Brazing vs. Arc Welding 304 CRES 2

[vmuro]

Hello all.

I have a task that requires me to decide on a method for joining two stainless steel thin walled tubes (see attachment). I am looking at either arc welding or brazing, but my concern is that I have only worked with mild steel material. I am leaning towards brazing because large batches can be done at a time and it is more economical. I would like to know from those with experience if one method has worked better than the other or presents more complications, etc. Also, if brazing, what filler material is best suited for CRES? Lastly, if arc welding, is it best to go with 304L as opposed to 304? Thanks in advance, your help is greatly appreciated.

-VM

 

[DVWE]

What are the service requirements and application of this part?

 

[TVP]

How many of these tube combinations need to be joined? Are you outsourcing this joining, or using in-house personnel? Is distortion an issue? Depending on the answer to these questions and that of DVWE, TIG brazing may be an option as well.

 

[vmuro]

Unfortunately, I do not know exactly what the intended use is, but the customer does R&D for vehicle emissions. I assume either combustion gases or fuels will flow through. At the moment, I am unsure of the quantity, but it could be in the thousands per year. If I go with brazing, I would have to outsource as I do not have the capabilities for this, and then I'd finish the assembly once I receive it brazed. I would still like to be well informed as to what filler material would work best. If I decide arc welding, I would like to use in-house personnel to accomplish this. Also, distortion is an issue as the customer specified tight tolerances on the tube OD's as well as circularity form tolerance.

Thanks for your feedback.
-VM

 

[unclesyd]

If you go the welding rout I would seriously look at Orbital
Welding. It is extremely fast once you have the setup. Links
number6 & 7 have information on Micro-Welding. I"m using two
diferent weding processes to solicit information. If your quanity

Is quanity is high you, can rent an orbital welding head.

If by some chance you go the brazing rrout I would seriously look
at Nicro-Braze materials. Nicro-Braze was was uswd in nearly all the beazing of SS we accomplished

http://www.arcmachines.com/

http://www.weldplus.com/orbital.asp

http://weldlogic.com/index.html

http://www.magnatech-lp.com/

http://www.weldplus.com/orbital.asp

http://www.microweldingproducts.com/index.html

http://www.novatechinc.com/default.html

http://www.wallcolmonoy.com/Products/brazing/index.html

 

[DVWE]

One thing to keep in mind that is when the temperature reaches approximately 800F then your braze metal is pretty much worthless. Without knowing any more than what you've told us, I am biased towards welding as there are plenty of options to automate a welding process if your quantities get large, and you should be able to hold your tolerances if the process is set up properly as well. I agree with unclesyd on the orbital welding for small to medium size quantities as it is a very simple and reliable process once set up.

 

[btrueblood]

We used to furnace braze with filler alloys that melted well above 1800 F. 800F is the standard transition temperature from "solder" processes to "brazing" processes, but is certainly not an upper temperature limit for brazing alloys.

 

[TVP]

Since this is for elevated temperature exhaust application, I would go with welding vs. brazing. Use 304L instead of 304, as it will minimize/eliminate sensitization. If you decide on a manual arc welding process, use GTAW (TIG) instead of GMAW (MIG) in order to have better control with thin tubes, distortion, etc.

 

[DVWE]

btrueblood, that's great, but the OP is not going to require a braze material that melts at those temperatures for this application. Not only would that distort the hell out of the parts, but it would also be close to re-solution annealing them.

 

[vmuro]

btrueblood, thanks for the advice, but I have had experiences where the temperature was too high and the parts came back fully distorted, so I definitely want to avoid that.

Thanks to inclesyd (great links by the way), I have been looking into orbital welding and it seems like it will work well with my application, after all it is TIG welding. I have seen a few videos online, and my concern is that the electrode is perpendicular to the pipe centerline and it rotates about that axis, which is fine because the two tubes being joined have the same OD...but the tubes I am working with have one tube with OD .375" over the other tube with OD .3125" (see image on initial post) and I believe the electrode must come in at an angle to weld in between. Can this be accomplished? Sorry if this is a silly question, but I am still relatively unfamiliar with orbital welding and I want to get it straight. Thanks again...

