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Thread: Welding chomoly tubing

  1. #11
    Aaron Novak's Avatar
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    No insult intended:
    For future reference, when I use the term "cowboy" , I am using in in the context of vernacular common to people working in engineering fields.

    Definition: People or Persons who defy common practice, engineering data and historical data when designing or fabricating something, due to sheer laziness or some strange desire to fight a perceived "establishment". Typically persons in this category have an inability to fill out paperwork, and suffer from extreme cases of selective hearing ( especially in the presence of field experts ). The decisions made by such persons mentioned above as "cowboys" are referred to as being "shot from the hip".

    No implied connection is made to the above mentioned "cowboys" of the engineering vernacular set, and of persons who live, or have lived in what is generally thought of as the "western" half of the USA from the middle 1800's to the 1960's, working cattle, getting saddle sores and allowing John Wayne to earn a sizeable fortune.

  2. #12
    prasmussen's Avatar
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    Nice man. Gotta love him. He built on his full-scale, slightly banana-shaped warbird replica for at least 14 years. Everyone stood around hoping for the best but frankly expecting disaster when he taxied it for the first time. We all agreed it must be wind shear that caused it to crab sideways down the taxiway. Lost track of him. Really hope all turned out well. But that brings up a third category beside the cheap and the cowboys: the enthusiastically inept.


    Although the chances of those intrepid aviators hurting anyone but themselves (and I sincerely certainly hope not) is slight, your point about over-flight is well taken. The real danger, in my opinion though, is that if the builder/pilot community does not educate/advise and prevent the somewhat predictable results from these mostly well-meaning groups of fellow aviators, the regulatory structure of homebuilding will become unmanageable for everyone.


    That's away from welding tubing though......


    Here's what I have been thinking:



    • Charts I've gotten from the forums/workshops at AirVenture indicate some new brazing alloys are as “strong” as welds.
    • If heat is the enemy of steel, wouldn't it be better to look in the direction of the lowest temperature process?

    • Maybe lower temperature oxy-hydrogen welding could be used to produce cleaner welds and shops (who wears a mask after all?).
    • I've worked with silicone bronze enough to know that alloy can be bent many more times than steel before cracking.
    • If the Eurozone uses brazing for a quick fix on racing cars, wouldn't it make easier relocating parts as I “refine” my aircraft design?



    Problem is my ignorance of a way to test my joints to see if the idea is really worth pursuing.



    Q: What is the tensile strength of brazed joints
    A: It all depends
    No manufacturer lists the tensile strength of their brazing alloys. This is not to make life difficult for the ultimate consumer. It's because people tend to place to much emphasis on any number that might be published. Design engineers sometimes base designs on a number that's not appropriate for the ultimate use.

    In fact, the strength of a brazed joint depends more on the design and the brazing procedure then on the filler metal used.
    Furthermore, tensile strength numbers apply to material in the wrought state. When the filler metal is used in brazing, it is effectively recast. Recast metal has different properties from the wrought metal.
    Empirical testing of various brazed joints has shown that the PSI of the alloy does not correlate directly to the strength of the tested joint. We know some of the factors that influence this process. For example, if the alloy is overheated, the lower melting elements are burned off to a higher degree. This effectively changes the composition of the deposited metal. Thus our advice is to encourage customers to do their own testing of the brazed joint.

    But there are some rules of thumb:
    If customers insist on a certain PSI number, we suggest a number ranging from 60,000-70,000 PSI when tested in the wrought state.


    Another rough guideline is that joints properly brazed with silver alloys have a shear strength that exceeds three times the shear strength of the thinner, joined metal.



    Any thoughts? BTW no offense taken.
    The journey is the reward.

  3. #13
    Mike Switzer's Avatar
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    Quote Originally Posted by prasmussen View Post
    I've worked with silicone bronze enough to know that alloy can be bent many more times than steel before cracking.
    Sort of correct, but... For steel to bend you must exceed the yield strength. If you do not exceed the yield strength it will not bend, and therefore will not fatigue. Engineers design steel structures so that the structural members and the welds will not exceed the yield strength, which for the 4130 currently available for homebuilding is 70 - 75,000 psi depending on size & wall thickness. The yield strength of the hardest commonly available hard commercial bronze is approx 55,000 psi.

    A properly designed steel structure with properly designed welds will not bend at all and should last forever.

  4. #14
    Aaron Novak's Avatar
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    I guess I dont see what is to gain by brazing your structure? You cant improve upon something that does not fail, and there would be no weight savings or fatigue life gain. Is ability an issue? Because I will tell you this, if you cant weld, you cant braze. Oxyhydrogen is unsuitable for steel welding. Who says heat is the enemy of steel? If you want to design a brazed structure ( lug or fillet ), and develop the process, do the research and testing, then by all mean go for it. But for all the work involved....Im struggling to see the gain????

  5. #15
    prasmussen's Avatar
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    No, I chose to gas weld my SA-3A which is on the gear. Just wondering about stuff. Guess the gain is that we have given candgp something to think about.

    These forums are such a great idea!
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    Last edited by prasmussen; 10-25-2012 at 10:15 AM. Reason: Wrong picture, sorry
    The journey is the reward.

  6. #16

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    candgp

    Quote Originally Posted by prasmussen View Post
    No, I chose to gas weld my SA-3A which is on the gear. Just wondering about stuff. Guess the gain is that we have given candgp something to think about.

    These forums are such a great idea!
    Thanks to all who have replied to my question, it has been helpful and informative.

  7. #17

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    Quote Originally Posted by candgp View Post
    Thanks to all who have replied to my question, it has been helpful and informative.
    I've seen 4130 that's been brazed and it looks ugly. It cracks on the underside. Go try it on some plate and see what happens. Mild steel can be brazed without any problems, but I would caution you against doing it on 4130.

  8. #18
    prasmussen's Avatar
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    Interesting. Any idea what's happening chemically to the metal that causes it to crack?
    The journey is the reward.

  9. #19

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    Not sure, I was going to replace some mild steel pars with 4130. They had brazed fittings, so I brazed the new ones and got ugly cracks. I also saw some samples from someone else that does welded repairs all the time. Same type cracks with his samples..

  10. #20
    prasmussen's Avatar
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    Cracks on the back side only? I have gotten the steel too hot and had it try to do a half braze and half weld where I can see it on the metal surface. That is truly ugly. Don't know what the alloy of brass and steel is called but it is a disaster when it happens because it is brittle junk and has to be ground down to bare steel before starting over. Can't weld it, can't braze it, just have to play it again Sam. Anyone a metalurgist?
    The journey is the reward.

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