Metallurgy learnin's and other questions- Mtbr.com
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  1. #1
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    Metallurgy learnin's and other questions

    Hello,
    With a lot of help from the knowledge dispensed on this forum, I just finished my first hardtail frame and I have a few questions regarding the materials I'm working with. My buddy who's got a background in materials asked me if I was planning on heat treating and it made me realize how little I know about the chemistry of welding alloy steel.
    Metallurgy learnin's and other questions-part_1526266278395.jpg
    In my frame I used:
    -House-brand Nova Cycles tubes
    -Basic normalized 4130 tubes
    -Vari-wall Thermlx down tube (only 850mm tube I could find)
    -er312 filler rod

    Here's the bike cad drawing if you're curious.

    So here are some questions for you:
    1) Is bad to mix and match alloys like that?
    2) Is there any benefit to having a frame like this heat treated? Is that something that can even be done affordably in a one-off scenario?
    3) How does a lay person go about learning their metallurgy? Is there a good book you can recommend? I've read Carroll Smith's metallurgy chapters in his books, which are pretty cool, but don't really get into details about steel alloys.
    4) Not having tube-bending capability at this point, I offset the seat tube 10mm forward on the BB shell for tire clearance. There's about 5mm of seat tube hanging in front of the shell. Is this going to be a problem for crack-starting issues at the edge of the unsupported tube? I didn't get a picture of it before I welded the downtube on, but here's a picture of the cluster so you can see what I'm trying to describe.
    Metallurgy learnin's and other questions-018.jpg

    unrelated question:
    4) Is there a trick to using the advanced search function, or am I just being a techno-ignoramus? I haven't figured out how to limit my search results to a specific forum, nor even how to learn about such things.
    Metallurgy learnin's and other questions-019.jpg

    Thanks for the generosity of knowledge. It's great. I'm new to posting on forums, so please let me know if I breach etiquitte [sic] or anything. I've read the FAQ.

    Pleased to make your acquaintance,
    Trevor

  2. #2
    Location: 10 ft from Hell Moderator
    Reputation: life behind bars's Avatar
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    Searching here is less than ideal, google makes it easier, search term/mtbr.
    "These things are very fancy commuter bikes or really bad dirt bikes, but they are not mountain bikes." - J. Mac

  3. #3
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    Reputation: Eric Malcolm's Avatar
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    Having constructed a bike is a very satisfying experience and I hope you enjoy the ride.

    You must be a somewhat tall person because that is one very stretched out bike!!

    But to address your main concern re: heat treatment, this is not necessary. You only do heat treatment on 6061 type aluminium alloys as the weld heat reduces tube strength and you need to re-establish this strength. Steel does not behave like this, (though there is a small loss, it is not enough to get concerned about).

    Eric
    If I don't make an attempt, how will I know if it will work?

  4. #4
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    1) Mix'n'matching steel is fine. You can even weld stainless to 4130.
    2) Like Eric said, no need to heat treat steel.
    3) I don't have a great place to start. I audited a metallurgy of welding course in grad school, but that's far from being a normal situation. From my little experience, commonly-used bicycle steels are more esoteric than stainless when it comes to academic knowledge. Start with google? I'll add that I'm not sure there's all that much you need to know that's going to be terribly helpful in making bikes. (Beyond basic stuff like you can't braze stainless with brass.)
    4) My guess is the seat tube will be fine, but I guess time will tell. FWIW, if I were doing that, I'd probably fully join the DT to the BB shell and miter the ST to the DT (though my MO is to fully join the ST to the BB and miter the DT to the ST). But it's probably not a big difference either way. Just keep an eye on it if you're worried.
    4)

  5. #5
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    First of all, great looking frame! Nice work.

    Steel frames made of 4130 or the related air-hardening steels (such as ThermLX) aren't usually heat treated after being welded into a bike frame. To heat treat these steels requires bringing them to really high temperatures and then quenching. When quenching the tubes, it's very important to make them go into the quenchant straight, because they'll warp if they go in sideways. So you can imagine what would happen to a finished frame if you tried to heat treat it.

    Usually steel tubes are available as cold drawn (industry term for not having been heat treated after drawing) or heat treated, and you build with them in that state. For Vari-wall tubes, the prefixes TXD and CMD refer to cold-drawn tubes and TXH and CMH refer to heat treated tubes. Mixing and matching different steels isn't a problem, as far as I know. They all have the same modulus of elasticity since they're all steel alloys, and so should flex similarly.

    I wish I knew more about the specifics of different filler rod on the hardness of the weld zone, and how that affects the fatigue life of joints. Specifically, I'd like to know if fusion welds in 4130 are super hard and to be avoided or not. I have a carbide lathe insert that lost a fight with some fusion welded 4130, and it's made me wary of that ever since.

    Aluminum however is a different animal. 6061 Alu becomes annealed at welding temperatures, and must be heat treated as a complete unit to reach its full strength. From what I understand, bike companies will even build special jigs that hold the frames straight to minimize warping during quenching (since 6xxx aluminums also need to be quenched for heat treatment). This is why repairing aluminum frames is so tricky, because the act of welding them weakens the zone you're trying to repair, and re-heat treating them is basically not an option.

    So you can search a specific site from google using "site:" without the quotes. So to search the framebuilding forum, I go to google and put in:
    site:forums.mtbr.com/frame-building/
    then a space, then whatever words you're searching for. It works really well, about a million times better than this site's search function.
    Myth Cycles handbuilt bike frames
    Durango, CO
    http://www.mythcycles.com

  6. #6
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    I had a steel frame that was heat treated after welding- a 2010? NS society. I never broke it, which is a credit to it. My goober understanding is that with MTB frames if you have a heavy enough tube for impact resistance you have plenty of tensile strength so it's not really that valuable to heat treat. Or something like that.

    I've done 2 frames with the seat tube offset from the BB like that. I offset them by ~8mm so that i can bend the end of the tube so it fully contacts the BB, but since that zone is supported by the DT it's hard to believe there are large stresses there.

    That bike looks like a beast! I'd love to ride something like that, but not enough to pay for the experiment.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  7. #7
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    Wow thanks for all the good info!

    Eric: Yes, I'm really tall (or long-boned as I prefer to say) 203cm.

    Erichimedes: That answers a lot of my questions! Thanks.

    Does anyone here have experience with the big time Asian production facilities? How do they efficiently notch tubes to fit the weird-shaped hydroformed tubes? Laser cutting?

    Is post-weld heat treatment used by bicycle manufacturers who do huge runs of steel frames at a time? Maybe I'm a bit obsessed with the perceived benefits of heat treatment. I've just had multiple engineer-types expound the benefits of heat treating alloy steel weldments. Particularly if you could "temper" (not sure if that's an accurate word) hard spots left by welding that could lead to fatigue failure.

  8. #8
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    Quote Originally Posted by Trevor Schweald View Post
    Is post-weld heat treatment used by bicycle manufacturers who do huge runs of steel frames at a time?
    No, I've never seen it used for steel frames.

    There are some applications where it's done, usually with a oxy-acetylene torch. I'd look at home-built aircraft and the frames for things like Baja racers.

  9. #9
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    I think you're being wound up by theorists frankly. None of the bike people I know do this heat treatment thing because its totally unneccessary with a steel frame. The industry and real life attests to the fact that bikes go the distance without heat treatment.

    I have bike frames still running around that are over 35yrs old and they were never heat treated.

    Eric
    If I don't make an attempt, how will I know if it will work?

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