-VM

 

[btrueblood]

"Not only would that distort the hell out of the parts, but it would also be close to re-solution annealing them. "

Hey, don't listen to old farts like me and syd, what do we know? Distortion doesn't come from the temperature, as much as the process. Which is why one would use a controlled atmosphere, controlled cycle braze (e.g. a furnace, not some knucklehead with a torch and a brush), and decently designed fixturing. Not sure why solution annealing would be a bad thing for this part, but yes, that is a concern. One could easily pick an alloy and temperature cycle that would avoid annealing. The Nioro or Nicro alloys suggested by unclesyd are just as corrosion-resistant and temperature-resistant as the base metal if you pick the right ones, and that is not very hard to do. The back end of a lot of Rocketdyne's rocket engines use brazed tube assemblies, and the exhaust temp's. there are a heckuva lot hotter (and a fair bit more corrosive) than a typical vehicle exhaust. The benefit of a brazed joint for concentric tubes is that the braze metal, which is generally softer and more ductile, can "give" to take up thermal expansion strains, an issue if the assembly is thermally cycled a number of times.

Regarding welding, I have no problem with the idea, but orbital welding and other fixtured, controlled-input (vs. manual) welding would be a good idea here, too, for the same distortion concerns. Also, 304L material doesn't eliminate the risk of sensitization, just minimizes it somewhat. If the part is going to operate within the sensitization temperature range for stainless steels, use a stabilized alloy, i.e. 321 or 347. It costs a bit more, but if long term stability and lack of sensitization corrosion/cracking is of any concern, the stabilized alloys are worth the expense. Also hard won knowledge from several year's experience in the design and manufacture of cogenration heat exchangers (operating on IC engine exhaust systems).

 

[metengr]

btrueblood
Nicely stated.

 

[DVWE]

Hey, don't listen to old farts like me and syd, what do we know?

btrueblood, I surely don't mean any disrespect or come off as lack of listening to you "old farts", OK? But let's get back to the point, the OP is not going to take a joined piece of 304 tubes up to 1800F, and in a controlled environment, and still be cost effective. Especially when welding is a more viable method.

I believe the electrode must come in at an angle to weld in between. Can this be accomplished? Sorry if this is a silly question, but I am still relatively unfamiliar with orbital welding and I want to get it straight.

The angle of the torch can be adjusted in the machines that I have seen and used. The manufacturers should be able to give you exact details.

 

[Tmoose]

As I myself am a smug old fart I try to keep in mind something I read about Napoleon's mule. The late great Carroll Smith tells it pretty well.

Dan T

 

[btrueblood]

Cost effective? Maybe, but it would depend on the production quantities we are talking about, which we don't know. 1800 F is not a necessary braze temp, just as neither is 800F a very reliable braze temp., my point is that there are lots of possibilities between 800 and 1800 where an acceptable braze joint could be designed.

The OP wrote:

"I am leaning towards brazing because large batches can be done at a time and it is more economical."

Which is quite correct, for some given batch or lot size. In a fairly typical commercial furnace, that part could be easily brazed in quantities of 100's if not 1000's at a time, probably two or three batches per day. And those numbers are conservative, there are continous feed processes for brazing that I am less familiar with, but could probably be used here. You could spend a lot of money and time on orbital welding equipment and still not approach that production rate, and certainly not the per part cost.

Even in small quantities, with a cover-gas and purge approach, an induction coil braze could probably compete quite well with a similar, fixtured and controlled welding process.

"Viable" processes are those which produce acceptable parts, at the desired quantities, for the least cost. We have no idea, given quantities stated, what a viable process might be, but to rule out brazing given that lack of data is pretty short sighted.

 

[DVWE]

Fair enough, btrueblood.

vmuro, you have some thinking to do.

 

[vmuro]

Absolutely, I will consult with some local suppliers regarding equipment specific concerns. Also, I am also looking into the Nicrobraz brazing alloy as a solution to this as well, so DVWE I do have a lot of thinking to do here. I suppose it will come down to when I actually have a concrete number on the quantities required, but I will need to do a few prototype sample runs first and provide to the supplier, then we'll see.

One more thing, TIG welding & brazing will require a chemical passivation process afterwards to remove any surface contamination and restore the protective passive film, correct? Thanks again all...

-VM