Tig welding 304 stainless sheet

[geoffno6]

Hi, I've been tig welding for a few months and can now do butt welds on stainless down to 1mm and regularly butt weld 2mm sheet and polish it so that you can't see the weld but the method i use seems to be long winded and I wondered if anyone might have a better method.

On say a 6" butt weld I clamp the sheets to a flat steel table, tack weld one side 2 or three times, turn the piece over and tack again say 6 times then turn it over again and do a series of 1/2" long welds with a 2" gap between them. I'm using about 80A, a red tungsten, 7 or 8 lpm (i think it's litres) of pureshield argon. I have auto HF on my welder and my 1/2" welds are a series of spots made by going on and off the foot pedal once every second. i'm not using any filler.

What concerns me is it's all a bit slow and fiddly and i have little tiny dots left in many of the little spot welds that although they are tiny take a lot of grinding out to get a smooth finish.

Any advice would be appreciated.

Thx Geoff

 

[Delgado]

Hi, I've been tig welding for a few months and can now do butt welds on stainless down to 1mm and regularly butt weld 2mm sheet and polish it so that you can't see the weld but the method i use seems to be long winded and I wondered if anyone might have a better method.

On say a 6" butt weld I clamp the sheets to a flat steel table, tack weld one side 2 or three times, turn the piece over and tack again say 6 times then turn it over again and do a series of 1/2" long welds with a 2" gap between them. I'm using about 80A, a red tungsten, 7 or 8 lpm (i think it's litres) of pureshield argon. I have auto HF on my welder and my 1/2" welds are a series of spots made by going on and off the foot pedal once every second. i'm not using any filler.

What concerns me is it's all a bit slow and fiddly and i have little tiny dots left in many of the little spot welds that although they are tiny take a lot of grinding out to get a smooth finish.

Any advice would be appreciated.

Thx Geoff

Personally (and I'm fairly new to tig myself), I would tack it together and lay it ontop of your bench, put a thin 10mm wide x 2mm ish thick SS flat bar along the join (between the sheet and the bench) and then clamp the whole thing together (effectively bending the sheet youre going to weld into a slight "C"). Once youve done this, weld it using the "back stepping" method, start at say 6 inchs away from one end, and weld to the end of the sheet, then weld the next run from 6inchs further back than the first weld, and run to where you started it, then repeat until you have finished the whole thing. with regards to the "black dots" that you mention, I think that they are either caused by not ramping down the arc at the end of a weld (if they are on the welded side) or because the weld is getting contaminated by the air or the mild steel bench (if they are on the other side).

By the way, better not to weld SS ontop of a mild steel bench, and if you are doing "T" fillets you could use a filler rod instead of a flat bar

Hope this helps!!!

 

[geoffno6]

Thanks for that.

One question, should i start the weld from where i've put a tack and use a lower current say 60A and just move the tungsten along or should i stay with a higher current and go on and off the pedal (i haven't got pulse). Also will a lens make much difference?

Personally (and I'm fairly new to tig myself), I would tack it together and lay it ontop of your bench, put a thin 10mm wide x 2mm ish thick SS flat bar along the join (between the sheet and the bench) and then clamp the whole thing together (effectively bending the sheet youre going to weld into a slight "C"). Once youve done this, weld it using the "back stepping" method, start at say 6 inchs away from one end, and weld to the end of the sheet, then weld the next run from 6inchs further back than the first weld, and run to where you started it, then repeat until you have finished the whole thing. with regards to the "black dots" that you mention, I think that they are either caused by not ramping down the arc at the end of a weld (if they are on the welded side) or because the weld is getting contaminated by the air or the mild steel bench (if they are on the other side).

By the way, better not to weld SS ontop of a mild steel bench, and if you are doing "T" fillets you could use a filler rod instead of a flat bar

Hope this helps!!!

 

[fingers99]

Lens will probably help. I wonder, though, whether the black spots are oxidisation because there's no easy way to back purge such a job? Pickling paste may help and/or if only one side is to be seen, weld and tack only on the side which will be visible.

 

[geoffno6]

If i weld and tack only on the visible side wont the sheets buckle up towards the weld i.e. the welded piece will end up with a slight v to it?

 

[fingers99]

If i weld and tack only on the visible side wont the sheets buckle up towards the weld i.e. the welded piece will end up with a slight v to it?

Possibly. Will depend to some extent on how well you clamp it and how well you tack it (as well as weld order) but it's pretty normal to tack and weld from one side only. Any warping can generally be dressed out.

Are these flat sheets? If they're not to stay flat (that is, if you're putting shape into them afterwards) you might try putting some shape into them first as that will help control any warping.

Some pics of the black spots might help identify what the cause may be.

 

[hotrodder]

If i weld and tack only on the visible side wont the sheets buckle up towards the weld i.e. the welded piece will end up with a slight v to it?

As Delgado said, you can 'preload' the work to help counter the distortion, stainless will move more than a 'slight v shape' given the chance! Using aluminium/copper as a backing strip will work better than stainless due to the heat sink effect and if the backing strip is a good fit it can sometimes be enough to keep air away from the back of the weld and avoid purging

Even partial pen welds on material this thin are going to result in the back of the weld getting hot enough to cause problems. If it's got hot enough to scale then the oxides will be drawn through to the puddle- the black specs in the craters. Purging flat sheet is easy enough- tape some aluminium foil to the back of the weldment if it's large enough to prevent burning the tape for example.

Pulsed welding... each pulse needs to overlap the previous sufficiently to remelt the inevitable craters that occur from rapid drops in current, ideally 70-80% overlap. 'Pumping the pedal' typically results in slower travel speeds or a sore ankle if you can pedal it quick enough. At the very least modify the pedal pumping to keep the arc lit, say, between half and full pedal (will help with the cratering) but personally i would ditch it altogether and just run with puddle, especially if the weld is being polished out afterwoods

 

[geoffno6]

Sorry for slow reply, been away for a few days and thnks for the info' v useful.

One last question, i dont seem to be getting any black spots (oxides ?) my problem is little pits which then require loads of grinding whichh causes a hollow in the sheets. Is it 'done to do a weld then if every couple of inches along a fused weld i have an indentation to do a little weld and put some filler just in that spot?

 

[busa]

Geoff
It can sometimes be difficult to weld thin stainless and get enough heat into it without causing a certain amount of oxidising or 'coking' of the back side. You can easily arrange a back purge by bending a narrow strip of material and securing it across the back of the butt with masking tape, then stick your purge pipe in one end. When welding something that is going to be dressed then you can afford to push more rod into the pool, this effectively cools things down, although it will result in a larger buildup. If making the weld on the flat then the slightly raised pressure caused by the back purge will help support the weld pool. With stainless, heat control is very important, when you feel that it's getting too hot you need to stop, sometimes when i'm capping thin walled stainless pipe at work i'll leave it for a while, go and have a smoke or do something else for a few minutes and then come back to it.

Dave

 

[geoffno6]

Mmmm interesting. Do i follow the advice where it's suggested clamping the sheets to copper to suck the heat away (just bought a piece of copper sheet to try this) or , do i back purge?

Spose I should try both....i'm a bit lazy so back purging sounds like hard work but it is a nice technical method.

Thx to u guys for the advice

One other thing on this subject,.I've played at chasing a pool along a horizontal butt weld and it gives a reasonable result but doesn't look like a proper weld as it doesnt give the little semi circular marks, is this a sensible way to weld

 

[dannyweld]

http://www.youtube.com/watch?v=dTbNGY_j8gI[/URL]

check out this youtube video, its stainless sheet being tacked and welded by TIG.