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Thread: Warping in Sheet Phosphor Bronze

  1. #1
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    Default Warping in Sheet Phosphor Bronze

    I have a problem I need solving. Not a boat project but rather one of my landscape architectural projects: I'm getting a set of corner brackets made up for a rhomboidal glass spa to hide the ugly butt joins of the 32mm glass (and no, toughened, immersion grade laminated glass cannot be bevelled), but the issue is the 1.2mm thick sheet bronze needs to be annealed prior to it being put into a brake press otherwise the bends split along the grain of the sheet. Thus the sheet will bend nicely, but the outer edges get a little bit buckled and warped, with the alloy acting a bit like spring steel.

    Here's the spa awaiting the brackets:

    IMG_9459.jpg

    Here's the set up of the brackets without the capping pieces to them:

    IMG_6605.jpg

    You might be able to make out a bit of a wobble down the edge of the out bracket section in this next shot:

    IMG_6606.jpg

    And you can certainly see how it's bowed over the length when set against a straight edge:

    IMG_6611.jpg

    IMG_6609.jpg

    continued...
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  2. #2
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    Default Re: Warping in Sheet Phosphor Bronze

    One last shot of the bendy edge:

    IMG_6610.jpg

    Here's the test piece that was not annealed and you can see the bend is nice and sharp on the inside but cracked on the outside:

    IMG_6607.jpg

    IMG_6608.jpg

    Without having to go out and get another sheet is there any way these admittedly small imperfections can be successfully ironed out? Maybe spot annealing and quenching? Or baking and repressing? Anything would be good if anyone has had to deal with this issue before. If the grain of the sheet was across the bend we wouldn't have this issue, but the grain ran the length of the sheet, so we have lots of tension in the sheet as it's pressed into the angle.

    My fabricator is at a loss. He's worked with copper and brass plenty of times, but this is his first trip down the bronze lane, although he says the material is sheer joy to TIG weld!

    Anyone had this issue and found the work-around?
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  3. #3
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    Default Re: Warping in Sheet Phosphor Bronze

    Never had to bend sheet bronze, Duncan, but that's one spectacular design you have going! I'd like to see photos of the finished project.

  4. #4
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    Default Re: Warping in Sheet Phosphor Bronze

    Does the bow matter? Seriously, it would be hardly discernible. I mean, if the bend is true, the edges shouldnít matter that much, eh? Thatís not much warp, and if the alignment isnít an issue, Iíll wager nobody will,ever notice the warped edges...

    Maybe weld up and grind the crack in the unannealed flavor? Would that look to gross?

    I donít know. Iím dumb. Why am I even answering? Haha.

    Peace,
    Robert

  5. #5
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    Default Re: Warping in Sheet Phosphor Bronze

    I can't make much sense out of your photos, but i think you can avoid the problem if first you cap the corners with wood thick enough to radius the corners then cap the wood with radiused cornered bronze. That will spread the tension enough that you won't get the cracking. A rounded corner won't be so hard on the body when someone collides with it also.

    As far as the straightness goes I think it's so insignificant that you can just pull it into shape.

  6. #6
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    Default Re: Warping in Sheet Phosphor Bronze

    Would a metalworkers stretcher or shrinker tweak the flanges of the angle section?

  7. #7
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    Default Re: Warping in Sheet Phosphor Bronze

    Make a longer one and chop the bent end(s) off
    Creationists aren't mad - they're possessed of demons.

  8. #8
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    Default Re: Warping in Sheet Phosphor Bronze

    Just to clarify:
    No annealing produces a sharp bend and cracking along the outside.
    Annealing stops the cracking but produces these wobbles along the edges.

    The only other way forward would be when gluing the brackets in place we use some kind of clamping method to even it out.

    Twisting and pulling doesn't appear to work either as the grain of the sheet has made the material like spring steel.

    I was hoping there might be some way of treating the sheet once bent to take out the wobbles?
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  9. #9
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    Default Re: Warping in Sheet Phosphor Bronze

    Hey Duncan,
    What happens if you cut it a bit wider than spec, anneal and bend and then grind the edges straight? Is the actual bend sufficiently straight?

