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Thread: Thinwall 4130 brazing, some questions

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    Thinwall 4130 brazing, some questions

    Hello,
    I am preparing to build a structure for which I am fillet brazing 4130 steel with wall thicknesses of 0.028" to 0.035" Today I was doing some practice work with scrap, and have the following questions.
    The structure will employ joints quite similar to fillet brazed bicycle frames of yesteryear, but is not a bike. Strength to weight, i.e., quality workmanship is rather important in this one.

    1) I am using a Turbo Torch (air) Acetylene #8A, which claims to move 8cu-ft per hr, it seems that the flame is too wide, yet appropriately hot. It's just unconcentrated for my work, any opinions if the #5A would be better, or do you have any mini-torch recommendations.
    2) Is there a mandate to clean,sand, and flux the inside of tubes as well as outside? Any recommendations for this if so?
    3) The tubes were sanded with 120grit sandpaper, I think I could have done better, any thoughts from the pic?

    I feel the first braze burnt off the flux early, and made a mess, #2 (in pic) was a lot better employing capillary action and less dirty. I feel that this forum is likely to yield some wisdom to point me in the right direction

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    Last edited by Insect; 04-19-2021 at 11:06 PM.

  2. #2
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    Re: Thinwall 4130 brazing, some questions

    If you have access to a oxy. acetylene torch I think you would be much happier with the results of using a small welding/brazing tip. More concentrated heat source over a smaller area.

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    Re: Thinwall 4130 brazing, some questions

    You can test them. Saw them apart to see in capillary action is occuring. Not real sure what you are trying to accomplish. Do you want the material to be ground away? Or are you going to build it up and profile it. How much strength do you need? What filler are you using?

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    Re: Thinwall 4130 brazing, some questions

    Thanks for the replies.

    The goal is a solid bond, I will groom the joints slightly before paint, but not as smoothly as a premium bicycle, etc, not needed for this. Strength should exceed the 97000psi that 4130 delivers, thru a relatively thick fillet.
    I am also considering TIG brazing, or TIg welding for some sections, brass brazing doesnt have to be the only choice.

    The structure will have 50+ joints, and with my experience level I rather not have defects accumulating that I cannot fix. This is a different methodology to a simple object that has fewer joints which can be discarded if it gets off to a bad start, thus why I am favoring brazing (and refining techniques) rather than just TIG the whole thing.

    I have stick welding and some brazing experience, not new to me, but I certainly am an amateur.


    I ordered a 3A torch tip, to replace this 8A, after calling the manuf and confirming that the 8A is a pretty wide one for what I am doing. Oxy-acetelene is an option, but I rather keep my shop simple... too many disciplines under one person and in one shop leads to disorder, so I try to explore simple paths when I can get away with it.

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    Re: Thinwall 4130 brazing, some questions

    Brazing a butt joint will never achieve a joint strength that you are specifying.

    Your maximum strength can be calculated as some percentage (50%,, 70% ,,??) of the tensile strength of the filler metal.

    If you want to braze, and achieve the strength of the steel, you will need to use some sort of overlapping joint, so that the filler metal will be in shear, rather than tension.
    The overlap joint will be a minimum length of several times the material thickness.

    Who cares how far the filler metal goes by capillary action,, you are gonna paint it. The filler metal is so thin, it will never be seen.

    So, either redesign for overlapping joints if you want to stay with brazing, or plan on full penetration TIG welding.

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    Re: Thinwall 4130 brazing, some questions

    Not gonna address the strength issue but if you want cleaner more controlled joints you need more flux. Paint it all over at least an inch on either side of the joint. Harris white flux and silver solder works great and they also have a black flux that is more tolerant of overheating. Slowly heat the joint until the flux melts and flows and becomes clear then start dabbing the filler.
    Lookup Mike Zancanato. He makes custom bikes and his braze joints are amazing.
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    Re: Thinwall 4130 brazing, some questions

    So what it the silver alloy you are using? That blend makes a huge difference in flow.
    Weld like a "WELDOR", not a wel-"DERR"
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    Re: Thinwall 4130 brazing, some questions

    Thanks for the replies, and the vid, definitely a lot of flux being used there!

    My brazing rod is 58/41/1% of Cu/Zn/Tin respectively, in 1.16" thick

    I have practiced a bit more, with the #8 and #3 turbo torch.
    It seems the turbotorch can deliver heat in a broader area than needed, it's not ideal, I may try silver rod and see how it goes.

    I seem to lack the ability to make deep fillets with this, it runs around, as the torch is on there a while to heat with the broad flame.

    My questions:
    1) Is it common practise to sand/clean/flux the inside of tubes when fillet brazing?
    2) How about inside tube cleaning for tig welding?

