Welding stainless sheet metal

By ssbn727 (*****)

Date 11-01-2003 07:26

Hi Jon20013!
16ga 304? or is it 304L? The 304L is a little more expensive but alot less carbon content which when it comes to welding minimizes the amount of carbide percipitation compared to 304. 308L filler wire works well here also but without knowing exactly what you're going to be fabricating none of "old timers" can really advise you more specifically...
Some of the questions I have are,; what joint types or configurations will be incorporated into this design? These questions will enable us old timers to help you choose the correct process(es) to use for joining...
How many of these items are to be welded and what is the approximate combined weld length of each item??? How much time do you have to produce these items??? You know where I'm getting I presume...
Are the welds to CJP = Complete Joint Penetration butts from one side only or are they mostly fillets or laps, corners, flanges in other words - we need more details!!!
I'll tell you this, minimizing the amount of heat input is paramount with such thin gage stainless and achieving the fastest travel speed is also important in minimizing heat input. Another important factor is fixturing whereby if possible, incorporating some sort of heat sink(s) to absorb the heat input out of the weld zone in order to minimize warpage adjacent to the welds... Prior to welding, tack the joints together with tack locations and spacing, no more than 1-1/2" preferrably less apart without filler (autogenous) if possible. the size of these tacks should not be the same as the weld size (about 2/3 or less) because you really want to think about heat input in those tacks and the fact that you will be going over them once you deposit the welds so, an even better or ideal scenario would be to not tack the parts together prior to welding if possible!!! Either way, if you choose GTAW otherwise known as TIG welding then, a gas lens is necessary for your torch and if your travel speed is fasT enough whereby there's not enough time allowed to shield the weld metal as your progressing then, if your automating - a trailing shield would also be necessary!!! I would'nt recommend fast travel speeds manually because us humans cannot do this consistently or as steady all the time with such thin sheet if we're talking about seams longer than 12", as well as a custom automated configuration.
If there are all different joint types in the design then, Robotics may have to be considered if there is to be an extremely heavy demand for this product whereby the end product will be produced the same way repeatedly for at least a year or so
Real adjustable pulsing and other programmable features in the welding equipment may also help control heat input and size of the HAZ = Heat Affected Zone which is also important with Stainless
This requires a considerably larger investment in fixturing and if you have a backorder that will never see the light of day if you're using a manual or semi-automatic process then you should consider instead,
Automation If you're making vessels of some sort and you're going to be welding alot of seams whereby CJP's are required then, back purging must be a necessary element in the welding procedure...
Going this route will also require a welding power source that has all the bells and whistles that really enhance the welding process and not something that you really do not need!!!
Now GTAW is'nt the only way to go here as far as process is concerned! You have Gas Metal Arc, Flux-cored, Plasma, Electron Beam and Laser welding also... However this all depends on whether or not these processes are cost effective for your application because after all you want to maintain profitability once you meet your ROI = Return On Investment from your initial start-up design/capital/labor costs!!! None or most of this cannot be achieved or should be thought about if you're talking about only a one time production run so, we need more details please!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!

By jon20013 (*****)

Date 11-03-2003 13:30

Thanks SSBN727; 304L base and ER308L filler is what we are intending to use. I've got loads of experience with heavy section stainless steels but I am looking for solutions to control distortion on light gage materials. The joints on my project are a combination of fillets and CJP (we are fabricating a thin wall vessel). For the equipment I have available in my shop, I think I will be limited to GTAW ~ the CJP joints must be of RT quality so GMAW, in my opinion, isn't really a good choice and the base materials are way too thin to use FCAW. Robotics also isn't an option because we're not set up for it and are only going to do a few of these. I do have an orbital TIG machine that I've thought of setting up for this but again, my number 1 concern is shrinkage and distortion and had planned on doing a backstepping technique with heat sinks if we do it manually. So, you see, it's not that I don't know what I'm doing, I'm just looking for some tips from those who have extensive experience with stainless sheet metal where oilcan free finish is required. Any other suggestions?