  10. #10
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    Default Re: Warping in Sheet Phosphor Bronze

    If this is being done in a press brake, perhaps the radius of the die is too small?
    Just guessing from here but after anealing, the press is actually "coining" the bronze making it slightly thinner (and longer) causing the rest of the piece (edges not coined) to buckle or curve.
    Larger dies that would coin the entire piece might work.

  11. #11
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    Default Re: Warping in Sheet Phosphor Bronze

    It looks like the bend radius is just too small. The minimum bend radius without cracking depends on the exact alloy (there is more than one phosphor bronze), the temper, direction of bend relative to the rolling direction and the thickness. Your sheet looks like it is over 1mm thick. Overall, phosphor bronze should bend well as long as the radius of the bend is right for the thickness and temper. The bend radius in the test piece looked very small. It looked like the test piece has two bends and the bend with the larger radius was OK.

    Phosphor bronze contains tin,which makes it harder and less ductile than pure copper. The direction that the sheet was rolled has a strong effect on the bend radius. The bad way is when bend axis is in the rolling direction. If you bend it that way it might take twice the radius to avoid cracking.

    Annealing should have worked, but if the dies were mismatched and the metal was compressed at the bend, it would tend to warp as you [edit canoeyawl] suggested about coining. I think the dies need to be matched to the thickness of the sheet. [Edit The radius of male die has to be large enough to avoid bottoming out?] That is something to ask about, and hopefully the shop can answer the question better than I can. If you clamp it straight and heat it to 250C for an hour, it might stay straight. Annealing at 400 C in a straightening fixture will definitely soften the metal and it will remain straight.
    Last edited by MN Dave; 06-14-2018 at 12:21 PM.
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  12. #12
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    Default Re: Warping in Sheet Phosphor Bronze

    Thanks Dave, I’ll take your suggestions to the fabricator. I suspected annealing and clamping up and letting it cool might have been the answer and we did talk about it.
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  13. #13
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    Default Re: Warping in Sheet Phosphor Bronze

    Duncan, how was the heating for the annealing done? In an oven, or just random heating with a gas torch?

    I used to do the heat-treatment oven calibration for all the RNZAF workshops. Aircraft aluminum is extremely fussy with heat treatment profiles, a few degrees can make an enormous difference to the hardness, so these things had pretty sophisticated controllers.
    Anyway, there was an issue with the replacment tail-fin skins for the Spitfire that Tim Wallis badly bent (the first time).
    They had been heat treated in one of our ovens that had large carbon rod elements, and the bits closest to these had a different temper from the rest of the skin, resulting in a similar effect to what you are seeing - the softer bits stretched more when bending them over the ribs.
    Doesn't solve your problem, but maybe something to consider next time. I would just about put money on the wonky edges being caused by the bend radius not being totally consistent due to slightly uneven annealing.
    If you really want the edges "perfect" and don't want to start over, is there enough depth to get these into a guillotine? Or failing that, true them up the old fashioned manual way by draw filing.

    Pete
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  14. #14
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    Default Re: Warping in Sheet Phosphor Bronze

    Ultimately it's up to the client and how much he wants to spend and what result he's going to be happy with. The feeling was that stainless would ultimately corrode in an immersed chlorinated environment leading to unsightly rust marks unless the entire bracket was pickled and polished, which would prove costlier than this bronze option. I did speak with a fabricator in Melbourne and he usually uses SB sheet which he reckons behaves quite differently to PB. The alloy is C519 by the way.

    My own feeling is that once the brackets are installed the overall effect will disguise these relatively small imperfections, particularly as the bronze ages and develops its patina, and that the glue up of the brackets might iron out the wobbles in particular and be further strengthened with a filler and bead of pool silicone.

