Help welding flanges on a pipe.

[CraigS]

I am adding a Z-axis ball screw to my Bridgeport and need to make a new pylon. The pylon is fabricated from a 10” length of 4” OD schedule 80 (.318 wall) iron pipe with half-inch thick cold rolled top and bottom flanges. The bottom flange is about 5.5” in diameter and the top flange is 4”, same as the pipe OD. I need to weld the flanges to the pipe.

It’s been a long time since I did any serious welding, I could really use some advise and recommendations for the welding task. I have an old but nice monster Miller 330A that will do AC or DC +/- and is also set up for TIG. My TIG skills? – let’s say I have done a lot more stick than TIG and that has mostly been burning holes in aluminum.

Since most all the load is axial compression, strength is not all that important, would like it fairly stiff but my main concern is to keep the welding distortion to a minimum. Like to keep the flanges flat and parallel and axial aligned.

In preparation I have machined a V at the upper flange/pipe weld area but the bottom weld will just be a standard fillet weld. The pipe buts to the flange and does not go through the flange. There are provisions for a couple of threaded rods that run flange to flange to clamp the assembly together for welding.

So how do I approach the welding to minimize the distortions? Pre heat? Multiple short beads letting things cool between sections? Alternate sides of the pipe ?

Should I stick weld or try the TIG?

What rod would be good? I have 3/32” and 1/8” 6011, 6013 and 7014 and most likely get others.

I assume I should use DC but not sure which polarity?

Thanks,
Craig

[Harold_V]

would like it fairly stiff but my main concern is to keep the welding distortion to a minimum. Like to keep the flanges flat and parallel and axial aligned.

If you'd like my opinion, you're expecting a little too much from a weldment. You'd be far better served to do the finish machining after welding. Regardless of the amount of care you take, you'll be fighting the roundness and straightness of the pipe, to say nothing of distortion from welding. Hoping to end up with parallel and concentric surfaces is quite a long shot. I'd say you'd be fortunate should it work out as you'd hope.

Harold

[CraigS]

Harold,
I have a plan to chuck up the welded assembly in the lathe using mandrel down the center hole to face off the flanges. This in theory should get them parallel.

Craig

would like it fairly stiff but my main concern is to keep the welding distortion to a minimum. Like to keep the flanges flat and parallel and axial aligned.

If you'd like my opinion, you're expecting a little too much from a weldment. You'd be far better served to do the finish machining after welding. Regardless of the amount of care you take, you'll be fighting the roundness and straightness of the pipe, to say nothing of distortion from welding. Hoping to end up with parallel and concentric surfaces is quite a long shot. I'd say you'd be fortunate should it work out as you'd hope.

Harold

[hammermill]

alot of welding is set arounding planning movement so use a tac weld each ,120 deg checking alignment as you go, then preheat and weld with oposing welds, after cooling do your final machining on surfaces that need to be parallel

[awake]

I share the concern about distortion. Is the load all axial compression, or is there some axial stretching? If only compression, I'd just put very short beads (just more than a tack) at 4 axes and call it good -- no need to weld all the way around.

If you are planning to machine afterwards, do yourself a favor and use 7018 -- or at least do NOT use the 6011. It will form very hard spots that are murder on your machining tool bits.

[Harold_V]

or at least do NOT use the 6011. It will form very hard spots that are murder on your machining tool bits.

That's an interesting observation. I've been told that the wire used for rod doesn't vary--it's the coating (flux) that makes a difference. Cellulose is reputed to be part of the coating on 6011 rod-----which, I assume, is reduced to carbon. That might account for the potential to harden upon rapid cooling. Your thoughts, please?

Harold

[hammermill]

i was always told the wire varried as to type ie all 60 series rods will have the same base metal and then alloy powders and minerals are added to coating

http://www.youtube.com/watch?v=1r0s23HL ... re=related

here is a quick video of rod making and related machinery

[Russ Hanscom]

After a quick look at the Lincoln Procedure Handbook, I can report that it depends.

No particular composition is listed for 60 series, but for the 70 and higher series rods, the alloy composition does change as you go from an xx10 to xx27 rod. Referenced to AWS A5.5

More specific compositions are indicated by welding rods with a suffix after the normal four digits - E7018-xx

Lots of info out there if you want all the details.

[awake]

or at least do NOT use the 6011. It will form very hard spots that are murder on your machining tool bits.

That's an interesting observation. I've been told that the wire used for rod doesn't vary--it's the coating (flux) that makes a difference. Cellulose is reputed to be part of the coating on 6011 rod-----which, I assume, is reduced to carbon. That might account for the potential to harden upon rapid cooling. Your thoughts, please?

Harold

I don't know what is happening, metallurgically speaking. It could be carbon from the cellulose, I suppose. 6011 is a "fast-freeze" rod, with very, very thin slag, so it presumably cools down very quickly. If there is carbon in the resulting weldment, that would give an opportunity for hardening. As I understand it, 6011 introduces hydrogen into the weld; I've heard the term "hydrogen embrittlement," and of course that is part of what 7018 is supposed to avoid, but I don't know whether "brittle" also means hard -- though I could certainly imagine that it does.

The bottom line is that my statement is based on painful experience, rather than metallurgical theory. The first time it happened, I thought maybe it was the base metal, or had something to do with technique. I tried just tacking with 6011, and then melting it out with 7018, but I still hit hard spots when machining the result. On the other hand, when I've machined something welded only with 7018, it has been just fine. One of these days I've thought about try a more systematic set of experiments, not only comparing the machinability of 6011 vs 6013 vs 7018, etc., but also whether one can draw the hardness of the 6011 welds down in an oven.

[Harold_V]

Thanks!
Regards drawing down hardness in an oven---assuming it's caused by carbon, yes, that works.

It's pretty hard to argue with real life experiences, so one must explore to find the reason for unexpected reactions. It would be interesting to get to what happened in your case. Near as I can tell, the use of 6011 (or 6010) is routine for a root pass when welding pressure vessels. I would expect that it shouldn't get hard, so there may be an explanation for your experience. Interesting, to say the least.

If memory serves, hydrogen doesn't make metal hard, just brittle (reputed to interrupt the slip plane)---but it isn't retained readily in soft materials, unlike hardened materials.

Harold

[CraigS]

Finished up the lower flange today and will be ready to do some welding tomorrow. I need to find some scrap and get some practice before I attack the pylon.

[awake]

Craig, with all the work you've put into machining these, is it too late to change your attachment to a mechanical system? It looks like the wall of the pipe is thick enough to use small machine screws. This would avoid the distortion issues of welding!

Note, however, that you still may face alignment problems. For perfect alignment, you're going to need to chuck the entire assembly -- welded or screwed together -- up on the lathe and bore the central hole.