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10-03-2008, 12:08 AM #1
Controlling distortion welding stainless sheet?
Studying the construction of some commercial custom NSF sinks & trying to figure out how to fab something similar in construction & appearance. These appear to be TIG welded from separate press-brake-bent sections - as opposed to the common deep-drawn single sheet stampings of "home" sinks.
On the visible surfaces, the weld beads are ground flush and then the material is brushed to a uniform finish where the joints are invisible. The outside or non-visible surfaces show conventional weld beads.
What I don't get is how they keep the sheets so flat. Why haven't all the weld lines shrunk and turned it into a wavy pretzel? How do you weld something like this, continuous beads, watertight, and keep it looking so good?
I'm no master of distortion control, but this is way beyond what I think I know about distortion control in typical structural projects. What techniques are used in a production shop? Fast automatic welding? Heat sinks? Annealing and/or flattening after welding?
10-03-2008, 02:58 PM #2
In my career as a welder, I welded stainless steel sheet in the 16-20ga range with TIG. I worked at a company making resturaunt tables and equipment (hoods, lockers etc) and at a company making equipment for corrosive material handling.
everything is generally cut via laser or shear then press braked followed by TIg welding outsides corners, square groove welds, Fillets or lap joints. The general weld procedure is Vertical down or flat, fuse welds with no filler. Following welding, joints are ground down and then "grain matched" if neccessary. Stamping would only be economical on mass produced items such as "home" sinks and as such most smaller manufacturers will not have the capital or volume to produce units like that.
Distortion control is actually extremely easy on stainless steel sheet metal. This is because there is very little restraint on the weld and the metal is relatively ductile in such thin sections. Finally while stainless steel is more prone to distortion than carbon steels. Distortion is more a function of time and heat input. sheet metal is usually welded very cool around 45-60 amps in the vertical down processes at around 10-20IPM I.e a very fast and cool weld. there is little chance for distortion as the base metal does not get that hot.
I'm not sure how familiar you are with sheet metal fabrication but I'll try to describe the general procedure for a box form
If you are trying to make a square box with 5 sides and an open top lets say 3' to a side
Most likely it would be pressed in to 2 L shapes with a square cut for the final side.
It's impossible to line up both joints at once, so you start with the top of one joint and line it up perfectly. Unlike other welds fit up is very critical in fuse welding. Once the top corner is lined up perfectly you'll tack it.
Tacks are generally done at hotter settings and the torch is cycled on and off as fast as you can flick the switch. (similar to arc spot welding).
Now you'll proceed to move about 2-4" down the joint line it up again and tack. You repeat this process all the way down the seam. In the end it's "stitched" with about 3-5 tacks a foot. The procedure is repeated on the other side and the unit is brought close to square.
To weld you would start at the top (if vertical down) and weld continously between the tacks which prevent the sheet metal from seperating, The numerous tacks mean the metal really won't warp or distortion while welding. Once the first (or both) joints are done, pipe clamps will bring the angles back to square. Now you have a continous water tight seam with little build up and little heat input.
does any of that make sense?
On a simple shape like a box or say welding a rolled section to make a tube, distortion shouldn't be much of an issue. Once you start welding lap joints on the top of flat sheets then distortion starts becoming evident. Sometimes over the course of a long seam (imagine a long U shaped trough) the whole assembly will want to distort. This is countered by clamping and fixturing, or post weld heat treatments and dummy beads.
Overal fuse welding stainless steel sheet is considered "easy" work by most of the welders I have run into, and making a 10 foot squared seam isn't hard for a single person to fabricate.
10-03-2008, 04:35 PM #3
Attached are a few pictures of some closups of long SS fused seams, ga would be 16-20. I probably built 20-30 of those squire cage blowers, and they did have some problems with distortion.
10-04-2008, 02:16 AM #4
Thank you, that was extremely helpful.