW or SMAW are not only much productive but will cause significantly greater distortion.

As regards the tank contents (water), it is vital that you eliminate ALL crevices resulting from welding or other processes. Also remove all base metal discoloration by mechanical means followed by pickling (contact your welding supplier for advice and products).

 

[Guest102023]

Manual GTAW or SMAW are not only much LESS productive, of course. And FCAW is another good process.