    I have a great client at any rate!
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    Default Re: Warping in Sheet Phosphor Bronze

    Duncan, the bend radius is way too sharp, particularly for PB sheet, which at its softest is probably half hard. I don't think its supplied in any other condition. To anneal PB, you have to heat until dull red (workshop, not daylight), then plunge into water. Trouble is the folded corner will be hard and the sides still soft, but at least it can be straightened easily. Some gentle taps on the side adjacent to one you want straightened (convex) or on the side you want straightened (concave). Use a planishing hammer and a flat dolly. If you work the inside surface then outside surface should be ok to polish. SB sheet is hard to come by in Australia, its also much nicer to work, if you hear of any suppliers, let me know. I do know what I'm talking about, check out the mast fittings on the Resurrection of Ghost thread, all made from PB sheet with a small amount of swearing involved. Good luck with the project. How are the sections going to be joined? Rivets (SB) or screws/nut, etc good, silver solder may not be strong enough.
    the invisible man........

  16. #16
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    Default Re: Warping in Sheet Phosphor Bronze

    Small corner caps will be TIGed to connect the inner and outer angles. The finish will be an 320 then 600 orbital. I’ll check out your thread on Ghost tomorrow Adrian! Should give you a tingle soon! JAH!
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  17. #17
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    Default Re: Warping in Sheet Phosphor Bronze

    Just had a squiz and couldn’t find it Adrian. Could you please post clicky linky link here? Cheers Duncanstein!
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    Default Re: Warping in Sheet Phosphor Bronze

    Geez. Who designed this?

    Peace,
    Helpy Helperton

  19. #19
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    Default Re: Warping in Sheet Phosphor Bronze

    Quote Originally Posted by Duncan Gibbs View Post
    Just had a squiz and couldnít find it Adrian. Could you please post clicky linky link here? Cheers Duncanstein!
    This is the way to squiz the forum: site:forum.woodenboat.com <key words>
    This will drill down straight to the thread: site:forum.woodenboat.com Resurrection of Ghost

    So bottom line, you might be able to straighten the existing pieces by heating to 250C on a fixture. If you try again, you need a larger radius if not annealed. You seem to have a tooling problem with the apparent coining but the fabricator should be able to give you better advice than I can.

    My heat treatment volume of the metals handbook is buried somewhere in the garage. The 1942 single volume says that heating and cooling rates are relatively unimportant. The only alloys that need quenching are the precipitation hardening ones that we don't use here. The points made in the book to remember are that the metal needs to be heated and cooled uniformly (as Pete said#13). All of the phosphor bronze alloys are annealed between 482 and 677įC (900 and 1250įF). The annealing temperature should be kept at the low side of the range to avoid grain growth, which makes for a rougher surface on the bend. The high end of the range is for mill processing early in the breakdown from ingot to sheet. Stress relief at 200-250C will help with straightening and the (unlikely in bronze) corrosion cracking problems that the residual stress in the bend may cause.
    Too much and too little specific information about 5% tin bronze. C519 is 6% Sn but the difference between 5 and 8% is not that big. Who said the exact aloy? Well, it's usually more important.

    Going straight from 320 to 600 might take a lot more effort than 320 to 400 to 600. This is for wood, but the principal is the same.
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  20. #20
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    Default Re: Warping in Sheet Phosphor Bronze

    Phosphor Bronze used to be avaiable in soft, half hard and hard. I have yet to find a current supplier who stocks it in anything other than hard. I have found that in shop methods of annealing it it to be less than good! However, once heated it should be left to cool slowly as quenching will not help. Better to leave the softening to a heat treating expert.
    Jay

  21. #21
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    Default Re: Warping in Sheet Phosphor Bronze

    Duncan, and with respect to the other contributors, this is why I don't offer any solutions to problems any more. I have worked PB a lot and I don't like it much but it's seems to be all you can get here in Oz. All the diagrams, charts, crystal growth.......... blah blah. I know what works and I know what doesn't. My rigging is probably more stressed than a pool fence (but still extremely important). Plus I don't like abusing metal. Where the science is important is in large industrial applications, what we have here is sheet metal work. The Ghost thread is at top of Building Repair now.
    the invisible man........

  22. #22
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    Default Re: Warping in Sheet Phosphor Bronze

    If you want to save the existing pieces, you need to move a bit of material. I would suggest finding and talking to someone with an English Wheel. They might be able to make the subtle corrections you require.

    Alan
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