By ssbn727 (*****)

Date 11-04-2003 08:39

Hi Jon20013!
Well - now that we've cleared that up, let's see if I can assist you in your search for some suggestions that could help in minimizing shrinkage and distortion for your stainless steel welding application that requires an "Oilcan free" overall geometric dimensional shape... I know that's what you really meant, especially adjacent to where the joints are concerned, correct??? I say this because distortion, shrinkage does'nt noticeably change the surface finish grade of the metal and for clarification purposes!!! In other words, no ripples, curves or bends where the surface lines or the surface contours should be flat, straight and continuous, correct???
Anywho, here are some other suggestions or options that might be useful for your application:
#1.) Having the capability of programming, upsloping, downsloping and pulsing your welding current, which is as you already know, dependant on the type of power source available to you would be helpful in controlling the heat input... Possibly renting or leasing the equipment could be an option here if none of these capabilities are readily available to you with your present welding equipment...

#2.) Generally speaking of course; If you're vessel requires both longitudinal and circumferential butt joints then, it would be logical to weld the longitudinal seams first and your circumferentials last as far as your overall sequence of operations, with everything else that's to be welded to the vessel in between... It's the "in between" part that's going to be difficult to control as far as distortion and shrinkage is concerned so;
#3.) If you're only going to produce a few then, shucks!!!
I apologize for not remembering the brand or manufacturer (Tempil?) of this heat sink (the name brand is HEAT) compound I used awhile back in the eighties which was available in in paint cans or caulking tubes... This compound would absorb the heat adjacent to the joints (both fillets and butts) once spread on to the material prior to welding so it would have to be applied to both members of any of the joints where other heat sinks may not be accessable! Once welding is completed, the compound readily peels right off if applied properly!!! (definitely read the instructions first with this stuff) If tee joints are to be welded, apply a coat of this stuff on the backside of the tees adjacent to the centerline of the tee and not directly on the opposite side of that centerline so that you do'nt burn the stuff because it stinks like heck when it burns, but works really great when properly applied!!!
#4.) Even when you tack the CJP's together, you must back purge the joints prior to welding or you'll never get acceptable RT's!!!
Of course, you must also backpurge the joints prior to and during welding so that the backside of the roots (penetrations) do'nt show contamination or porosity in the radiographs!!! Also, do'nt forget to cap/cover the openings and/or fittings prior to purging or else you'll be wasting alot of argon!!! Use common sense here!!!
#5.) Use an oxygen/nitrogen analyzer/indicator in order to constantly check for atmospheric contamination levels in the purge areas prior to,during and after both tacking and welding the vessel CJP joints together!!! A definite must for acceptable RT's!!! Then again, you do'nt want the welds to be just acceptable do you??? Of course not!!! You want them to be superlative!!!
#6.) use tempsticks or pyrometers to control your interpass temperatures while welding and as a welding engineer, you should have either of these items available to you. you should also calculate the interpass temps yourself for 16 gauge 304L ss...
#7.) After the welds are completed especially not too long after; Passivate and clean the welds, HAZ zones so that you minimize or even eliminate potential ISCC (Intergranual Stress Corrosion Crack) and pitting corrosion locations at or adjacent to the joints!!! Very Important!!! Especially with thin walled vessels!!! If you can, passivate and clean the joints prior to tacking and welding also, preferrably after the components are fabricated!!! Oh yeah - I almost forgot, deburr the components prior to passivation and/or assembly/tacking where the CJP's are!!! Also, if you must plasma cut openings; grind and polish kerfs and remove via polishing or preferrably passivation (because of possible inpregnation of dirt, grease film or other potential contaminants to the fusion/weld zones) any HAZ showing prior to welding if it's a CJP weld!!! In other words, use common sense here!!!

I'd be happy to assist you in greater detail but, as you know, since I do not know what Type of equipment you have available, what the actual design and/or specs. looks like; I'll have to stop here for now!!!
If you would like, e-mail me ( hanklive39@hotmail.com ) a detailed description (possibly a set of prints?) of the vessel in question, a list of your available equipment and whatever else you might feel necessary for me to assist you further unless that is, if you can take it from here!!!
Good luck you youngtimer!!! All the BEST!!!

Respectfully,

SSBN727 Run Silent... Run Deep!!!