    I dont plan on any butt joints brazed, no strength as indicated, but tube intersections with all 4 corners in a state of intersection, yes, that is planned, unless I go tig on the whole project.

    Thanks!

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    Re: Thinwall 4130 brazing, some questions

    It’s common practice to clean/sand/flux anywhere you want
    the filler material to go. That joint type may not be the best
    application for brazing, but if that’s how you decide to do it ,
    and strength is critical, then I’d be cleaning and fluxing the
    inside so filler flows inside and fills any gaps where a tear
    could start.
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    Re: Thinwall 4130 brazing, some questions

    Quote Originally Posted by Insect View Post
    Strength should exceed the 97000psi that 4130 delivers
    I don't think 97ksi is achievable through brazing or silver soldering. I'm not even sure it's achievable through welding, at least not in my shop!

    What are you making that requires such strength? (And isn't the UTS of 4130 about 80ksi, not 97ksi -- and yield strength almost 20% less than that?)
    Last edited by Kelvin; 04-24-2021 at 09:48 AM.

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    Re: Thinwall 4130 brazing, some questions

    Quote Originally Posted by Kelvin View Post
    I don't think 97ksi is achievable through brazing or silver soldering. I'm not even sure it's achievable through welding, at least not in my shop!

    What are you making that requires such strength? (And isn't the UTS of 4130 about 80ksi, not 97ksi -- and yield strength almost 20% less than that?)

    Yea, if the fillet is 4x the width of the predominantly 0.035" tubing, then at 0.14" of fillet width it is achieving more UTS/Yield strength than the 50k spec of the filler rod... in theory.
    It is an ultralight aircraft fuselage. My interest is in optimizing strength/weight, I don't really need extraordinary strength. There is a relationship between the process chosen, and the predictability of results / operator maintaining awareness of joint quality as the process goes on, which may point me to tig or brazing. I come from cycling, in the 90's we had a lot of fillet brazed frames that worked, so it intrigues me, normal people would just tig weld it and be done with it.

    I am open to, and will try next week, tig welding it.

    My ideas on why brazed joints were preferred initially by me:
    I'm dealing with over 50x joints on one structure, if some get cobbly welds, I feel that the fix is not always easy, with fillet brazing it's a little easier to clean it up and remelt. I had an intuition that for thinwall sections, one manages potential corrosion, and stress risers better with a soft metal fillet than with a tig weld.

    If you have access to a oxy. acetylene torch I think you would be much happier with the results of using a small welding/brazing tip. More concentrated heat source over a smaller area.

    It's becoming noticeable now, the lack of heat concentration. I don't want oxygen just yet, tempted to try a medical oxygen generator, but also not trying to reinvent the wheel.


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    Re: Thinwall 4130 brazing, some questions

    Working with that material, my first choice would be TIG brazing. Second choice would be oxy-acetylene brazing with a small tip and lots of flux.

    Years ago I did some work on Formula Ford race cars that were made by March in the UK. The entire frames were brazed together and the welding was gorgeous. I think that they used O/A torches too.
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    Re: Thinwall 4130 brazing, some questions

    Bicycle frames being mostly round tubing lend themselves to fillet welding moreso than square. With the square you have one fillet and 3 butts or grooves. 50 plus joints is not many. Limited access, part positioning and body position might need practice too.

    How will brazing lessen the work of 50 plus joints vs tig weld? Is brazing material lighter than tig filler?

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    Re: Thinwall 4130 brazing, some questions

    I would weld this up with oxy acetylene.
    Honestly with the wealth of info on the internet now its getting to where we spend more time researching how to do a project then actually doing it.
    Don't make yourself crazy looking for an aircraft torch, a victor torch with a small tip will work fine.
    If you look at pictures of the women welding airframe during ww2 they are using some big looking torch.
    Years ago homebuilts were welder with gas and old coat hangers.
    A guy by the handle of little scraper on honebuilt airplanes has put together some good videos on welding aircraft tubing.

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    Re: Thinwall 4130 brazing, some questions

    I did a job years ago that the customer wanted fillet brazing for the joints, this was purely a cosmetic choice on a nonstructural assembly. Took a lot of practice to get it looking good. We would first braze the joint together with a small brazed joint, relying on the capillary action to make the joint. Then we went back over the joint heating small spots quickly with an O/A torch and dabbing filler rod into the spot. This was a slow and tedious process but did produce a fillet bead that was higher then the base material. This was very difficult to do in position and we moved and rotated the frames to do everything in a flat and level position. I am not involved in the aircraft industry at all but do not think I am flying in a craft with brazed 4130 square tubing for the structure. I would think round 4130 tig welded with the proper procedures and skill set would be my choice. Just my cents worth.

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