Difficult TIG weld.

This week I almost got beat by a TIG weld project. The shop next door to my school is American Hose and Fittings. They build hoses and such for all sorts of things, but their main
products are custom stainless flex hoses.
They take stainless flex tube of many diameters, sheath it in stainless braid, and weld it to various pipe ends and flanges for engine exhausts and other high temp applications. They do this all day every day, using rotating positioners and a couple of TIG machines.
Anyway... on Monday their head welder came over to my shop to ask my opinion on a troublesome weld.
They had built these 316 SS flex hoses about 14 inches long and 3 inches diameter with heavy flanges on each end and stainless braid on the outside.
They were returned by the customer because they needed them welded on the inside as well as the outside to satisfy a "sanitary" requirement.
The joint is a 4 inch length of 3 inch sched 40 316 SS pipe butted against the end of the SS flex hose. Flex hose is made from 26 gauge sheet so it is paper thin.
So you have an inside, blind, lapped, outside corner weld of 26 ga to 1/4" wall pipe at the inner end of 4 inches of pipe.
I ended up taping an inspection mirror to my TIG torch so I could see my progress immediately.
If you tried to heat the edge of the pipe, add lots of filler and flow the bead over the edge to capture the flex hose edge, the edge would just vaporize away from the heat.
I wrestled with some practice pieces until I figured it out. The trick was to turn the machine down to 25 amps, and create a worm of weld metal on the edge of the 26 ga flex hose end. Once I had this 3/32" to 1/8" diameter worm of metal running completely around the inside of the pipe I could go back in and fuse the worm to the pipe wall using higher amps and a pulser. A 3rd pass flowed the worm over the edge and into the flex hose wall.
A complete pain in the $@#&@^ It worked and I figured out how to fill any pin holes I blew into it.
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snipped-for-privacy@stagesmith.com says...

Sounds almost like you could have used a consumable insert machined at a taper. Thin side welded to the 26 guage hose, thicker side welded to the pipe. Would something like that have worked?
If there are bunches of these to do, perhaps consumable inserts would prove to be an economical aid in this situation.
Were they looking for full penny welds, or would partial penny on both sides work?
--
Tin Lizzie
"Elephant: A mouse built to government specifications."-Lazarus Long
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With some pre-planning they could have bevelled the inside edge of the sched 40 pipe where it joined the flex, but they didn't realize they needed the inside bead until after they were welded on the outside.

They already had a full pen weld with the outside weld and the flange end weld.
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Ernie Leimkuhler wrote:

When does welding become additive fabrication?
-- Peter Fairbrother
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Ernie Leimkuhler wrote in sci.engr.joining.welding on Fri, 08 Oct 2010 00:55:23 -0700:

Cool Ernie. Would it have helped to machine the pipe to the same size and wall thickness as the 26ga flex section? That should make it a more strait forward butt joint. Unless you want ongoing work of course. : )
--

Dan H.
northshore MA.
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If they had known about the inside bead when they were building them they could have bevelled the edge of the sched 40 pipe down to something more reasonable for a butt weld to the flex. If the client wants another set they will push for sched 10 end pipes.
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I'll second Dan's suggestion to machine the pipe to the same thickness and diameter as the flex tube. Then the joint can be made with a tube welding head and an ID purge. Extra material can be added to the joint with an inverted tee insert if necessary. Like Tin Lizzie's insert only upside down with the stem of the tee towards the ID of the joint in contrast to the conventional mushroom shaped insert commonly used for pipe.
J

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For those of us with far less skill than you have is that a good general procedure to join thick and thin sections?
jsw, practicing TIG at night school.
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In article

Being aggressive with your filler rod is a standard trick when dealing with thin exposed edges. Keep the filler rod end between the tungsten and the edge, and melt the rod onto the edge so it thickens it. Using the filler rod to eat some heat is very common in thick to thin welds.
BTW that is also how you weld pop cans.
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Aside from any welding implications, if they were looking for a sanitary/hygienic application (maybe food industry) then the flex hose is no good as it has all kinds of ledges/voids along its length that trap product and prevent full cleaning....
Phil.
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    --FWIW I think the reason for using this tube form is so that a vacuum, rather than a pressure is drawn and the tube won't collapse. So in terms of sanitation whatever's in it goes away from the food-y sanitary sections, yes?     --Every once in a great while a pile of this corrugated flex tubing turns up at our local junkyard and one time I grabbed a bunch. Sure looks purdy on a steam engine! :-)
--
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Hacking the Trailing Edge! : for vindictive jerks!'
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